Mid-wavelength Infrared Range Research Articles

Design and fabrication of highly efficient antireflective coating in MWIR on germanium using ion-assisted e-beam deposition

We present the design, fabrication, and characterization of a multilayer anti-reflective (AR) coating on a Germanium (Ge) substrate for mid-wavelength infrared (MWIR) applications using the ion-assisted electron-beam evaporation (IAPVD) technique. The AR coating structure comprises five layers of thin films, alternating between low (nL) and high (nH) refractive index materials, specifically silicon dioxide (SiO2) and Ge. The total thickness of the multilayer structure is maintained below 2 μm. The design was meticulously optimized for the 3.6–4.9 μm MWIR range using OptiLayer software, achieving an impressive average transmission of 99.492 % and an average reflectance of merely 0.508 % over a broadband spectral width of 1300 nm. Following the fabrication process using the IAPVD technique, the empirical results demonstrated an average transmission of 98.56 % and a reflectance value of 0.177 % within the 3.6–4.9 μm range. A strong correlation was observed between the simulated and the fabricated results, with a deviation of only 0.331 % on the reflectance value, proving the accuracy and reliability of the design and fabrication process. This work introduces a highly efficient solution for AR coatings on Ge surfaces, enhancing the performance of electro-optical applications in the MWIR region. To the best of our knowledge, the newly designed Ge/SiO2 multilayer structure and the results achieved have not been previously documented in the literature, and offer substantial improvements in transmission efficiency and reflectance reduction for Ge-based optical systems.

Quantifying non-threshold Auger-recombination processes in mid-wavelength infrared range HgCdTe quantum wells

We study photoluminescence temperature quenching in HgTe/CdHgTe quantum wells (QWs) emitting at 3–4 μm wavelengths and recover temperature-dependent interband recombination rates. Recombination coefficients are determined for the process that we identify as a non-threshold ehh Auger process involving valence band continuum states in barriers. With the effective Auger coefficient CA reaching ∼10−13 cm4/s under resonant conditions, such a process is shown to determine stimulated emission thresholds in a wide temperature interval, while the contribution of conventional, activated Auger processes is presumably rather limited. Thus, threshold energy considerations should be used with caution for the optimization of HgCdTe QW lasers operating around 3 μm, and relatively low-barrier QWs may provide better performance than the high-barrier ones despite lower energy thresholds for thermally activated eeh-Auger recombination. It holds as long as the conduction band offset is detuned from the bandgap energy to avoid additional non-threshold eeh-processes and sufficient hole localization at elevated temperatures is maintained.

Ultra-thin midwavelength infrared absorber using bismuth based planar thin film metamaterials

We reveal the extraordinary potential of bismuth (Bi) based planar thin film metamaterials in achieving light perfect absorption for midwavelength infrared (MWIR) range from 3 to 6 μm. The proposed absorber is composed of an ultra-thin Bi film and a continuous metallic film separated by a dielectric spacer. Theoretical analyses show that the absorber exhibits narrowband absorption that can continuously span the whole MWIR range by varying the geometric parameters. Furthermore, it is found that the absorber displays wide-angle absorption up to 80° as well as polarization-insensitive properties. Experimental measurements are performed to corroborate the theoretical analyses.

Emerging Type‐II Superlattices of InAs/InAsSb and InAs/GaSb for Mid‐Wavelength Infrared Photodetectors

Mid‐wavelength infrared (MWIR) photodetectors (PDs) are highly essential for environmental sensing of hazardous gases, security, defense, and medical applications. Mercury cadmium telluride (MCT) materials have been the most used detector in the MWIR range. However, it is plagued by several challenges including toxicity concerns, a high rate of Auger nonradiative recombination, a large band‐to‐band (BTB) tunneling current, nonuniformity, and the need for cryogenic cooling. Theoretically, it is predicted that type‐II superlattice (T2SL) materials can emerge as an alternative with the potential to outperform the current state‐of‐the‐art MCT PDs due to suppression of Auger recombination associated with bandgap engineering and reduced BTB tunneling current caused by the larger effective mass. Based on this theoretical prediction, it is believed that T2SL have the potential to operate at high temperatures and overcome the size, weight, and power consumption limitations of MCT. Herein, a detailed review of the fundamental material properties of T2SL PDs is provided while providing a comparison of the optical and electrical performances of Ga‐free (InAs/InAsSb) and Ga‐based (InAs/GaSb) T2SL PDs. Finally, recent advances in IR detection technologies including focal plane arrays and quantum cascade infrared photodetectors are explored.

Enhancement in the extinction ratio of a wire grid polarizer in the mid-wavelength infrared range via hot electron diffused cold-annealing

Theoretical and experimental investigations were performed on the fabrication and polarization property enhancement of a single-layer Au grating serving as a wire grid polarizer (WGP) in the mid-wavelength infrared (MWIR) range. Femtosecond laser (FSL) annealing was performed to overcome the performance limitations of the WGP fabricated by nanotransfer printing (NTP), and the results were compared with those of the conventional thermal annealing methods using scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Consequently, the extinction ratio of the WGP annealed with FSL reached up to 1378 (∼31 dB) in the 3–5 μm wavelength range, which corresponds to a 2.18-fold improvement over the non-annealed WGP. This result suggests that it is possible to manufacture MWIR WGPs at a low cost and over large areas through a simple NTP and FSL annealing process, ultimately yielding WGPs with improved performances.

Large metasurface-based optical concentrators for infrared photodetectors

We demonstrate a modular design approach for large metasurface-based optical concentrators. In this approach, each concentrator is split into a collection of sublens modules. Each sublens module has an off-axis focal point, and this point is located between the concentrator center and the intended detector center. This reduces the necessary deflection angle, thus improving the concentrator design. Moreover, each concentrator module is designed individually, thereby reducing the required computational resources and improving the design versatility. We designed, fabricated, and tested 300-μm-diameter metasurface-based optical concentrators operating in the 3–5 μm mid-wavelength infrared range. These optical concentrators are fabricated on a gallium antimonide substrate, which can be used for epitaxial growth of infrared detectors. This allows future monolithic integration of these concentrators with detectors epitaxially grown on the front side of the substrate and concentrators fabricated on the backside. The optical concentrators enhanced the measured optical intensity at the intended detector position up to a factor of 6.4; in the future, this will improve the signal-to-noise ratio of detectors and increase their operating temperature.

Investigation of Radiation Recombination Channels in Long-Wavelength InAs/InAsSb/InAsSbP LED Heterostructures

This work presents the results of the investigation of optical properties of long-wavelength (~5 µm at 300 K) InAs/InAsSb/InAsSbP LED heterostructures. These heterostructures are used in various applications in mid-wavelength infrared range, such as environmental monitoring, etc. Electroluminescence was used to study the optical characteristics of the structures in the temperature range 4.2–300 K. Various radiative recombination channels in LED heterostructures were considered, including those associated with the InAs substrate and those related to the active layer, the latter competing depending on the temperature. The obtained results can be useful when designing optoelectronic devices with weak temperature dependence of the emission wavelength.

Poly(sulfur-random-(1,3-diisopropenylbenzene)) based mid-wavelength infrared polarizer: Optical property experimental and theoretical analysis

Development of polymer based mid-wavelength infrared (MWIR) optics has been limited mainly due to high optical loss of organic polymers used in general optical components. In this study, a MWIR polarization grating based on a sulfuric polymer poly(sulfur-random-(1,3-diisopropenylbenzene)) with a low loss in the MWIR range was fabricated using a simple two-step process: imprint and metal deposition. Fourier-transform infrared (FTIR) spectroscopy measurement showed that this polymeric MWIR polarizer selectively transmitted the polarized IR in transverse magnetic (TM) mode over the transverse electric (TE) mode at normal incidence. The measured extinction ratios (η = The ratio of transmissions in TM and TE) were 208, 176, and 212 at the wavelength of 3, 4, and 5 μm, respectively. The computational simulation and analytical model confirmed that the enhanced TM transmission efficiency and η followed a Fabry-Pérot (FP) resonance mode within the created sulfuric polymer film. This polymeric MWIR polarizer demonstrated a great potential for broader applications in IR photonics to realize low-cost and durable optical components.

Nanogap Engineering for Enhanced Transmission of Wire Grid Polarizers in Mid-Wavelength Infrared Region

Wire-grid polarizers (WGPs) have been widely used in various fields, such as polarimetry, imaging, display, spectroscopy, and optical isolation. However, conventional WGPs used in diverse mid-wavelength infrared (MWIR) applications show high reflection losses, which intrinsically arise from high refractive indices of their IR-transmitting substrates, such as silicon (Si) and germanium (Ge). This study demonstrated the enhanced transmittance of a transverse magnetic (TM) wave that surpassed ~80% over the entire MWIR range from 3000 to 5000 nm in a narrow air gap of a WGP, where aluminum (Al) was selectively deposited on a nanopatterned Si substrate using an oblique angle deposition method. Moreover, a higher TM wave transmittance was achieved by reducing the air gaps of the WGPs in the nanopatterns, which were distinctly different from the traditional WGPs comprising metal wires patterned directly on a flat substrate. A finite-difference time-domain simulation was performed to investigate optical properties of the proposed WGPs, which showed that the electric field in the air nanogap was remarkably enhanced. The characteristic performances were further investigated using a combination of an effective medium approximation and an admittance diagram, revealing that the broadband transmission enhancement could be attributed to a combined effect of a strong electric field and a better admittance matching. The approach and results described in this paper hold promise for the design and the fabrication of high-quality WGPs, as well as their numerous applications.

Large hh-lh splitting energy for InAs/AlSb/GaSb based N-structure photodetectors

We investigate the band properties of InAs/AlSb/GaSb (N-structure) and InAs/GaSb material based type II superlattice (T2SL) photodedectors. The superlattice empirical pseudopotential method is used to define band-structures such as the bandgap and heavy hole-light hole (hh-lh) splitting energies in the mid-wavelength infrared range (MWIR) and long wavelength range (LWIR). The calculations are carried out on the variation of AlSb/GaSb layer thickness for (InAs)10.5/(AlSb)x/(GaSb)9-x and the variation of InAs layer thickness for (InAs)x/(AlSb)3/(GaSb)6 T2SL structures at 77 K. For the same bandgap energy of 229 meV (5.4 μm in wavelength), hh-lh splitting energy is calculated as 194 meV for the (InAs)7.5/(AlSb)3/(GaSb)6 structure compared to the (InAs)10.5/(GaSb)9 structure with hh-lh splitting energy of 91 meV within the MWIR. Long wavelength performance of InAs/AlSb/GaSb structure shows superior electronic properties over the standard InAs/GaSb T2SL structure with larger hh-lh splitting energy which is larger than the bandgap energy. The best result is obtained for (InAs)17/(AlSb)3/(GaSb)6 with the minimum bandgap of 128 meV with hh-lh splitting energy of 194 meV, which is important for suppressing the Auger recombination process. These values are very promising for a photodetector design in both MWIR and LWIR in high temperature applications.

Electrical and optical performances with extracted minority carrier lifetimes of InAs/GaSb SL photodetector operating in the mid wavelength infrared range

We report a study on the temperature dependence of electrical and optical performance of InAs/GaSb based type-II superlattice (T2SL) pin photodetectors in the mid wavelength infrared range (MWIR). The SL structure exhibits an optical response of 50% cut-off wavelength at 4.9 μm at 79 K. Deduced from current density–voltage (J–V) measurements, dark current density under 0.1 V reverse bias is measured as 7.6 × 10−6 A/cm2 with a corresponding differential–resistance–area product (R0A) of 3.3 × 104 Ωcm2 at 100 K. Minority carrier lifetimes of the T2SL detectors are analysed by Shockley's Model where experimental data for dark current densities are fitted by diffusion and generation-recombination (GR) components at different temperatures.

Demonstration of a mid-wavelength infrared narrowband thermal emitter based on GaN/AlGaN quantum wells and a photonic crystal

We experimentally demonstrate a thermal emitter with a narrow-bandwidth and low background emission, operating in the mid-wavelength infrared (MWIR) range, based on a combination of intersubband transitions in GaN/AlGaN multiple quantum wells and optical resonance in a photonic crystal slab. The fabricated device exhibits single-peak narrowband thermal emission with a Q factor of 93 at a wavelength of 4.0 μm. Stable operation at high temperatures over 700 °C has been demonstrated owing to the good thermal stability of GaN/AlGaN, which enables the generation of strong peak emission intensity as high as 93 mW/μm/sr/cm2. Such a narrow-band and low-background emitter in the MWIR range has been difficult to realize by metal or heavily doped semiconductor-based emitters due to the broadband emission characteristics of the materials and by GaAs/AlGaAs-based emitters due to the thermal instability of the materials. Our device can be applied to various MWIR applications including CO2 and NOx gas sensing systems.

Theoretical utmost performance of (100) mid-wave HgCdTe photodetectors

HgCdTe detectors designed to detect mid-wavelength (3–5 μm) infrared radiation must be cooled to reach the required performance. The cooling requirement makes the sensor system both expensive and bulky and the fundamental goal is to reach higher operating temperature condition preserving near background limited performance with high detectivity and high speed response at the same time. In order to reach higher operating temperature condition the thermal generation rate must to be suppressed under the photon generation rate. Except Auger 7 generation-recombination process, p-type HgCdTe is mostly limited by technology dependent Shockley-Read-Hall generation-recombination mechanism. One of the ways to reduce the trap density is a growth of the (100) HgCdTe on GaAs substrates. That orientation allows reaching lower carrier concentration ~5 × 1014 cm−3 in comparison to the commonly used (111) orientation ~5 × 1015 cm−3 in mid-wavelength infrared range. In addition, it was presented that Shockley-Read-Hall traps density could be reduced to the level of ~4.4 × 108 cm−3. The theoretical simulations related to the utmost performance of the (100) HgCdTe Auger suppressed structures are presented. Dark current is reported to be reduced by more than one order of magnitude within the range ~6 × 10−2–3 × 10−3 A/cm2. Detectivity increases within range ~3–12 × 1011 cm Hz1/2/W (wavelength ~5 μm) at temperature 200 K and voltage 200 mV.

Modifications of spheroid plasmonic particle geometry for enhancement of ultrathin semiconductor infrared detectors

We consider the enhancement of the specific detectivity of semiconductor infrared detectors utilizing a redshifted plasmonic light concentrator. To this purpose submicrometer-sized doped transparent conductive oxide particles with different shapes were used, embedded within high dielectric permittivity medium. The whole structure was topped with a graded antireflective layer. Localized surface plasmon resonance ensures high field localization around the plasmonic particles and directly beneath them, which translates into high density of optical energy within the detector active region. We investigate the possibilities to tune the particle shape and size in order to maximize the photodetector optical energy input, i.e. to maximize the detector external quantum efficiency. We perform ab initio simulation of the optical response utilizing the finite element method, starting with spherical particles and then extending this to include spheroids and sub-micrometric plates. As an illustrative example, we analyze a mercury cadmium telluride infrared detector with an ultrathin active epitaxial active layer. An increase in external quantum efficiency is obtained for non-spherical particles compared to the spherical ones. This can be used to offset a decrease in the optical path/internal quantum efficiency. Plasmonic localization is convenient for this since optical energy density is strongly localized close to the illuminated surface, thus ensuring a large decrease of the total detector volume and therefore suppressing the generation-recombination noise levels which are proportional to the device volume. Our results show that it is possible by this approach to realize an uncooled semiconductor detector for mid-wavelength infrared range with its specific detectivity strongly increased compared to the conventional devices.

Directional emittance of dry and moist paper

Abstract The directional emittance of dry and moist paper and board samples was measured in two wavelength ranges; the mid-wavelength infrared (MWIR) range and the long-wavelength infrared (LWIR) range. The influence of pulp type, pulp drying, pulp refining, fibre orientation, additives, coating, and observation angle on the emittance of dry paper was examined. The influence of sample moisture and observation angle on the emittance of moist samples was also investigated. The emittance in the LWIR range was higher than the MWIR emittance. The emittance varied with pulp type, especially for TMP, which had a significantly lower emittance compared to the samples made of chemical pulp. The impact of different properties, such as pulp type, refining or coating, was much smaller in the LWIR range than in the MWIR range. Observation angle was found to significantly impact the emittance at angles larger than 60° from the normal direction in the MWIR range, and angles larger than 70° in the LWIR range. The emittance increased with increasing moisture ratio. This increase was most pronounced at low absolute moisture ratios, where an addition of an already small amount of water could impart a large change in emittance. It was found that the emittance and sample moisture could be correlated well using a linear combination of the emittance of dry paper and pure water.

Low-Frequency Noise Characteristics of HgCdTe Infrared Photodiodes Operating at High Temperatures

We present experimental results of 1/f noise characterization in n+/p mercury-vacancy-doped photodiodes. The 1/f noise was measured as a function of temperature and diode bias voltage for different photodiode configurations. We have investigated the 1/f noise of devices with various geometries and structures, in different spectral bands, and produced using different growth processes. Contrasting 1/f noise behavior was observed in mid-wavelength infrared range (MWIR) and long-wavelength infrared range (LWIR) devices. The results indicate that the 1/f noise in MWIR photodiodes is proportional to the diode area, which implies the presence of a previously unreported noise-generating mechanism occurring in bulk material. The 1/f noise in the LWIR photodiodes was found to be independent of diode geometry, indicating the presence of a diffusion current modulation at specific points of the junction.

Design and fabrication of a mid-wavelength infrared Fresnel lens via liquid poly(methyl methacrylate)

Most popular materials for lenses, such as glass, have high absorption in the infrared range. Due to material restriction, infrared lenses are usually much more expensive. In this paper, we discussed a ubiquitous polymer material, poly(methyl methacrylate) (PMMA), for mid-wavelength infrared (MWIR) applications. PMMA is a low cost material and is widely used in daily life. We examined its optical properties in the mid-infrared range and found that poly(methyl methacrylate) is a highly promising material for MWIR lenses. Besides, liquid PMMA can be formed and solidified easily. Utilizing these characteristics, we proposed a novel way to fabricate PMMA lenses for MWIR range (wavelength from 3.6 to 5 µm). The fabrication process is much easier and less expensive compared with traditional machining processes. We have designed a PMMA Fresnel lens, which has f-number of 1.40, diameter of 10 mm and focal length of 14 mm. We also successfully fabricated the PMMA Fresnel lens using the molding process. Both structure and optical analyses show that the PMMA Fresnel lenses could meet the design parameters.

Overview of antimonide based III-V semiconductor epitaxial layers and their applications at the center for quantum devices

The properties of Sb-based III-V semiconductor compounds for optoelectronic applications in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) range were reviewed. The growths of the Sb-based binary, ternary and quaternary were studied by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). The structural, optical and electrical characterizations were carried out. Focal plane array, photoconductors and photodiodes were fabricated for the MWIR and LWIR range. Doublehetero structure (DH), multi-quantum well (MQW) and strained superlattice (SSL) lasers in the 3–5 μ m range were fabricated. InAs-GaSb type-II superlattices were designed, grown and fabricated into photodetectors for the MWIR and LWIR range.