A method for quantum control of upconversion luminescence of rare earth-doped materials based on micro-nano resonant structure

Abstract

The invention discloses a method for realizing quantum control of up-conversion luminescence of rare earth-doped materials based on a micro-nano resonance structure. By modulating the resonance mode, the radiation transition of the luminescence channel is suppressed, and the particles in each excited state are redistributed, including energy transfer and non-radiation transition of particles between different excited states. When the resonance wavelength of the microcavity is at the emission wavelength of the nanoparticle, the excited state of an allowed channel is made to gather more particles, thereby enhancing its luminescence. By correcting the light field mode in the micro-nano resonance cavity, the multi-step electron transition process (including radiation and non-radiation transition) between multiple energy levels is quantum controlled through the resonance mode. When the resonance wavelength of the microcavity is at the emission wavelength of the nanoparticle, the up-conversion luminescence intensity at that location is enhanced, while the up-conversion luminescence of other unnecessary emission wavelengths is suppressed, thereby suppressing the unnecessary luminescence process, and promoting the luminescence process at the required specific wavelength, thereby realizing monochromatic luminescence.

Description

Technical Field

[0001] The invention relates to a luminescence technology, in particular to a method for controlling up-conversion luminescence of rare earth-doped materials. Background Art

[0002] In recent years, due to the characteristics of rare earth lanthanide elements such as long excited state energy level lifetime and energy level ladder distribution, upconversion luminescent materials doped with lanthanide ions have attracted the attention of a large number of researchers, and gradually revealed their application value in bio-labeling and imaging, lighting, three-dimensional display, laser, solar cells, etc. In the upconversion luminescence process, the activator is usually an ion that is doped as a luminescence center. The upconversion luminescence process refers to the luminescence center absorbing two or more low-energy photons, exciting the ground state electrons to a lower excited state, and through excited state absorption or non-radiative energy transfer and other processes, the electrons jump from a high energy level to the ground state or low energy level, thereby radiating high-energy photons. Figure 1 Yb 3+ / Er 3+ The energy level diagram of the ion shows the main upconversion luminescence processes involved in the visible light range. The solid line, dotted line, and broken line in the figure represent the radiation transition and excitation, energy transfer, and radiationless relaxation processes respectively. Under the excitation of near-infrared light (λ~980nm), Yb 3+ The ion absorbs a photon from the ground state 2F 7 / 2 Transition to the excited state 2F 5 / 2 , which then transfers the energy to Er 3+ Different energy levels of ions, Er in various excited states 3+ The ion finally returns to the ground state through multiple steps of multiphoton radiationless relaxation and radiative relaxation, and emits visible light. 3+ Ions from 4F 9 / 2 To 4I 15 / 2 The energy level transition emits red fluorescence with a central wavelength of 650nm, from 4S 3 / 2 or 2H 11 / 2 To 4I 15 / 2 The energy level transition emits green fluorescence with a central wavelength of 520nm or 540nm. Usually under 980nm excitation, Yb 3+ The sensitization effect of Yb 3+ , Er 3+ The interaction between 3+ From 4I 13 / 2 The transition to the ground state emits light at 1530nm.

[0003] Since upconversion luminescence is a nonlinear process, its luminescence process often involves multi-step electronic transitions and multi-wavelength photon emission between multiple energy levels of excited rare earth ions, as well as energy transfer and quenching effects between adjacent ions (limiting the doping concentration of rare earth ions), so that the luminescence efficiency of rare earth-doped materials (especially its upconversion luminescence) is generally low, and it is difficult to meet its application in micro-optical devices. How to improve the upconversion luminescence efficiency has become a major research hotspot. In addition to optimizing the composition and crystal structure of nanomaterials and encapsulating nanocrystals to form core-shell nanostructures, various other measures have been taken to improve the efficiency of upconversion luminescence. Among them, enhancing the upconversion luminescence efficiency by structured plasmon metal nanostructures has been widely studied. In these studies, strongly localized plasmons are usually non-resonant or low-order resonances with wide resonance bands. Therefore, the local field enhancement of surface plasmons is indiscriminately applied to all emission bands and excitation light of interest. This results in an almost uniform enhancement of upconversion luminescence at the emission wavelength. However, in many applications of upconversion luminescence, multicolor luminescence is unnecessary, and strong monochromatic luminescence is more desired. Summary of the invention

[0004] The object of the present invention is to provide a method for converting up-conversion luminescence into timbre light by regulating a micro-nano resonant cavity.

[0005] To achieve the above-mentioned purpose, the present invention provides a method for realizing quantum control of upconversion luminescence of rare earth-doped materials based on micro-nano resonant structure. Under the excitation of near-infrared light, the sensitizer of the upconversion luminescent material can effectively absorb external radiation energy and quickly transfer it to the activator ion. The activator ion absorbs photons and transitions from the ground state to the excited state, and then transfers the energy to different energy levels of the upconversion luminescent material ion. The activator ions in each excited state finally return to the ground state through multi-step multi-photon radiationless relaxation and radiation relaxation, and emit visible light. The activator ions and sensitizer ions of the upconversion luminescent material are made into nanocrystalline particles and arranged in a micro-nano resonant cavity to adjust the resonant wave of the micro-nano resonant cavity. The length of time it takes to be at the emission wavelength of the activator ion will cause the multi-step transition of the activator ion, enhance the up-conversion luminescence intensity at that location, and suppress the radiative transition of the luminescence channel through the modulation of the resonant mode, prompting the redistribution of particles in various excited states, including the energy transfer and non-radiative transition process between particles in different excited states. When the resonant wavelength of the microcavity is at the emission wavelength of the nanoparticles, at the resonant wavelength, the mode density increases, the spontaneous radiation rate increases, and the optical resonant cavity in the resonant state can allow photons that meet certain frequency matching conditions to pass through, so that the excited state of a channel that is allowed gathers more particles, thereby suppressing the up-conversion luminescence of other emission wavelengths and realizing the up-conversion luminescence as monochromatic light. The activator ion and sensitizer ion are Er, respectively. 3+ and Yb 3+, to obtain NaYF4:Yb 3+ / Er 3+ Nanocrystalline particles; Yb-doped 3+ / Er 3+ The nanocrystalline particles are sandwiched in a Fabry-Parrot resonant cavity between DBR multilayer dielectric reflectors. The upper and lower DBR multilayer dielectric reflectors are composed of two layers of dielectric films with different refractive indices. 3+ / Er 3+ Nanocrystalline particle layer; adjust the thickness and resonant wavelength of the Fabry-Parrot resonant cavity between the DBR multilayer dielectric reflectors to form a defect mode on the bandgap of the DBR multilayer dielectric reflector. When the defect mode is at about 540nm, the green fluorescence is enhanced and can be emitted, while the red fluorescence is still in the bandgap range and cannot be emitted; adjust the thickness and resonant wavelength of the resonant cavity. When the defect is at 650nm, the red fluorescence can be emitted, thus achieving the emission of monochromatic light; doping Yb 3+ / Er 3+ The nanocrystalline particles are arranged on a composite metal grating with a dual surface plasmon resonance mode. When the dual resonance mode matches the green and red upconversion luminescence UCL bands at the same time, both the green and red UCL emissions can be enhanced; when the dual state resonance mode matches both the excitation wavelength and the green or red UCL band, the UCL of the emission band matching the resonance mode can be particularly enhanced, and the enhancement factor is greater than that of the single plasmon resonance mode of the single period metal grating when it matches the excitation wavelength or the emission wavelength. 3+ / Er 3+ Enhancement of UCL of nanocrystalline particles.

[0006] The micro-nano resonant cavity is a Fabry-Perot resonant cavity, or FP cavity. The resonant cavity is a layer of metal film thermally evaporated on a glass substrate, a dielectric film is deposited on the metal film, and a Yb-doped layer is arranged in the middle of the dielectric film. 3+ / Er 3+ Nanocrystalline particle layer, and then deposit a metal film on the dielectric film; by adjusting the thickness of the dielectric film between the metal films, the resonant light field mode of the Fabry-Parrot microcavity structure formed above is adjusted, so as to achieve the effect of Yb-doped 3+ / Er 3 + Quantum control of the luminescence of nanocrystal particles; the resonant wavelength of the microcavity is at the emission wavelength of the nanoparticles, which enhances the upconversion luminescence intensity at this location, and uses the anti-resonance mode of the microcavity to suppress the upconversion luminescence of other emission wavelengths. By internally adjusting the Er in the resonant cavity mode 3+ Multi-step transition of ions to achieve single-color upconversion luminescence and enhancement;

[0007] After adopting the above scheme, by modifying the light field mode in the micro-nano resonant cavity, the multi-step electronic transition process between multiple energy levels (including radiative and non-radiative transitions) is quantum controlled through the resonant mode; when the resonant wavelength of the microcavity is at the emission wavelength of the nanoparticles, the up-conversion luminescence intensity at this location will be enhanced, and the enhancement factor will reach several orders of magnitude, while suppressing the up-conversion luminescence of other unnecessary emission wavelengths to suppress the unnecessary luminescence process, and promote the luminescence process at the required specific wavelength, thereby achieving monochromatic luminescence and greatly improving the luminescence efficiency at this wavelength. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 Yb 3+ / Er 3+ Energy level diagrams of ions and the main upconversion luminescence processes involved in the visible light range;

[0009] Figure 2 Schematic diagram of the cross-section of the Fabry-Perot resonant cavity structure of Example 1 of the present invention;

[0010] Figure 3 are the sample reflection spectra with the 0th-order resonance positions at 540 nm and 650 nm in Example 1 of the present invention;

[0011] Figure 4 (a) is an up-conversion fluorescence spectrum of the sample with a 0th order resonance wavelength at 650nm in Example 1 of the present invention, and (b) is an up-conversion fluorescence spectrum of the sample with a 0th order resonance wavelength at 540nm;

[0012] Figure 5 This is an up-conversion fluorescence spectrum of a sample with a first-order resonance wavelength of 540 nm under 975 nm laser excitation in Example 1 of the present invention;

[0013] Figure 6 Schematic diagram of the cross-section of the DBF resonant cavity structure of Example 2 of the present invention;

[0014] Figure 7 are the reflectance spectra of the samples at 540nm and 650nm respectively;

[0015] Figure 8 The fluorescence spectra of the DBF resonant cavity band gap defect at 540nm and 650nm in Example 2 of the present invention are converted respectively;

[0016] Fig. 9 The surface profile of a double-line multiplexed grating formed by superimposing two sinusoidal gratings with periods p1 and p2.

[0017] Fig.10 Schematic diagram of UCNPs-coated multiplexed metal grating;

[0018] Fig.11Reflection spectra of the prepared UCNPs-coated multiplexed metal gratings with (p1, p2) = (462600) and (600947) nm;

[0019] Fig.12 (a, c) Reflection (a, c) and UCL spectra (b, d) of singlet (a, b) and multiplexed doublet (c, d) metal gratings coated with UCNPs; (e) is the enhancement factor of the UCL integrated intensity of the sample relative to UCNPs on bare flat glass. DETAILED DESCRIPTION

[0020] In order to explain the technical content, structural features, achieved objectives and effects of the present invention in detail, the following is a detailed description in conjunction with the implementation methods and the accompanying drawings.

[0021] In order to achieve monochromatic luminescence (with a certain bandwidth) of rare earth ions and further enhance their luminescence efficiency, we propose a new approach and method: based on the enhancement of plasmon light field, we focus on the "inside" of the luminescence process of rare earth ions, and use resonant modes to quantum control the multi-step electron transition process between multiple energy levels (including radiative and non-radiative transitions); thereby suppressing unnecessary luminescence processes and promoting luminescence processes at the required specific wavelength, thereby achieving monochromatic luminescence and greatly improving the luminescence efficiency at this wavelength.

[0022] In simple terms, Figure 1 Er-doped 3+ / Yb 3+ The luminescent system, after excitation by near-infrared light (~980nm), energy transfer and multi-photon, multi-step upward transition, the excited Er 3+ There are three main radiative relaxation channels for luminescence (red, green, and IR), of which red and green are upconversion luminescence. Erbium atoms convert into Er by losing one electron in the 4f orbit and two electrons in the 6s orbit. 3+ Ion (5s5p energy level electrons remain unchanged), Er 3+ The electronic structure of the ion is [Xe]4f N-1 6s 0 ,[Xe] is the full layer structure of xenon. Since the remaining N-1 inner electrons are shielded by the 5s and 5p layers, the absorption and emission of photons during the 4f-4f energy level transition are not disturbed by the external environment. The different arrangements of the 4f layer electrons lead to the generation of rich energy levels, and the transitions between different energy levels are very important for Er 3+ The upconversion luminescence of ions and even the entire lanthanide ions plays a decisive role.

[0023] For Er 3+Ion 4f energy level, the ground state 4I is split into four states through the interaction between spin and orbit: J = 9 / 2, 11 / 2, 13 / 2, 15 / 2. According to Hund's rule, 4f 11 If the number of electrons is greater than half full, then for the four split states, the larger the total angular momentum quantum number J, the lower the energy level.

[0024] By modulating the resonance mode of the plasmon light field, the radiation transition of two luminescence channels is suppressed, and the particles in each excited state are redistributed (including energy transfer and non-radiative transition between particles in different excited states), so that the excited state of one allowed channel gathers more particles, thereby enhancing its luminescence. In this process, due to the mutual coupling of various physical parameters, the transitions between energy levels involved, as well as other related relaxation and energy transfer processes will also be regulated accordingly.

[0025] In free space, since the electromagnetic field contains various continuous modes, any excited atom is allowed to emit photons. However, when the excited atom is in a resonant cavity, only specific cavity modes will selectively enhance or suppress the spontaneous emission of the excited atom. The multiple of the spontaneous emission probability enhancement can be expressed by the Purcell factor:

[0026]

[0027] Where n is the refractive index of the cavity, V is the cavity mode volume, and (λ0 / n) is the resonant wavelength in the cavity.

[0028] The Purcell effect refers to the change in the mode density of the electromagnetic field in the cavity under the influence of the external environment, which affects the spontaneous radiation of the luminous particles in the cavity. The Purcell effect can be explained by Fermi's golden rule.

[39] To explain:

[0029] For a two-level system, the spontaneous emission rate of atoms in the resonant cavity can be expressed as:

[0030]

[0031] Among them, |<f|H|i> | represents the transition matrix element of the H operator acting on the ground state and excited state under the interaction between atoms and electromagnetic fields, |f> represents the low-energy ground state, |i> represents the high-energy excited state, and ρ(w) is the electromagnetic field mode density, that is, the number of modes contained in a unit frequency interval. Since the mode density at the resonant frequency is

[0032]

[0033] Where V is the cavity mode volume; Δυ is the spectrum width of the resonance. The electromagnetic field mode density in free space is

[0034]

[0035] Then the ratio of the electromagnetic field mode density at the resonance point to that in free space is

[0036]

[0037] When we express the quality factor Q as follows,

[0038]

[0039] The ratio of the two mode densities can be simplified to

[0040]

[0041] That is to say, the multiple by which the photon state density increases under the action of the microcavity is proportional to the quality factor and inversely proportional to the cavity mode volume.

[0042] In quantum systems, the spontaneous emission rate (and excited state lifetime) is not an intrinsic property of atoms. Under the action of an optical microcavity, the external electromagnetic field can control the spontaneous emission by regulating the local photon mode density. The higher the quality factor of the cavity structure and the smaller the cavity mode volume, the larger the purcell factor. It is worth noting that the photon mode density not only affects the spontaneous emission rate, but also has a profound impact on other processes of the interaction between light and matter.

[0043] Embodiment 1 of the present invention uses a Fabry-Perot (FP) resonant cavity.

[0044] The FP cavity is a common multi-beam interference optical resonant cavity in modern optics. It is mainly composed of two parallel reflectors. Generally, the surfaces facing each other are coated with silver film or other materials with high reflectivity. The incident light is repeatedly reflected in the cavity, so that the outgoing light interferes and produces a filtering effect.

[0045] The Fabry-Perot (FP) metal microcavity of the present invention adopts NaYF4:Yb 3+ (18%) / Er 3+ (2%) nanocrystal particles to up-convert luminescence, such as Figure 2 To prepare the nanoparticle-doped FP resonant cavity of the present invention, a thin silver film of about 45 nm and a MgF2 dielectric layer with adjustable thickness are first evaporated on a clean glass substrate by thermal evaporation, and then NaYF4:Yb 3+ (18%) / Er 3+(2%) nanocrystalline particles are evenly spin-coated on the substrate, and finally the MgF2 dielectric layer and 200nm Ag film of the same thickness as the lower layer are evaporated in sequence. The top 200nm Ag film acts as a reflector, reflecting the light entering the FP resonant cavity back along the original path. By adjusting the thickness of the MgF2 dielectric layer, that is, the FP cavity length, the reflection resonance wavelength in the visible and near-infrared spectrum range is changed, thereby achieving luminescence modulation of the nanocrystalline particles.

[0046] Due to the absorption loss of the metal, the resonant cavity quality factor is relatively low (low Q value), firstly due to Er 3+ / Yb 3+ The energy levels of ions are known, NaYF4:Yb 3+ / Er 3+ The upconversion luminescence emission bandwidth of nanocrystalline particles is around 20-30nm, and a resonant mode with a high quality factor (high Q) is not required. Secondly, high-Q modes usually have a narrow free spectrum range, and adjacent resonant modes may not want to overlap with other emission bands or excitation wavelengths, so it is unlikely to control the upconversion luminescence enhancement through resonant modes. High Q values ​​cannot control the upconversion luminescence at the resonant mode, but sufficient quality factors are also required to offset adjacent upconversion emission bands. In the invention, the lowest-order resonant mode of the metal cavity we prepared has a bandwidth of about 30-50nm, which can selectively control Er 3+ The red and green upconversion luminescence of the ions. Since the cavity length of the resonant cavity is small, in the range of 100nm-200nm, although the quality factor is low, the Purcell factor (Fp~Q / V) of this mode can still be very high.

[0047] Resonant cavities with different MgF2 thicknesses are prepared by thermal evaporation, thereby changing the cavity length of the resonant cavity and achieving the purpose of controlling the resonant position. Figure 3 The resonant wavelength of the resonant cavity is 540nm (λ c (0) =540nm) and 650nm(λ c (0) =650nm), the reflection spectrum has the lowest reflectivity at the resonant position, and the light is trapped in the cavity and oscillates back and forth. Figure 4 This is the up-conversion fluorescence spectrum of the corresponding resonance position under 975nm laser excitation (power 330mW). It can be seen from the figure that when λ c (0) = 650nm, the red up-conversion fluorescence is significantly enhanced compared to the green fluorescence, such as Figure 4 (a); and when λ c (0)=540nm, the red fluorescence intensity and green fluorescence intensity are greatly weakened, but we can see that the green-red light intensity ratio is still increased compared with the nanocrystalline particle sample in MgF2. Figure 4 (b). When we continue to increase the thickness of the MgF2 dielectric layer so that the first-order resonance is located at 540nm (λ c (0) =1075nm,λ c (1) =540nm), it can be clearly seen that the fluorescence intensity of 540nm green light is much greater than the red fluorescence intensity of 650nm and the green fluorescence intensity of 520nm. Figure 5 After up-conversion fluorescence tests on samples at three different resonant positions, the fluorescence phenomenon fully confirmed the regulation effect of the resonant mode of the microcavity on the luminescence intensity of NaYF4 nanocrystal particles.

[0048] Usually, due to the strong attenuation of incident and outgoing light when passing through a thin metal layer, the luminescence intensity of nanoparticles in a metal cavity measured externally should be significantly weaker than that of nanoparticles coated with MgF2 alone. However, when the cavity resonant wavelength is 650nm, the red upconversion fluorescence is enhanced exponentially. The reason is that the emission wavelength of green light is between the 0th and 1st order resonant wavelengths of the FP cavity, resulting in Er 3+ Ions in addition to the red light excitation energy level 4 F 9 / 2 Other 4 S 3 / 2 and 2 H 11 / 2 The saturation distribution of the two green light excitation energy levels makes a larger portion of Er 3+ The ions are excited to 4 F 9 / 2 The energy level increases, which enhances the red up-conversion fluorescence, while the green up-conversion fluorescence is suppressed or even prohibited.

[0049] Similarly, when the first-order resonance wavelength of the FP cavity is 540nm (i.e., λ c (1) =540nm,λ c (0) =1075nm), more Er 3 + The ions are excited to 4 S 3 / 2 The energy level leads to a significant enhancement of the 540nm green fluorescence and the suppression or even prohibition of the red fluorescence. When the 0th order resonance wavelength of the cavity is 540nm, the red up-conversion luminescence is not prohibited because the 650nm fluorescence emission wavelength is greater than the resonance wavelength of the cavity and does not cause destructive interference to the emitted photons. Furthermore, the corresponding 540nm green fluorescence emission 4 S3 / 2 Ions of energy levels can also transition non-radiatively to 4 F 9 / 2 Energy level, so the red upconversion fluorescence cannot be effectively suppressed. Due to the attenuation from the metal film layer, in the 0th order resonance 540nm fluorescence spectrum, we can only observe weak upconversion luminescence from nanocrystal particles.

[0050] The second embodiment of the present invention adopts a Distributed Bragg Reflector (DBR), such as Figure 6 NaYF4:Yb 3+ / Er 3+ Nanoparticles (NP) are sandwiched in a Fabry-Perot cavity between DBR multilayer dielectric reflectors. DBR multilayer dielectric is set on a glass substrate, with four layers of MgF2 evenly distributed in ZnS on the top and bottom, and NaYF4:Yb in the middle layer. 3+ / Er 3+ Nanocrystalline particle layer. Figure 7 As shown, in the experiment, when there is no resonant cavity, under the excitation of 975nm laser (power 330mW), at 650nm, the red up-conversion fluorescence is significantly enhanced, and at 540nm the green fluorescence is significantly enhanced. Figure 8 As shown in the figure, the corresponding relationship between the bandgap and the up-conversion light in the Fabry-Parrot resonant cavity between the DBR multilayer dielectric reflectors of the present invention changes. We found that under normal circumstances, the 650nm red fluorescence and 540nm green fluorescence of the up-conversion light are both within the bandgap range. However, by adjusting the thickness and resonant wavelength of the Fabry-Parrot resonant cavity between the DBR multilayer dielectric reflectors of the present invention, a defect mode is formed on the bandgap of the DBR multilayer dielectric reflector. When the defect is at 540nm, the green fluorescence is enhanced and emitted from the defect, while the red fluorescence is still unable to be emitted within the bandgap range, and its energy is converted into green fluorescence energy. By adjusting the thickness and resonant wavelength of the resonant cavity, the red fluorescence can be emitted from the defect when the defect is at 650nm, thereby achieving the emission of monochromatic light.

[0051] A third implementation of the present invention is that a "multiple resonance" microcavity structure has at least two groups (m=1, 2, ...; n=1, 2, ...; ...) of resonance modes, and the resonance frequencies of the modes belonging to different groups can be adjusted independently.

[0052] The upconversion luminescence (UCL) of NaYF4:Yb,Er nanoparticles is resonantly controlled and enhanced using multiplexed metal gratings by designing dual-state plasmon resonance modes located at either the green and red UCL emission wavelengths, or the excitation and emission wavelengths, to improve the excitation / emission efficiency of the nanomaterials. When the dual-state plasmon resonance mode is matched to both the excitation wavelength and the green or red UCL emission wavelength, the UCL enhancement can be further improved, accompanied by a substantial modification of the green to red UCL intensity ratio.

[0053] Localized plasmons in metallic nanoelement usually have low quality factor (Q), broadband dipole resonance characteristics, but can strongly enhance the local field. On this basis, localized SPs enhance UCL by enhancing the local field of the excitation light as well as the Purcell factor (Fp~Q / V) of the UCL emission light due to their ultrasmall mode volume (V). However, the stronger confinement of the excitation field by the localized SP modes is usually accompanied by the quenching of the emitted photons. As a result, the enhancement factor is usually low, ranging from 0.3 to 21, as reported in the literature review. SP resonances in periodic metallic structures usually exhibit narrowband Bloch wave-like resonance characteristics and are highly tunable in the resonance position by controlling the structure dimensions (e.g., period). In the literature reports, Bloch-type SP resonant modes are usually designed to be located at the excitation wavelength to enhance the UCL by enhancing the excitation field. Therefore, the Purcell coefficient of the UCL emission is usually very low, such as about one to two, while simultaneous resonant enhancement of excitation and emission has always been desired. This requires spatially overlapping, multiple sets of independent resonant modes to match the frequencies of the excitation and emission light. Nevertheless, ordinary plasmon resonators or conventional resonators cannot easily provide such resonance modes. For example, the serial multiple modes of conventional resonators (e.g., Fabry-Perot cavities, distributed feedback resonators, and whispering-gallery mode resonators, etc.) are intrinsically correlated, so their resonance frequencies cannot be independently and arbitrarily tuned to match the resonance frequencies of the excitation and emission light, if not by chance. Cascaded resonators cannot be used either because they lack the spatial overlap of the resonant fields. In the field of plasmons and metamaterials, there have been some reports demonstrating multiple resonance modes. However, these plasmon modes are either coupled to each other in the near field or combined with common structural features. Therefore, they are also correlated and their resonance positions cannot be adjusted independently.

[0054] Fig. 9 The surface profile of a two-line multiplexed grating formed by superimposing two sinusoidal gratings with periods p1 (dark solid line) and p2 (light solid line) is shown. In general, the surface profile of a multiplexed grating can be expressed as:

[0055]

[0056] where pi and ai are the period and amplitude of the ith grating component, respectively, representing the average level of the surface profile. M is the multiplexing number, usually a maximum of two or three is sufficient in most applications. Based on multiple exposure holographic lithography (see Methods), doublet and triplet multiplexed gratings were fabricated.

[0057] Fig.10 Schematic diagram of UCNPs-coated multiplexed metal gratings. Fig.11 The reflection spectra of the prepared UCNPs-coated multiplexed metal gratings with (p1, p2) = (462600) and (600947) nm, whose double-peak resonance positions are located in the green and red UCL emission bands, or one in the red UCL emission band and the other at the excitation wavelength.

[0058] Fig.10 Schematic diagram of the multiplexed metal gratings of the UCNPs coating in the study. Fabrication of the UCNPs coating. The multiplexed gratings were fabricated using multiple exposure holographic lithography. In the holographic exposure setup, a 325nm He–Cd laser was used and the laser beams were expanded, collimated, split and recombined as usual to interfere on the photoresist-coated substrate. By changing the crossing angle between the interfering beams, the photoresist will be exposed to UV light with different periodic sinusoidal distribution intensities. In the experiment, we used a thin layer of photoresist (about 120nm thick) spin-coated on a clean silicon substrate. In the multiple exposures, it was assumed that the directions of the grating lines were well aligned. After careful exposure and development of the photoresist, a silver film (about 150nm thick) was conformally deposited on top, followed by a MgF2 layer (about 20nm thick) using a thermal evaporator. UCNPs were coated on gratings or flat substrates by immersing a UCNP solution dispersed in cyclohexane (molar concentration 0.01 mmol / mL) on its surface and spin coating at 3000 rpm for 30 seconds. In order to reduce the quenching effect, a spacer layer (MgF2) of about 20 nm thick was deposited on the corrugated metal (Ag) layer (about 150 nm thick) before coating the UCNPs. As shown in TEM images, the diameter of the UCNPs is about 25 nm. The constitutive period is (p1, p2) = (462600) nm. The UCNPs are mainly located in the grooves of the multiplexed grating, and the maximum depth of the grooves is about 35 nm.

[0059] Fig.11The measured reflection spectra of the above sample (p1, p2) = (462600) nm and another sample (p1, p2) = (600947) nm under normal incidence of transverse magnetic (TM) polarized light are shown. In the spectra, the Double-SP resonant modes indicated by the reflection dips are located in the green and red UCL emission bands, or in the red UCL emission band and the excitation wavelength (λ = 975 nm). Similar to the case of singlet metallic gratings, the positions of the doublet SP resonant modes can be approximately estimated by the following, where the constitutive grating period is the effective refractive index of the SP mode on the quasi-planar metallic surface and m is the resonance order (m = 1 is usually used). Note that due to the nature of the SP resonant modes being Bloch waves at the band edges,34 the reflection dips may usually split under non-ideal conditions of slightly tilted incident angles in experiments. For longer wavelengths (corresponding to larger grating periods) the resonant reflection dip appears more prominent (see Supporting Information for numerical verification).

[0060] Fig.12 (ad) Reflection (a, c) and UCL spectra (b, d) of singlet (a, b) and multiplexed doublet (c, d) metal gratings coated with UCNPs. (e) Enhancement factor of the UCL integrated intensity of the samples relative to UCNPs on bare flat glass. The green and red columns represent green and red UCL, respectively.

[0061] exist Fig.12 In this paper, various combinations of UCLs with UCNPs-coated multiplexed metal gratings located in the green and / or red UCL emission bands and excitation wavelengths and UCNPs-coated metal gratings with singlet SP resonance modes were investigated. The samples have the same structure, as Fig.10 As shown, except for different surface profiles, they were prepared under the same conditions. Fig.12 Reflection spectra of single-period UCNPs-coated metal gratings at p = 462,600 and 947 nm under normal incidence of TM polarized light are shown. Their singlet SP resonance modes, indicated by reflection dips at λ = 539, 651 and 982 nm, are located in the green or red UCL emission bands, or overlap with the excitation wavelength. Fig.12 c shows that for the UCNPs-coated multiplexed metal gratings with constitutive periods of (p1, p2) = (462600), (462947) and (600947) nm, doublet SP resonance modes appear at the reflection dip positions of (λ1, λ2) = (545659), (548983) and (661980) nm, respectively. Therefore, the doublet SP resonance modes are located in the green and red UCL emission bands, or one is located in the green or red UCL emission band and the other is located at the excitation wavelength. The UCL spectra of the above samples are shown in Figure 2. Fig.12b and 12d. In the following, we first examined the enhancement factor (EF) of the integrated UCL intensity of the samples compared with that of UCNPs on bare flat glass, as shown in Fig.12 As shown in e, the green UCL is integrated within the range of λ=515-570nm, and the red UCL is integrated within the range of λ=635-685nm.

[0062] Fig.12 e shows that for UCNPs on a single-period metal grating, the SP resonance mode at the excitation wavelength (p = 947nm) has a significantly stronger effect on the enhancement of green and red UCL than the SP resonance in the UCL emission band (p = 462 and 600nm). The enhancement factor of the former is as high as more than 200 (i.e., EF = 206 for green UCL and EF = 247 for red UCL); while for the latter, the values ​​are only within a few dozen times, i.e., for the SP resonance in the green UCL band, EF = 44 at the green UCL and EF = 12 at the red UCL (for p = 462nm), and for the SP resonance in the red UCL band, EF = 17 at the green UCL and EF = 48 at the red UCL (for p = 600nm). The results show that under the singlet SP resonance, the enhancement factor of UCL emission under resonant conditions is several times larger than that under non-resonant conditions (e.g., about 2.8 or 3.7).

[0063] For UCNPs on double reused metal gratings, the UCL enhancement factor is much higher when one of the dual SP resonance modes is located at the excitation wavelength [for (p1, p2) = (462, 947) and (600, 947) nm], compared with the sample with the dual SP resonance mode located at the green and red UCL bands [for (p1, p2) = (462, 600) nm]. Here, even if one of the dual SP resonance modes is located at the excitation wavelength, the enhancement factor in the resonant UCL emission band [i.e., EF = 302 at the green UCL of (p1, p2) = (462, 947) nm and EF = 333 at the red UCL of (p1, p2) = (600, 947) nm] is always greater than the enhancement factor of the other one under the non-resonant condition [i.e., EF = 221 at the red UCL of (p1, p2) = (462, 947) nm and EF = 113 at the green UCL of (p1, p2) = (600, 947) nm].

[0064] In contrast, for doublet SP resonances (e.g., EF=302 for green UCL and 333 for red UCL), the enhancement factors in the UCL emission band under resonant excitation are much larger than those of the single-period sample (p=947 nm) under resonant excitation alone (e.g., EF=206 for green UCL and 247 for red UCL). In addition, although the enhancement factors of the UCL emission of the doublet SP resonance mode in both emission bands [for (p1, p2)=(462600) nm] are not very significant, they are still larger than the enhancement factors of the UCL emission of the sample with only singlet SP resonance mode in the green or red UCL bands (for p=462 and 600 nm).

[0065] In general, it can be concluded that for UCNPs on single-periodic and double-line multiplexed metal gratings, the SP resonant modes at the excitation wavelength enhance the UCL more profoundly than the SP resonant modes in the green and / or red UCL emission bands. Compared with UCNPs on planar metal films (e.g., UCNPs / MgF2 / Ag), the SP resonant modes in the UCL emission band moderately enhance the corresponding green or red UCL. Since the dual-state SP resonant modes in the multiplexed gratings match both the excitation wavelength and the UCL emission band, the UCL emission can be more strongly enhanced than the emission from the single-state SP resonant modes located only in one of the excitation wavelength or the UCL emission band.

[0066] To understand the influence of the double-peak SP resonance mode on the intermediate transition of the UCL process, the dependence of the UCL integrated intensity of the samples on the excitation power was characterized. The slope of the linear fit of the double logarithmic dependence is related to the path of the intermediate transition process and the competition between the transition processes. UCL is usually considered to be a two-photon process; therefore, the linear fit slope is usually close to but less than 2 in the "low power limit"; when the intermediate state is exhausted, upconversion dominates and linear decay dominates, and the slope decreases to 1 in the "high power limit". The appropriate linear fit for each set of data can be divided into two sections, which are significantly different in arrangement and slope value. Here, we define them as the "low power region" and the "high power region", respectively. In some cases, slopes greater than 2 indicate that three-photon processes are also involved,40,41 where a combination of ETU processes (k1, k2, and k3) through non-radiative decay leads to green and red UCL.

[0067] In particular, for the sample with (p1, p2) = (462600) nm, both its green and red UCL emissions are coupled to the dual SP resonance mode, and the large slopes in the low-power region (2.42 for green UCL and 2.41 for red UCL) imply that their large populations in the excited state are transferred to the low intermediate state in a nonradiative manner, which contributes to the UCL through the three-photon process (as described in the Supporting Information). This indicates that in the low-power region, the SP resonance mode at the emission wavelength is not very effective in enhancing the UCL. In the high-power region, however, the ETU process is enhanced, resulting in the generation of a large number of excited-state ions and UCL enhancement through the two-photon process, which causes the slope to drop below 2 (i.e., 1.28 for green UCL and 1.42 for red UCL). For samples with (p1, p2) = (462, 947) and (600, 947) nm, the excitation light and its green or red UCL emission are both coupled to the two-state SP resonance mode, but there is an obvious difference, that is, the UCL of the sample with SP resonance in the red UCL band [i.e., (p1, p2) = (600, 947) nm] always shows a two-photon process for a smaller slope less than 2. This indicates that under the two-state resonance condition, the SP resonance in the red UCL band is more effective in enhancing the green and red UCL emission rates than the SP resonance in the green UCL band, thereby promoting more efficient ETU to fill the population in the UCL excited state under resonant excitation. For samples with SP resonance modes in the green UCL band [i.e., (p1, p2) = (462947) nm], the three-photon process has a relatively large part in the UCL. Since the efficiency of the three-photon process is lower than that of the two-photon process, this leads to a lower degree of resonant enhancement of the UCLs. Compared with UCNPs on planar glass substrates, UCNPs on both single-period and multiplexed metal gratings show reduced UCL lifetimes in SP resonant modes, corresponding to larger radiative decay rates of the UCL. In particular, significantly larger reductions in UCL lifetimes occur for red UCL in cases where the SP resonant modes match the excitation wavelength and / or the red UCL emission band, e.g., for UCNPs on single-period gratings with p = 600 or 947 nm, or for multiplexed gratings with (p1, p2) = (462947) and (600947) nm. We attribute this to the enhanced nonradiative decay of excited-state populations in the intermediate state 4I11 / 2 (i.e., level 2→1) at the SP resonance, which favors red UCL emission over subsequent ETU (i.e., level 2→5) for green UCL emission. In addition, higher absorption losses in metallic structures at shorter wavelengths (e.g., in the green UCL band) may also affect the enhancement of the green UCL. Larger plasmon enhancements of the red UCL are often observed in the experimental results, even when the SP resonant modes are at the excitation wavelength.

[0068] UCL of UCNP (NaYF4:Yb,Er) on multiplexed metallic gratings with dual SP resonant modes that can be independently and arbitrarily tuned to simultaneously match two excitation wavelengths, green UCL, and red UCL emission bands. Experiments show that when the dual state resonant modes match both the green and red UCL bands, both green and red UCL emissions can be enhanced, stronger than the emission from UCNP on a single period metallic grating whose singlet SP resonant mode is located only at the green or red UCL band. Since the dual state resonant mode matches both the excitation wavelength and the green or red UCL band, the UCL in the resonant emission band can be specifically enhanced with an enhancement factor greater than that of UCNP on a single period metallic grating whose singlet SP resonant mode is located only at the excitation wavelength. These conclusions are also supported by the UCL power dependence and lifetime measurements. This work demonstrates that multiple independent resonant modes in multiplexed gratings can be flexibly applied to control light-matter interactions involving photons of multiple frequencies. This realization provides a way to resonantly control and enhance the UCL of Ln3+-doped materials for the development of efficient on-chip upconversion devices.

[0069] The doping method of the present invention is not limited to rare earth materials, but is applicable to any multi-level material involving up-conversion luminescence or multi-channel and multi-color luminescence.

[0070] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent shape or structural transformation made using the contents of the present invention's specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for quantum control of upconversion luminescence of rare earth-doped materials based on micro-nano resonant structure. Under the excitation of near-infrared light, the upconversion luminescent material sensitizer can effectively absorb external radiation energy and quickly transfer it to the activator ion. The activator ion absorbs photons and transitions from the ground state to the excited state, and then transfers energy to different energy levels of the upconversion luminescent material ion. The activator ions in each excited state finally return to the ground state through multi-step multi-photon radiationless relaxation and radiation relaxation, and emit visible light, characterized in that: The activator ions and sensitizer ions of the up-conversion luminescent material are made into nanocrystalline particles and arranged in a micro-nano resonant cavity. The resonant wavelength of the micro-nano resonant cavity is adjusted to be at the emission wavelength of the activator ions, which will activate the multi-step transition of the activator ions and enhance the up-conversion luminescence intensity at that location. By modulating the resonant mode, the radiative transition of the luminescent channel is suppressed, and the particles in each excited state are redistributed, including the energy transfer and non-radiative transition process between particles in different excited states. When the resonant wavelength of the microcavity is at the emission wavelength of the nanoparticles, at the resonant wavelength, the mode density increases, the spontaneous radiation rate increases, and the optical resonant cavity in the resonant state can allow photons that meet certain frequency matching conditions to pass through, so that the excited state of a channel that is allowed gathers more particles, thereby suppressing the up-conversion luminescence of other emission wavelengths and realizing the up-conversion luminescence as monochromatic light. The activator ions and sensitizer ions are Er, 3+ and Yb 3+ , to obtain NaYF4:Yb 3+ / Er 3+ Nanocrystalline particles; Yb-doped 3+ / Er 3+ The nanocrystalline particles are sandwiched in a Fabry-Parrot resonant cavity between DBR multilayer dielectric reflectors. The upper and lower DBR multilayer dielectric reflectors are composed of two layers of dielectric films with different refractive indices. A Yb-doped 3+ / Er 3+ Nanocrystalline particle layer; adjust the thickness and resonant wavelength of the Fabry-Parrot resonant cavity between the DBR multilayer dielectric reflectors to form a defect mode on the bandgap of the DBR multilayer dielectric reflector. When the defect mode is at about 540nm, the green fluorescence is enhanced and can be emitted, while the red fluorescence is still in the bandgap range and cannot be emitted; adjust the thickness and resonant wavelength of the resonant cavity. When the defect is at 650nm, the red fluorescence can be emitted, thus achieving the emission of monochromatic light; doping Yb 3+ / Er 3+ The nanocrystalline particles are arranged on a composite metal grating with a dual surface plasmon resonance mode. When the dual resonance mode matches the green and red upconversion luminescence UCL bands at the same time, both the green and red UCL emissions can be enhanced; when the dual state resonance mode matches both the excitation wavelength and the green or red UCL band, the UCL of the emission band matching the resonance mode can be particularly enhanced, and the enhancement factor is greater than that of the single plasmon resonance mode of the single period metal grating when it matches the excitation wavelength or the emission wavelength. 3+ / Er 3+ Enhancement of UCL of nanocrystalline particles.

2. A method for quantum control of upconversion luminescence of rare earth-doped materials based on a micro-nano resonant structure as claimed in claim 1, characterized in that: The micro-nano resonant cavity is a Fabry-Perot resonant cavity, or FP cavity. The resonant cavity is a layer of metal film thermally evaporated on a glass substrate, a layer of dielectric film is deposited on the metal film, and a layer of Yb-doped 3+ / Er 3+ Nanocrystalline particle layer, and then deposit a metal film on the dielectric film; by adjusting the thickness of the dielectric film between the metal films, the resonant light field mode of the Fabry-Parrot microcavity structure formed above is adjusted, so as to achieve the effect of Yb-doped 3+ / Er 3+ Quantum control of the luminescence of nanocrystal particles; the resonant wavelength of the microcavity is at the emission wavelength of the nanoparticles, which enhances the upconversion luminescence intensity there, and uses the anti-resonance mode of the microcavity to suppress the upconversion luminescence of other emission wavelengths. By internally adjusting the Er in the resonant cavity mode 3+ Multi-step transitions of ions achieve single-color upconversion luminescence and enhancement.

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