Long-wavelength Infrared Research Articles

Study of subjective and objective quality assessment of infrared compressed images

Abstract Given the lack of accessible infrared compressed images’ benchmarks annotated by human subjects, this work presents a new database with the aim of studying both subjective and objective image quality assessment (IQA) on compressed long wavelength infrared (LWIR) images. The database contains 20 reference (pristine) images and 200 distorted (degraded) images obtained by application of the most known compression algorithms used in multimedia and communication fields, namely: JPEG and JPEG-2000. Each compressed image is evaluated by 31 subjects having different levels of experience in LWIR images. Mean opinion scores (MOS) and natural scene statistics (NSS) of pristine and compressed images are elaborated to study the performance of the database. Five analyses are conducted on collected images and subjective scores, namely: analysis by compression type, analysis by file size, analysis by reference image, analysis by quality level and analysis by subject. Moreover, a wide set of objective IQA metrics is applied on the images and the obtained scores are compared with the collected subjective scores. Results show that objective IQA measures correlate with human subjective results with a degree of agreement up to 95 %, so this benchmark is promising to improve existing and develop new IQA measures for compressed LWIR images. Thanks to a real-world surveillance original images based on which we analyze how image compression and quality level affect the quality of compressed images, this database is primarily suitable for (military and civilian) surveillance applications. The database is accessible via the link: https://github.com/azedomar/compressed-LWIR-images-IQA-database. As a follow-up to this work, an extension of the database is underway to study other types of distortion in addition to compression.

Growth of high material quality InAs/GaSb type-II superlattice for long-wavelength infrared range by molecular beam epitaxy

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.

New insights into the ultimate performance of HgCdTe photodiodes

Proposal of a new metric, so-called “Law 19”, for estimation of the HgCdTe performance of photodiodes forces the verification of the ultimate properties of HgCdTe photodiodes, in particular in the long wavelength infrared (LWIR) spectral range. The present work addresses this challenge. Assuming a new benchmark, the HgCdTe photodiodes performance predictions are compared with “Rule 07” metric and confronted with the experimental data available in the literature. The main comparative efforts concern such photodiode parameters as dark currents and the R0A products.

Dark current and noise analysis for Long-wavelength infrared HgCdTe avalanche photodiodes

In this paper, the temperature-dependent current–voltage (I-V) characteristics (30∼100 K) of planar (n+/n-/p) long-wavelength infrared (LWIR, cutoff-wavelength λc = 11.5 μm at 80 K) HgCdTe avalanche photodiodes (APDs) device were measured and numerical simulated. The parameters in the Okuto-Crowell model were obtained as a function of bandgap and operation temperature, and the results showed that our theoretical model fit well with the experiment. Considering the difficulty of evaluating excess noise factor F at high gains due to the rapid increase in band-to-band tunneling (BBT) current, the effects of BBT on the F measurements were discussed, by means of noise power spectral density (PSD) and noise figure meter. We obtained a low F = 1–1.27@gain (M) = 6 at −2 V and extended the F to a higher gain (M = 23 at −3 V). The bias-dependent dark noise was further studied, and we supposed that the BBT component might partially undergo avalanche multiplication in our LWIR devices. Therefore, suppressing the BBT current by an appropriate annealing process was one possibility to reduce dark current and noise in devices. The method in this paper can provide a reference for performance evaluation in HgCdTe APDs, as well as guidance for designing or optimizing the structure.

Glass formation and optical properties of Sn modified GeS2-Ga2S3-CsCl chalcogenide glasses

Chalcogenide glass is emerging as important IR materials for advanced athermal infrared (IR) optical systems because of its low thermal coefficient of refractive index. New chalcogenide glasses based on 75GeS2-15Ga2S3-10CsCl were developed through the substitution of GeS2 by SnS2. The introduction of heavier tin (Sn) results in the red shift of transmission window, both of visible and long-wavelength IR cut-off edges, the elevation of refractive index dispersion curves, and the decreasing of glass transition temperature. The formation of [SnS4-xClx] structural units, instead of [GeS4-xClx], should be responsible for the variation of these structural properties. In addition, homogeneous Sn modified GeS2-Ga2S3-CsCl chalcogenide glass of Φ30 mm was also prepared successfully, showing it of high potential for commercial applications.

Spectral tuning in quantum rings by magnetic field: detection from NIR to FIR regime

In this work, we have proposed a new method for absorption spectrum tuning of the quantum ring (QR) in a wide interested range of IR (1.24–103 µm) using an external vertically magnetic field. We have used a tight binding method (TBM) to investigate the optical properties of the structure. The effect of the external magnetic field on the absorption spectrum has been investigated as the post-processing method. Also, the effect of different geometrical parameters like the number of QWs, QR radius, and constructed material considered as processing methods for tuning absorption. The results show for GaAs based QR with the radius of 100/π nm consist of 4 QWs, absorption peak linearly increase from 100 meV (12.4 um) in long wavelength infrared (LWIR) regime and shifts to 200 meV (6.2 um) in mid-infrared (MIR) regime with increasing external magnetic field. Also for a QR with a radius of 200/π nm, by changing the magnetic field different absorption peaks appear and disappear in an absorption spectrum and a multi-color detector converts to single-detector in the near-infrared (NIR) regime. This results help experimentalist to select proper materials with optimized geometrical parameters to achieve desired wavelength for various detection applications.

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.

Modulation transfer function measurements of HgCdTe long wavelength infrared arrays for the Near-Earth Object Surveyor

The modulation transfer function (MTF) is a useful measure in image quality analysis and performance budget determination. Sensitive long wavelength infrared (LWIR) detectors for astronomical space telescopes require slight modifications to the existing MTF measurement methods due to the increased prevalence of high dark current pixels. Presented here are the specifics of a modified slanted edge method to determine the MTF in λc > 10 μm HgCdTe detectors to be used with the planned Near-Earth Object Surveyor Mission. The measured MTF at Nyquist using 6 μm light is 0.22 ± 0.02 and is 0.25 ± 0.02 using 10 μm light for both 250 and 350 mV of applied reverse bias. These measurements are from edge spread functions with median signal values around 50% of the well depth, as the MTF is expected to change with signal value due to two brighter-fatter type effects. The expected trends caused by the influences of these two effects and the expected trends with wavelength of absorbed photons are all observed.

High-Performance, Polarization-Sensitive, Long-Wave Infrared Photodetection via Photothermoelectric Effect with Asymmetric van der Waals Contacts.

Long-wavelength infrared (LWIR) photodetection is important for heat-seeking technologies, such as thermal imaging, all-weather surveillance, and missile guidance. Among various detection techniques, photothermoelectric (PTE) detectors are promising in that they can realize ultra-broadband photodetection at room temperature without an external power supply. However, their performance in terms of speed, responsivity, and noise level in the LWIR regime still needs further improvement. Here, we demonstrated a high-performance PTE photodetector based on low-symmetry palladium selenide (PdSe2) with asymmetric van der Waals contacts. The temperature gradient induced by asymmetric van der Waals contacts even under global illumination drives carrier diffusion to produce a photovoltage via the PTE effect. A responsivity of over 13 V/W, a response time of ∼50 μs, and a noise equivalent power of less than 7 nW/Hz1/2 are obtained in the 4.6-10.5 μm regime at room temperature. Furthermore, due to the anisotropic absorption of PdSe2, the detector exhibits a linear polarization angle sensitive response with an anisotropy ratio of 2.06 at 4.6 μm and 1.21 at 10.5 μm, respectively. Our proposed device architecture provides an alternative strategy to design high-performance photodetectors in the LWIR regime by utilizing van der Waals layered materials.

Passive near-field imaging via grating-based spectroscopy.

Passive scattering-type scanning near-field optical microscopy (s-SNOM) has recently been developed for studying long-wavelength infrared (LWIR) waves. It detects surface-localized waves without any external illumination or heating and enables the imaging of hot-electron energy dissipation and nanoscale Joule heating. However, the lack of a wavelength selection mechanism in the passive LWIR s-SNOM makes it difficult to perform a thorough analysis of the surface-localized waves. Here, we develop a novel passive scanning near-field optical spectroscopy with a diffraction grating. The spectroscopic optics are designed to exhibit a high signal efficiency and mechanical performance at the temperature of liquid helium (4.2K). Using the developed passive LWIR near-field spectroscopy, the spectral information of thermally excited evanescent waves can be directly obtained without any influence from the external environment factors, including environmental heat. We have detected the thermally excited evanescent waves on a SiC/Au micropatterned sample at room temperature with a spatial resolution of 200nm and a wavelength resolution of 500nm at several wavelengths in the range of 14-15 µm. The obtained spectra are consistent with the electromagnetic local density of states calculated based on the fluctuation-dissipation theorem. The developed passive LWIR near-field spectroscopy enables the spectral analysis of ultrasmall surface-localized waves, making it a high-performance surface analysis tool.

Influence of oxygen incorporation on infrared transmission of ternary Ge-Sb-Se chalcogenide glass in the compositional range for use in infrared-imaging applications

Optical transmission window of ternary Ge-Sb-Se glass extends up to ∼15 μm, thus being transparent across the long-wavelength infrared (LWIR; 8–12 μm) range. Along with its superior thermal stability as to the precision glass molding (PGM) process, this Ge-Sb-Se glass is most well-known as the material of choice for LWIR-imaging lens applications. Presence of oxygen atoms on the surface and/or in the interior of LWIR-transmitting chalcogenide glass needs to be allowed to a tolerable amount; otherwise, its LWIR transmittance is deteriorated due to the vibrational excitations of oxygen-related bonds. In this study, we take into consideration ternary Ge-Sb-Se glasses as an exemplary case to get a clear grasp on correlations between oxygen incorporation and LWIR transmittance of chalcogenide glass: Surface oxidation turns out to be insignificant to the LWIR transmittance, but presence of oxygen inside the glass is reconfirmed to exert a strong influence. It is verified that the oxygen-related absorption peak located at ∼800 cm -1 is mainly dominated by Ge–O vibrations; however, its position tends to redshift with increasing Sb content, implying indirect involvement of Sb atoms. Also verified is that the surface of Sb powders chosen as a starting material is more vulnerable to oxidation than the surface of Ge powders. This observation makes it reasonable to assume that Ge–O bonds are preferentially formed, at the expense of Sb–O bonds, during melting of the starting materials.

Dust-enshrouded AGNs Can Dominate Host-galaxy-scale Cold Dust Emission

Abstract It is widely assumed that long-wavelength infrared (IR) emission from cold dust (T ∼ 20–40 K) is a reliable tracer of star formation even in the presence of a bright active galactic nucleus (AGN). Based on radiative transfer (RT) models of clumpy AGN tori, hot dust emission from the torus contributes negligibly to the galaxy spectral energy distribution (SED) at λ ≳ 100 μm. However, these models do not include AGN heating of host-galaxy-scale diffuse dust, which may have far-IR (FIR) colors comparable to cold diffuse dust heated by stars. To quantify the contribution of AGN heating to host-galaxy-scale cold dust emission at λ ≳ 100 μm, we perform dust RT calculations on a simulated galaxy merger both including and excluding the bright AGN that it hosts. By differencing the SEDs yielded by RT calculations with and without AGNs that are otherwise identical, we quantify the FIR cold dust emission arising solely from reprocessed AGN photons. In extreme cases, AGN-heated host-galaxy-scale dust can increase galaxy-integrated FIR flux densities by factors of 2–4; star formation rates calculated from the FIR luminosity assuming no AGN contribution can overestimate the true value by comparable factors. Because the FIR colors of such systems are similar to those of purely star-forming galaxies and redder than torus models, broadband SED decomposition may be insufficient for disentangling the contributions of stars and heavily dust-enshrouded AGNs in the most IR-luminous galaxies. We demonstrate how kiloparsec-scale resolved observations can be used to identify deeply dust-enshrouded AGNs with cool FIR colors when spectroscopic and/or X-ray detection methods are unavailable.

Flexible smart photovoltaic foil for energy generation and conservation in buildings

Building-integrated solar cells not only generate electricity but also impact the envelope thermal characteristics, thus changing the micro-climate of buildings. Therefore, the collective effects of energy conservation and generation on buildings should be considered in one design. In this work, a smart photovoltaic window foil with near-infrared (NIR) modulation and low long-wavelength IR emissivity has been fabricated by combining organic perovskite and inorganic tungsten doped vanadium dioxide nanoparticles (W-VO2 NPs). The W-VO2 offers solar modulation and promotes extraction and transport of photogenerated charges, giving peak power conversion efficiency of 15.4% at 25 °C and 16.1% at 45 °C and average visible transmission of 25.5%, comparable to the best reported semitransparent perovskite solar cell with an additional favorable NIR modulation of 10.7%. This new strategy to integrate energy-saving functionality into photovoltaic foil initiates a new design rule for smart window development.

Fast and accurate polarimetric calibration of infrared imaging polarimetric sensors.

Polarimetric imaging cameras require polarimetric calibration to accurately estimate the incident Stokes vector of incoming radiation. This calibration establishes a relationship between changes in the sensor signal and incident Stokes vector. In the standard procedure, an imager is presented with a set of input Stokes vectors with two different radiance values. In the long-wavelength infrared (LWIR) and mid-wavelength infrared bands, blackbodies with different temperatures are used for each set of Stokes vectors. The radiometric offset is subtracted, and standard radiometric or nonuniformity correction procedures are performed in a separate step. This paper proposes an alternative all-in-one approach that combines radiometric calibration, nonuniformity correction, and polarimetric calibration. The standard and proposed methods are compared for a division-of-time LWIR polarimeter. The proposed calibration method achieves an RMS error of 0.34% compared with the conventional technique's error of 0.83%, yielding a factor of 2.4 improvement in the reconstructed accuracy of a linear Stokes vector; in addition, it is less time-consuming and less prone to ambient temperature fluctuations than the typical two-point method. The method also accounts for beam wander and narcissus effects and enables simple, straightforward polarimetric measurement.

Micromachined porous silicon Fabry-Pérot long wavelength infrared filters

Porous silicon Fabry-Pérot (PS-FP) filters operating in the long wavelength infrared (LWIR) range are demonstrated and studied. Porous silicon (PS) offers many advantages for such filters, including improved optical and mechanical properties compared to other LWIR filter materials, and the ability to modulate porosity to form multilayer structures that are compatible with conventional semiconductor technologies. This work discusses the optical and the mechanical characterizations of what are the first MEMS-based suspended PS-FP filters operating in the LWIR range. Methods developed to model and characterize PS-FP filters are discussed, with characterization a challenge for such porous structures considering their low refractive index and low Young’s modulus. The transmittance measurements show that introducing notches at the four corners of the top mirror minimizes the bowing of the top mirror significantly. The stress measurements of attached films show that the lattice mismatch at the PS/silicon interface is likely the key factor controlling the stress in multilayer PS thin films. The micromachining techniques developed to form a resonance cavity within PS-FP filters can also be tuned to reduce the stress mismatch and the absorption loss in the LWIR range. This study shows exciting promise for the use of micromachined PS-based filters for LWIR operation.

Thermal Image Restoration Based on LWIR Sensor Statistics.

An imaging system has natural statistics that reflect its intrinsic characteristics. For example, the gradient histogram of a visible light image generally obeys a heavy-tailed distribution, and its restoration considers natural statistics. Thermal imaging cameras detect infrared radiation, and their signal processors are specialized according to the optical and sensor systems. Thermal images, also known as long wavelength infrared (LWIR) images, suffer from distinct degradations of LWIR sensors and residual nonuniformity (RNU). However, despite the existence of various studies on the statistics of thermal images, thermal image processing has seldom attempted to incorporate natural statistics. In this study, natural statistics of thermal imaging sensors are derived, and an optimization method for restoring thermal images is proposed. To verify our hypothesis about the thermal images, high-frequency components of thermal images from various datasets are analyzed with various measures (correlation coefficient, histogram intersection, chi-squared test, Bhattacharyya distance, and Kullback–Leibler divergence), and generalized properties are derived. Furthermore, cost functions accommodating the validated natural statistics are designed and minimized by a pixel-wise optimization method. The proposed algorithm has a specialized structure for thermal images and outperforms the conventional methods. Several image quality assessments are employed for quantitatively demonstrating the performance of the proposed method. Experiments with synthesized images and real-world images are conducted, and the results are quantified by reference image assessments (peak signal-to-noise ratio and structural similarity index measure) and no-reference image assessments (Roughness (Ro) and Effective Roughness (ERo) indices).

Cooling performance of a bioinspired micro-crystal-bars coated composite fabric with solar reflectance

Inspired by the cooling performance of the micro-hair from the Saharan silver ants (SSant), we fabricated a novel solar heat shielding fabric with synthesized zinc oxide (ZnO) micro-crystal-bar (MCB) composite. The ZnO MCB composite coating can significantly improve the solar reflectivity of fabric from 69% to 95% (0.4–1.8 μm). A cooling performance experiment reveals that the surface temperature of the modified fabric reduced by 7.6%–17.7% compared to that of an unmodified reference. Furthermore, the results also showed that the modified fabrics have a high far infrared emittance, which can promote effective long wavelength infrared (LWIR) radiation (5–14 μm) and lead to heat dissipation. More so, other important attributes, such as thermal conductivity, solar power intensity and transmittance, as well as mechanical property, also improved significantly after the treatment. In summary, this work provides a unique strategy for the design and fabrication of thermal management fabric for solar heat shielding.

Analysis of temperature dependent dark current mechanisms for long-wavelength infrared p-on-n HgCdTe detectors

The dark current density versus voltage (J–V) curve of long-wavelength infrared (LWIR) p-on-n mercury cadmium telluride (HgCdTe, MCT) photodiode is measured in the temperature range of 40 K–130 K in this study. The dark current mechanisms, namely diffusion, generation–recombination (GR), trap-assisted tunneling, band-to-band tunneling (BBT) and shunt current, are investigated for LWIR p-on-n MCT diode at different temperatures. The results show that the dark current is dominated by diffusion current at low reverse bias when the operating temperature is above 65 K. As the operating temperature decreases, GR current and shunt current gradually become the main ingredient of dark current. In the high reverse bias region, the contribution of BBT mechanism accounts for the largest proportion when the operating temperature is below 75 K. The extracted fitting parameters show that the donor concentration in the n-type region of p-on-n MCT diode display better temperature stability compared with that of n-on-p diodes. Because the thermal excitation effect increases with the increase of temperature, the equivalent trap level moves from shallow acceptor level to deep acceptor level, and the trap density increases from 1.1 × 1010 to 6.0 × 1012 cm−3 when the operating temperature is increased from 80 K to 130 K.

AVERAGE TOTAL HEMISPHERIC EMISSIVITY MEASUREMENT IN THE LWIR SPECTRUM FOR ADHESIVE TAPES USED IN THE THERMOGRAPHY TAPE TEST

Thermographers often use comparative methods to estimate surfaceemissivity. Among the most used is the tape method. In this method a knownemissivity tape in the LWIR (Long Wavelength Infrared) spectrum is placedon the surface to be inspected. After thermal equilibrium, the temperature ofthe tape and the surface under inspection must be the same. In this case, thetemperature observed on the tape is the reference temperature. The emissivityof the surface must then be changed until the reference temperature isreached. It is common practice to admit the value of the emissivity of theadhesive tape as 0.95, there are few studies that present these data withmetrological rigor, which leads to doubts about the emissivity of thecommercial tapes. In this work, experiments were performed on Tekbond,Double A, 3M 101, Rapix, Altape, adhesive tapes for temperatures of 50, 55,60, 65 and 70ºC. An experimental apparatus was developed through which itwas possible to estimate surface reflection, transmission and atmosphericemission for one and two layers of tapes, in order to make emissivitymeasurements possible. Through the data it was possible to statisticallyestimate the LWIR average total hemispheric emissivity as well as theacceptance range to 95% certainty, being therefore equal to ε=0.94±0.03. Itis possible to arm, therefore, that the value of 0.95, usually used as emissivityof the adhesive tape, is extremely reasonable because it is 0.01 of the averagevalue of the Gaussian distribution calculated by this work.

Ultrathin, broadband, omnidirectional, and polarization-independent infrared absorber using all-dielectric refractory materials

Abstract Broadband long-wavelength infrared (LWIR) optical absorbers have important applications in thermal emission and imaging, infrared camouflaging, and waste heat and biothermal energy utilization. However, the practical application of broadband LWIR optical absorbers requires low-cost and facile fabrication of large-area structures with limited thickness. This paper reports the design and fabrication of an ultrathin, broadband, omnidirectional, and polarization-independent LWIR optical absorber composed of anodized aluminum oxide and highly doped Si using the gradient refractive index strategy. The average absorption of the broadband optical absorber is higher than 95% in the 8–15 μm wavelength range, and it has wide incident angle and polarization tolerances. More than 95% of the optical energy in the wavelength range from 8 to 13 μm was absorbed within a depth of 8 μm, making this absorber the thinnest broadband LWIR dielectric absorber so far. The absorption remained above 90% after annealing at 800 °C in air. The infrared camouflage of the proposed absorber was successfully demonstrated with a human body background. With the advantages of facile fabrication, low-cost materials, restricted absorption thickness, and excellent thermal stability, the developed broadband LWIR optical absorber is very promising for the practical applications mentioned above.