Passive mid-infrared hydrogen sensors based on phonon- polaritonic structures

Abstract

Sensors configured to detect gas phase molecules, e.g., H2, are provided. In an embodiment, such a sensor comprises a polar material and a transducer material that forms an interface with the polar material, wherein the polar material is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material induces a change in the resonance wavelength upon dissociation of H2 on the transducer material; a sample delivery system configured to introduce a sample to the sensor; and a detector configured to detect output light from the sensor.

Description

Atty. Dkt. No. 00100-0421 -PCTPASSIVE MID-INFRARED HYDROGEN SENSORS BASED ON PHONON- POLARITONIC STRUCTURES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 790,855 that was filed April 18, 2025, the entire contents of which are incorporated herein by reference.BACKGROUND

[0002] The mid-infrared (mid-IR) region of the electromagnetic spectrum is of significant interest for chemical sensing as it overlaps with the molecular fingerprint region (approximately 500 to 1500 cm’1or 6-20 pm), where molecules exhibit characteristic vibrational modes that provide distinct spectral signatures for different chemical compounds. However, chemical sensing in the mid-IR is often constrained by the inherently long photon wavelengths and the low absorption cross-sections of molecules in the mid-IR (approximately IO’20cm2). Mid-IR detection is particularly challenging for molecular gases due to their low number densities at atmospheric pressure.SUMMARY

[0003] Provided are sensors configured to detect gas phase molecules, e.g.. H2. by leveraging phonon polariton modes supported by a heterostructure or a metasurface of the sensor using infrared (IR) light. Methods of using the sensors to detect gas phase molecules are also provided.

[0004] The present disclosure is described by reference to an Example below, which describes an illustrative low-loss phonon polariton platform using a Pd / SiC heterostructure and a Pd / SiC metasurface for H2 detection. Upon H2 absorption, Pd undergoes a phase transition to PdHx, altering the local dielectric environment and modulating the phonon polariton response. By leveraging phonon polariton modes, including localized phonon polariton modes in the metasurface, narrowband, highly sensitive, and reversible detection of H2 in the mid-IR was achieved. The platform advances passive optical H2 sensing technologies beyond the visible spectrum and into the mid-IR, enabling integration with other advanced IR spectroscopic techniques for dynamic chemical process monitoring, withAtty. Dkt. No. 00100-0421 -PCTbroader implications for gas-phase sensing, environmental monitoring, and in situ reaction studies.

[0005] In one aspect, a sensor configured to detect H2 is provided. In an embodiment 1, the sensor comprises a polar material and a transducer material that forms an interface with the polar material, wherein the polar material is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material induces a change in the resonance wavelength upon dissociation of H2 on the transducer material; a sample delivery system configured to introduce a sample to the sensor; and a detector configured to detect output light from the sensor.

[0006] An embodiment 2 is according to embodiment 1, wherein the polar material and the transducer material form a heterostructure comprising a layer of the transducer material on a surface of the polar material.

[0007] An embodiment 3 is according to embodiment 1, wherein the polar material and the transducer material form a metasurface comprising an array of subwavelength structures arranged according to a pattern.

[0008] An embodiment 4 is according to embodiment 3, wherein the subwavelength structures comprise the polar material and the transducer material is in a form of a layer on a surface of each subwavelength structure in the array.

[0009] An embodiment 5 is according to embodiment 4, wherein the subwavelength structures are cylindrical pillars.

[0010] An embodiment 6 is according to any of embodiments 1-5, wherein the output light is reflected light from the sensor.

[0011] An embodiment 7 is according to any of embodiments 1-6, wherein the polar material comprises a polymorph of silicon carbide and the transducer material comprises Pd.

[0012] An embodiment 8 is according to embodiment 7, wherein the polymorph of silicon carbide is 4H-SiC.

[0013] An embodiment 9 is according to any of embodiments 1-8, wherein the sensor is passive and is not operably coupled to a power source.Atty. Dkt. No. 00100-0421 -PCT

[0014] An embodiment 10 is according to any of embodiments 1-9. wherein the polar material does not form an interface with a metal other than that formed with the transducer material.

[0015] An embodiment 11 is a sensor configured to detect a gas phase molecule, the sensor comprising: a polar material and a transducer material that forms an interface with the polar material, wherein the polar material comprises a polymorph of silicon carbide and is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material comprises Pd and induces a change in the resonance wavelength upon absorption or dissociation of a gas phase molecule on the transducer material; a sample delivery system configured to introduce a gas phase sample to the sensor; and a detector configured to detect output light from the sensor.

[0016] An embodiment 12 is according to embodiment 11, wherein the polymorph of silicon carbide is 4H-SiC.

[0017] An embodiment 13 is according to any of embodiments 11-12, wherein the polar material does not form an interface with a metal other than that formed with the transducer material.

[0018] An embodiment 14 is according to any of embodiments 11-13, wherein the transducer material induces a change in the resonance wavelength upon dissociation of the gas phase molecule on the transducer material.

[0019] In another aspect, a method of detecting H2 is provided. In an embodiment 15, the method comprises: delivering a sample to a sensor comprising a polar material and a transducer material that forms an interface with the polar material, wherein the polar material is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material induces a change in the resonance wavelength upon dissociation of H2 on the transducer material; coupling infrared light into the sensor; and detecting output light from the sensor.

[0020] An embodiment 16 is according to embodiment 15, wherein the polar material and the transducer material form a heterostructure comprising a layer of the transducer material on a surface of the polar material.Atty. Dkt. No. 00100-0421 -PCT

[0021] An embodiment 17 is according to embodiment 15, wherein the polar material and the transducer material form a metasurface comprising an array of subwavelength structures arranged according to a pattern.

[0022] An embodiment 18 is according to embodiment 17, wherein the subwavelength structures comprise the polar material and the transducer material is in a form of a layer on a surface of each subwavelength structure in the array.

[0023] An embodiment 19 is according to any of embodiments 15-18, wherein the polar material comprises a polymorph of silicon carbide and the transducer material comprises Pd.

[0024] An embodiment 20 is according to embodiment 19, wherein the polymorph of silicon carbide is 4H-SiC.

[0025] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings.

[0027] FIGS. 1A-1B. Phonon polariton sensors for hydrogen gas detection and broad-spectrum chemical sensing. FIG. 1 A shows a schematic representation of a 4H-SiC resonator unit cell topped with a thin Pd layer, serving as the transducer material for H2 detection. The inset shows a magnified view of the Pd lattice, illustrating the reversible phase transition between Pd and PdHxduring H2 absorption / dissociation and desorption cycles. FIG. IB shows an overview of spectral ranges covered by the Reststrahlen bands of various polar materials and the associated quality factors of localized phonon polariton modes. The mid-IR spectral region highlights characteristic infrared fingerprint vibrations of selected molecules, which are indicative of their chemical identity. The molecular fingerprint region is also marked as the shaded region in the mid-IR spectrum (approximately 500 to 1500 cm’1or 6-20 pm).

[0028] FIGS.2A-2G. Mid-IR Response of Pd / SiC Heterostructure Upon H2 Absorption. FIG. 2A shows an Atomic Force Microscopy (AFM) image of a 25 nm thick Pd film deposited on a 4H-SiC substrate via e-beam evaporation (top panel, scale bar: 3 pm) andAtty. Dkt. No. 00100-0421 -PCTa cross-sectional line profile illustrating the thickness variations across the Pd / SiC heterostructure and the bare 4H-SiC substrate (bottom panel). FIG. 2B shows a schematic of the gas-phase FTIR measurement setup featuring an FTIR microscope, a gas chamber equipped with an IR-transparent KBr window, sources of H2 and N2 controlled by mass flow controllers (MFCs), and a vent system. FIG. 2C shows a real part of the dielectric function of 4H-SiC, showing variations for in-plane (x, y) and out-of-plane (z) components. FIG. 2D shows experimental reflectance spectra of the Pd / SiC heterostructure at varying H2 concentrations. FIG. 2E shows simulated reflectance spectra of the Pd / SiC heterostructure at different H2 concentrations. FIG. 2F shows dielectric function changes in Pd (and PdEk) at different H2 concentrations, derived from the experimental reflectance spectra in FIG. 2D. FIG. 2G shows comparative analysis of the relative change in reflectance spectrum of the Pd / SiC heterostructure at 4% H2 concentration and the energy loss spectrum of 4H-SiC for in-plane (x, y) and out-of-plane (z) components.

[0029] FIGS.3A-3E. Pd / SiC metasurface featuring near-unit absorption and high quality factors for H2 sensing. FIG. 3A shows a scanning electron microscope (SEM) image of a top view of an array of 4H-SiC metasurfaces with a 25 nm Pd layer on top of the pillars . FIG. 3B shows SEM images of a single 4H-SiC pillar, shown with a Pd cap (top panel) and without a Pd layer (bottom panel) on top of the pillar. FIG. 3C shows measured reflectance spectra of the 4H-SiC metasurface with and without Pd caps, illustrating the localized phonon polariton mode featuring near-unit absorption and high quality7factor (shaded region). The inset illustrates the simulated electric field profile of the localized phonon polariton mode supported in the Pd / 4H-SiC metasurface. FIG. 3D shows real-time relative change spectra of the Pd / SiC metasurface in FIG. 3C upon 4% H2 exposure, highlighting the sharp spectral response to gas exposure. FIG. 3E shows time-series reflectance spectra of the Pd / SiC metasurface in FIG. 3C upon 4% H2 exposure, showing changes over a period from 0 to 15 minutes.

[0030] FIGS.4A-4D. Performance of Pd / SiC metasurface and heterostructure for H2 sensing. FIG. 4A shows relative change in reflectance of the Pd / SiC metasurface at 953 cm'1and 968 cm'1in response to repeated cycles of 4% H2 exposure. FIG. 4B shows relative change in reflectance of the Pd / SiC metasurface in response to stepwise increasing concentrations of H2 (diluted in N2) at room temperature. FIG. 4C shows relative change in reflectance of the Pd / SiC heterostructure in response to stepwise increasing concentrations of H2 (diluted in N2) at room temperature. FIG. 4D shows measured relative change inAtty. Dkt. No. 00100-0421 -PCTreflectance as a function of H2 concentration for both the Pd / SiC metasurface and Pd / SiC heterostructure, derived from experimental data in FIG. 4B and FIG. 4C.DETAILED DESCRIPTION

[0031] Provided are sensors configured to support a phonon polariton mode upon illumination of the sensor with infrared (IR) light. As described in detail below, the phonon polariton mode may be used to detect gas phase molecules, e.g., H2. The sensors comprise a polar material and a transducer material forming an interface with the polar material. In some embodiments, such a sensor comprises a heterostructure comprising the polar material and the transducer material. In other embodiments, such a sensor comprises a metasurface comprising the polar material and the transducer material.

[0032] The polar material is a crystalline solid having a polar lattice structure in which coherent vibrations of the lattice structure (i.e., phonons) induce a net dipole moment and which are capable of coupling with the IR light, thereby generating phonon polaritons therein. An illustrative polar material is the 4H polymorph of silicon carbide (i.e., 4H-SiC). Other polar materials may be selected depending upon the application for the sensor, including the gas phase molecules to be detected. Illustrative polar materials include hexagonal boron nitride (hBN), cubic boron nitride (cBN), molybdenum trioxide (MoOs), silicon dioxide (SiO2), calcite, gallium nitride (GaN), aluminum nitride (AIN), aluminum oxide (AI2O3), vanadium oxide (V2O5) (see FIGS. 1 A-1B). The IR light refers to light having a wavelength within the IR region of the electromagnetic radiation, including mid-IR light having a wavelength in a range of from 6 pm to 20 pm. The particular range of wavelengths to be coupled into the sensor depends upon the selected polar material and generally is selected to overlap with the Reststrahlen band of the polar material (or vice versa).

[0033] The transducer material is a material capable of inducing a change in the dielectric properties of the polar material (and / or of the interface formed between the transducer material and the polar material) in the presence of a gas phase molecule to be detected. This change in dielectric properties may occur upon absorption or upon subsequent dissociation of the absorbed gas phase molecule on the transducer material. The change in the dielectric properties results in a detectable shift in a resonance wavelength of the sensor, specifically, the heterostructure or the metasurface thereof. Thus, the composition of the transducer material to be used can depend upon the selected polar material as well as the gas phaseAtty. Dkt. No. 00100-0421 -PCTmolecule to be detected. An illustrative transducer material is palladium (Pd) or an alloy thereof. Pd is capable of absorbing H2, which dissociates and induces a phase transition to PdHx, resulting in a detectable shift in a resonance wavelength of a sensor comprising the Pd and 4H-SiC.

[0034] In embodiments, such as those illustrated by the inset image of FIG. 4C, the polar material and the transducer material are in the form of a heterostructure. The heterostructure may comprise a layer of the transducer material on a surface of the polar material, forming the interface therebetween. In such embodiments, the transducer material may be characterized as an unpattemed, continuous layer and the surface of the polar material may also be characterized as being unpattemed and continuous. The thickness of the layer of the transducer material may vary, but the thickness may be subwavelength, i.e., less than the wavelength of the light being coupled into the heterostructure.

[0035] In other embodiments, such as those illustrated in FIGS. 1A, 3A, and 4B, the polar material and the transducer material are in the form of a metasurface. In such embodiments, the polar material may be in the form of an array of subwavelength structures arranged according to a pattern and the transducer material may be in the form of a layer on a surface of each subwavelength structure, forming the interface therebetween. In such embodiments, the polar material may be characterized as being nanostructured and / or patterned due to the subwavelength structures.

[0036] The sub wavelength structures of the metasurface refer to elements which are composed of (or defined within) the polar material and have at least one dimension (e.g., 1, 2, or all 3) that is less than the wavelength of light being coupled into the metasurface. In embodiments, the subwavelength structures are projections extending away from a plane defined by an underlying surface of the sensor. Various shapes may be used for the subwavelength structures, but an illustrative structure is a cylindrical pillar having a diameter and height, both of subwavelength dimensions. (See FIG. 1A.) The array may be a two-dimensional (2D) array in which a plurality of individual, separated subwavelength structures extend along two dimensions. (See FIG. 3 A.) The term “pattern” refers to geometric parameters including the dimensions, shapes, arrangement, and periodicity (for periodic arrays) of the subwavelength structures that make up the metasurface.As noted above, the transducer material may be in the form of a layer on a surface of each of a plurality of subw avelength structures (e.g., cylindrical pillars). Thus, the transducer materialAtty. Dkt. No. 00100-0421 -PCTmay also be characterized as being nanostructured and / or patterned. The thickness of these layers of the transducer material may vary, but is generally sufficiently small so as to maintain the subwavelength dimensions described above.

[0037] As noted above, the coupling of the IR light with phonons of the polar material excites one or more phonon polariton modes of the sensor, specifically, of the heterostructure or the metasurface. In general, the sensor may support multiple phonon polariton modes, each of which may be characterized by different optical properties, e.g., different resonance wavelengths (i.e., the wavelength at which the IR light and the particular mode are resonant), different quality (Q) factors, and / or different electric field enhancement profiles. The sensor may be configured such that it supports a particular mode having particular optical properties, depending upon the desired application (including the desired gas phase molecules to be detected). This may be accomplished by appropriate selection of the polar material, the transducer material, as well as appropriate selection of the geometric parameters of the heterostructure or the metasurface described above. Similarly, characteristics of the light illumination (e.g., wavelength, polarization, incidence angle, etc.) may be selected so as to excite a particular mode supported by the sensor.

[0038] In embodiments, the present sensors configured as described above (to support a particular phonon polariton mode(s)) are not also configured to support a plasmon polariton mode which would require a different configuration, e.g., a different selection of polar material, transducer material, and / or different geometric parameters. Relatedly, aside from the interface formed between the transducer material and the polar material, in embodiments, the present sensors do not comprise another interface formed between the polar material and a metal.

[0039] The Example below describes the coupling of IR light to excite a transverse dipole phonon polariton mode of a metasurface comprising Pd and 4H-SiC (the metasurface is shown in FIG. 3A). The reflectance spectrum showing the resonance wavelength (~ 953 cm’1) of the metasurface is shown in FIG. 3C. The electric field enhancement profile of the mode along the x, y dimensions of an individual subwavelength cylindrical pillar is shown in the inset of FIG. 3C. The Example below further describes the coupling of IR light to excite an epsilon-near-zero polariton mode having a different resonance wavelength (~ 968 cm’1). (See FIG. 4A.) Depending upon the desired application for the sensor (including the gas phase molecules to be detected) other phonon polariton modes may be excited. Similarly, theAtty. Dkt. No. 00100-0421 -PCTExample below describes the coupling of IR light to excite phonon polariton modes of a heterostructure comprising Pd and 4H-SiC (the heterostructure is shown in the inset image of FIG. 4C). (See FIGS. 2A-2G.)

[0040] The Example below describes thin film deposition techniques and lithographic techniques that may be used to fabricate the heterostructures and metasurfaces of the present sensors.

[0041] The present sensors may include other components operably coupled to the heterostructure or the metasurface, including a detector. The detector is configured to detect output light from the sensor, which may be reflected light as shown in FIG. 3C (versus transmitted light). Detection includes detecting the output light as a function of, e.g.. wavelength, time, or both. Absorption (and subsequent dissociation) of gas phase molecules can generate a change in the detected output light due to the shift in resonance frequency of the sensor, specifically, the heterostructure or the metasurface as described above. Changes in the detected output light may be reported as a relative change with respect to output light detected in the absence of the gas phase molecules. (See FIGS. 2G, 3D, 4A, 4B, 4C.) Changes in detected output light may be analyzed to extract information, e.g., identity, concentration of the gas phase molecules. This may involve use of calibration plots generated from detecting output light from samples comprising known gas phase molecules and / or known concentrations thereof. (See FIG. 4D.)

[0042] The present sensors may comprise other components, e.g., a light source providing the IR light; optoelectrical components configured to couple the IR light into the sensor, specifically, the heterostructure or the metasurface; a sample delivery' assembly configured to introduce a sample to the sensor; and a controller configured to control one or more components of the sensor. The controller may comprise an input interface, an output interface, a communication interface, a computer-readable medium, a processor, and an application. The application performs operations associated with components of the sensor. These operations may include receiving and / or processing detector signals so as to extract information (e.g., identity, concentration, etc.) about gas phase molecules in the sample as well as outputting received and / or processed detector signals and extracted information, e.g.. to a display. The sensors may be integrated with other spectroscopy systems configured to carry' out other spectroscopic techniques. Finally, although a variety' of components may beAtty. Dkt. No. 00100-0421 -PCTincluded in the present sensors as described above, they do not require use of an external power source. As such, the sensors are passive systems.

[0043] The present sensors may be used to detect molecules present in (or suspected of being present in) a sample. The samples are gaseous in nature. A variety of gas phase molecules may be detected using the present sensors, including hydrogen (H2). Other gas phase molecules include, e.g., CO, CO2, CH4, NH3, NO, and N2O. In an embodiment, a method of detecting a molecule comprises introducing a sample to any of the disclosed sensors; illuminating the sensor with IR light; and detecting output light from the sensor. A change in detected output light is indicative of the presence of the gas phase molecule.EXAMPLE

[0044] Introduction

[0045] This Example explored the use of phonon polaritons for gas-phase sensing, with a focus on hydrogen (H2) gas. H2 is already used extensively in industrial chemical processes, such as in Haber-Bosch ammonia synthesis and in Fischer-Tropsch synthesis of methanol. H2 is also considered crucial for global sustainability goals as a promising energy carrier in the transition away from fossil fuels. As the prevalence of the use of H2 outside of heavy industry increases, it will be increasingly important to monitor and quantify its presence in the environment. H2 is highly explosive when mixed with air at volumetric concentrations above 4%, making optical sensing of H2 highly desirable for industrial and automotive applications due to its ability to minimize the risk of explosion. As a homonuclear diatomic molecule, H2 has no dipole moment and therefore is IR inactive. Consequently, most nanophotonic optical sensing of H2 has been conducted in the visible spectral region. Despite the success of H2 sensing in the visible spectrum, there is a mismatch between these working wavelengths and the molecular fingerprint region of a wide range of gases in the mid-IR, limiting the potential for integration with current infrared spectroscopy techniques for monitoring dynamic chemical processes involving H2.

[0046] This Example bridges this gap by introducing a low-loss phonon polariton platform that uses Pd as a transducer material for mid-IR hydrogen detection at room temperature and pressure and 4H-SiC, a polar material compatible with standard large-scale fabrication processes. First, the mid-IR response of a thin Pd film on 4H-SiC upon H2Atty. Dkt. No. 00100-0421 -PCTabsorption was investigated, and phonon-enhanced hydrogen detection was demonstrated using the unpattemed polaritonic substrate.

[0047] Additionally, the dielectric function of Pd in the mid-IR was presented, derived from reflectance measurements of Pd / 4H-SiC heterostructures under various levels of H2 loading. This provided accurate mid-TR properties of Pd for applications in IR nanophotonics.

[0048] Furthermore, 4H-SiC metasurfaces were fabricated as periodic arrays, capped with a thin Pd layer (FIG. 1 A) using standard photolithographic techniques. These Pd / 4H-SiC resonators exhibited near-unit absorption for unpolarized IR light and high quality factors (Q > 70) before hydrogen loading. Upon hydrogen absorption, Pd underwent a phase transition to palladium hydride (PdHx), altering the local dielectric environment of the 4H-SiC resonators and modifying the resonance of the localized phonon polariton modes. Using this platform, enhanced and reversible hydrogen detection was demonstrated across concentrations ranging from 0.1% to 4.0%, meeting the performance targets set by the U.S. Department of Energy (DoE) and other key stakeholders for H2 sensor technologies. The low-loss phonon polariton platform extends current visible-light-based hydrogen sensors into the mid-IR range. The platform may be integrated into surface-enhanced infrared spectroscopy techniques for in situ monitoring of various EE-related chemical processes.

[0049] Methods

[0050] Sample fabrication

[0051] Pd / SiC metasurfaces were fabricated using soft contact photolithography. To remove all organic residues, an undoped 4H-SiC wafer was cleaned with acetone, Isopropyl alcohol (IP A) and Methanol, followed by a N2 blow dry and was baked for 5 minutes at 100 °C. Next, two layers of positive photoresist (LOR5b / Azl505) were used: the first as a lift-off resist and the second as a photoresist, both uniformly applied to the undoped 4H-SiC using a spin coater to ensure uniform coverage. Following exposure, the sample was developed in Az300k solution for 1 minute. Subsequently, thin films were deposited, starting with 25 nm of palladium (Pd), followed by 15 nm of titanium (Ti) and 100 nm of nickel (Ni) using electron beam evaporation in a high vacuum chamber. The Ni acted as an etch mask, and the Ti then acted as an etch stop for Aqua Regia removal of the Ni mask. The SiC pillars were etched using reactive ion etching (RIE) using a mixture of O2 and SFe at an etching rate of 375 nm / min. The lift-off process was done by soaking the samples in propylene glycol (PG). After liftoff, the metals were etched using aqua regia for Ni, followed by the Transene Ti-Atty. Dkt. No. 00100-0421 -PCTEtchant for Ti layer. The Pd / SiC heterostructure shown in FIGS. 2A-2G was fabricated using the same procedure as Pd / SiC metasurfaces but no photolithography was involved in the process.

[0052] Gas-phase FTIR microscope measurements

[0053] The room-temperature reflection spectra for the fabricated samples were collected using a Hyperion 2000 IR microscope attached to a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer with a liquid-nitrogen-cooled HgCdTe (MCT) detector. The microscope objective used was a 15x Cassegrain objective, which illuminated the sample with an average incident angle of 16°. All the spectra presented were collected in 64 scans with spectral resolution set to 2 cm-1. The fabricated sample was placed in a gas chamber (Linkam) equipped with an IR-transparent KBr window with sources of H2 and N2 controlled by mass flow7controllers (MFCs, Alicat).

[0054] Mid-IR dielectric function of Pd

[0055] The dielectric function of Pd consists of both the intraband transitions associated with free electrons and interband transitions of bound electrons in Pd metal. The Brendel-Bormann model was applied to describe the mid-IR dielectric function of Pd in terms of Drude and Lorentz contributions. In the context of this model, the dielectric function of Pd is given byEPd(m) = £D(m) + £LW)

[0056] with Drude term= 1 — / "&>(<u + JFQ)

[0057] and interband transitionsA l r+“ / ;n2e A £, (u>) = > , _ — 7 - r-dx^211(1] J-00 (%2— co2— iuTj)

[0058] where u>pis the plasma frequency. r0is the intrinsic collision or scattering frequency, K is the number of Lorentz oscillators, <Jj is the absorption bandwidth, and coj is the resonance frequency in the Lorentz model. The parameters for the fitted dielectric functions of Pd presented in FIGS. 2A-2G were obtained but not shown here.Atty. Dkt. No. 00100-0421 -PCT

[0059] Electromagnetic simulations

[0060] Finite-difference time-domain (FDTD) simulations were performed in a Lumerical 2021 using Northwestern’s Quest high-performance computing cluster. The geometry of the simulated structure was defined according to the SEM image of the fabricated device. The periodic boundary conditions and a perfectly matched layer for the substrate were used in the simulations. The dielectric function used for 4H-SiC was derived from Paarmann A, et al., Physical Review B 94, 134312 (2016).

[0061] Results

[0062] Phonon polaritons for chemical sensing in the mid-IR

[0063] As introduced above, phonon polaritons arise from optical phonons in the polar lattices of materials such as 4H-SiC. In these materials, the coherent vibrations of the lattice induce a net dipole moment, resulting in strong absorption of light at the transverse optical (TO) phonon frequency. This absorption also leads to a breaking of the degeneracy of the optical phonons at the gamma (y) point, causing the longitudinal optical (LO) phonon frequency to shift to higher values. The spectral band between the TO and LO phonon frequencies, known as the Reststrahlen band, is where phonon polaritons can be excited, facilitating highly confined optical fields. The Reststrahlen band is a material-specific spectral window, with crystal anisotropy playing a significant role in light confinement and light manipulation. A wide range of polar materials can support phonon polaritons from the mid-IR to the terahertz (THz) regions. Notably, as depicted in FIG. IB, phonon polaritonic materials span the molecular fingerprint region, making them excellent platforms for chemical sensing in the mid-IR.

[0064] By patterning those polar materials into subwavelength nanoresonators or nanoantennas, localized phonon polaritons emerge. Their resonant frequencies are highly tunable across the Reststrahlen band of a certain polar material by varying the resonator’s shape, size, and periodicity, as well as through coupling with other polaritonic modes. The low loss nature of phonon polaritons contributes to narrower resonance linewidths in these resonators, resulting in quality factors approximately an order of magnitude higher than those of their plasmonic counterparts in the mid-IR while offering similar or even improved field confinements. Such high quality' factors are critical for mid-IR chemical sensing, as even slight changes in the surrounding environment can lead to significant changes in resonance conditions. Notably, among phonon polaritonic materials with Reststrahlen bands in theAtty. Dkt. No. 00100-0421 -PCTmolecular fingerprint region as illustrated in FIG. IB, SiC-based resonators stand out, demonstrating quality factors theoretically approaching 1000 and experimentally demonstrating as high as 320.

[0065] Phonon-enhanced mid-IR H2 detection in unpatemed Pd / SiC heterostructures

[0066] Having selected SiC as the phonon polaritonic medium, the mid-IR response of a heterostructure consisting of a 25 nm thick Pd film deposited on a semi-insulating 4H-SiC substrate was first examined. The sample is depicted in the atomic force micrograph (AFM) shown in FIG. 2A. The fabrication details are provided above. An FTIR microscope equipped with an environmental gas chamber, supplied with H2 and N2 gases controlled by mass flow controllers (MFCs) was utilized to study the mid-IR properties of the Pd / 4H-SiC heterostructure under various H2 concentrations, as illustrated in FIG. 2B. This setup allowed for the evaluation of the dynamic mid-IR spectral responses of the heterostructure driven by changes in hydrogen concentration.

[0067] The w urtzite unit cell of the 4H-SiC cry stal caused the material to exhibit an anisotropic dielectric function, with the real parts for in-plane (x, y) and out-of-plane (z) components presented in FIG. 2C. Within its in-plane and out-of-plane Reststrahlen band, w hich spans from the TO to the LO phonon frequency, the real part of the dielectric function becomes negative, creating the conditions for phonon polaritons with high field enhancement. At the LO phonon frequency, where the real part of the permittivity approaches zero, the spectral region supports the formation of Berreman and epsilon-near-zero (ENZ) polariton modes. The Berreman mode, appearing near the LO phonon frequency within the light line (on the low- momentum side), exhibits a leaky mode character. Conversely, the ENZ mode, occurring outside the light line (on the high momentum side) near the LO phonon frequency, is characterized by its evanescently confined nature.

[0068] With a thin layer of Pd deposited on the 4H-SiC substrate, the highly reflective Reststrahlen band of 4H-SiC was significantly altered, as demonstrated in FIG. 2D. The reflectance spectrum of the Pd / 4H-SiC heterostructure, in the absence of H2 and under an N2 environment, showed a pronounced reflectance peak near the TOx,yphonon frequency, coupled with a distinct reflectance dip near the LOX,yphonon frequency. These spectral features highlighted the influence of underlying phonon modes on the heterostructure's overall optical response. For comparison, the reflectance spectrum of bare 4H-SiC versus the heterostructure w as obtained (data not shown). Upon H2 exposure to up to 4% H2 in balanceAtty. Dkt. No. 00100-0421 -PCTof N2, the reflectance of the heterostructure decreased; the higher the H2 concentration, the lower the reflectance, although the general shape of the spectrum remained consistent.

[0069] The reflectance spectrum of the Pd / 4H-SiC heterostructure was further simulated under different H2 concentrations using the transfer-matrix method to model the reflectance of the multilayer structure. To describe the dielectric function of Pd at different H2 concentrations, the Brendel-Bormann model was employed, which accounted for both Drude and Lorentz contributions (see above). The simulated reflectance spectra in FIG. 2E showed good agreement with the experimental spectra, displaying the same spectral features and trends upon H2 loading. Additionally, the fitted dielectric function of Pd in the mid-IR at various H2 concentrations was obtained as shown in FIG. 2F. The dielectric function clearly illustrates the transition of Pd to PdHxupon H2 absorption, characterized by a reduction in the values of both the real and imaginary components, indicating a less metallic nature in the hydride phase. Here, the dielectric function of Pd and PdHxwas provided based on experimental mid-IR measurements within the technically relevant H2 concentrations, which is valuable for the future development of IR nanophotonic devices using Pd.

[0070] When examining the relative change in the reflectance spectrum of the Pd / 4H-SiC heterostructure upon H2 absorption, the phonon modes from the underlying polaritonic substrate showed an enhanced response for H2 detection. As illustrated in the experimental time-dependent relative change spectra (data not shown), defined as Ro— R) / Ro, where Rois the reflectance before H2 loading and R is the reflectance upon H2 loading, the relative change occurred more rapidly and was more pronounced near the LO phonon frequency. This phonon-enhanced H2 detection was even more apparent in the steady -state spectrum presented in FIG. 2G, derived from both experimental and simulated data. The relative change spectrum exhibited an opposite shape when compared to the absolute reflectance spectrum presented in FIG. 2D, a trend that qualitatively agreed with the energy loss spectrum of 4H-SiC, defined as Im(— esic), where esicis the dielectric function of 4H-SiC. Notably, this enhancement was more prominent near the LOx. yphonon frequency, consistent with the higher amplitude in the energy loss spectrum for the x, y components. This enhancement near the LO phonon of 4H-SiC was attributed to the free-space excitation of the leaky Berreman mode in the polaritonic medium, which amplified the relative change in the top Pd layer. Thus, it was demonstrated that even an unpattemed phonon polaritonic medium can enhance H2 detection in the mid-IR spectral region.Atty. Dkt. No. 00100-0421 -PCT

[0071] Pd / SiC metasurface featuring near-unit absorption and high quality factors

[0072] Building on the findings of phonon-enhanced H2 detection in the mid-IR, 4H-SiC metasurfaces consisting of periodic arrays of subwavelength pillars were designed and fabricated to further explore localized phonon polariton modes for enhanced H2 sensing (FIG.3A). These metasurfaces were fabricated using standard photolithography techniques with varying diameters and periodicities, as detailed above. A 25 nm layer of Pd was defined on top of each pillar, with these Pd caps serving as the transducer material for H2 detection. To further compare the optical response with Pd / 4H-SiC metasurfaces, bare 4H-SiC metasurface arrays were also fabricated without Pd, as shown in FIG. 3B.

[0073] The two types of localized phonon polariton modes include the monopole resonance and the transverse dipole resonance. The monopole resonance arises from a modified longitudinal dipole. In this mode, a uniform near-field charge distribution along the height of the pillar is balanced by an opposing phase on the surface of the substrate. This monopole resonance is excited when the electric field has a component perpendicular to the substrate plane, and as such, only -polarized light can excite this mode under off-normal incidence. The transverse dipole resonance occurs at higher energies within the Reststrahlen band of 4H-SiC near the LO phonon frequency. This mode is excited when the electric field has a component parallel to the substrate plane, leading to lateral charge separation and resulting in two intensity maxima at the edges of the pillars. Under oblique illumination, both p- and .s’-polarized light can excite this mode. In this Example, the transverse dipole mode was of primary interest, as it fully exploits unpolarized light illumination, which is crucial for enhancing H2 detection.

[0074] With the addition of Pd on top of the 4H-SiC metasurface, the localized phonon polariton modes still maintained a narrow resonance linewidth, as shown in the measured unpolarized reflectance spectra in FIG. 3C. Notably, the transverse dipole mode around 955 cm'1exhibited increased absorption due to coupling with the lossy Pd layer, achieving nearzero reflectance (or near-unit absorption, as no transmission occurred within the Reststrahlen band of 4H-SiC). Despite this increase in absorption, the quality' factor remained around 70. This low reflectance is advantageous for H2 sensing applications, as it could translate to a higher relative change as has been defined ((Ro— R) / Ro). Furthermore, the experimental results of 4H-SiC metasurface with and without Pd were consistent with the simulated spectra (data not shown). The simulated field profile of the narrowband, near-unit absorptionAtty. Dkt. No. 00100-0421 -PCTtransverse mode is presented in the inset of FIG. 3C showing the field enhancement at the edge of pillars with asymmetric distribution due to the oblique illumination. It is also important to note that the ENZ polariton mode near the LO phonon frequency was excited for both the Pd-coated and bare metasurfaces, as the subwavelength pillars provided the necessary additional momentum.

[0075] Upon exposure to H2, the transverse dipole mode around 955 cm'1further redshifted and broadened the linewidth as illustrated in the time-series reflectance spectra presented in FIG. 3E. This resonance shift was the direct result of the local dielectric environment change for the localized phonon polariton mode. Based on the mid-IR dielectric function of Pd, the reflectance spectra when PdHxformed in the metasurface were also simulated (data not shown). When looking at the real-time relative change spectra presented in FIG. 3D, such resonance shift of the transverse dipole mode led to a sharp spectrum change with more than 20% change upon 4% H2 absorption. Such narrowband changes in response to gas exposure are beneficial for practical H2 sensing applications employing single wavelength excitation and detection schemes, for example using CO2 or quantum cascade laser sources. It should also be noted that at the frequency near the ENZ polariton mode, there was also broader amplitude change that extended outside the Reststrahlen band of 4H-SiC upon H2 absorption when compared to the transverse dipole mode. Nevertheless, such relative change already enhanced the mid-IR H2 detection when compared to the Pd / 4H-SiC heterostructure presented in FIGS. 2A-2G.

[0076] Performance of Pd / SiC metasurface for H2 sensing

[0077] Building on the findings of enhanced mid-IR H2 detection using localized phonon polariton modes, the performance of the Pd / 4H-SiC metasurface for H2 sensing was further evaluated. First, a cycling study was conducted, exposing the devices to repeated cycles of 4% H2 and recording the time-dependent relative change in reflectance spectra (FIG. 4A). These real-time spectra distinctly showcased the narrowband detection capability around the transverse dipole mode upon H2 absorption, offering enhanced sensitivity7compared to regions outside the Reststrahlen band of 4H-SiC, where phonon polariton modes were unsupported. Additionally, the ENZ polariton mode also exhibited a pronounced response compared to other modes within the Reststrahlen band. Consequently, the relative change was specifically analyzed at two critical frequencies: the transverse mode at 953 cm'1and the ENZ mode at 968 cm'1, as presented in FIG. 4A. These traces confirmed that the H2 detectionAtty. Dkt. No. 00100-0421 -PCTusing the Pd / 4H-SiC metasurface was highly reversible and repeatable. It should be noted, however, that a slight baseline shift in FIG. 4A can be attributed to the inherent thermal drift of the liquid-nitrogen-cooled Mercury Cadmium Telluride (MCT) detector used during extended measurement periods.

[0078] Next, the dynamic range of the H2 sensor was assessed by evaluating its response to stepwise increasing concentrations of H2 (balance N2), ranging from 0.1% to 4%, at room temperature. To facilitate a comparison relevant to typical H2 sensing applications, where resonance shift based on spectral fitting may not always be practical, the mode with the most significant relative amplitude change was instead used. The ENZ polariton mode for the metasurface was compared with the Berreman mode for the heterostructure, which are depicted in FIGS. 4B and 4C, respectively. This comparison revealed that the metasurface not only exhibited a higher amplitude change but also consistently detected across the full H2 concentration range from 0.1% to 4%, meeting the performance targets set by the DoE and other key stakeholders for H2 sensor technologies. Moreover, as shown in FIG. 4D, the relative change in reflectance for both the metasurface and the heterostructure scaled linearly with Eh concentration. Nevertheless, the metasurface platform demonstrated superior sensitivity compared to a thin film of Pd on 4H-SiC.

[0079] Finally, regarding the response time of the Fh sensor (currently on the scale of minutes), it is noted that this limitation was primarily due to the FTIR measurement scheme used for reflectance measurements and the fact that the gas cell was not optimized for factors such as mass diffusion of Fh molecules. As mentioned earlier, practical Fh sensing applications employing single-wavelength lasers would be better suited to assess the true response time of the sensor that leverages low-loss phonon polaritons. Nevertheless, the results highlight the robustness and sensitivity of the phonon polariton platform for gas-phase Fh sensing in mid-IR, enabling integration of this platform into practical applications, including real-time monitoring of Eh on catalytic surfaces, as well as enhanced chemical sensing for other gases with low-loss polaritonic systems.

[0080] Discussion

[0081] This Example introduced a low-loss phonon polariton platform for mid-IR Eh detection, leveraging Pd / 4H-SiC metasurfaces with localized phonon polariton modes. The findings demonstrate the capability of phonon polaritons to enhance gas-phase chemical sensing, achieving narrowband detection with high sensitivity and repeatability'. Importantly,Atty. Dkt. No. 00100-0421 -PCTthis platform provides a novel solution to overcoming the limitations of existing H2 sensors in the visible spectrum by extending detection capabilities into the molecular fingerprint region of the mid-IR, enabling integration with other IR spectroscopy techniques for dynamic chemical process monitoring on catalytic surfaces. Furthermore, the phonon-enhanced detection demonstrated may be used for sensing beyond hydrogen, including for gas-phase chemical sensing such as carbon monoxide, ammonia, or NOx.

[0082] Additional information, including that indicated as being not shown may be found in supplemental, may be found in the U.S. provisional patent application number 63 / 790,855 that was filed April 18, 2025, the entire contents of which are incorporated herein by reference.

[0083] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0084] The foregoing description of illustrative embodiments of the disclosure has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principles of the disclosure and as practical applications of the disclosure to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents.

[0085] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term “about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

[0086] Terms such as “comprising” and the like may be replaced by “consisting” and the like.

Claims

Atty. Dkt. No. 00100-0421 -PCTWHAT IS CLAIMED IS:

1. A sensor configured to detect H2, the sensor comprising:a polar material and a transducer material that forms an interface with the polar material, wherein the polar material is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material induces a change in the resonance wavelength upon dissociation of H2 on the transducer material;a sample delivery' system configured to introduce a sample to the sensor; and a detector configured to detect output light from the sensor.

2. The sensor of claim 1, wherein the polar material and the transducer material form a heterostructure comprising a layer of the transducer material on a surface of the polar material.

3. The sensor of claim 1, wherein the polar material and the transducer material form a metasurface comprising an array of subwavelength structures arranged according to a pattern.

4. The sensor of claim 3, wherein the subwavelength structures comprise the polar material and the transducer material is in a form of a layer on a surface of each sub wav elength structure in the array.

5. The sensor of claim 4, wherein the subwavelength structures are cylindrical pillars.

6. The sensor of claim 1, wherein the output light is reflected light from the sensor.

7. The sensor of claim 1, wherein the polar material comprises a polymorph of silicon carbide and the transducer material comprises Pd.

8. The sensor of claim 7, wherein the polymorph of silicon carbide is 4H-SiC.

9. The sensor of claim 1, wherein the sensor is passive and is not operably coupled to a power source.Atty. Dkt. No. 00100-0421 -PCT10. The sensor of claim 1, wherein the polar material does not form an interface with a metal other than that formed with the transducer material.

11. A sensor configured to detect a gas phase molecule, the sensor comprising: a polar material and a transducer material that forms an interface with the polar material, wherein the polar material comprises a polymorph of silicon carbide and is configured to support a phonon polariton mode having a resonance wavelength that is excited upon coupling infrared light into the sensor, and the transducer material comprises Pd and induces a change in the resonance wavelength upon absorption or dissociation of a gas phase molecule on the transducer material;a sample delivery system configured to introduce a gas phase sample to the sensor; anda detector configured to detect output light from the sensor.

12. The sensor of claim 11 , wherein the polymorph of silicon carbide is 4H-SiC.

13. The sensor of claim 11, wherein the polar material does not form an interface with a metal other than that formed with the transducer material.

14. The sensor of claim 11, wherein the transducer material induces a change in the resonance wavelength upon dissociation of the gas phase molecule on the transducer material.

15. A method of detecting Tb, the method comprising:delivering a sample to a sensor comprising a polar material and a transducer material that forms an interface with the polar material, wherein the polar material is configured to support a phonon polariton mode having a resonance w avelength that is excited upon coupling infrared light into the sensor, and the transducer material induces a change in the resonance wavelength upon dissociation of H2 on the transducer material;coupling infrared light into the sensor; anddetecting output light from the sensor.Atty. Dkt. No. 00100-0421 -PCT16. The method of claim 15, wherein the polar material and the transducer material form a heterostructure comprising a layer of the transducer material on a surface of the polar material.

17. The method of claim 15, wherein the polar material and the transducer material form a metasurface comprising an array of subwavelength structures arranged according to a pattern.

18. The method of claim 17, wherein the subwav elength structures comprise the polar material and the transducer material is in a form of a layer on a surface of each subwav elength structure in the array.

19. The method of claim 15, wherein the polar material comprises a polymorph of silicon carbide and the transducer material comprises Pd.

20. The method of claim 19, wherein the polymorph of silicon carbide is 4H-SiC.

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