Mid-infrared Range Research Articles

Sensing of gas mixtures via nonlinear interferometry

Abstract We demonstrate the use of a nonlinear interferometer with bulk lithium niobate crystal for sensing of gas mixtures over broadband mid-infrared (mid-IR) range of wavelengths. Our method utilizes spontaneous parametric down-conversion to generate correlated photon pairs in the visible and mid-IR ranges. We show that by leveraging the phase-matching properties of lithium niobate crystal, it is possible to access the ‘fingerprint’ region of greenhouse gases in the mid-IR range by detection of correlated visible light signal. Our technique enables simultaneous detection of gas mixtures with high spectral resolution and fast readout without the need for specialized mid-IR equipment. The experimental results show its applicability to molecular gas sensing, paving the way for new advancements in environmental monitoring.

NIM diffuse reflectance scale to super black level (0.0001)

Abstract The realization of the super black level diffuse reflectance scale of the National Institute of Metrology (NIM, China) is presented. Two facilities were used to achieve the reflectance scale under 0/d condition covering 0.0001-1.0, the first one based on the supercontinuum light source was used to realize the diffuse scale in the wavelength range 500 nm to 2000 nm, while the other one based on QCL was applied for the mid infrared range from 7.5 μm to 10.6 μm. The relative expanded uncertainty(coverage factor k = 2) of the reflectance scale was evaluated to be from 2.2 % to 8.4 % varying with range and wavelength in visible band to near infrared band while the reflectance is down to 0.0001. In the mid infrared band, the uncertainty of the reflectance scale was up to 8.8% while reflectance is 0.001, and up to 42.4% while reflectance is down to 0.0001. This low reflectance level scale can meet the calibration requirement of super black material or system such as carbon nanotube or blackbody.

Dual-band spin-dependent perfect absorber with anisotropic thin films in mid-infrared range

Abstract Motivated by the imperative demand for chirality manipulation within mid-infrared (mid-IR) range, this study presents an innovative solution to obtain selective perfect absorption and chiroptical responses with conventional Otto configuration. Optical responses of the structure have been calculated numerically by the 4×4 anisotropic transfer matrix method (ATMM). The results show that the proposal supports the chiral response with dual-band resonance in absorption spectra by displaying different properties for left-hand circular polarization (LCP) and right-hand circular polarization (RCP) light, its circular dichroism (CD) and working wavelength could be tuned by adjusting the layer thickness and incidence angle. It is demonstrated that the proposed device exhibits an extraordinary absorption efficiency of unity for RCP light and zero for LCP light at a wavelength of 9.32 μm, accompanied by a maximum CD in absorption of -1. The strategy in manipulating dual-band selective chiroptical (CO) responses holds great potential in realizing mid-IR devices capable of chirality manipulation.

Tailoring the optical properties of holey Si thin films

Abstract Si thin films with holes are composite materials with interesting optical properties that can be fabricated and modified by state-of-the-art Si process technologies. Adjusting the volumetric air fraction of these films allows control over their effective refractive indices. This work demonstrates a novel scalable method that combines charged sphere colloidal lithography (CSCL) and dry etching to pattern spatially disordered nanoholes in Si thin films. The method can also be adapted to dielectric materials other than silicon. We show controlled tuning of the effective refractive index by lateral dry-etching of the holes. Utilizing this process, a progressive widening of the average hole diameter was obtained, expanding from initial diameters of 60 nm and 100 nm to dimensions reaching 118 nm and 168 nm, respectively. Consequently, the refractive index of the holey films decreased to approximately 2.1 determined via ellipsometry and Bruggeman's model in the near-infrared (NIR) - mid-infrared (MIR) range, in contrast to the unstructured Si refractive index of 3.42. The systematic optical modification was also observed in the reflection spectra of the fabricated films using Fourier-transform infrared spectroscopy (FTIR). Dielectric holey thin films can be attractive for potential applications in MIR photonic devices such as filters and waveguides.

Infrared nearly complete absorption in hexagonal boron nitride crystals based on guided mode resonance

The near-complete absorption effect in hexagonal boron nitride (hBN) crystals for mid-infrared spectra with guided mode resonance (GMR) is studied. It is achieved by placing an hBN film on a substrate, which is separated from a subwavelength grating by a dielectric film. Under normal incidence, the absorber demonstrates approximately 100% absorption in the reststrahlen upper-frequency band of light. The underlying physical mechanism responsible for this near-complete absorption effect is examined through visualization of electric field distributions at the resonant frequency. Additionally, to provide practical guidance for fabrication, the influence of structural dimensions on the absorption spectra is also explored. These findings reveal that guided mode resonance can be utilized to achieve perfect absorption in hBN films, thereby enhancing their potential applications in the mid-infrared range.

Mid-infrared characterization of NbTiN superconducting nanowire single-photon detectors on silicon-on-insulator

Superconducting nanowire single-photon detectors are widely used for detecting individual photons across various wavelengths from ultraviolet to near-infrared range. Recently, there has been increasing interest in enhancing their sensitivity to single photons in the mid-infrared spectrum, driven by applications in quantum communication, spectroscopy, and astrophysics. Here, we present our efforts to expand the spectral detection capabilities of U-shaped NbTiN-based superconducting nanowire single-photon detectors, fabricated in a 2-wire configuration on a silicon-on-insulator substrate, into the mid-infrared range. We demonstrate saturated internal detection efficiency extending up to a wavelength of 3.5 μm for a 5 nm thick and 50 nm wide NbTiN nanowire with a dark count rate less than 10 counts per second at 0.9 K and a rapid recovery time of 4.3 ns. The detectors are engineered for integration on waveguides in a silicon-on-insulator platform for compact, multi-channel device applications.

Visible-frequency hyperbolic plasmon polaritons in a natural van der Waals crystal

Controlling light at subwavelength scales is crucial in nanophotonics. Hyperbolic polaritons, supporting arbitrarily large wavevectors, enable extreme light confinement beyond the diffraction limit. Traditional hyperbolic metamaterials suffer from high losses due to metallic components, while natural low-loss hyperbolic phonon polaritons are limited to the mid-infrared range. Some hyperbolic materials at visible frequencies have been studied, but they are either very lossy or only feature out-of-plane hyperbolicity. Here, we demonstrate the presence of low-loss, in-plane hyperbolic plasmon polaritons in the visible and near-infrared in thin-films of MoOCl2, a natural van der Waals crystal. The polariton dispersion is predicted based on the framework of light propagation in biaxial media and experimentally confirmed by real space nano imaging on exfoliated flakes. MoOCl2 constitutes an ideal material platform for visible range applications leveraging the unboundedness of hyperbolic modes, such as hyperlensing, Purcell factor enhancement, and super-resolution imaging, without the drawbacks of metamaterials.

Development of FTIR-ATR spectra and PLS regression combination model for discrimination of pure and adulterated acacia honey

A Fourier Transform Infrared Spectroscopy and attenuated total reflectance (FTIR-ATR)-based chemometric model was evaluated for the rapid identification and estimation of adulterants in honey. Two types of honey, bee honey and stingless bee honey, were adulterated with acid adulterants (acetic acid, citric acid, and tamarind extract) at different concentrations of 1%, 3%, 5%, and 7% and sugar adulterants (liquid corn syrup, cane sugar, palm sugar, and inverted sugar) at different concentrations of 1%, 2%, and 3%. The physicochemical properties (Brix, moisture content, pH, and free acidity) and sugar contents (glucose, fructose, and sucrose) of each type of honey and their adulterated samples were determined before FTIR analysis. For all the samples, FTIR spectra were acquired in the mid-infrared range (400-3600 cm−1) and the spectra obtained were subjected to partial least squares (PLS) analysis to develop the model for the determination of adulterants in honey samples. The PLS model produced high coefficient of determination (R2) for bee honey adulterated with acetic acid (0.95), citric acid (0.98), tamarind extract (0.97), and stingless bee honey adulterated with liquid corn syrup (0.99), inverted sugar (0.91), cane sugar (0.86), and palm sugar (0.77). A recognition model was developed for mixture detection and verified through the physicochemical analysis. The combination of FTIR-ATR spectra and the PLS regression model can be a fast, reliable, nondestructive method to detect honey adulteration.

Fluoride and chalcogenide glass fiber components for mid-infrared lasers and amplifiers: Breakthroughs, challenges, and future perspective

Since the early 1990s, when researchers began to explore rare-earth-doped mid-infrared glass fibers, fiber laser systems have emerged as promising high-brightness light sources with wavelengths beyond 2.5 μm for applications in spectroscopy and sensing, optical communications and ranging, and processing of complex materials and bio-tissues, to name a few. Despite a substantial research effort over the years, mid-infrared fiber lasers and amplifiers have yet to reach the maturity required for widespread and/or industrial use. The well-known advantages of fiber lasers over their bulk counterparts, namely superior stability and beam quality, compactness, cost-efficiency, flexibility, and maintenance-free operation, can only be fully harnessed in the mid-infrared wavelength range with the development of non-existent yet essential fiber-based components made of advanced fluoride or chalcogenide-glass materials. This Perspective reports on the recent significant achievements that have been made in the design and fabrication of in-fiber and fiber-pigtailed components for fully integrated mid-infrared fiber laser systems. Building upon a comprehensive overview of the mechanical, thermodynamic, and optical properties of fluoride and chalcogenide glass fibers, as well as their interaction with light, we aim to highlight current challenges and opportunities and provide an informed forecast of future advancements in mid-infrared all-fiber laser research.

In situ high-temperature emissivity measurements of heat-treated, silicon coated stainless steel for solar thermal applications

Understanding thermal emissivity at high temperatures is crucial for developing efficient materials for solar thermal applications. We present a new approach for creating an efficient material for solar absorber by developing a nano structured surface on stainless steel substrate through Si deposition and annealing. We prepare five samples by annealing them at five temperatures between 700 °C and 1100 °C. Afterwards, we perform a systematic study of the spectral emissivity at elevated temperatures, focusing on different parameters: angle dependence, wavelength dependence, and temperature dependence. The spectral directional emissivity experiments performed in the mid-infrared range reveal a dielectric behavior of the samples in the short wavelength region (λ < 6 μm) and metallic behavior in the long wavelength region (λ > 12 μm). The results indicate an increase in hemispherical and total normal emissivity with measurement temperature (from 200 °C to 700 °C), influenced by oxide/silicide formation due to interdiffusion, and by surface roughness. Notably, samples annealed at 900 °C and 1000 °C demonstrate enhanced thermal stability at 700 °C, showcasing promising characteristics for high-temperature applications. Consequently, this study presents a viable method for developing cost-effective silicon-based solar absorber coatings on stainless steel with tailored properties for solar thermal applications along with its real time high temperature emissivity details.

Active infrared tuning of metal–insulator-metal resonances by VO2 thin film

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal–insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3–5 μm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 μm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Comparing silicon mineral species of different crystallinity using Fourier transform infrared spectroscopy

In soils, various solid silica (Si) species exhibit different weathering behaviors and surface reactivities, which are among other characteristics related to the crystallinity of the silicate tetrahedral network. Amorphous species exhibit faster weathering and generally possess a larger specific surface area in comparison to crystalline species. However, the characterization of these different species is commonly based on wet chemical extraction methods, which lack selectivity. While Fourier transform infrared spectroscopy (FTIR) in the mid-infrared range can differentiate between short-range ordered aluminosilicates (SROAS) and pure amorphous silica (ASi), few systematic studies are found on the IR spectral features that distinguish solid Si species by crystallinity. This study aims to identify FTIR absorption bands that can differentiate Si species based on their crystallinity. Our data clearly indicate that ASi can be distinguished from very crystalline silica (quartz) and sea sand. The absorption band at approximately 800 cm−1 in the FTIR spectra allows determining the degree of crystallinity of the studied ASi species since the band becomes smaller and the band maximum shifted toward lower wavenumbers with increasing degree of crystallinity. Hence, FTIR spectra may be used to differentiate certain Si species in complex samples like soils, allowing the estimation of weatherability and surface reactivity of those species.

Ultrabroadband Optical Properties of 2D Titanium Carbide MXene.

MXenes have rapidly ascended as a prominent class of two-dimensional (2D) materials, renowned for their distinctive optical and electrical properties. Despite extensive exploration of MXenes' optical properties, existing studies predominantly focus on the near-infrared (NIR) to the ultraviolet spectral range, leaving the mid-infrared (mid-IR) range relatively uncharted. In this study, we conducted a comprehensive characterization of the intrinsic optical properties of Ti3C2Tx MXene across an ultrabroadband spectral range, spanning from mid-IR (28 meV) to vacuum ultraviolet (VUV, 6.4 eV). For this purpose, Ti3C2Tx MXene films of varying thicknesses were coated on quartz substrates, resulting in two distinct categories: thin film samples with thicknesses below 50 nm and bulk-like samples with thicknesses exceeding 500 nm. Using spectroscopic ellipsometry, we analyzed the optical properties of films of various thicknesses and extracted detailed information on their dielectric functions. Our findings reveal resonances in the mid-IR to VUV range. Employing the Lorentz-Drude model to examine these resonances has uncovered the optical resistivity of MXene films and led to the identification of multiple plasmonic modes active in the visible to NIR range, as well as broad band-to-band transition-like resonances in the mid-IR range. This ultrabroadband optical versatility of Ti3C2Tx MXene is anticipated to bring about a wide range of thermal and optical applications.

Freestanding Germanium Photonic Crystal Waveguide for a Highly Sensitive and Compact Mid-Infrared On-Chip Gas Sensor.

The performance of mid-infrared (MIR) on-chip gas sensors, operating via laser absorption spectroscopy, hinges critically on light-matter interaction dynamics, significantly influenced by external confinement and the effective light path length. Conventional on-chip sensors, however, face challenges in achieving the required limit of detection for highly sensitive applications, primarily due to their intrinsically short effective light path. Furthermore, these sensors are limited in their spectral range coverage within the MIR spectrum by the constraints of standard silicon-based platforms. To overcome these limitations, our research presents a novel approach to fabricate a freestanding germanium (Ge) photonic crystal waveguide (PCW) on a germanium-on-insulator (Ge-OI) platform, utilizing yttrium oxide (Y2O3) as the buried oxide layer. This device leverages the broad transparent windows of Ge and Y2O3, broadening the spectral coverage across the MIR range. The introduction of the PCW and its slow light effect significantly elevate external confinement and light-matter interactions, enabling a notable reduction in waveguide length, which traditionally limits on-chip configurations. The freestanding structure not only expands the sensing region and enhances external confinement but also prevents the emergence of leaky modes within the PCW. As a result, our compact sensor achieves an exceptionally low LoD of 7.56 ppm for carbon dioxide (CO2) sensing at the operational wavelength of 4.23 μm, with a compact waveguide length of only 800 μm.

Fabrication of optimized partial-reflection coatings for mid-infrared quantum cascade lasers

This study presents both numerical modeling and experimental fabrication of three different partial-reflection (PR) coatings each optimized for quantum cascade lasers (QCLs) that emit radiation in the mid-infrared range. A novel double-layer PR coating comprising silicon dioxide (SiO2) and silicon nitride (Si3N4) was proposed as a potential solution for compatibility with QCLs fabrication processes. Subsequently, the PR coating was compared with two well-known PR coatings: a single layer of aluminum oxide (Al2O3) and a single layer of yttrium oxide (Y2O3). The coatings were designed to reduce the reflectivity of the front laser mirror from 30% to approximately 13%. The thickness of the dielectric layers was optimized for lasers emitting at 4.4 μm, with applicability in the 2.5–6 μm range. The proposed double-layer coating achieved the desired reflectivity while reducing the total coating thickness by 120 nm. By using the presented coatings it will be possible to increase the optical power of Mid-Infrared QCLs.

Mass-Producible Transparent Flexible Passive-Cooling Film.

Passive cooling is regarded as the desired method for interior temperature regulation due to its advantage of energy consumption reduction. An ideal passive-cooling window is expected to exhibit a high emissivity in the mid-infrared (MIR, 8-13 μm) spectral range for cooling and a low transmittance in the near-infrared (NIR, 780-2100 nm) spectral range for reducing heat flow, while presenting a high transmittance in the visible (VIS, 400-780 nm) spectral range for daylighting. However, material structures that meet these requirements often come with high demands for precision in manufacturing and elevated processing costs, which have limited their potential for large-scale mass production. Here, we propose a mass-producible transparent flexible passive-cooling film that is relatively easy to process and low-cost and meets all of the requirements mentioned above. The film is made of poly(methyl methacrylate) mixed with Cs0.33WO3 nanoparticles, and it shows a high absorptance (80%) in NIR for blocking solar radiation penetration and a high emissivity (93%) in MIR for radiative cooling as well as a reasonable transmittance (40%) in VIS for visibility. Under solar intensity of ∼900 W/m2, a maximum temperature reduction of 8.4 and 7.8 °C has been achieved for a window coated by the film compared to the uncoated window in the condition of the absorbing chamber and car, respectively. Such a mass-producible transparent flexible passive-cooling film holds promising applications in large windows, such as those used in automobiles and buildings, where there is a high demand for both daylighting and cooling.

Tunable graphene plasmonic metadevice for infrared polarization resolved spectroscopy detection

Polarization-resolved spectroscopy imaging holds significant promise in various fields. However, conventional polarization-resolved spectroscopy imaging systems often reply on separate dispersion elements, polarization splitting components, and mechanical scanning or rotating structures, resulting in bulky systems with intricate designs. The pursuit of lightweight and seamlessly integrated polarization spectroscopy imaging systems presents a critical challenge. Here, we propose an integrated polarization spectroscopy detector operating in the mid-infrared range based on a tunable graphene metasurface. Within the graphene metasurface, plasmonic resonances of graphene are excited by a gold nano-grating, allowing simultaneous modulation of spectral and polarization transmission characteristics for infrared light by tuning the Fermi energy of graphene. With the help of neural network based recovery algorithms, this system achieves pixel-level spectral detection (with a resolution below 200 nm), multi-color polarization Stokes vector analysis (with a monochromatic angular error of less than 0.1 rad), and concurrent polarization and spectroscopy detection. This research paves the way for the development of the next generation integrated infrared polarization resolved spectroscopy imaging systems.

Mid-infrared hyperspectral microscopy with broadband 1-GHz dual frequency combs

Mid-infrared microscopy is an important tool for biological analyses, allowing a direct probe of molecular bonds in their low energy landscape. In addition to the label-free extraction of spectroscopic information, the application of broadband sources can provide a third dimension of chemical specificity. However, to enable widespread deployment, mid-infrared microscopy platforms need to be compact and robust while offering high speed, broad bandwidth, and high signal-to-noise ratio. In this study, we experimentally showcase the integration of a broadband, high-repetition-rate dual-comb spectrometer (DCS) in the mid-infrared range with a scanning microscope. We employ a set of 1-GHz mid-infrared frequency combs, demonstrating their capability for high-speed and broadband hyperspectral imaging of polymers and ovarian tissue. The system covers 1000 cm−1 at νc = 2941 cm−1 with 12.86 kHz spectra acquisition rate and 5 µm spatial resolution. Taken together, our experiments and analysis elucidate the trade-off between bandwidth and speed in DCS as it relates to microscopy. This provides a roadmap for the future advancement and application of high-repetition-rate DCS hyperspectral imaging.

Growth and optical properties of high-concentration Ho3+ ion-doped Ba(Y1−xLux)2F8 crystal for mid-infrared laser application

30mol% Ho:BaY2F8 (Ho:BYF) and 30mol% Ho, 20mol% Lu:BaY2F8 (Ho:BYLF) crystals with monoclinic phase were prepared by the Bridgman technique. After introducing Lu3+ ions, the thermal property of Ho:BYLF crystal was improved without significantly shortening the 5I5 level lifetime of Ho3+ ion compared with Ho:BYF crystal. Under laser diode pumping at 889 nm, the emission peaks of both Ho:BYF and Ho:BYLF crystals in the range of 3–5 μm could be detected. The spectral parameters of the crystal are calculated in detail, and the cross sections of absorption, emission and gain are given. During laser pumping, the two crystals showed low pumping threshold, and Ho:BYF crystal obtained a maximum energy output of 5.22 mJ, while Ho:BYLF crystal tended to be saturated at 1 mJ due to the high concentration of Lu3+ ion. These two crystals show great application potential for laser operating in the mid-infrared range.

Mid-infrared Measurements of Ion-irradiated Carbonaceous Meteorites: How to Better Detect Space Weathering Effects

Remote sensing study of asteroids will soon enter a new era with an increasing amount of data available thanks to the JWST, especially in the mid-infrared (MIR) range that allows identification of mineral species. It will then be possible to establish a taxonomy, as is currently available in the visible–near-infrared range, based on MIR spectral parameters. It had been previously shown that the MIR range is very sensitive to space weathering (SpWe) effects. Thus, it is crucial to determine which spectral changes are involved to disentangle initial composition from surface aging and provide tools to interpret future remote sensing data of asteroids. We present here MIR measurements of a wide variety of ion-irradiated carbonaceous chondrites as a simulation of the solar wind SpWe component. We evaluate several parameters (the Christiansen feature and Reststrahlen band positions, the width of the main Si–O band) and test different measurement conditions (ion energy and geometry of observation). We highlight a dependency of the spectral changes with the initial composition, as hydrated samples are more affected than anhydrous ones. We confirm the role of the geometry in the detection of SpWe effects as already shown in the near-infrared, with a competition effect between the depth probed by photons and the implantation depth of ions (function of the energy used). We will discuss the results in the framework of future observations and Ryugu’s and Bennu’s samples studied in the laboratory.