Long-wavelength Infrared Region Research Articles

Long-wavelength infrared upconversion time-stretch spectroscopy

High-speed spectroscopy in the molecular fingerprint spectral region (≈6–12 μm) is essential for the detection of ultrafast molecular dynamic processes, rapid combustion analysis, and biological diagnostics. However, ultrafast spectroscopy in the long-wavelength infrared (LWIR) region remains a challenge due to the limitations of laser sources and the lack of ultrafast and sensitive detectors in this wavelength region. Here, we demonstrate broadband LWIR time-stretch spectroscopy, which can realize a single-shot high-speed spectral measurement in a 8–10 μm region, by combining the LWIR femtosecond (fs) light generation and upconversion time-stretching detection with specific dispersive fiber. Broadband tunable fs light generated in the 8–10 μm region is upconverted to the 1.1–1.2 μm near-infrared wavelength via difference-frequency generation with the 1 μm chirped pump pulse. Time-stretch detection of the upconverted light can then be realized by adopting dispersion shifted fiber, which has a superior dispersion-to-loss ratio in the 1.1–1.2 μm wavelength region, as the dispersive medium. As a result, we experimentally demonstrate LWIR time-stretch spectroscopy in the 8–10 μm region with a spectral resolution of 1.07 cm−1, at a rate of 200 kSpectra s−1, which is only limited by the repetition rate of the 1 μm pump source. The demonstration of high-speed time-stretch spectroscopy in the LWIR region would open the possibility in exploring the transient dynamics of molecular fingerprint spectroscopy.

Compact all-dielectric metasurface for full polarization detection at the long-wavelength infrared region.

Metasurfaces have been extensively demonstrated in engineering and detection of polarization of light from the visible to terahertz regions. However, most of the previous metasurfaces for polarization detection are spatially divided into different parts, and each of the parts focuses on different polarization components, resulting in large metasurface size and hindering their integration development. In this paper, a compact all-dielectric metasurface is proposed and numerically demonstrated to achieve full polarization detection at the long-wavelength infrared region (LIR). First, we design the metasurface at a wavelength of 10µm, which can converge incident beams to specific positions corresponding to different polarization states. In this design, the metasurface is based on an oblique alternant double-phase modulation method, which arranges meta-atoms with the ability to control as many as possible different polarizations in a limited region, ensuring the high efficiency of polarization detection while giving more freedom and flexibility to the metasurface. Second, the intensity distributions of the electric field of different polarization components are simulated at wavelengths of 9.4µm and 10.5µm, verifying the broadband performance of the proposed metasurface. The proposed method has potential applications in integrated multifunctional devices and multispectral polarization imaging.

A transparent zinc oxide based metamaterial for perfect absorption of long- wavelength and mid-wavelength infrared spectral bands

A high absorption capabilities in atmospheric transparency windows lies at mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) region is important for various applications, particularly, for passive thermal cooling and camouflage technology compatible with infrared (IR) lasers. In this paper, we proposed a simple and cost-effective design for a tri-layered metamaterial based on zinc oxide (ZnO) which exhibits almost perfect absorption in both the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions with excellent transmission in the visible range. It consists of a metal-dielectric-metal sandwiched structure with a two-dimensional periodic pattern of gallium doped ZnO circular discs at the top, ZnO in the middle, and an aluminum doped ZnO at the bottom. The proposed metamaterial exhibits almost perfect absorption in both the LWIR (∼ 10 µm) and MWIR (∼ 5 µm) regions, while maintaining excellent transmission (∼ 75%) in the visible region. Further, it also exhibits a better angular (0–50°) insensitivity with respect to the incident angle and to the polarization (TE and TM) of electromagnetic wave. The proposed metamaterial has potential applications in thermal cooling/emission and infrared camouflage technology.

Linear polarization-separating metalens at long-wavelength infrared.

We designed and fabricated a linear polarization-separation metalens (PSM) made of single-crystal silicon (sc-Si) for long-wavelength infrared (LWIR) imaging. The PSM comprises sc-Si dielectric waveguide pillar meta-atoms with rectangular cross-sections, providing a full 2π phase delay range for two orthogonal linear polarization components with high transmittances (>70%). Electron beam lithography and deep reactive ion etching were used to fabricate the PSM. Polarization-separation imaging of elevated and ambient temperature objects was demonstrated with high extinction ratios of 21.8 dB and 12.8 dB for the x- and y-polarizations, respectively. Additionally, polarization-sensitive imaging was demonstrated by distinguishing the surfaces of a hand and toy windows. Our work enables the visualization of invisible information in the LWIR region and has widespread applications.

Uncooled two-microbolometer stack for long wavelength infrared detection

We have investigated an uncooled infrared (IR) detector utilizing a dual level architecture. This was achieved by combining two-microbolometer stack in the vertical direction to achieve high IR absorption over two distinct spectral windows across the long wavelength infrared region (LWIR). In addition, we have studied amorphous silicon germanium oxide (SixGeyO1−x−y) as an IR sensitive material, and metasurface to control IR absorption/reflection in interaction with standard Fabry–Perot cavity. The bottom microbolometer uses a metasurface to selectively absorbs a portion of the spectrum and reflects radiation outside this window range. At the same time, the top microbolometer uses a conventional Fabry–Perot resonant cavity to absorb a different portion of the spectrum and transmit any unabsorbed radiation outside this window. This device can be used to measure the absolute temperature of an object by comparing the relative signals in the two spectral bands. The spectral responsivity and detectivity, and thermal response time were > 105 V/W, > 108 cm Hz1/2/W, and 1.13 ms to filtered blackbody infrared radiation between (2–16) µm. The microbolometer voltage noise power spectral density was reduced by annealing the microbolometers in vacuum at 300 °C.

Interparticle-Coupled Metasurface for Infrared Plasmonic Absorption

Plasmonic metasurfaces operating in the long-wavelength infrared region (LWIR) are primarily employed for bio-sensing and imaging applications. The key factor affecting the capability of LWIR metasurfaces is absorption efficiency, whereas significant advances have been made to attain higher absorption intensities, using metal-insulator-metal (MIM) stack structures, which require a complex nanofabrication process and repetition of the patterning process to develop each layer. This study experimentally demonstrates the integration of metasurfaces with plasmonic resonators. The metasurfaces and plasmonic resonators are developed through electron beam lithography (EBL) on the same plane of the Silicon substrate. The fabricated prototype has broad incident angle stability at a bandwidth of 2μm with near-perfect absorption at 8μm resonance wavelength when characterized through the Attenuated Total Reflectance - Fourier Transforms Infrared Spectroscopy (ATR-FTIR) setup. The metasurface device achieves tunable resonance behavior when the incident angles reach up to 70°. The proposed integration of metasurface with plasmonic resonators are well-suited for enhanced biosensing applications with metasurface devices.

High absorption and a tunable broadband absorption based on the fractal Technology of Infrared Metamaterial Broadband Absorber

This paper proposes a dual-band tunable broadband metamaterials absorber, achieved absorption in 3-5 μm mid-wavelength infrared (MWIR) and 8-12 μm long-wavelength infrared (LWIR). The absorber is composed of periodic units, and each unit is composed of a bottom ideal electric conductor-dielectric layer-graphene layer-metal fractal cross resonator in which a graphene layer-metal fractal cross is embedded. The metal structure uses the multi-scale self-similar characteristics of fractal geometry to create multiple resonances for the same element structure. Numerical simulations were carried out using the finite element method (FEM). The optimization results show that the designed single-layer 3-Level graphene-metal fractal cross absorber produces broadband absorption of 2.86 μm (absorption >90%) in the LWIR region and an average absorption rate of 92.1%. By adding the bottom layer 3 × 3 graphene-metal fractal cross array structure, a resonant absorption peak is generated in the MWIR region, and the broadband absorption of 0.38 μm (absorption >80%), and it achieves broadband absorption of 2.48 μm (absorption >90%) in the LWIR region. For the double-layer graphene-metal structure, the resonance bandwidth and absorptivity of the absorber can be tuned by adjusting the Fermi energy of each layer of graphene. Due to its broadband, polarization insensitivity, high absorption, flexible tunable, and other characteristics, the proposed metamaterials absorber is beneficial for fabricating nano-optoelectronic devices and can be applied in thermal imaging and infrared detection.

InAsSb pillars for multispectral long-wavelength infrared absorption

InAsSb pillars were investigated for multispectral photodetection in the long wavelength infrared (LWIR) region. An InAs0.19Sb0.81 thin film was successfully grown on Si (100) substrate, utilizing an AlSb buffer layer to alleviate the large lattice mismatch. X-ray diffraction studies showed a majority [100] orientation of the as-grown films, with minor orientations arising as a result of twinning. Arrays of InAsSb pillars with diameters ranging from 1700 nm to 4000 nm were fabricated by a top-down reactive ion etching process. The arrays showed resonant optical absorption peaks in the LWIR region from 8 to 16 μm wavelength, dependent on the pillar diameter. The peak absorptance wavelength increased by 0.46 μm for each 100 nm increase in pillar diameter, demonstrating the multispectral tunability of such arrays.

Citrate-assisted hydrothermal synthesis of vanadium dioxide textured films with metal-insulator transition and infrared thermochromic properties

The present study reports the fast synthesis of VO2 (М1) oriented films on the r-Al2O3(012) substrates by the hydrothermal method using citric acid and vanadium (V) oxide as precursors with post deposition annealing in inert atmosphere. The effects of synthesis parameters (solution concentration and autoclave filling factor) on the films composition, morphology and electric properties are considered using XRD, Raman spectroscopy, AFM and SEM. The obtaining VO2(M1)/r-Al2O3 films show a metal-insulator transition with resistivity change of ~3.5 orders of magnitude and excellent thermochromic properties in the long-wavelength IR region. This work demonstrates a promising technique to promote the commercial implementation of VO2 as material for design of infrared (IR) switch devices.

High refractive index dispersion of compositionally optimized Ge-Ga-Sb-S sulfide glass for use as molded lens in the long-wavelength infrared range

In an effort to endow Ge-Sb-S sulfide glass with capability to transmit the long-wavelength infrared (LWIR) photons as well as to possess highly dispersive refractive indexes over the LWIR spectral range, we have compositionally adjusted ternary Ge-Sb-S glass prior to tuning quaternary Ge-X-Sb-S (X = Ga, Bi, In or Sn) glass. Among the constituent atoms of ternary Ge-Sb-S glass, Ge content turned out to exert the strongest influence not only on thermal and mechanical properties but also on infrared transmission edge. Specifically, glass transition temperature and Vickers hardness tended to be directly proportional to Ge content, but infrared transmission edge was blue-shifted upon increase of Ge content, simultaneously producing an additional multiphonon absorption peak at ~ 9 µm. As such, Ge-Sb-S glass containing Ge atoms less than ~ 15 at% was considered to feature applicability in LWIR range, which on the other hand suffers from relatively poor thermal and mechanical properties. In our experimental attempts on quaternary sulfide glasses performed to mitigate such deteriorations, presence of relatively small amount of Ga appeared to be most effective among the elements employed in this study as the fourth constituent. Moreover, only Ga atoms improved thermal stability with significantly increasing crystallization temperature. Quaternary Ge-Ga-Sb-S glass compositionally optimized in this study was then verified to be much more dispersive than typical Se-based chalcogenide glasses in the LWIR region.

Tunable Fabry-Perot Interferometer Designed for Far-Infrared Wavelength by Utilizing Electromagnetic Force.

A tunable Fabry-Perot interferometer (TFPI)-type wavelength filter designed for the long-wavelength infrared (LWIR) region is fabricated using micro electro mechanical systems (MEMS) technology and the novel polydimethylsiloxane (PDMS) micro patterning technique. The structure of the proposed infrared sensor consists of a Fabry-Perot interferometer (FPI)-based optical filter and infrared (IR) detector. An amorphous Si-based thermal IR detector is located under the FPI-based optical filter to detect the IR-rays filtered by the FPI. The filtered IR wavelength is selected according to the air etalon gap between reflectors, which is defined by the thickness of the patterned PDMS. The 8 μm-thick PDMS pattern is fabricated on a 3 nm-thick Al layer used as a reflector. The air etalon gap is changed using the electromagnetic force between the permanent magnet and solenoid. The measured PDMS gap height is about 2 μm, ranging from 8 μm to 6 μm, with driving current varying from 0 mA to 600 mA, resulting in a tunable wavelength range of 4 μm. The 3-dB bandwidth (full width at half maximum, FWHM) of the proposed filter is 1.5 nm, while the Free Spectral Range (FSR) is 8 μm. Experimental results show that the proposed TFPI can detect a specific wavelength at the long LWIR region.

Surface plasmon-enhanced dual-band infrared absorber for -based microbolometer application**Project supported by the One Hundred Talents Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 61376083 and 61307077), the China Postdoctoral Science Foundation (Grant Nos. 2013M530613 and 2015T80080), and the Guangxi Key Laboratory of Precision Navigation Technology and Application (Grant Nos. DH201505, DH201510, and

We propose a periodic structure as an extra absorption layer (i.e., absorber) based on surface plasmon resonance effects, enhancing dual-band absorption in both middle wavelength infrared (MWIR) and long wavelength infrared (LWIR) regions. Periodic gold disks are selectively patterned onto the top layer of suspended SiN/VO2/SiN sandwich-structure. We employ the finite element method to model this structure in COMSOL Multiphysics including a proposed method of modulating the absorption peak. Simulation results show that the absorber has two absorption peaks at wavelengths and with the absorption magnitudes more than 0.98 and 0.94 in MWIR and LWIR regions, respectively. In addition, the absorber achieves broad spectrum absorption in LWIR region, in the meanwhile, tunable dual-band absorption peaks can be achieved by variable heights of cavity as well as diameters and periodicity of disk. Thus, this designed absorber can be a good candidate for enhancing the performance of dual band uncooled infrared detector, furthermore, the manufacturing process of cavity can be easily simplified so that the reliability of such devices can be improved.

Modeling of Quantum Well Infrared Photo Detector for Long Wavelength Infrared Detection

ABSTRACTThis paper presents the detection of long wavelength (LW) infrared using quantum well infrared photo detector (QWIP). A theoretical modeling of GaAs/AlxGa1 − xAs QWIP has been developed for astronomical application such as morphological study of planets and stars. The parameters, namely high responsivity, absorption efficiency, and low dark current have been obtained on the basis of this model with the fine variation of aluminum mole fraction and well width that lies in wavelength range 9.7–11.6 µm. It reveals that this photo detector is best suited for detection in long wavelength infrared region (LWIR). Here, the obtained capture probability is almost constant with respect to wavelength in LWIR region.

Fano resonance in asymmetric-period two-dimensional plasmonic absorbers for dual-band uncooled infrared sensors

The spectral discrimination function of uncooled infrared (IR) sensors has significant advantages for applications such as fire detection, gas analysis, and biological analysis. We have previously demonstrated wavelength-selective uncooled IR sensors using two-dimensional plasmonic absorbers (2-D PLAs) over a wide range spanning the middle- and long-wavelength IR regions. 2-D PLAs are highly promising in terms of practical application due to the ease of fabrication and robustness for structural fluctuations. However, dual-band operation based on this concept has not yet been investigated, even though the ability to absorb in two different wavelength bands is extremely important for object recognition. Thus, a dual-band uncooled IR sensor was developed that employs Fano resonance in the plasmonic structures. To achieve dual-band detection, asymmetric periods in the orthogonal x- and y-directions were introduced into 2-D PLAs. Theoretical investigations predicted an asymmetric absorbance line shape dependent on the polarization attributed to Fano resonance. The spectral responsivity of the developed sensor demonstrated that selective detection occurred in two different wavelength bands due to polarization-dependent Fano resonance. The results obtained in this study will be applicable to the development of advanced sensors capable of multiband detection in the IR region.

A Fabry–Perot interferometer-based long-wavelength infrared spectrometer utilizing a novel PDMS patterning technique

We report a Fabry–Perot interferometer (FPI)-based infrared (IR) spectrometer designed for long-wavelength infrared (LWIR) region. To fabricate the proposed FPI-based LWIR spectrometer, we developed a novel poly(dimethylsiloxane) (PDMS) patterning technique by combining photolithography and dry etching. The proposed PDMS patterning technique has advantages such as clear shape of PDMS pattern and no damage to the silicon (Si) substrate. Especially, the final height of PDMS pattern is easily adjusted by controlling the thickness of spin-coated photoresist. The proposed FPI-based IR spectrometer, which is composed of upper and lower substrate were fabricated separately utilizing newly proposed PDMS patterning technique. Experimental results show that maximum transmittance of filtered wavelength component by the proposed FPI-based IR spectrometer lies at the wavelength of 16 µm. The proposed IR spectrometer can detect a specific wavelength at LWIR region by controlling the air etalon gap that is adjusted by utilizing the proposed PDMS patterning technique.

IR optical system design of uncooled thermal imaging camera in long band (8—12µm)

For 384×288 pixel uncooled micro bolometer (a-Si) focal plane array (FPA) detector an infrared optical system (lenses) has been designed in this paper for work in long wavelength infrared region (LWIR) 8— 12 µm. the design results show that the optimum optical performance can be obtained by our proposed approach, so the modulation transfer function (MTF) at Nyquist frequency above 0.5 and closed to diffraction limit, and diameter of geometrical blur image less than detector unit cell (pixel) and concentrated on it. The main advantages of this design beside quality of it optically, it's simple in structure (consists of one conic surface - ellipse; and five spherical surfaces), lightweight (about 77.59g), small volume (entrance pupil diameter=40mm), and short length (total track=58.5mm). These mechanical parameters in addition to chosen tolerance level for manufacture of lenses have a positive affect on the cost of production for this designed system. So the designed optical system should good enough to meets the technical performance requirements for: high image quality, small size, light weight, and low cost thermal camera. This system can be applied in

ENVIRONMENTAL SENSING OF CHEMICAL AND BIOLOGICAL WARFARE AGENTS IN THE THz REGION

Discrimination between hazardous materials in the environment and ambient constituents is a fundamental problem in environmental sensing. The ubiquity of naturally occurring bacteria, plant pollen, fungi, and other airborne materials makes the task of sensing for biological warfare (BW) agents particularly challenging. The spectroscopic properties of the chemical warfare (CW) agents in the long wavelength infrared (LWIR) region are important physical properties that have been successfully exploited for environmental sensing. However, in the case of BW agents, the LWIR region affords less distinction between hazardous and ambient materials. Recent studies of the THz spectroscopic properties of biological agent simulants, particularly bacterial spores, have yielded interesting and potentially useful spectral signatures of these materials. It is anticipated that with the advent of new THz sources and detectors, a novel environmental sensor could be designed that exploits the peculiar spectral properties of the biological materials. We will present data on the molecular spectroscopy of several CW agents and simulants as well as some THz spectroscopy of the BW agent simulants that we have studied to date, and discuss the prospectus with regard to detection probabilities through the application of sensor system modeling.