Systems and methods for improving upconversion

Abstract

Systems and methods for upconverting near-infrared wavelengths to higher-energy wavelengths are provided. Various material combinations can be implemented to upconvert the near-infrared to higher-energy wavelengths including visible wavelengths via TTA-UC. The TTA-UC systems can be further combined with plasmonic structures and / or thin films to enhance absorption and emission.

Description

Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCTSYSTEMS AND METHODS FOR IMPROVING UPCONVERSIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 693,628, titled “Systems and Methods for Improving Upconversion”, filed September 11 , 2024, which is hereby incorporated by reference in its entirety.STATEMENT OF FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under Grant No. HR00112220010, awarded by the Defense Advanced Research Projects Agency. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The disclosure is generally directed to systems and methods for optimizing upconversion efficiencies of the process converting light at near-infrared wavelengths to higher-energy wavelengths including (but not limited to) visible wavelengths.BACKGROUND

[0004] Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at a shorter wavelength. Various organic and inorganic materials can perform upconversion through various mechanisms. Organic molecules can achieve photon upconversion through triplet-triplet annihilation upconversion (TTA-UC), which is an energy transfer mechanism between two molecules in their triplet state. To achieve photon upconversion through triplet-triplet annihilation, two types of species are often combined: a sensitizer and an emitter (annihilator). The sensitizer absorbs the low energy photon and populates its first excited triplet state (T 1 ) through intersystem crossing. The sensitizer then transfers the excitation energy to the emitter, resulting in a triplet excited emitter and a ground state sensitizer. Two tripletStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT excited emitters then can undergo triplet-triplet annihilation, and if a singlet excited state (S1 ) of the emitter is populated fluorescence results in an upconverted photon.SUMMARY

[0005] Several embodiments are directed to systems and methods optimizing upconversion efficiencies of near-infrared to higher-energy wavelengths including visible wavelengths. In many embodiments, the TTA-UC systems use various sensitizer and annihilator pairs to upconvert near-infrared wavelengths to higher-energy wavelengths including visible wavelengths. Several embodiments attach ligands to the sensitizers to improve the upconversion efficiencies. Various sensitizer and annihilator pairs can be incorporated in bulk heterojunction (BHJ) structures. In various embodiments, the BHJ structures are preferred to the bilayer structures. In some embodiments, the TTA-UC systems can incorporate various resonant structures and / or dichroic backreflectors with thin film coatings to enhance and optimize absorption and emissive properties. In some embodiments, the resonant structures can include plasmonic structures or dielectric resonators (also referred as Mie-resonant structures). The upconversion systems in accordance with many embodiments can achieve passive upconversion without external power or electronics. The upconversion systems can improve upconversion efficiencies in various wavelength ranges. In many embodiments, the upconversion systems can be integrated into various devices and / or systems such as (but not limited to) imaging systems, vision systems, photodetectors, cameras, CCD cameras, night vision systems, photovoltaics, anti-counterfeit systems, vein imaging systems, drug delivery systems, and / or optogenetics.

[0006] Some embodiments include a triplet-triplet annihilation photon upconversion (TTA-UC) system, comprising a sensitizer, an annihilator, and an emitter; wherein the sensitizer, the annihilator, and the emitter are mixed to form a bulk heterojunction structure; and wherein the TTA-UC system upconverts a near-infrared wavelength to a higher-energy wavelength.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0007] In some embodiments, the sensitizer is selected from the group consisting of: PbS quantum dots, HgTe quantum dots, Ag2Se quantum dots, AgAuSe quantum dots, MoTe2 monolayers, near-infrared absorbing non-fullerene acceptors; wherein the annihilator is selected from the group consisting of: TES-ADT and di-fluorinated derivative of TES-ADT; wherein the emitter is selected from the group consisting of: DBP, BTZ- DMAC, IR-786, TXO-TPA, HIDC iodide, K1295, CdSe quantum dots, and perovskite nanocrystals.

[0008] In some embodiments, the sensitizer are quantum dots comprising an organic ligand.

[0009] In some embodiments, the organic ligand is 5-tetracene carboxylic acid.

[0010] In some embodiments, the sensitizer is PbS quantum dots with 5-tetracene carboxylic acid ligands, the annihilator is TES-ADT, and the emitter is DBP.

[0011] In some embodiments, the near-infrared wavelength is greater than or equal to 800 nm.

[0012] In some embodiments, the near-infrared wavelength is greater than or equal to 1000 nm.

[0013] In some embodiments, the near-infrared wavelength is greater than or equal to 1200 nm.

[0014] In some embodiments, the higher-energy wavelength is a visible wavelength.

[0015] In some embodiments, the near-infrared wavelength is greater than or equal to 800 nm, and the visible wavelength is greater than or equal to 600 nm.

[0016] Some embodiments include an upconversion system, comprising a plurality of resonant structures, and a TTA-UC system; wherein the plurality of resonant structures are embedded in the TTA-UC system; wherein the TTA-UC system is deposited on a substrate; wherein the upconversion system upconverts a near-infrared wavelength to a visible wavelength.

[0017] In some embodiments, the substrate is transparent to an incident near-infrared wavelength.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0018] In some embodiments, the substrate comprises a material selected from the group consisting of: silicon, glass, NBK7 glass, quartz, and sapphire.

[0019] Some embodiments further comprise an electrically conductive layer between the substrate and the plurality of resonant structures.

[0020] In some embodiments, the electrically conductive layer comprises ITO and has a thickness greater than or equal to 100 nm and less than or equal to 300 nm.

[0021] Some embodiments further comprise a low loss dielectric layer between the substrate and the plurality of resonant structures.

[0022] In some embodiments, the low loss dielectric layer comprises silicon oxide and has a thickness greater than or equal to 500 nm and less than or equal to 5 microns.

[0023] In some embodiments, at least one of the plurality of resonant structures has a shape selected from the group consisting of: a cuboid, a cube, a pillar, a nanopillar, a circular pillar, a square pillar, a rectangular pillar, a rod, a nanorod, a plus shaped pillar, a bowtie pillar, a cylinder, an elliptical cylinder, a trapezoid, a triangular prism, a polygonal prism, a pyramid, a sphere, and a nanosphere.

[0024] In some embodiments, the plurality of resonant structures comprise a material selected from the group consisting of: a metal, a metal oxide, gold, silver, aluminum, a semiconductor, silicon, germanium, gallium phosphide, gallium arsenide, silicon carbide, titanium oxide, silicon nitride, barium titanate, lithium niobate, tantalum pentoxide, a nanoparticle, a gold nanoparticle, a silver nanoparticle, and an aluminum nanoparticle.

[0025] In some embodiments, the plurality of resonant structures comprise a metal, wherein the upconversion system further comprises a passivation layer on the plurality of resonant structures.

[0026] In some embodiments, the passivation layer comprises a material selected from the group consisting of: an oxide, silicon oxide, aluminum oxide, and hafnium oxide.

[0027] In some embodiments, the TTA-LIC system is the TTA-UC system described above.

[0028] Some embodiments include an upconversion system, comprising a plurality of thin films; and a TTA-UC system; wherein a first side of the plurality of thin films is inStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT contact with a substrate; wherein a second side of the plurality of thin films is in contact with the TTA-UC system; and wherein the system upconverts a near-infrared wavelength to a higher-energy wavelength.

[0029] Some embodiments further comprise a second plurality of thin films deposited between the TTA-UC system and a second substrate.

[0030] In some embodiments, the plurality of thin films comprise alternating layer of a high index material and a low index material.

[0031] In some embodiments, a thickness of each of the plurality of thin films is greater than or equal to 10 nm and less than or equal to 1000 nm.

[0032] In some embodiments, the high index material is titanium oxide or niobium oxide, and the low index material is silicon oxide.

[0033] In some embodiments, the substrate is transparent to an incident near-infrared wavelength.

[0034] In some embodiments, the substrate comprises a material selected from the group consisting of: silicon, glass, NBK7 glass, quartz, and sapphire, wherein the substrate has a thickness greater than or equal to 1 mm and less than or equal to 10 mm.

[0035] In some embodiments, a plurality of plasmonic structures are embedded in the TTA-UC system, wherein the plurality of resonant structures are deposited on a second substrate.

[0036] In some embodiments, at least one of the plurality of resonant structures has a shape selected from the group consisting of: a cuboid, a cube, a pillar, a nanopillar, a circular pillar, a square pillar, a rectangular pillar, a rod, a nanorod, a plus shaped pillar, a bowtie pillar, a cylinder, an elliptical cylinder, a trapezoid, a triangular prism, a polygonal prism, a pyramid, a sphere, and a nanosphere.

[0037] In some embodiments, the plurality of resonant structures comprise a material selected from the group consisting of: a metal, a metal oxide, gold, silver, aluminum, a semiconductor, silicon, germanium, gallium phosphide, gallium arsenide, silicon carbide, titanium oxide, silicon nitride, barium titanate, lithium niobate, tantalum pentoxide, a nanoparticle, a gold nanoparticle, a silver nanoparticle, and an aluminum nanoparticle.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0038] In some embodiments, the plurality of resonant structures comprise a metal, a metal oxide, gold, silver, or aluminum; wherein the system further comprises a passivation layer comprising a material selected from the group consisting of: an oxide, silicon oxide, aluminum oxide, and hafnium oxide.

[0039] In some embodiments, the upconversion system comprises the TTA-UC system as described above.

[0040] Some embodiments include an optical system, comprising a vision system and an upconversion system as described above.

[0041] Some embodiments include an optical system, comprising a CCD array and an upconversion system as described deposited on top of the CCD array.

[0042] Some embodiments include an optical system, comprising a CMOS array and an upconversion system as described deposited on top of the CMOS array.

[0043] Some embodiments include an optical system, comprising an upconversion system as described and photovoltaic cell adjacent to the upconversion system.

[0044] Some embodiments include a system, comprising an upconversion system as described embedded in a platform for anti-counterfeiting; wherein the platform is selected from the group consisting of: a currency paper, a document, and a computer chip.

[0045] Some embodiments include a drug delivery system, comprising an upconversion system as described positioned adjacent to a tissue.BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The description and claims will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention.

[0047] Figure 1A provides an example schematic of upconversion utilizing the TTA- UC mechanism.

[0048] Figure 1 B provides an example schematic of material components of a TTA- UC system.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0049] Figure 2 provides an example schematic of a bulk heterojunction structure.

[0050] Figure 3A provides an example of plasmonic structures comprising metal circular pillars.

[0051] Figure 3B provides an example of plasmonic structures comprising square pillars.

[0052] Figure 3C provides an example of plasmonic structures comprising rectangular pillars or rods.

[0053] Figure 3D provides an example of narrowband tunable absorption of rectangular pillars.

[0054] Figure 3E provides an example of plasmonic structures comprising plus shaped pillars.

[0055] Figure 3F provides an example of plasmonic structures comprising bowtie pillars.

[0056] Figure 3G provides an example of plasmonic structures comprising bowtie pillars.

[0057] Figure 4A provides an example of plasmonic structures comprising semiconductor circular pillars.

[0058] Figure 4B provides an example of plasmonic structures comprising semiconductor circular pillars.

[0059] Figure 4C provides an example of plasmonic structures comprising semiconductor circular pillars.

[0060] Figure 5 provides an example of plasmonic structures comprising metal nanoparticles.

[0061] Figure 6 provides an example of plasmonic structures arranged in alternating arrays.

[0062] Figure 7A provides an example of plasmonic structures arranged as spatially separated pixels.

[0063] Figure 7B provides an example of metasurface comprising plasmonic structures.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0064] Figure 8 provides an example of thin film coatings integrated with upconverting systems.

[0065] Figure 9A provides an example of alternating H and L materials as thin film coatings.

[0066] Figure 9B provides an example of reflectivity design of the alternating thin film coatings for a TTA-LIC system.

[0067] Figure 9C provides an example of reflectivity design of the alternating thin film coatings for a TTA-UC system.

[0068] Figure 10 provides an example of integrating the thin film coatings on both sides of the upconverting systems.

[0069] Figure 11 provides an example of integrating the thin film coatings and the plasmonic resonators with the upconverting systems.

[0070] Figure 12 provides an example of integrating the upconversion system with a vision system.

[0071] Figure 13 provides an example of integrating the upconversion system with a CCD array.

[0072] Figure 14 provides an example of integrating the upconverting system with photovoltaics.

[0073] Figure 15 provides an example of integrating the upconverting system in anticounterfeit.

[0074] Figure 16 provides an example of applying the upconverting system for bioimaging.

[0075] Figures 17A through 17G provide examples of NIR to visible upconversion imaging using a bulk heterojunction thin film.

[0076] Figures 18A through 18G provide examples of using nanophotonic structures to enhance upconversion efficiency.

[0077] Figures 19A through 19D provide examples of measured power enhancements and imaging preservation.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0078] Figures 20A through 20G provide examples of power and imaging results through integrating gold pillars with backreflector to the bulk heterojunction thin film.

[0079] Figures 21 A through 21 C provide examples of light extraction enhancement simulations.

[0080] Figures 22A through 22D provide examples of backreflector resolution loss simulations.

[0081] Figures 23A and 23B provide examples of gold nanopillars FDTD simulations.

[0082] Figures 24A through 24C provide examples of Zemax simulations.

[0083] Figure 25 provides an example of the effect of passivation.

[0084] Figure 26 provides an example of power measurement using control sample for fully integrated upconverter.

[0085] Figure 27 provides an example of the log-log plots of the input NIR power vs output visible power within and outside gold region.DETAILED DESCRIPTION

[0086] Turning now to the drawings and data, systems and methods for upconversion of near-infrared light are described. In many embodiments, upconversion systems convert incoherent near-infrared light (NIR) (wavelengths from about 800 nm to about 2500 nm) into higher-energy wavelengths (wavelengths from about 380 nm to about 800 nm). In some embodiments, upconversion systems can upconvert near-infrared light without the use of external power input. In some embodiments, the absorbed near-infrared light has a wavelength from about 800 nm to about 1300 nm. In some embodiments, the emitted higher-energy wavelengths light is in the wavelength ranges from about 450 nm to about 800 nm. The upconverting systems in accordance with several embodiments can include various components to enhance NIR photon upconversion, such as (for example) TTA- UC system, resonant structures, plasmonic structures, dielectric resonators, and / or dichroic backreflectors with thin film coatings. The TTA-UC systems can be enabled by triplet fusion in bulk heterojunction structures with linear power dependency. The TTA- UC systems can incorporate various combinations of sensitizers and annihilators (alsoStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT referred to as emitters) to upconvert near-infrared to higher-energy wavelengths including (but not limited to) visible ranges. In some implementations, a plurality of plasmonic nanostructures is utilized with the TTA-UC systems, which can enhance absorption of the sensitizer. In some implementations, a dichroic thin film is utilized with the TTA-UC systems, which can increase the collection of upconverted light. In some embodiments, the TTA-UC systems incorporate both plasmonic nanostructures and dichroic thin films.

[0087] In prior systems and methodologies, the materials utilized in these systems are limited for NIR upconversion due to an inability to upconvert photons with wavelengths greater than about 1100 nm and having very low efficiencies for wavelengths between 800 nm and 1100 nm. Here, the various embodiments upconversion systems of the current disclosure are able to upconvert photons greater than or equal to about 1200 nm and up to about 1250 nm or 1300 nm, with much improved efficiencies in wavelengths between 800 nm and 1100 nm.

[0088] The upconversion systems in accordance with several embodiments can have various applications such as (but not limited to) improvements to photovoltaics, night vision, anti-counterfeiting, cameras, vein imaging, drug delivery or any system that can be enhanced by converting NIR into visible light. In one illustrative example, the upconversion systems can be inserted at the intermediate image plane of a Keplerian telescope system, with near-infrared optics in the front and visible optics at the back, which preserves the relative directionality of rays between the incident near-infrared light and output visible light. The resulting system can take low intensity incoherent nearinfrared light and passively generate visible light at human-eye sensitivity.Compositions and Compounds for TTA-UC Systems

[0089] Components for performing TTA-UC comprise a sensitizer and annihilator. Conventional NIR sensitizers (such as lead sulfide (PbS) quantum dots) and paired annihilators (such as rubrene) can suffer from challenges such as low absorption and / or efficiency when upconverting NIR photons greater than or equal to about 800 nm. Furthermore, rubrene is limited by a relatively high triplet energy (about 1.14 eV), whichStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT makes rubrene unable to upconvert wavelengths greater than about 1050 nm. Previous studies have used PbS quantum dots as sensitizer with 5,11 - bis(triethylsilylethynyl)anthradithiophene (TES-ADT) as annihilator and 5,10,15,20- Tetraphenylbisbenz[5,6]indeno[1 ,2,3-cd:1 ',2',3'-lm]perylene (DBP) as emitter in bulk heterojunction structures to achieve upconversion of photons of about 980 nm. (See, e.g., N. Tripathi, et al., ACS Appl. Nano Mater. 2024, 7, 3, 2950-2955; the disclosure of which is incorporated by reference.)

[0090] In many embodiments, various TTA-UC systems utilize optimized components that upconvert NIR photons with greater efficiency, including upconverting photons greater than or equal to about 800 nm; greater than or equal to about 1200 nm; up to about 1300 nm. In some embodiments, the TTA-UC systems can include sensitizers and annihilators. In several embodiments, the TTA-UC systems can include sensitizers, annihilators, and emitters. Some embodiments include emitters when the annihilators cannot emit photons efficiently. Once the annihilator has a high energy excitation, it can transfer the energy to an emitter which is much better at emitting a photon. The annihilators are not always good at emitting because the reverse of TTA (singlet fission) is a feasible pathway for annihilators. The sensitizers can include various types of quantum dots and / or monolayers such as (but not limited to) transition metal dichalcogenide monolayers. The quantum dots can be modified with organic ligands to improve the upconversion efficiency in near-infrared wavelengths. Various types of annihilators and / or emitters can be implemented in the TTA-UC systems to enhance the near-infrared to higher-energy wavelengths upconversion efficiency.

[0091] The TTA-UC systems in accordance with many embodiments use various sensitizers. Examples of sensitizers for the TTA-UC systems include (but are not limited to) PbS quantum dots, HgTe quantum dots, silver selenide (Ag2Se) quantum dots, silver gold selenide (AgAuSe) quantum dots, MoTe2 monolayers, and non-fullerene acceptors. In some embodiments, sensitizers for use in NIR TTA-UC systems include (but are not limited to) PbS, HgTe, MoTe2, and non-fullerene acceptors. The compounds can be provided in any appropriate form, such as monolayers, quantum dots, etc. For example,Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT various systems can utilize PbS quantum dots, HgTe quantum dots, MoSe2 monolayers as sensitizers. In several embodiments, various types of quantum dots sensitizers can be modified with ligands to upconvert photons greater than or equal to about 1200 nm and / or enhance upconversion efficiencies. As can be readily appreciated, any of a variety of sensitizers can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0092] Several embodiments of TTA-UC systems utilize quantum dots modified with organic ligands as near-infrared absorbers. Organic ligands can assist in extracting the triplet energy that is absorbed by the quantum dots. Because energy transfer generally propagates downward, organic ligands with a triplet energy level lower than the bandgap of the quantum dots can be utilized to extract the triplet energy from the quantum dots. However, against that conventional wisdom, several embodiments of TTA-UC systems utilize PbS quantum dots modified with 5-tetracene carboxylic acid (TCA) ligands as nearinfrared absorbers. The triplet energy level of (TCA) is generally accepted to be about 1 .3 eV (see, e.g., X. Luo, et al., A / at. Commun. 11 , 28 (2020), the disclosure of which is hereby incorporated by reference). And the bandgap of the PbS quantum dots is on the order of about 1.1 eV. Because the triplet energy level of TCA is greater than the bandgap of PbS quantum dots, one would not expect TCA to be able to extract energy from PbS quantum dots. Based on experimentation performed, TCA can extract the triplet energy absorbed by PbS quantum dots, greatly improving TTA-UC systems that utilize PbS quantum dot sensitizers and provides proof of principle that TCA can be utilized in TTA-UC systems with sensitizer bandgaps as low as at least 1.1 eV (and potentially lower). In some embodiments, quantum dots modified with TCA ligands can enable TTA-UC systems to upconvert photons greater than or equal to about 1200 nm; or from about 1100 nm to about 1130 nm; or from about 1100 nm to about 1200 nm; or from about 1200 to about 1250 nm; or from about 1200 to about 1300 nm. In several embodiments, quantum dots modified with TCA ligands can enhance upconversion efficiencies of TTA-UC systems. Using an 808 nm laser, some embodiments can achieve upconversion efficiencies up to about 0.609% external quantum efficiency (EQE). Using an 1208 nm laser, someStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT embodiments can achieve upconversion efficiencies up to about 0.0028% EQE. Using an 1130 nm laser, some embodiments can achieve upconversion efficiencies up to about 0.0321 % EQE using PbS quantum dots without additional optical setups (such as mirrors, cavities, plasmonic nanostructures, etc.). Using an 1130 nm laser, some embodiments can achieve upconversion efficiencies up to about 0.018% EQE using HgTe quantum dots without additional optical setups (such as mirrors, cavities, plasmonic nanostructures, etc.). The TCA ligand modified sensitizers can improve film homogeneity. As can be readily appreciated, any of a variety of organic ligands can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0093] The TTA-UC systems in accordance with many embodiments use various compounds as annihilators. Examples of annihilators for the TTA-UC systems include (but are not limited to) TES-ADT and di-fluorinated derivative of TES-ADT (diF-TES-ADT). As can be readily appreciated, any of a variety of annihilators can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0094] The TTA-UC systems in accordance with many embodiments use various compounds as emitters. Examples of emitters for the TTA-UC systems include (but are not limited to) DBP, BTZ-DMAC, IR-786, TXO-TPA, HIDC iodide, diBr-DBP, CdSe quantum dots, and perovskite nanocrystals. As can be readily appreciated, any of a variety of emitters can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0095] The TTA-UC systems can be incorporated in bulk heterojunction (BHJ) structures in accordance with many embodiments. Conventionally, BHJ structures are considered ineffective for upconversion devices due to increased back transfer from the annihilator singlet to the sensitizer. However, against conventional wisdom, several embodiments can enhance upconversion efficiency by mixing high concentrations of sensitizers in emitter and annihilator blends. In some embodiments, PbS quantum dots modified with TCA ligands can be used as sensitizers and TES-ADT doped with DBP canStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT be used as annihilators in the TTA-UC systems. In various embodiments, fabrication methods such as (but not limited to) spin coating and / or drop casting can be used to form the TTA-UC systems.

[0096] The TTA-UC systems use various combinations of sensitizers, emitters, and annihilators. Some embodiments use PbS quantum dots modified with TCA ligands as the near-infrared absorbers, TES-ADT as the upconverting material, and DBP as emitters in a bulk heterojunction architecture.

[0097] Many embodiments utilize material combinations in TTA-UC systems to upconvert photons at wavelengths greater than or equal to about 800 nm; greater than or equal to about 1200 nm. In several embodiments, the TTA-UC systems can be integrated with semiconductor devices, imaging systems, and / or light detection systems to enhance device functions. Most near-infrared imaging systems rely on external power sources. Photon upconversion, a passive technique involving sequential energy transfer from two or more low-energy photons to a high-energy photon, offers an alternative that can operate with incoherent incident near-infrared light. TTA, as a photon upconversion technique, stands out as a suitable option for imaging in low-light situations, exhibiting high efficiency at low-intensity irradiance and greater absorption across broadband light, as compared to rare earth-doped ions, which is a different material platform for upconversion. Many embodiments utilize material combinations in TTA-UC systems to upconvert photons at wavelengths greater than or equal to about 800 nm.

[0098] Fig. 1 A provides a schematic of upconversion utilizing the TTA-UC mechanism. The TTA-UC systems can comprise one or more sensitizers and one or more annihilators; the sensitizer and annihilator are paired to perform photon upconversion via TTA-UC. Accordingly, the sensitizer is capable of receiving longer-wavelength photons then transferring the energy of the photons to the annihilator such that it emits photons of desired wavelengths.

[0099] Fig. 1 B provides a schematic of material components of a TTA-UC system. The sensitizers absorb low-energy photons, undergo triplet energy transfer, transfer the excitation energy to the annihilators and / or emitters, and the annihilators and / or emittersStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT emit high energy photons. Quantum dots modified with organic ligands can be used as sensitizers. Various types of quantum dots can be modified with TCA ligands to improve upconversion in the NIR wavelengths. Examples of quantum dots that can be modified with organic ligands include (but are not limited to) PbS quantum dots, HgTe quantum dots, and any other appropriate quantum dots species that can be used as sensitizers for NIR upconversion. Although TCA is shown in Fig. 1 B as the organic ligand, as can readily be appreciated, any of a variety of organic ligands can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. In Fig. 1 B, TES-ADT can be used as annihilators and DBP can be used as emitters. Although TES-ADT and DBP are shown in the figure, as can readily be appreciated, any of a variety of annihilators and / or emitters can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0100] Fig. 2 provides a schematic of a bulk heterojunction structure. The sensitizers 201 (such as quantum dots, ligand modified quantum dots, or monolayers) can be dispersed in a mixture 202 comprising the annihilators and / or the emitters. The sensitizers, annihilators, and / or emitters can form a solution, and the solution can be deposited onto a substrate via suitable methods such as (for example) drop casting or spin coating to form the desired morphologies and / or geometries.Resonant Structures

[0101] Several embodiments are directed to TTA-LIC systems comprising resonant structures, which can enhance absorption, emission, and collection.

[0102] Resonant structures (such as plasmonic structures, dielectric resonators) and / or nanostructures in accordance with some embodiments can be utilized with the upconverting TTA-UC systems to enhance the absorption of incident light. The resonant structures can have various shapes, dimensions, and / or material compositions, which can be selected based on the applications. The resonant structures can be designed to have optimized parameters (such as dimensions, period, shapes, material compositions,Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT configurations, position of structures) in order to achieve the desired resonance peak(s) and / or functionalities.

[0103] The resonant structures can be on a transparent substrate. The substrate can be transparent to wavelengths from about 400 nm to about 1500 nm. In some embodiments, the substrate can be partially transparent in the wavelengths from about 400 nm to about 1500 nm. Examples of transparent substrate materials include (but are not limited to) glass, NBK7 glass, quartz, sapphire, and silicon. As can be readily appreciated, any of a variety of transparent substrate can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. The substrate can have various dimensions to accommodate the plasmonic structures, for example (but not limited to) at least one dimension in a range from about 1 mm to about 10 mm; or at least one dimension in a range less than or equal to about 2 mm; or at least one dimension in a range greater than or equal to about 10 mm; or at least one dimension in a range greater than or equal to about 50 microns. In some embodiments, the substrate can have a thickness from about 1 mm to about 10 mm; or a thickness less than or equal to about 2 mm; or a thickness from about 1 mm to about 2 mm; or a thickness greater than or equal to about 100 microns.

[0104] In several embodiments, an electrically conductive layer can be positioned adjacent to the transparent substrate. In some embodiments, the electrically conductive layer is also transparent to incident light. The electrically conductive layer can be made of various materials such as (but not limited to) indium tin oxide (ITO). As can be readily appreciated, any of a variety of conductive layers can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. The electrically conductive layer can have various dimensions to accommodate the plasmonic structures, for example (but not limited to) at least one dimension in a range from about 100 nm to about 300 nm; at least one dimension in a range less than or equal to about 100 nm; at least one dimension in a range greater than or equal to about 300 nm. In some embodiments, the electrically conductive layer can have a thickness from about 100 nm to about 300 nm; or a thickness less than or equal to about 100 nm; or aStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT thickness greater than or equal to about 300 nm.

[0105] In some embodiments, a low loss (in visible to NIR region) dielectric or insulator layer can be added below the electrically conductive layer. In some embodiments, the electrically conductive layer can be replaced by a low loss (in visible to NIR region) dielectric or insulator layer. In such embodiments, resonant structures can be deposited on the low loss dielectric layer. In certain embodiments, silicon resonant structures can be deposited on silicon substrates. The thickness of the dielectric layer can be between about 0.5 and 5 microns. Examples of the low loss dielectric layer materials include (but are not limited to) silicon dioxide, aluminum oxide, hafnium oxide, silicon nitride, and lithium fluoride. In some embodiments, the dielectric / insulator layer can also be omitted. It is not necessary for the functioning of the resonant structures. It is there to provide an extra boost in absorption through cavity effects inside the dielectric layer.

[0106] In some embodiments, resonant structures can be on the electrically conductive layer. The resonant structures can be a plurality of repeating units or a plurality of units of various shapes and / or dimensions. The resonant structures can be arranged in an array having a regular or irregular pattern; or in parallel lines; or in straight lines; or in curved lines; or in an aperiodic manner. A repeating unit of the plasmonic structures can be referred to as a unit cell. A unit cell can include at least one nanostructure; or at least two nanostructures; or at least three nanostructures; or at least four nanostructures; or at least five nanostructures; or at least ten nanostructures; or at least fifteen nanostructures; or at least twenty nanostructures. The repeating unit cells can have a periodicity. The length, the width, and the height of each of the resonant structures can be the same or different. In several embodiments, each of the resonant structures can have a symmetrical shape or a non-symmetrical shape. Each of the resonant structures can have various a shape such as (but not limited to) cuboids, cubes, pillars, nanopillars, circular pillars, square pillars, rectangular pillars, rods, nanorods, plus shaped pillars, bowtie pillars, cylinders, elliptical cylinders, trapezoids, triangular prisms, polygonal prisms, pyramids, spheres, nanospheres, nanoholes, and any combinations thereof. The plasmonic structures can have anisotropic shapes for such that the upconversionStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT response is different for different polarization of light. As can readily be appreciated, any of a variety of shapes of the resonant structures can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. In several embodiments, the resonant structures can be made of various materials such as (but not limited to) metals, metal alloys, metal oxides, metal nanoparticles, semiconductors, or dielectric materials. The resonant structures can be referred to as plasmonic structures when made with metals, metal alloys, metal oxides, metal nanoparticles, and / or semiconductors. The resonant structures can be referred to as dielectric resonators and / or Mie-resonant structures when made with dielectric materials. Examples of the resonant structure materials include (but are not limited to) gold, silver, aluminum, silicon, germanium, gallium phosphide, gallium arsenide, silicon carbide, titanium oxide, silicon nitride, barium titanate, lithium niobate, tantalum pentoxide, gold nanoparticles, silver nanoparticles, or aluminum nanoparticles. As can readily be appreciated, any of a variety of materials can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. Adhesion layers (such as titanium, or chromium) may be needed for some metal plasmonic structures (such as gold). The plasmonic structures can be fabricated using various lithography processes such as (but not limited to) electron beam lithography or deep-ultraviolet lithography.

[0107] In many embodiments, the plasmonic structures can be incorporated with the upconverting components. The upconverting components can be formed in layers, films, mixtures, or bulk heterojunction structures. Any of the suitable material components of the upconversion systems in accordance with various embodiments can be combined with the plasmonic structures.

[0108] Several embodiments implement a passivation layer between the plasmonic structures and the upconverting components. In some implementations, passivation of the plasmonic structures can improve functionalities of the upconversion systems. It has been found that a passivation layer can reduce quenching of emitted visible light of some TTA-UC systems that utilize metallic plasmonic structures. When electromagneticStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT simulations were performed, these simulations did not show any quenching, but some quenching was empirically observed when some systems were tested. Determining optimal thickness of passivation layer without experimentally testing several devices can be difficult. Many embodiments determine the optimal thickness of the passivation layers experimentally through a range of thickness and deposition techniques. When the passivation layer is too thin, it can lead to pinholes and quenching of output. When the passivation layer is too thick, the upconversion efficiency is reduced because the enhancement due to plasmonic resonator is mostly in the near-field. The passivation layer can comprise various materials including (but not limited to) oxides, silicon oxide (SiO2), aluminum oxide (AI2O3), hafnium oxide (HK ), and mixed oxides. As can be readily appreciated, any of a variety of passivation layers can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention. In some embodiments, the optimal thickness of the passivation layer can be determined to accommodate the plasmonic structures and the TTA-LIC material systems. In certain embodiments, the passivation layer made of silicon oxide with an optimal thickness of about 7 nm for gold nanopillar plasmonic structures and Y6 and rubrene upconverting systems is sufficient for preventing quenching. Several embodiments use plasma-enhanced chemical vapor deposition (PECVD) to form the passivation layers. Passivation layers formed by atomic layer deposition (ALD) against gold surfaces may have pinholes. Modified ALD processes can be adapted to form the passivation layers to eliminate pinholes.

[0109] Provided in Fig. 3A is an example of plasmonic structures comprising metal circular pillars. Fig. 3A shows a profile view, a top view, and a side view of the upconversion system comprising the metal circular-pillar plasmonic structures embedded in the TTA-LIC systems (for example, PbS / TES-ADT). The upconversion system includes a transparent substrate, a conducting layer deposited on the transparent substrate, a passivation layer deposited between the conducting layer and the upconverting material. The substrate can be a transparent substrate in the wavelength range of about 400-1500 nm, such as silicon, glass, quartz, Sapphire, or NBK7 glass. The conducting layer canStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT comprise indium tin oxide. The passivation layer can be silicon oxide, aluminum oxide, or hafnium oxide. The upconverting material can be PbS / TES-ADT. A plurality of plasmonic structures such as circular pillars are embedded in the upconverting material. The circular pillars can have circular cross sections from the top view. The metal circular pillars can be made of gold, silver, or aluminum. The passivation layer electrically passivates the plurality of plasmonic structures from the upconverting material. The substrate has a thickness of about 1-2 mm. The conducting layer has a thickness of about 100-300 nm. The passivation layer has a thickness of about 5-15 nm. The metal pillars can have a height of about 20-500 nm, a diameter of about 50-700 nm, and a center-to-center distance of about 50-1000 nm. The circular-pillar plasmonic structures can be used for broadband polarization-insensitive absorption enhancement.

[0110] Provided in Fig. 3B is an example of plasmonic structures comprising square pillars. Fig. 3B shows a profile view, a top view, and a side view of the upconversion system comprising the square-pillar plasmonic structures embedded in the TTA-LIC systems. The square pillar plasmonic structures can be used for broadband polarizationinsensitive absorption enhancement. The upconversion system in Fig. 3B can have similar structures in Fig. 3A. The plurality of plasmonic structures can be square pillars. The square pillars can have square cross sections from the top view. The square pillars have advantages such as 1 ) easier and faster to fabricate using electron beam lithography; 2) the corners of the square have high intensities shown in 301 ; 3) the high- intensity points are useful for reducing the intensity threshold for the TTA-UC process and increasing the external quantum efficiency of the upconverting material.

[0111] Provided in Fig. 3C is an example of plasmonic structures comprising rectangular pillars or rods. Fig. 3C shows a profile view, a top view, and a side view of the upconversion system comprising the rectangular-pillar plasmonic structures embedded in the TTA-UC systems. The upconversion system in Fig. 3C can have similar structures in Fig. 3A. The plurality of plasmonic structures can be rectangular pillars or rods. The rectangular pillars or rods can have rectangular cross sections from the top view and the side view. The rectangular-pillar plasmonic structures are polarization sensitive and canStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT be tuned to be more narrowband compared to the circular or the square pillars. The corners of the rectangular have high intensities shown in 302. Tunable narrowband absorption enhancement can be used for applications in multispectral imaging and sensing.

[0112] Provided in Fig. 3D is an example of narrowband tunable absorption of rectangular pillars. 303 shows absorption in metal for different rod lengths Lx of about 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, and 100 nm. 304 shows absorption enhancement in metal for different rod lengths. 305 shows absorption in BHJ for different rod lengths Lx of about 80 nm, 90 nm, and 100 nm, and clean BHJ. The input polarization is in the X direction. Fig. 3D shows peak shifts to right (red shift) with increasing X-span. Period is about500 nm. Y span is about 50 nm. Z span is about 50 nm. X span is about 50-100 nm.

[0113] Provided in Fig. 3E is an example of plasmonic structures comprising crossshaped pillars. Fig. 3E shows a profile view, a top view, and a side view of the upconversion system comprising the cross-shaped-pillar plasmonic structures embedded in the TTA-UC systems. The upconversion system in Fig. 3E can have similar structures in Fig. 3A. The plurality of plasmonic structures can be cross-shaped pillars. The crossshaped pillars can have cross-shaped cross sections from the top view. The cross- shaped-pillar plasmonic structures have similar polarization insensitive properties as the square pillars. The cross-shaped-pillar plasmonic structures can be useful for cases where narrowband absorption enhancement is needed with polarization insensitivity. In addition, these structures have more corners, hence more high electric field intensity areas.

[0114] Provided in Fig. 3F is an example of plasmonic structures comprising bowtie pillars. Fig. 3F shows a profile view, a top view, and a side view of the upconversion system comprising the bowtie-pillar plasmonic structures embedded in the TTA-UC systems. The upconversion system in Fig. 3F can have similar structures in Fig. 3A. The plurality of plasmonic structures can be bowtie pillars. The bowtie shaped pillars can have bowtie cross sections from the top view. The bowtie-pillar plasmonic structures can generate very high intensity regions in the gap (center of the bowtie). Such structures canStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT be useful for cases where localized high intensity spots are desired, e.g. at very low intensity input. In some embodiments, each triangle of the bowtie is symmetric or isosceles. In some embodiments, the corners towards the gap can be flat as shown in 306. The electric field can resonate between the bowtie for a long period of time.

[0115] Provided in Fig. 3G is an example of plasmonic structures comprising rectangular pillars with tapered corners. Fig. 3G shows a profile view, a top view, and a side view of the upconversion system comprising the rectangular pillars with tapered comers plasmonic structures embedded in the TTA-UC systems. The upconversion system in Fig. 3G can have similar structures in Fig. 3A. The plurality of plasmonic structures can be rectangular pillars with tapered corners. The rectangular shaped pillars can have rectangular cross sections with tapered corners from the top view. In some embodiments, the tapered corners are shown in 307. The electric field can resonate between the bowtie for a long period of time.

[0116] Provided in Fig. 4A is an example of plasmonic structures comprising semiconductor circular pillars, where the semiconductor has low loss at the incident and upconverted wavelengths. Fig. 4A shows a profile view, a top view, and a side view of the upconversion system comprising the semiconductor circular-pillar plasmonic structures embedded in the TTA-UC systems. The upconversion system includes a transparent substrate, a conducting layer deposited on the transparent substrate, and upconverting material deposited on the conducting layer. The substrate can be a transparent substrate in the wavelength range of about 400-1500 nm, such as silicon, glass, quartz, Sapphire, or NBK7 glass. The conducting layer can comprise indium tin oxide. The upconverting material can be PbS / TES-ADT. A plurality of semiconductor structures such as circular pillars are embedded in the upconverting material. The circular pillars can have circular cross sections from the top view. The semiconductor circular pillars can be made of silicon or germanium. The substrate has a thickness of about 1-2 mm. The conducting layer has a thickness of about 100-300 nm. The pillars can have a height of about 50-1500 nm, a diameter of about 50-1000 nm, and a center-to-center distance of about 100-3000 nm. For semiconductor plasmonic structures, passivation layer(s) may not be needed. TheseStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT semiconductor pillars can be useful for narrowband applications and / or broadband applications (such as LEDs), especially with thick upconverting layers, as these pillars tend to be larger than their metal counterparts for the same resonance wavelength.

[0117] Provided in Fig. 4B is an example of plasmonic structures comprising semiconductor circular pillars on an insulating substrate. The silicon pillars are embedded in the TTA-UC systems. The upconversion system includes a transparent substrate, a dielectric or insulator layer deposited on the transparent substrate, and upconverting material deposited on the dielectric or insulator layer. 401 shows silicon pillars on a dielectric or insulator substrate (for example, silicon oxide). The dielectric or insulator substrate is on a transparent substrate (for example, silicon). The upconverting material can be PbS and TES-ADT. The silicon pillars can have a period of about 735 nm. Each pillar can have a radius of about 110 nm and a height of about 445 nm. The dielectric oxide layer can have a thickness of about 3.5 microns. The upconverting material layer can have a thickness of about 500 nm. The XY profile in 402 and the XZ profile in 403 show that the silicon substrate acts as a partial backreflector for visible light, and also partially reflects 1200 nm light leading to more absorption in BHJ. Through sweeps and optimization, a pillar configuration is determined that can provide high absorption in a about 50 nm wavelength band around 1200 nm wavelength as shown in 404. The configuration can be further optimized by a metasurface configuration as shown in Figure 4C.

[0118] Provided in Fig. 5 is an example of plasmonic structures comprising metal nanoparticles. Fig. 5 shows various shapes of metal nanoparticles can be incorporated as plasmonic nanostructures embedded in the TTA-UC systems as the upconversion systems. The upconversion system includes a transparent substrate and upconverting material deposited on the substrate. The substrate can be a transparent substrate in the wavelength range of about 400-1500 nm, such as silicon, glass, quartz, Sapphire, or NBK7 glass. The upconverting material can be PbS / TES-ADT. Various metal nanoparticles can be embedded and distributed in the upconverting material. The metal nanoparticles can be made of various metals such as gold, silver, or aluminum. VariousStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT shapes of the nanoparticles such as (but not limited to) spheres, shells, rods, or cubes) in different sizes can be added to the upconverting solution before it is spin-coated and annealed. Surface functionalization such as using ligands (for example carboxylic acid) to gold nanoparticles can be used to prevent aggregation. Silica-coated metal nanoparticles or nanoparticles with suitable dielectric coatings can be used to prevent quenching.

[0119] Provided in Fig. 6 is an example of nanostructures arranged in alternating arrays. Fig. 6 shows a profile view, a top view, and a side view of the alternating periodic array of nanostructures embedded in the TTA-LIC systems as the upconversion systems. The upconversion system can include a transparent substrate, a conducting layer on the substrate, and upconverting material deposited on the conducting layer. The substrate can be a transparent substrate in the wavelength range of about 400-1500 nm, such as glass, quartz, Sapphire, or NBK7 glass. The upconverting material can be PbS / TES-ADT. The nanostructures can be embedded and distributed in the upconverting material. The nanostructures can be made of metal or semiconductors. If the nanostructures are metal plasmonic structures, a passivation layer is deposited between the nanostructures and the upconverting material. In Fig. 6, pillars of different geometries and / or materials can be arranged in alternating periodic patterns. The different geometries and / or materials pillars are shown in Fig. 3A through Fig. 4. Alternating patterns can be with 2 different pillar structures (shown in Fig. 6); or 3 different pillar structures; or 4 different pillar structures. The alternating periodic arrays of pillar plasmonic structures can be used for real-time multispectral imaging. The structures shown in Fig. 6 can have separate resonance peaks at different wavelengths.

[0120] Provided in Fig. 7A is an example of plasmonic structures arranged as spatially separated pixels. Fig. 7A shows a profile view, a top view, and a side view of the combination of groups of pillars as plasmonic nanostructures embedded in the TTA-UC systems. In Fig. 7A, pillars of different geometries and / or materials can be arranged in various combination of groups. Each group can be viewed as a pixel. The different geometries and / or materials pillars are shown in Fig. 3A through Fig. 4. The groups canStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT have from about 2 to about 20 different pillar designs. Fig. 7A shows 4 different pillar designs. Such structures can be used for filtering different spectral input into different pixels or bins, which can be useful for multispectral imaging and / or sensing. Tiling regions can have absorption enhancement at different wavelengths such that it can convert spectral information into spatial information. The alternating periodic pillars can be read off with high resolution visible sensors.

[0121] Provided in Fig. 7B is an example of a metasurface comprising plasmonic structures. Fig. 7B shows a profile view, a top view, and a side view of the metasurface embedded in the TTA-UC systems. In Fig. 7B, pillars of different geometries and / or materials can be placed in various locations. Fig. 7B shows that the metasurface comprises a plurality of pillars. As can be readily appreciated, any of a structure shown in Fig. 3A through Fig. 4 can be implemented in the metasurface.Thin Films

[0122] Several embodiments are directed to TTA-UC systems incorporating dichroic backreflectors using thin film coating, which can counter the isotropic emission properties of the upconverting material and increase collection efficiency. In some embodiments, the thin film coating is transmissive in the near-infrared and reflective in the higher-energy wavelengths including visible wavelength band, which allows the system to redirect emitted light in a single (forward) direction. The thin film coatings can effectively double the collection efficiency. In some embodiments, the thin film structures can redirect transmitted unabsorbed near-infrared back into upconverter, while letting visible light through, enabling an absorption boost.

[0123] In many embodiments, thin film coatings can be on a transparent substrate. The substrate can be transparent to incident light such as incident near-infrared light with wavelengths from about 400 nm to about 1500 nm. Examples of substrate materials include (but are not limited to) silicon, glass, NBK7 glass, quartz, sapphire. As can be readily appreciated, any of a variety of transparent substrate can be utilized as appropriate to the requirements of specific applications in accordance with variousStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT embodiments of the invention. The substrate can have various dimensions to accommodate the plasmonic structures, for example (but not limited to) at least one dimension in a range from about 1 mm to about 10 mm; at least one dimension in a range less than or equal to about 2 mm; at least one dimension in a range greater than or equal to about 10 mm; at least one dimension in a range greater than or equal to about 100 microns; a thickness from about 1 mm to about 10 mm; a thickness less than or equal to about 2 mm; a thickness from about 1 mm to about 2 mm; a thickness greater than or equal to about 100 microns.

[0124] In several embodiments, thin film coatings can comprise a plurality of thin films. In some embodiments, the thin film coatings can comprise a plurality of alternating layers of high index (H) and low index (L) materials. The alternating layers of H and L materials can interfere together to provide the desired reflectivity. The alternating layers of H and L materials can have various number of layers such as (but not limited to) from about 2 to about 40 layers. The desired number of layers can be selected to achieve optimal structures and functions of the thin film coatings. Each of the alternating layers of H and L materials can have the same thickness or different thickness. The thickness of each layer can range from about 10 nm to about 1000 nm. Various materials can be used for the H and L materials. Examples of H materials include (but are not limited to) titanium oxide (TiO?) and niobium oxide (Nb2Os). Examples of L materials include (but are not limited to) silicon oxide (SiCh). As can be readily appreciated, any of a variety of high index or low index materials can be utilized as appropriate to the requirements of specific applications in accordance with various embodiments of the invention.

[0125] In many embodiments, the upconverting materials can be coated on the thin films. The upconverting materials can be formed in layers, films, mixtures, or bulk heterojunction structures. Any of the suitable material components of the upconversion systems in accordance with various embodiments can be combined with the plasmonic structures.

[0126] Provided in Fig. 8 is an example of thin film coatings integrated with upconverting systems. The thin film coatings on the glass substrate are transparent to theStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT incident near-infrared light. The upconverting materials can be deposited on the thin film coatings. The isotopically emitted light in the visible wavelengths from the upconversion systems can be reflected by the thin film coatings in order to effectively double the light collection on one end.

[0127] Provided in Fig. 9A is an example of alternating H and L materials as thin film coatings. Fig. 9A shows a profile view and a side view of the thin film coatings integrated with the TTA-UC systems (for example, PbS / TES-ADT). A plurality of alternating dielectric layers of high dielectric constant (H material) and low dielectric constant (L material) can be deposited on a transparent substrate. The upconverting material can be deposited on the plurality of alternating dielectric layers. The substrate can be a transparent substrate in the wavelength range of about 400-1500 nm, such as glass, quartz, Sapphire, or NBK7 glass. The upconverting material can be PbS / TES-ADT. The H materials can be TiCh or Nb20s. The L materials can be SiO2. The substrate can have a thickness of about 1-2 mm. Each of the alternating dielectric layer can have a thickness from about 10 nm to about 1000 nm. Fig. 9A shows 8 alternating H and L materials as thin film coatings. The number of alternating layers can vary depending on the specific applications.

[0128] Provided in Fig. 9B is an example of reflectivity design of the alternating thin film coatings for a TTA-UC system. The thin film coatings have the alternating H and L material structures as shown in Fig. 9A, although the number of layers may vary. The TTA-UC material systems can include Y6 (as sensitizer) and rubrene (doped with DPB as annihilator) in a BHJ structure. The TTA-UC material systems can include any sensitizer, annihilator, and / or emitter as described above in accordance with various embodiments. The reflectivity shown in 701 can be used for collection efficiency enhancement as a backreflector. In 701 , the thin film coatings can redirect visible light in a single (forward) direction with Y6 and rubrene. To achieve the reflectivity in 701 , the thin film coatings can have a total of 12 alternating H and L layers using SiO2 as L materials and Nb20s as H materials. The total thickness of the thin film coatings can be about 900 nm. 701 is plotted using transfer matrix multiplication.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0129] The reflectivity shown in 702 can be used for absorption enhancement. In 702, the thin film coatings can redirect near-infrared back into the upconverting materials. To achieve the reflectivity in 702, the thin film coatings can have a total of 8 alternating H and L layers using SiO2 as L materials and Nb2Os as H materials. The total thickness of the thin film coatings can be about 800 nm. 702 is plotted using transfer matrix multiplication.

[0130] Provided in Fig. 9C is an example of reflectivity design of the alternating thin film coatings for a TTA-UC system. The thin film coatings have the alternating H and L material structures as shown in Fig. 9A, although the number of layers may vary. The intended TTA-UC material systems are PbS and TES-ADT in a BHJ structure. The reflectivity shown in 703 can be used for collection efficiency enhancement as a backreflector. In 703, the thin film coatings can shift to longer wavelengths in near-infrared for new materials system. To achieve the reflectivity in 703, the thin film coatings can use SiO2 as L materials and Nb2Os as H materials. 703 is generated using the transfer matrix method.

[0131] The reflectivity shown in 704 can be used for absorption enhancement. In 704, the thin film coatings can shift to longer wavelengths in near-infrared for new materials system. To achieve the reflectivity in 704, the thin film coatings can use SiO2 as L materials and Nb2Os as H materials. 704 is plotted using the transfer matrix method.

[0132] In several embodiments, the thin film coatings can be integrated together to form a cavity like structure. Several embodiments are directed to the combination of thin film coatings on both sides of the upconverting material. In such embodiments, one side of the thin film coatings can let input near-infrared light through and reflect emitted higher- energy wavelengths including light. The other side of the thin film coatings can reflect unabsorbed input near-infrared light and let emitted light pass through. These integrations can multiply enhancements and maximize efficiency.

[0133] Provided in Fig. 10 is an example of integrating the thin film coatings on both sides of the upconverting systems. Fig. 10 shows a profile view and a side view of the integrated system. The integrated system can include a transparent substrate, a pluralityStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT of alternating dielectric layers deposited on the transparent substrate, upconverting material deposited on the plurality of alternating dielectric layers, a second plurality of alternating dielectric layers deposited on the upconverting material, and a second transparent substrate deposited on the second plurality of alternating dielectric layers. The plurality of alternating dielectric layers and / or the second plurality of alternating dielectric layers can have similar structures shown in Fig. 9A.

[0134] In many embodiments, the thin film coatings can be integrated with the resonant structures. Several embodiments integrate the backreflector thin films on one side of the upconverting materials and the plasmonic and / or dielectric resonators on an opposite side of the upconverting materials. Such integration can multiply the enhancements. These integrations can maximize the efficiency of the upconverting device. Detailed descriptions and results can be found in the appendix. As can be readily appreciated, any of the plasmonic or dielectric or semiconductor resonators as shown in Fig. 3A through Fig. 7, and any of the thin film coatings as shown in Fig. 9A, can be implemented in such integrated structures in accordance with various embodiments.

[0135] Provided in Fig. 11 is an example of integrating the thin film coatings and the plasmonic resonators with the upconverting systems. Fig. 11 shows a profile view and a side view of the integrated system. The integrated system can include a transparent substrate, a conducting layer deposited on the transparent substrate, a plurality of nanostructures embedded in the upconverting material deposited on the conducting layer, a plurality of alternating dielectric layers deposited on the upconverting material, and a second transparent substrate deposited on the plurality of alternating dielectric layers. The plurality of nanostructures can have similar structures shown in Fig. 3A through Fig. 7. If the nanostructures are metal plasmonic nanostructures, a passivation layer is deposited between the nanostructures and the upconverting material. If the nanostructures are semiconductor structures, the passivation layer may not be needed. The plurality of alternating dielectric layers can have similar structures shown in Fig. 9A.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCTUpconversion System Integration

[0136] Many embodiments are directed to integrating the upconversion systems with photonic techniques or devices to achieve various applications. The upconversion systems refer to any combinations of the TTA-UC material components, the plasmonic structures, and / or the thin films.

[0137] Several embodiments combine the upconversion systems with nanophotonic techniques to achieve all-passive high-quality upconversion imaging using near-infrared light with nightglow intensities. The system efficiency can be enhanced several folds by increasing the collection efficiency and absorption, which is necessary to push the materials system to upconvert nightglow intensities to visible intensities approaching human-eye sensitivity. These advancements will aid the development of passive, lightweight infrared imaging devices tailored for night vision applications.

[0138] Provided in Fig. 12 is an example of integrating the upconversion system with a vision system. Lens 1 is optimized for infrared wavelengths, and Lens 2 is optimized for visible wavelengths. The upconverting systems can be any combination of the TTA-UC material components, the plasmonic structures, and / or thin films as described above. Lens 1 and 2 can be any type of lens such as (but not limited to) planoconvex, aspheric, objective, relay, doublet, triplet, or a metasurface. This system can be used for imaging via naked eye or by a camera such as (but not limited to) charge-couple device (CCD) sensor or complementary metal oxide semiconductor (CMOS) sensor. Details regarding integrating vision system with the upconversion system can be found in the appendix.

[0139] In several embodiments, the upconversion systems can be combined with a CCD sensor or a CMOS sensor. The upconversion systems can be integrated on top of a CCD array or a CMOS array. This configuration may enable the visualization of “beyond silicon” by upconverting wavelengths beyond the responsivity curve of a Si detector. Note that the thin film coating for a dichroic backreflector (if used in this case) would block the transmission of visible light to the CCD. In this case the device will solely act as an infrared detector / camera.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0140] Provided in Fig. 13 is an example of integrating the upconversion system with a CCD array. Fig. 13 shows a similar upconversion system shown in Fig. 11 integrated with a CCD array with the CCD array being a substrate. Incoming IR light from an object or the environment can be upconverted by the upconversion system to the desired wavelength before it reaches the CCD array. Although the plasmonic resonators and thin films with the TTA-UC materials are shown in Fig. 13, any upconversion systems can be combined with the CCD array to achieve the desired functions.

[0141] In some embodiments, the upconversion systems can be combined with photovoltaic cells. Provided in Fig. 14 is an example of integrating the upconverting system. Photovoltaics generally are able to capture light energy from wavelengths between 400 nm and 1100 nm, out to their bandgap energy. This configuration together with all integrations would allow upconverting at least some of the unabsorbed near infrared, increase the band of wavelengths from which energy can be captured, and increase the efficiency of capturing NIR bands. Although the plasmonic resonators and thin films with the TTA-UC materials are shown in Fig. 1 , any upconversion systems can be combined with the photovoltaic cell(s) to achieve the desired functions.

[0142] In several embodiments, the upconversion systems can be used for anticounterfeit applications. The upconverting systems can be used as a part of currency paper as invisible ink for anticounterfeit applications. The upconverting portion of such currency papers will be visible (or, at least, stand out) only when illuminated by certain infrared wavelengths.

[0143] Provided in Fig. 15 is an example of using the upconverting system in anticounterfeit applications. When illuminated with near-infrared light, the ink comprising the upconversion components becomes visible. Such features can be integrated in anticounterfeit applications. Although the Y6 / rubrene BHJ systems are shown in Fig. 15, any upconversion systems can be integrated to achieve the desired anti-counterfeiting functions.

[0144] In several embodiments, the upconversion systems can be used for bioimaging, drug delivery, or optogenetics. The upconverting systems can be miniaturizedStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT and inserted as a chip underneath the skin for imaging, drug delivery, or optogenetics. The infrared light, which more-easily transmits through tissues, can trigger visible light emission via the upconversion systems, which can trigger drug release and / or delivery. In some embodiments, the upconversion systems can be used to measure water content in tissue (e.g., edema, bruising, internal bleeding), oxygen content in blood / tissue, ischemia, tissue perfusion.

[0145] Provided in Fig. 16 is an example of applying the upconverting system for bioimaging or vein visualization. As can be appreciated, any upconversion systems can be integrated to achieve the desired functions.EXAMPLES

[0146] The various embodiments of the disclosure will be better understood with the several examples provided below. Many examples and results of systems and methods for upconverting near-infrared wavelengths to higher-energy wavelengths including visible wavelengths are described. In particular, the many examples and results show that various TTA-UC material combinations can be integrated with plasmonic structures and / or thin films to enhance adsorption and emission properties.Example 1. All-passive upconversion imaging of the near infrared at near nightglow intensities

[0147] Many embodiments provide a high-resolution upconversion imaging system that converts low-intensity incoherent near-infrared light into the visible, without the use of external power input. The upconverting element includes the following: (1 ) photon upconversion, with linear power dependency, is enabled by triplet fusion in a bulk heterojunction of the organic molecule Y6 and organic semiconductor rubrene; (2) plasmonic nanostructures are used to enhance absorption in the thin heterojunction layer, pushing the upconversion into the linear regime; (3) a dichroic thin-film assembly is used to increase the collection of upconverted light in a transmission configuration. This upconverting element is inserted at the intermediate image plane of a Keplerian telescopeStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT system, with near-infrared optics in the front and visible optics at the back, which preserves the relative directionality of rays between the incident near-infrared light and output visible light. The resulting system can take near-infrared input as low as 50 nW / cm2, within an order of nightglow illumination, and passively generate visible light at human-eye sensitivities.

[0148] Near-infrared (NIR) imaging has recently emerged as an important tool across a diverse fields, as the NIR holds a wealth of information not available to the human eye. This has led to its use in the biomedical industry as a non-invasive imaging technique for biological tissues, in agricultural and environmental monitoring, food quality and safety assessment, defect detection, and face recognition. Another notable application of NIR imaging is in night vision. A key phenomenon of interest in this scenario is nightglow (or airglow), the emission of light by various processes in the upper atmosphere at night. Nightglow is particularly advantageous for night vision as it emits more strongly in the 900-1700 nm region than in the visible spectrum, enhancing visibility in low-light conditions during nighttime.

[0149] Most NIR imaging systems are reliant on external power sources. Commercial NIR imaging systems employed in night vision applications typically rely on active power sources, necessitating batteries to operate image intensifiers and power separate screens for displaying NIR images. Organic upconverting devices and non-linear crystals for NIR to visible imaging require a power source, or in the case of non-linear crystals, at least a coherent high intensity source to upconvert efficiently. Photon upconversion, a passive technique involving sequential energy transfer from two or more low-energy photons to a high-energy photon, offers an alternative that can operate with incoherent sources. Among the most common photon upconversion techniques are TTA and d and f subshell transitions in rare earth-doped ions. TTA, particularly in the solid thin film state, stands out as the more suitable option for nightglow imaging, exhibiting high efficiency at low-intensity irradiance and greater absorption across broadband light, as compared to rare earth-doped ions.

[0150] Several embodiments combine TTA materials with two nanophotonicStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT techniques to achieve all-passive high-quality upconversion imaging using NIR light with nightglow intensities. System efficiency can be improved by nearly 4 times by increasing the collection efficiency and absorption, which is necessary in order to push the materials system to upconvert nightglow intensities to visible intensities approaching human-eye sensitivity. These advancements will aid towards the development of passive, lightweight infrared imaging devices tailored for night vision applications.

[0151] Many embodiments provide upconversion systems built on TTA systems without nanophotonic enhancement. Several embodiments use a ~150 nm bulk heterojunction (BHJ) comprised of two organic semiconductors, Y6 and rubrene, the latter doped with organic semiconductor tetraphenyldibenzoperiflanthene (DBP). Y6 functions as the sensitizer molecule, absorbing photons broadly within the 700-900 nm range, and following several energy transfer processes illustrated in Fig. 17A-17C, TTA occurs within the DBP-doped rubrene, which functions as the annihilator molecule. The resulting emitted photons are centered on 610 nm.

[0152] Fig. 17A shows the upconverting device comprising of a bulk heterojunction of organic molecules rubrene and Y6, sealed between two glass pieces using epoxy to prevent direct contact with oxygen, which would degrade the upconverting material. Fig. 17B shows the molecular diagram and Fig. 17C shows the energy diagram of TTA-LIC in Y6 / rubrene BHJ. Electrons in Y6 in ground singlet state (So) absorb incident NIR photons to become excited singlets (Si ), which diffuse to the interface between Y6 and rubrene. At this interface, singlets transform into free charges and recombine in rubrene to form excited triplets (Ti ). These triplets combine via TTA in rubrene to produce a high energy singlet and then emit a photon via photoluminescence. Fig. 17D shows images of the upconverting thin film in its original state under ambient light are shown with (bottom) and without (top) NIR laser illumination. Fig. 17E shows the upconverter is placed at the focal point of the Keplerian lens system. This configuration conserves incident angle 6, and therefore allows upconversion imaging. Fig. 17F shows a schematic of the imaging setup used for transmission mode imaging of an Airforce resolution target. Fig. 17G shows image captured using the setup in (c) with nearly 1 :1 magnification between the objectStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT and the image. Resolution groups 4 and higher are highlighted on the right.

[0153] Following TTA, the emitted visible light is both isotropic and incoherent, hence a mechanism is needed to preserve imaging through the upconversion process. To accomplish this, many embodiments position the upconverter BHJ at the mutual focal plane of a NIR imaging system on one side, and a visible imaging system on the other, reminiscent of a Keplerian telescope. The imaging experiment is set up as shown in Fig. 17F. The NIR lens (L1 ) is achromatically corrected for the 700-900 nm absorption band of our Y6 / rubrene / DBP BHJ. Because the lenses convert the direction of incident rays to position, and the TTA process preserves position, the resulting optical system enables upconversion imaging.

[0154] Captured images indicate that through the upconversion imaging system, the resolution can reach down to the level of nearly 100 line pairs / mm on the upconverter under broadband NIR illumination. This resolution level is comparable to that of commercial NIR viewing cameras utilizing image intensifiers. The resolution is not limited by the upconverter and it is instead due to achromatic aberration of L1 with the current broadband source, as the focal length shift in L1 due to wavelength range of the LED source is nearly 10 pm.

[0155] Despite its ability to produce high-fidelity images under broadband incoherent light conditions, the upconverter faces limitations in achieving the necessary power efficiency for nightglow imaging since the external quantum efficiency (EQE) reaches a maximum of 0.035%. Considering nightglow irradiance levels in the NIR (800 - 1800 nm) are in the range of 1 nW / cm2and the human eye sensitivity threshold is around 0.01 nW / cm2, there arises a demand for a more efficient system. System integrations that have been used in solid state upconverters to improve efficiency include mirrors and cavities. While these are effective at power enhancement, these integrations do not preserve imaging through the upconverter and render them useless for most vision applications. Many embodiments use nanophotonic integrations to achieve this goal while conserving the capability to perform high resolution imaging in transmission mode.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCTExample 2: Increasing collection efficiency through a beaming dichroic backreflector

[0156] The isotropic emission of the Y6 / rubrene / DBP BHJ results in the loss of at least 50% of visible light in a transmission imaging system. To recover this backward-emitted light, many embodiments provide a thin-film coating (Fig. 18A and Fig. 18B), using alternate layers of the dielectrics Niobium Pentoxide (Nb20s) and Silicon Dioxide (SiO2), to act as a beaming backreflector. The thin film coating operates similarly to a dichroic mirror, allowing transmission of NIR light while efficiently reflecting visible light within a half angle of up to 30° - close to the critical emission angle from the upconverting device to air. The total thickness of the coating is under 1 pm, which simulations show, is small enough to prevent any degradation in imaging resolution. Isotropic emission simulations using finite-difference time domain (FDTD) reveal an enhancement of approximately 2.3 at the peak emission wavelength (610 nm) of the upconverter. The enhancement beyond a factor of 2 is due to the phenomena of Purcell enhancement. This dielectric coating design is deposited on 1-inch diameter glass substrates and is shown in Fig. 18B.

[0157] Fig. 18A shows a thin film coating on the glass substrate acts as a dichroic mirror, allowing NIR light to pass through while reflecting visible light. Fig. 18B shows calculated reflection spectrum of the coating on 1 mm thick glass. The left inset shows the fabricated coating on a 1-inch diameter glass substrate, and the right inset is the simulated light extraction enhancement using FDTD simulations. Fig. 18C shows periodic gold nanopillars are designed to be embedded in the upconverting material. The gold pillars are symmetrically spaced in x and y axes, (Px and Py), with diameter d, thickness / height H, and a total length D in both axes. A thin SiO2 passivation layer is deposited on the gold pillars to prevent direct contact with spin-coated Y6 / rubrene / DBP. Fig. 18D shows scanning electron microscope (SEM) images of the gold array with a passivation layer, and a 3D atomic force microscopy (AFM) image in the inset. Fig. 18E shows simulated absorption using FDTD of the upconverter in its original state, with periodic gold disks, and with gold disks and a 7 nm layer of SiO2. Fig. 18F and Fig. 18G show field enhancement profiles in the XY (top) and XZ (side) axes of the gold pillar, demonstrating increased field strength near the edges of the pillar.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0158] To experimentally validate the power enhancement resulting from the thin film coated substrates, many embodiments use the power measurement setup illustrated in Fig. 19A. A broadband NIR LED source is used and focus a 1 mm2circular beam onto the upconverter plane using NIR focusing lenses. In the field of upconversion, it is a well- established concept that EQE exhibits a quadratic to linear relationship with input intensity, hence the beam area is kept constant for all measurements. Collimation optics and infrared (IR) filters are used to capture the photoluminescence on the opposite end.

[0159] To quantify the enhancement from the thin film substrate, the output power from the upconverter is measured in its original state (i.e. , with glass on both sides), and then replaced this with the dichroic backreflector version. This procedure is repeated for different samples and the results of these experiments are plotted in Fig. 19B. The enhancement is calculated by fitting the two sets of data points to a line and then determining the slope ratio. Through this analysis, the addition of the beaming dichroic backreflector leads to a 2.5-fold increase in power, closely aligning with the values at the measured wavelength (610 nm). In addition, the resolution target is imaged again using this upconverter, and a close comparison between Fig. 17G and Fig. 19D (i) reveals that brighter patterns can be obtained without apparent compromise in imaging quality or resolution.

[0160] Fig. 19A shows a schematic of the power measurement setup: An NIR broadband source with three LEDs is used for power measurements, as depicted in the inset at the bottom left. This setup is used to measure visible power with the upconverter under the following conditions: 1 ) in its original state, 2) with the beaming dichroic backreflector, and 3) with gold nanopillars. Fig. 19B shows visible power measured against input NIR power for upconverters with and without a thin film substrate. Fig. 19C shows measured visible power against input NIR power within and outside the gold region. Inset shows the log-log plot for one of the measured gold array samples, with x axis changed to NIR intensity on the upconverter. Dotted lines show the estimated value of the threshold, which decreases within the gold region. Fig. 19D shows photos captured using the imaging setup in Fig. 17F under the following conditions: (i) with the upconverterStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT employing our beaming dichroic backreflector, (ii) with the upconverter utilizing the fabricated gold array (2mm by 2mm, highlighted by a white square), (iii) with the target removed from (ii).Example 3: Absorption enhancement using gold nanopillars

[0161] Simulations show that in a 100 nm film of Y6 / rubrene / DBP BHJ, the NIR absorption reaches a maximum of 25% at best. This provides an opportunity to boost the EQE by using metal nanostructures which resonate at NIR frequencies near the absorption band (700-900 nm) of our upconverting material. Many embodiments use periodically spaced gold nanopillars on indium tin oxide (ITO) coated glass substrate, as illustrated in Fig. 18C, with periodicity (P = Px = Py) of 340 nm, a diameter (d) of 170 nm, and a height (H) of 40 nm. Upon NIR excitation, these nanopillars resonate, resulting in enhanced electric field intensity in their proximity, thereby increasing absorption in the immediate vicinity. The nanopillar parameters are optimized to provide the highest absorption in the NIR range, while also tolerating small fabrication errors. Furthermore, the period and diameter of these pillars is well below the wavelengths of interest, hence no loss in resolution is expected. These gold pillars are fabricated using electron beam lithography (EBL), with several different grid sizes, in the range of 2-6 mm.

[0162] Note that, to avoid quenching, a thin dielectric (SiO?) layer is deposited on top of the gold pillars before spin coating with Y6 / rubrene, serving as a passivation layer. This passivation layer acts as a barrier, preventing direct contact between the emitting material and gold, thereby lowering the risk of quenching of the emitted visible light. FDTD simulations show a substantial enhancement in absorption using these gold pillars, more than 2-fold at 850 nm even with the passivation layer. The addition of a thin 7nm passivation layer results in a negligible drop in absorption and is demonstrated in Fig. 18E.

[0163] The setup in Fig. 19A is used to measure the power within the gold square region and compared it to the power measured outside this region. Using the emissivity of the LED source, the expected absorption enhancement is to be around 2.1 . The resultsStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT across several upconverting samples are illustrated in Fig. 19C, demonstrating a slope enhancement of 2.2 through the gold nanopillars. In addition, the resulting enhancement is measured without any passivation layer and it drops to nearly 1 .5, which demonstrates that the 7nm SiO2 layer prevents quenching, and allows to reach the desired enhancement. Resolution target imaging results show that there is no loss in resolution inside the gold region (forming a bright square patch, D: 2 mm by 2 mm), proving that the integration of gold nanopillars does not adversely impact the imaging quality of the system.

[0164] The quadratic to linear threshold of the upconverter is lowered due to the presence of the gold pillars. About 35% decrease is measured in the threshold value (in NIR power intensity) within the gold region across several upconverting devices. The effect of plasmonics in lowering threshold for upconversion has been previously reported for organic upconverting devices. Although this decrease in threshold aids in boosting the EQE and allowing to upconvert at lower intensities, it is a byproduct of using plasmonic resonators, which increase field enhancement in the vicinity, thereby increasing photon intensity in the upconverting material.Example 4: High-efficiency upconversion imaging with integrated nanophotonic upconverter

[0165] Many embodiments integrate the nanostructures into a single upconverting device. In the integrated upconverter, the beaming dichroic backreflector is on one side, and the fabricated gold array is on the other side, with Y6 / rubrene / DBP BHJ in the middle. This fully integrated upconverter is tested using the same power measurement setup as shown in Fig. 19A. Plotting the results for this upconverter against a bare BHJ upconverter (Fig. 20A) shows that the dichroic backreflector region lends an enhancement factor of 2.2, and the combination with gold plasmonic structures provides an extra factor of 1 .8, leading to an overall enhancement of 3.9. This is close to what our FDTD simulations show (~3.5) with the added unpredictability of several small (~1 pm) airgaps inside the upconverting device, which are unavoidable with the current upconverter material andStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT preparation method.

[0166] Using this fully integrated upconverter, nightglow intensities are simulated in the power measurement setup by lowering the NIR power and increasing beam size before L1 in Fig. 19A. This wide and weak input beam is fed into the system, and the intensity input to system versus the output visible light intensity captured is plotted in Fig. 20B. The results for a bare glass (no upconverter) shows that NIR light leaks through the power collection setup and hitting the power meter despite the IR filters. At lower intensities, the bare BHJ data points are below the bare glass data, hence at these intensities it is absorbing light, but not emitting. Whereas the fully integrated upconverter is emitting well beyond 2 nW / cm2even at the lowest intensities that could be illuminated at, which were slightly below 50 nW / cm2.

[0167] Fig. 20A shows visible power measured against NIR power for different regions on the fully integrated upconverter, compared to a bare BHJ upconverter. Fig. 20B shows NIR intensity to the system, before L1 in Fig. 19A, against the power intensity measured on the power meter. Closeup on the right shows the lower intensity region. Fig. 20C shows photos captured using the imaging system in Fig. 17F using the fully integrated upconverter with grid size of 6 mm. Zoomed in insets on the right show that the resolution of the system is approaching 100 line pairs / mm. Fig. 20D and Fig. 20E show images captured with the same setup as (c), but with the Airforce target replaced with a paper cut-out of logos. Fig. 20F and Fig. 20G show images captured with NIR diffused light shining on f) white hortensia, and g) birds foot trefoil. We use the fully integrated upconverter with the 6mm gold square (as highlighted by a white square).

[0168] For imaging purposes, the gold array has a grid size of 6mm to use in the fully integrated upconverter. Using this upconverting device, no loss in resolution through the integration of the gold array and beaming backreflector by imaging the Airforce target in full. Moreover, NIR imaging of small flowers in Fig. 20F and Fig. 20G by illuminating them with diffused NIR light and imaging the scattered NIR light using the fully integrated upconverter.

[0169] Many embodiments enhance passive frequency upconversion imaging. AStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT beaming dichroic backreflector is able to tackle losses due to isotropic nature of the photoluminescence and double the collection efficiency by redirecting emitted light. In addition, plasmonic nanostructures that broadly resonate in the range of NIR frequencies that the upconverting material absorbs in allows to increase the EQE by more than double and decrease the quadratic to linear threshold as well. These two nanostructures integrate well together and that these enhancements multiply, allowing to capture visible light at human eye sensitivities at nightglow intensities. In addition, these integrated nanostructures do not lower the imaging resolution of the upconverter, and imaging through weak scattered NIR light from flowers.

[0170] The methods are very broad and are applicable to many different material systems. The current material system, Y6 / rubrene / DBP BHJ, with all its integrations, is a useful device for imaging through human eye at low ambient NIR environments such as nightglow. However, since the absorption of Y6 is limited to below 900 nm, it does not fully capture nightglow and cannot replace current state of art silicon detectors. The same techniques can be applied to upconverting materials which can upconvert beyond 1 pm, for example, utilizing PbS quantum dots as the sensitizer. These quantum dots are tunable across a large range of wavelengths in NIR, extending up to 1.2 pm.Example 5: Y6 / Rubrene / DBP BHJ upconverter preparation

[0171] The Y6 / rubrene / DBP mixture is prepared using 14 mg rubrene, 2.5 mg Y6, and 1 % DBP is stirred in 1 mL chloroform until fully dissolved. All substrates and encapsulation pieces are first treated for 15 minutes in UV-ozone. Then these are brought inside a nitrogen-filed glovebox, where the substrate is placed on the spin-coater and spun at 1000 rpm for 30 seconds, with a 0.6 rpm / second ramp, using a solution of nearly 60 pL. Then this coated substrate is annealed at 70 °C for 5 minutes. The encapsulation piece is placed on top, and all sides of the upconverting material are thoroughly encapsulated using epoxy. Rubrene is especially sensitive to contact with oxygen and requires utmost care in order to prevent degradation. However, once all sources of air are thoroughly eliminated, these upconverting samples can survive for weeks.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT

[0172] For bare BHJ samples 1-mm thick glass pieces, ranging from 10 mm by 10 mm soda-lime glass to 1 -inch round B-270 glass are used as substrates, and a 1 mm thick microscope slide for encapsulation. For the backreflector upconverters, spin coat on top of the substrate, and encapsulate with a standard microscope slide. Similarly, for the gold patterned upconverters, spin coat on the gold pillar side, and encapsulate with a standard microscope slide again. For the final fully integrated upconverter, the BHJ needs to be fully in contact with the backreflector and gold pillars, hence spin coat on both sides. Half the concentration of the Y6 / rubrene / BHJ mixture is used on the dichroic backreflector side, and full concentration on the gold patterned substrate. Spinning full concentration on both sides leads to quenching of emitted light and hence lower enhancement. On the other hand, spinning half concentration on both sides is detrimental to the functioning of the gold pillars, since they are not submerged fully in the BHJExample 6: Imaging setup

[0173] The transmission imaging setup used is shown in Fig. 17F. The IR mounted broadband LED with peaks at 770 nm, 860 nm, and 940 nm are used. Collimate and spatially filter the LED light before passing it through a negative USAF resolution test target. The input beam to the imaging system is on the order of ~30 mW, and then reduce it down to ~20 mW for images with fully integrated upconverter. An achromatic 1 :1 lens pair (L1 in Fig. 17F) with an effective focal length of 100 mm is used to form an intermediary image plane on the upconverter. This lens pair is preferred for L1 because of its low chromatic focal shift in the NIR, reducing chromatic aberrations in the captured images. L1 is placed such that it is a single focal length away from both the target and upconverter and adjusted slightly to bring the target in focus on camera. The image on the upconverter is magnified by ~1 .17. A short-pass filter with cutoff wavelength 700 nm is used in order to filter out the NIR light from our upconverted light. For the imaging lens (L2 in Fig. 17F), a plano-convex lens with a focal length of 100 mm is used, and this lens is adjusted to provide the best focus and largest magnification on the digital camera. This camera is used together with the 100mm f / 2.8 2X Ultra Macro APO lens and is set to:Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT f / 2,8, ISO 100.

[0174] For the flower images shown in Fig. 20F and Fig. 20G, an imaging setup which is similar to the transmission setup. NIR light from the LED is passed through a 1500 grit diffuser and onto the flower at around an angle of 45 degrees. A 20 mm achromatic aspheric lens is used in order to focus the scattered light from the flower onto the fully integrated upconverter. The distance between the flower and lens, and the lens and the upconverter are all kept nearly 20 mm (focal length of the lens) for 1 :1 imaging and adjusted very slightly to bring the flower into focus on the camera. After the upconverter a short-pass filter with cutoff wavelength 700 nm is used in order to filter out the NIR light from the upconverted light. Then plano-convex lens with a focal length of 50.8 mm is used for imaging on the digital camera.Example 7: Thin film coating design and deposition

[0175] In order to design a thin film coating using Nb2Os and SiO2, the transfer matrix method in MATLAB is implemented. An arbitrary number of alternate layers N is used and initialize the thicknesses di randomly. Then propagate plane waves with incident angle 0opt under two different configurations: 1 ) Starting from air to glass substrate, through the thin film stack, thin film Y6 / rubrene / DBP BHJ, glass, and air again. 2) Starting in Y6 / rubrene / DBP, passing through thin film stack, glass substrate and then air. In each configuration the transmission values are sum up. In the first case, the NIR region (800- 900 nm) is sum over and maximized, and in the second case, the visible region (500-600 nm) is sum over and minimized. A regularization parameter p is used to balance between the two objectives and create a final objective function which is then fed into the MATLAB nonlinear optimization function ’optimproblem’. Note that the incident angle 0opt describes the angle to optimize till, since any angle 0 < 0opt is automatically optimized for.

[0176] The optimization problem is run for several values of N, di, and p, with 0opt set to 20°. With larger values of N, it is easy to satisfy the objective function. However, to maintain the imaging resolution, the total thickness is kept below 1 pm and hence 8 layers design is used. The resulting design is tweaked using the regularization parameter untilStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT good performance across both NIR and visible wavelengths. The final thicknesses are given by: [61 ,69,69,103,62,107,60,104,60,100,62,55] nm, where the first layer is Nb2Os, the second is SiCh and repeated again and so forth. The Y6 / rubrene / DBP BHJ is designed to go after the last layer in this stack. This design is provided to the thin film and is deposited on 1 mm thick 1” diameter B-270 glass substrates using deposition method.Example 8: Gold nanopillars

[0177] The gold nanopillars are designed to provide a broadband resonance in the NIR range centered around 850 nm. To find the optimal parameters, a sweep across radii (50-110 nm), period (300-450 nm), and height (20-60 nm) of pillars is done and measured absorption inside Y6 / rubrene / DBP BHJ using FDTD. The absorption enhancement profiles are analyzed for all of sweeps and the parameters that provided the highest broadband absorption enhancement and stable performance on slight variance from set values are chosen, in order to account for fabrication errors.

[0178] The gold pillars are fabricated on ITO on glass squares (1.1 mm thick, 12-15 Ohm / Sq, 15 or 25 mm length). First clean the substrates by sonicating in acetone and then isopropanol (IPA). Then spin two layers of resist on the substrates. First spin PMMA 495K A3 at 2000 rpm, 60 sec, 500 rpm / sec and bake at 180°C for 3 minutes, and then repeat this process with PMMA 950K A6 at 4000 rpm, 60 sec, 500 rpm / sec. A bilayer resist is used in order to improve the resolution of our patterning. These samples are then exposed to the electron beam with current 4 nA and a total pattern size of D, where D was either 2, 4, or 6 mm. The exposed samples are then developed by sonicating in a 1 :3 ratio of MIBKJPA followed by plasma descum. Gold is deposited using electron beam evaporation. A 5 nm layer of titanium is deposited before gold for adhesion. These samples are then soaked in 99% anisole at 70°C overnight for liftoff. To complete the liftoff, they are sonicated in clean anisole, and then acetone to remove all gold. The last step is the SiO2 passivation layer, which is deposited using PECVD. The thickness of SiO2 is to be close to 7 nm, as measured on homogenous gold thin films using ellipsometry.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCTExample 9: Minimum intensity measurement

[0179] For the intensity-intensity plot shown in Fig. 20B the same power measurement system is used as Fig. 17A, with a few alterations. In order to measure upconversion yield at nightglow intensities (~107W / cm2), the beam is allowed to diverge after the beamsplitter until it was nearly 4 cm2in area, then use the same focusing lens setup in order to focus it down to nearly 0.004 cm2on the upconverter. Then the input power is measured at the upconverter plane using the thermal power sensor. Turn the ND wheel to the highest OD setting, and then decrease the input power by adding ND filters before the beam splitter, right until able to get measurable signal on the thermal power sensor. Record a minimum light intensity approaching 50 nW / cm2. Following the same method as before, record the input power as a function of reflected beam splitter power. Then several data measurements are done with bare glass, bare BHJ, and the fully integrated device. Measure the beam spot at the power meter plane to be nearly 0.005 cm2. All beam spot measurements are done using a monochrome cMOS camera.Example 10: Light extraction enhancement due to backreflector coating

[0180] To measure light extraction enhancement due to the addition of the backreflector coating in the upconverting material, 3D FDTD simulation shown in Fig. 21 A is used. The far field electric field for nested parameter sweeps going over 1 ) all three dipole orientations, and 2) five different dipole locations across the thickness of the upconverting layer. Then the far-field emission is integrated over all angles, which gives average far-field emission vs wavelength. Then the same process is run for the FDTD set up without the backreflector coating (i.e. , replace with glass), and divide the far-field vs wavelength values for these two cases (with coating / without coating) to get the light extraction enhancement vs wavelength plot shown in Fig. 18B.

[0181] Fig. 21 A shows FDTD simulation used to measure light extraction enhancement with the upconverter and our designed dichroic backreflector coating. To simulate isotropic emission from the upconverting material, the average far field emissionStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT from all three dipole positions is calculated independently. Then to make it more accurate, the dipole location across different thickness values is swept and average these out. Fig. 21 B and Fig. 21 C show the far field emission pattern from the 2D FDTD simulation, with and without the dichroic backreflector coating.Example 11 : Gaussian beam shift analysis with backreflector coating

[0182] To calculate the resolution loss due to reflections from the backreflector, the FDTD simulation is set up as shown in Fig. 22A. A Gaussian beam with 1 pm beam width and wavelength 610 nm, is incident at 20 degrees from Y6 / rubrene onto the thin film stack upon glass substrate. The reflected beam for s and p-polarized light is measured and the beam shift due to the backreflector coating is calculated. This beam shift turns out to be 0.23 pm for s-polarized light, and 0.12 pm for p-polarized light. Both of which are below the current imaging resolution and the ideal diffraction limited resolution for our system. At angles greater than 20 degrees, start approaching the Brewster’s angle between the Y6 / rubrene / DBP and air. Hence the reflection for p-polarized light starts dropping until all p-polarized light is transmitted at 30 degrees, while the s-polarized light is still reflected fully. After around 38 degrees, all light emitted from Y6 / rubrene / DBP layer is totally internally reflected at the glass-air interface - setting the angle of the escaping light cone from the upconverter. This is the case even without the presence of the backreflector coating.

[0183] Fig. 22A shows FDTD simulation at the interface between the upconverting material and the backreflector coating. A tilted Gaussian beam is incident on the thin film coating and a field monitor is used to measure the reflected beam at the imaging wavelength. Fig. 22B shows the reflected beam for s and p-polarized input with reference to incident beam. Fig. 22C and Fig. 22D show XY field profile of the simulation for s and p-polarization.Example 12: Absorption enhancement due to gold pillars

[0184] In order to measure the absorption due to gold pillars embedded in theStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT upconverting material, the FDTD simulation is set up as shown in Fig. 23A and Fig. 23B. To measure absorption inside Y6 / rubrene, a filter that separates the electric field values based on the index inside the absorption monitor is used. Then the absorption can be integrated and calculated separately for gold and Y6 / rubrene / DBP BHJ using these two sets of field data. For calculation of expected enhancement using gold pillars with the NIR LED, multiply the absorption profile with LED emissivity. Then sum over the 700 - 1000 nm range and divide by the sum for bare BHJ to obtain the absorption enhancement of ~2.1 with this LED source.

[0185] Fig. 23A and Fig. 23B show XZ (side) and XY (top) profile of the FDTD simulation used to measure the absorption enhancement inside the upconverting material due to embedded gold nanopillars.Example 13: Zemax simulation of lens around upconverter

[0186] Fig. 24 shows the lens setup used for the minimum intensity measurement results shown in Fig. 20B. This lens setup is consistent with Fig. 19A power measurement setup, the only difference is the higher degree of focusing (controlled by positioning of L1 and L2) and resulting smaller beam spot size on the upconverter. We use an achromatic doublet (L1 , Fig. 19A) and aspheric lens (L2, Fig. 19A) to focus a wide NIR beam on the upconverter, down to a 0.38 mm2. After the upconverter, a configuration of three singlet lenses (L3-L5, Fig. 19A) is used to capture light from the upconverter and focus it down to a 0.47 mm2beam spot on the power meter.Example 14: Quadratic to linear threshold lowering with gold pillars and the effect of passivation on upconversion yield

[0187] Fig. 27 shows the log-log plots of the input NIR power vs output visible power within and outside gold region for 5 separate upconverting devices show the quadratic- linear threshold for the upconversion devices with gold patterned regions. The threshold for outside and inside the gold region specifically are: 88 mW / cm2and 140 mW / cm2; 90 mW / cm2and 134 mW / cm2; 88 mW / cm2and 160 mW / cm2; 90 mW / cm2and 150 mW / cm2;Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT93 mW / cm2and 147 mW / cm2Additionally, the x-axis is scaled with the input beam spot (~1 mm2) in order to get input intensity to upconverter. Instead of a sharp kink at the threshold point, a gentle curve is shown. This is because the input wavelength range is very broad, hence, near the threshold value, there is a mix of photons in quadratic region versus linear region, leading to more smoothed over kink at the threshold point. However, even with this smoothing, it is very easy to observe that the upconversion within the gold region is consistently lower than outside.

[0188] In addition, the quenching of visible output without the presence of a passivation layer between the Y6 / rubrene BHJ and gold nanopillars is shown. A 1 .5 factor of increase in slope is observed with non-passivated gold pillars. After depositing 7 nm of SiO2 through PECVD, a 2.2 enhancement is achieved. Fig. 25 shows the measured visible power from a non-passivated gold nanopillar upconverter, compared with the passivated upconverters, demonstrating the quenching of visible output with the absence of a passivation layer.Example 14: Control sample for fully integrated upconverter

[0189] The control upconverter for the fully integrated upconverter is prepared by spin coating full concentration of the Y6 / rubrene / DBP mixture on one glass substrate, and spin coating half concentration on another glass substrate. These coated glass substrates are allowed to anneal separately and then they are clamped together and encapsulated using epoxy. Fig. 26 shows the visible power measured from this control sample, compared to the bare BHJ upconverter, which is prepared normally. The increase in upconversion yield by a factor of ~1 .6 is due to absorption from a total thicker film, plus some cavity effects due to airgap between the two layers of Y6 / rubrene / DBP inside the control upconverter. Fig. 26 shows the plot of the measured visible power against the input NIR power to the control upconverter, with the bare BHJ data plotted as reference.DOCTRINE OF EQUIVALENTS

[0190] This description of the invention has been presented for the purposes ofStanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.

[0191] As used herein, the singular terms “a,” “an,” and “the,” may include plural referents unless the context clearly dictates otherwise. Reference to an object in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.”

[0192] As used herein, the terms “approximately” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ± 10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1 %, less than or equal to ±0.5%, less than or equal to ±0.1 %, or less than or equal to ±0.05%.

[0193] Additionally, amounts, ratios, and other numerical values may sometimes be presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. Where ranges are described, the range should be understood to include the endpoints of the ranges, and the endpoints of such ranges are also contemplated to stand on their own as inventive, individual data points and to form the endpoints of other ranges. For example, a ratio in the range of about 1 to about 200Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, sub-ranges such as about 1 to about 10, about 10 to about 50, about 20 to about 100, about 100 to about 200, and so forth, and related ranges such as greater than about 1 or less than about 200.

Claims

Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCTWHAT IS CLAIMED IS:1 . A triplet-triplet annihilation photon upconversion (TTA-UC) system, comprising: a sensitizer, an annihilator, and an emitter; wherein the sensitizer, the annihilator, and the emitter are mixed to form a bulk heterojunction structure; and wherein the TTA-UC system upconverts a near-infrared wavelength to a higher- energy wavelength.

2. The TTA-UC system of claim 1 , wherein the sensitizer is selected from the group consisting of: PbS quantum dots, HgTe quantum dots, Ag2Se quantum dots, AgAuSe quantum dots, MoTe2 monolayers, near-infrared absorbing non-fullerene acceptors; wherein the annihilator is selected from the group consisting of: TES- ADT and di-fluorinated derivative of TES-ADT; wherein the emitter is selected from the group consisting of: DBP, BTZ-DMAC, IR-786, TXO-TPA, HIDC iodide, K1295, CdSe quantum dots, and perovskite nanocrystals.

3. The TTA-UC system of claims 1 or 2, wherein the sensitizer are quantum dots comprising an organic ligand.

4. The TTA-UC system of claim 3, wherein the organic ligand is 5-tetracene carboxylic acid.

5. The TTA-UC system of claim 4, wherein the sensitizer is PbS quantum dots with 5-tetracene carboxylic acid ligands, the annihilator is TES-ADT, and the emitter is DBP.

6. The TTA-UC system of any one of claims 1 -5, wherein the near-infrared wavelength is greater than or equal to 800 nm.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT7. The TTA-UC system of any one of claims 1 -6, wherein the near-infrared wavelength is greater than or equal to 1000 nm.

8. The TTA-UC system of any one of claims 1 -7, wherein the near-infrared wavelength is greater than or equal to 1200 nm.

9. The TTA-UC system of any one of claims 1 -8, wherein the higher-energy wavelength is a visible wavelength.

10. The TTA-UC system of claim 9, wherein the near-infrared wavelength is greater than or equal to 800 nm, and the visible wavelength is greater than or equal to 600 nm.

11. An upconversion system, comprising: a plurality of resonant structures, and a TTA-UC system; wherein the plurality of resonant structures are embedded in the TTA-UC system; wherein the TTA-UC system is deposited on a substrate; wherein the upconversion system upconverts a near-infrared wavelength to a visible wavelength.

12. The upconversion system of claim 11 , wherein the substrate is transparent to an incident near-infrared wavelength.

13. The upconversion system of claims 11 or 12, wherein the substrate comprises a material selected from the group consisting of: silicon, glass, NBK7 glass, quartz, and sapphire.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT14. The upconversion system of any one of claims 11 -13, further comprising an electrically conductive layer between the substrate and the plurality of resonant structures.

15. The upconversion system of claim 14, wherein the electrically conductive layer comprises ITO and has a thickness greater than or equal to 100 nm and less than or equal to 300 nm.

16. The upconversion system of any one of claims 11 -13, further comprising a low loss dielectric layer between the substrate and the plurality of resonant structures.

17. The upconversion system of claim 16, wherein the low loss dielectric layer comprises silicon oxide and has a thickness greater than or equal to 500 nm and less than or equal to 5 microns.

18. The upconversion system of any one of claims 11-17, wherein at least one of the plurality of resonant structures has a shape selected from the group consisting of: a cuboid, a cube, a pillar, a nanopillar, a circular pillar, a square pillar, a rectangular pillar, a rod, a nanorod, a plus shaped pillar, a bowtie pillar, a cylinder, an elliptical cylinder, a trapezoid, a triangular prism, a polygonal prism, a pyramid, a sphere, and a nanosphere.

19. The upconversion system of any one of claims 11-18, wherein the plurality of resonant structures comprise a material selected from the group consisting of: a metal, a metal oxide, gold, silver, aluminum, a semiconductor, silicon, germanium, gallium phosphide, gallium arsenide, silicon carbide, titanium oxide, silicon nitride, barium titanate, lithium niobate, tantalum pentoxide, a nanoparticle, a gold nanoparticle, a silver nanoparticle, and an aluminum nanoparticle.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT20. The upconversion system of claim 19, wherein the plurality of resonant structures comprise a metal, wherein the upconversion system further comprises a passivation layer on the plurality of resonant structures.

21. The upconversion system of claim 20, wherein the passivation layer comprises a material selected from the group consisting of: an oxide, silicon oxide, aluminum oxide, and hafnium oxide.

22. The upconversion system of any one of claims 11-21 , wherein the TTA-LIC system is a system in any one of claims 1 -10.

23. An upconversion system, comprising: a plurality of thin films; and a TTA-UC system; wherein a first side of the plurality of thin films is in contact with a substrate; wherein a second side of the plurality of thin films is in contact with the TTA-UC system; and wherein the system upconverts a near-infrared wavelength to a higher-energy wavelength.

24. The upconversion system of claim 23, further comprising a second plurality of thin films deposited between the TTA-UC system and a second substrate.

25. The upconversion system of claims 23 or 24, wherein the plurality of thin films comprise alternating layer of a high index material and a low index material.

26. The upconversion system of claim 25, wherein a thickness of each of the plurality of thin films is greater than or equal to 10 nm and less than or equal to 1000 nm.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT27. The upconversion system of claims 25 or 26, wherein the high index material is titanium oxide or niobium oxide, and the low index material is silicon oxide.

28. The upconversion system of any of claims 23-27, wherein the substrate is transparent to an incident near-infrared wavelength.

29. The upconversion system of claim 28, wherein the substrate comprises a material selected from the group consisting of: silicon, glass, NBK7 glass, quartz, and sapphire, wherein the substrate has a thickness greater than or equal to 1 mm and less than or equal to 10 mm.

30. The upconversion system of claim 23, wherein a plurality of plasmonic structures are embedded in the TTA-UC system, wherein the plurality of resonant structures are deposited on a second substrate.

31. The upconversion system of claim 30, wherein at least one of the plurality of resonant structures has a shape selected from the group consisting of: a cuboid, a cube, a pillar, a nanopillar, a circular pillar, a square pillar, a rectangular pillar, a rod, a nanorod, a plus shaped pillar, a bowtie pillar, a cylinder, an elliptical cylinder, a trapezoid, a triangular prism, a polygonal prism, a pyramid, a sphere, and a nanosphere.

32. The upconversion system of claims 30 or 31 , wherein the plurality of resonant structures comprise a material selected from the group consisting of: a metal, a metal oxide, gold, silver, aluminum, a semiconductor, silicon, germanium, gallium phosphide, gallium arsenide, silicon carbide, titanium oxide, silicon nitride, barium titanate, lithium niobate, tantalum pentoxide, a nanoparticle, a gold nanoparticle, a silver nanoparticle, and an aluminum nanoparticle.Stanford: S24-290WARE Ref. P250012US01KPPB: S31-09320.PCT33. The upconversion system of claims 30 or 31 , wherein the plurality of resonant structures comprise a metal, a metal oxide, gold, silver, or aluminum; wherein the system further comprises a passivation layer comprising a material selected from the group consisting of: an oxide, silicon oxide, aluminum oxide, and hafnium oxide.

34. The system of any one of claims 23-33, wherein the TTA-UC system is a system in any one of claims 1 -10.

35. An optical system, comprising a vision system and an upconversion system in any one of claims 11 -34.

36. An optical system, comprising a CCD array and an upconversion system in any one of claims 11 -34 deposited on top of the CCD array.

37. An optical system, comprising a CMOS array and an upconversion system in any one of claims 11 -34 deposited on top of the CMOS array.

38. An optical system, comprising an upconversion system in any one of claims 11-34 and photovoltaic cell adjacent to the upconversion system.

39. A system, comprising an upconversion system in any one of claims 11-34 embedded in a platform for anti-counterfeiting; wherein the platform is selected from the group consisting of: a currency paper, a document, and a computer chip.

40. A drug delivery system, comprising an upconversion system in any one of claims 11 -34 positioned adjacent to a tissue.

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