Middle Wavelength Infrared Research Articles

Cryogenic etching and characterization of nano-sized silicon metadevice

The dielectric materials were usually deposited as hard masks for etching in the fabrication of dielectric metasurfaces, which increased fabrication steps and the distortion of the devices. We present a simplified process for preparing nano-sized silicon structure. The samples after photolithography were cooled and directly etched without deposited the hard mask. The cryogenic etching process reduce the fabrication steps and distortion caused by the transfer of photolithographic patterns. Here, a transmission middle-wavelength infrared (MWIR) metalens array was designed and fabricated as a demonstration. The optimal temperature for cryogenic etching was proved and found at 163 K by comparing the etching effects at different temperatures. The side walls of metalens prepared by the proposed process are steep, and the depth-width ratio meets the requirements. The metalens with operating at λ=4μm, can focus the light to a spot with a full-width at half-maximum (FWHM) of 20.308μm at the focal plane. The converging efficiency of the metalens reaches 95%, and the focal length is 385μm. Considering the increasingly complex and finer size of metasurface, this process may have great potentials in the preparation of nanostructures.

High performance InAs0.91Sb0.09 MWIR detectors with an AlAs1-ySby graded barrier

InAs0.91Sb0.09 barrier detectors are an important candidate in realizing high operating temperature middle wavelength infrared (MWIR) photodetectors. Here, an AlAs1-ySby graded barrier layer with linear Sb composition varying from y = 1 to 0.92 is utilized to improve the extraction of photo-generated minority carriers from an InAs0.91Sb0.09 absorber layer. With the modification of valance band misalignment between the absorber and barrier layers, although a hole barrier between AlAs0.08Sb0.92 and InAs0.91Sb0.09 layers exists, the photo-generated holes can be effectively extracted. At 150 K and under zero bias voltage, the maximum quantum efficiency of the devices with a graded barrier reaches ∼56 %, which is much higher than that of the device with a uniform AlAs0.08Sb0.92 barrier. Benefiting from the low dark current density at small reverse bias voltages, a maximum specific detectivity (D*) ∼6.19 × 1011 cm⋅Hz1/2/W is realized at zero bias voltage, which is 50 % higher than the maximum D* ∼4.26 × 1011 cm⋅Hz1/2/W of the uniform barrier device operated at −150 mV bias voltage.

Design of MWIR hyperspectral imagers based on acousto-optic tunable filters

Thermal infrared hyperspectral imagers have significant application values in the field of space optics. However, it is very difficult to obtain the spectral data cube and temperature field information of fast moving targets in a short time. An acousto-optic tunable filter (AOTF) has many advantages, such as fast tuning speed, light weight and no moving parts. It can obtain thermal infrared hyperspectral images of a fast-moving target in a very short integration time. In this paper, a collimating optical system is obtained by splicing optical path a telescope and a secondary imaging mirror. A middle wavelength infrared (MWIR) AOTF is introduced into the collimating optical system to modulate the spectrum and obtain thermal infrared hyperspectral images. The system with a focal length of 18 mm, a relative aperture of 1/2, a field of view (FOV) of 8°, a spectral resolution of 30.08 nm at 4 µm and a 100% cold diaphragm efficiency achieving is taken as a design example. The design results show that the modulation transfer function (MTF) of the system is greater than 0.6 at the Nyquist space frequency of 17 lp/mm, and the imaging quality is close to the diffraction limit. Finally, we develop a prototype of MWIR hyperspectral imager based on AOTFs and do experiments to prove its feasibility.

Room-temperature InAsSb pBin detectors for mid-infrared application

InAs-based compound is a direct, narrow bandgap III-V semiconductor with high carrier mobility and an attractive optoelectronic device candidate for application in the infrared region. Herein, a pBin InAsSb photodetector structure has been grown successfully by cost-effective Liquid Phase Epitaxy (LPE) technique. The introduction of high quality InAsSbP barrier can effectively reduce dark current, which leads to good room-temperature detector performances. A clear room-temperature photoresponse (1.6 A/W, corresponding to 50% external quantum efficiency) in the middle wavelength infrared (MWIR) range has been achieved. In addition, due to nearly zero valence band offset at zero bias, InAsSbP barrier blocks the electron dark current, while allowing the photogenerated holes to flow to the contact, which dramatically improved the performance of the detector without a turn-on operating bias. This self-powered device can reduce the size and weight of the system, and can be further developed towards the optoelectronic integrated nanosystems. The combination of the good room-temperature performance, self-powered design, and cost-effective fabrication suggests that this pBin InAsSb photodetector has wide application prospect in both military and civilian field.

In-process monitoring of the melt-pool motion during continuous-wave laser metal deposition

Abstract In the field of additive manufacturing (AM), the implementation of real-time monitoring systems is essential to guarantee the quality and repeatability of the manufactured parts. Particularly in the laser metal deposition technique, the dynamics and stability of the molten pool, both geometrically and thermally, have an impact on the microstructural properties and the formation of defects. This article presents a novel technique to track the molten pool in real-time. A middle wavelength infrared (MWIR) camera is used, with a spectral range between 3 and 5 μm. The acquired frames are processed to reduce their dimension using a new strategy based on Superpixels. Traditional pixels are grouped into block-structures that are similar in temperature, optical flow, and spatial position. This guarantees a better adherence to the edges improving the segmentation task. Then, the optical flow of the molten pool is calculated. Throughout this tracking, geometric and thermal features are extracted. The proposed method was evaluated using qualitative and quantitative criteria. The performance of the proposal is compared against benchmark algorithms using standard metrics such as Sub-segmentation Error, Compactness, and Achievable Segmentation Accuracy obtaining better results. The proposal presented is robust to the number of superpixels generated as well as the size of the images. Which provides better performance for in situ monitoring tasks. Our proposal has significant industrial potential for monitoring additive manufacturing processes, as it requires minimal modifications to commercially available industrial machines.

3D infrared thermospectroscopic imaging

This work reports a multispectral tomography technique in transmission mode (called 3DITI for 3D Infrared Thermospectroscopic Imaging) based on a middle wavelength infrared (MWIR) focal plane array. This technique relies on an MWIR camera (1.5 to 5.5 μm) used in combination with a multispectral IR monochromator (400 nm to 20 μm), and a sample mounted on a rotary stage for the measurement of its transmittance at several angular positions. Based on the projections expressed in terms of a sinogram, spatial three-dimensional (3D) cubes (proper emission and absorptivity) are reconstructed using a back-projection method based on inverse Radon transform. As a validation case, IR absorptivity tomography of a reflective metallic screw is performed within a very short time, i.e., shorter than 1 min, to monitor 72 angular positions of the sample. Then, the absorptivity and proper emission tomographies of a butane-propane-air burner flame and microfluidic perfluoroalkoxy (PFA) tubing filled with water and ethanol are obtained. These unique data evidence that 3D thermo-chemical information in complex semi-transparent media can be obtained using the proposed 3DITI method. Moreover, this measurement technique presents new problems in the acquisition, storage and processing of big data. In fact, the quantity of reconstructed data can reach several TB (a tomographic sample cube of 1.5 × 1.5 × 3 cm3 is composed of more than 1 million pixels per wavelength).

Carrier density modulation and photocarrier transportation of graphene/InSb heterojunction middle-wavelength infrared photodetectors

The photoresponse mechanism of graphene/InSb heterojunction middle-wavelength infrared (MWIR) photodetectors was investigated. The devices comprised a graphene/InSb heterojunction as a carrier-injection region and an insulator region of graphene on tetraethyl orthosilicate (TEOS) for photogating. The MWIR photoresponse was significantly amplified with an increase in the graphene/TEOS cross-sectional area by covering the entire detector with graphene. The graphene-channel dependence of the MWIR photoresponse indicated that the graphene carrier density was modulated by photocarrier accumulation at the TEOS/InSb boundary, resulting in photogating. The dark current of the devices was suppressed by a decrease in the graphene/InSb carrier-injection region and the formation of the heterojunction using an n-type InSb substrate. The results indicate that photocarrier transportation was dominated by the formation of a Schottky barrier at the interface of the graphene/InSb heterojunction and a Fermi-level shift under bias application. The high-responsivity and low-dark-current photoresponse mechanism was attributed to the graphene/InSb heterojunction diode behavior and the photogating effect. The devices combining the aforementioned features had a noise equivalent power of 0.43 pW / Hz1/2. The results obtained in our study will contribute to the development of high-performance graphene-based IR image sensors.

Photogating for small high-responsivity graphene middle-wavelength infrared photodetectors

We demonstrated a middle-wavelength infrared (MWIR) graphene photodetector using the photogating effect. This effect was induced by photosensitizers situated around a graphene channel that coupled incident light and generated a large electrical charge. The graphene-based MWIR photodetector consisted of a top graphene channel, source–drain electrodes, an insulator layer, and a photosensitizer, and its photoresponse characteristics were determined by current measurements. Irradiation of the graphene channel of the vacuum cooled device by an MWIR laser generated a clear photoresponse, as evidenced by modulation of the output current during irradiation. The MWIR photoresponse with the photogating effect was 100 times greater than that obtained from conventional graphene photodetectors without the photogating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The effect of the graphene channel size on the responsivity was evaluated to assess the feasibility of reducing the photodetector area, and decreasing the channel area from 100 to 25 μm2 improved the responsivity from 61.7 to 321.0 AW − 1. The results obtained in our study will contribute to the development of high-performance graphene-based IR imaging sensors.

IR-Band Conversion of Target and Background Using Surface Temperature Estimation and Error Compensation for Military IR Sensor Simulation.

Military infrared (IR) imaging systems utilize one or more IR wavelength-bands, among short wavelength IR (SWIR), middle wavelength IR (MWIR), and long wavelength IR (LWIR) band. The IR image wavelength-band conversion which transforms one arbitrary IR wavelength-band image to another IR wavelength-band image is needed for IR signature modeling and image synthesis in the IR systems. However, the IR wavelength-band conversion is very challenging because absorptivity and transmittance of objects and background (atmosphere) are different according to the IR wavelength band and because radiation and reflectance characteristics of the SWIR are very different from the LWIR and MWIR. Therefore, the IR wavelength-band conversion in this paper applies to only IR targets and monotonous backgrounds at a long distance for military purposes. This paper proposes an IR wavelength-band conversion method which transforms one arbitrary IR wavelength-band image to another IR wavelength-band image by using the surface temperature estimation of an object and the error attenuation method for the estimated temperature. The surface temperature of the object is estimated by an approximated Planck’s radiation equation and the error of estimated temperature is corrected by using the slope information of exact radiance along with the approximated one. The corrected surface temperature is used for generating another IR wavelength-band image. The verification of the proposed method is demonstrated through the simulations using actual IR images obtained by thermal equipment.

Low dark current and high-responsivity graphene mid-infrared photodetectors using amplification of injected photo-carriers by photo-gating.

Low dark current, high-responsivity middle-wavelength infrared (IR) graphene photodetectors using photo-gating amplification of injected photo-carriers are demonstrated. A graphene/p-indium antimonide (InSb) heterojunction and graphene/insulator region were formed. The injected photo-carriers from InSb to graphene were amplified by photo-gating induced in the graphene/tetraethyl orthosilicate (TEOS) region, resulting in the high responsivity and low dark current performance. A responsivity of 14.9A/W and an ON/OFF ratio of 2.66×104 were achieved. The photoresponse is shown to be determined by the cross-sectional area between the graphene and the TEOS-SiO2, in which the injected photo-carriers into graphene were modulated and amplified by the photo-gating effect. Our results indicate that high-performance IR photodetectors based on the developed graphene photodetectors can be realized.

High responsivity middle-wavelength infrared graphene photodetectors using photo-gating

In this work, high-responsivity graphene photodetectors operating in the middle-wavelength infrared (MWIR) spectral band were fabricated by taking advantage of the photo-gating effect. Graphene-based field effect transistors were fabricated on indium antimonide (InSb) substrates. The InSb generated photo-carriers in response to incident IR light modulated the graphene channel gate voltage and induced a large photocurrent. These graphene-based photodetectors exhibited a clear photoresponse during irradiation with 4.6 μm MWIR laser light and an ultrahigh responsivity of 33.8 A/W was achieved at 50 K due to the photo-gating effect. These devices were found to maintain an MWIR photoresponse up to 150 K. Our graphene-based photodetector design is expected to contribute to the development of high-performance MWIR image sensors.

Room temperature plasmon-enhanced InAs0.91Sb0.09-based heterojunction n-i-p mid-wave infrared photodetector

Middle wavelength infrared (MWIR) photodetectors have a wide range of applications, but almost all of them operate at low temperature due to the limit of materials and device structures. The capability of plasmonic structures to localize electromagnetic wave on the deep subwavelength scale provides the possibility for MWIR photodetectors operating at room temperature. Here, we report a high sensitivity room temperature MWIR photodetector which is an InAs0.91Sb0.09-based heterojunction n-i-p photodiode integrated with a Au-based two-dimensional subwavelength hole array (2DSHA). A room temperature detectivity of 0.8 × 1010 cm Hz1/2 W−1 and a response time of 600 ns are achieved. The non-cooling high performance of 2DSHA-InAs0.91Sb0.09 based heterojunction photodetectors will make their applications easier, broader, and economic.

High quality InAsSb-based heterostructure n-i-p mid-wavelength infrared photodiode

Abstract We present the effect of interface quality on the performance of InAsSb based hetero n-i -p middle wavelength infrared (MWIR) photodiodes. By adopting heavily doping wide bandgap p- and n-type layers and inserting a thin layer between the two doped layers and the absorbing InAsSb region, the interface quality can be improved. We also employed proper fabrication processes in device fabrication to improve surface quality. It is found that the improved interface and surface quality can reduce the noise current and enhance detection performance. A detectivity of ∼1.5 × 10 9 cmHz 1/2 W −1 can be achieved at room temperature, and it can be increased to ∼4.0 × 10 9 cmHz 1/2 W −1 at 250 K.

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.

Theoretical investigation of all-metal-based mushroom plasmonic metamaterial absorbers at infrared wavelengths

High-performance wavelength-selective infrared (IR) sensors require small pixel structures, a low-thermal mass, and operation in the middle-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions for multicolor IR imaging. All-metal-based mushroom plasmonic metamaterial absorbers (MPMAs) were investigated theoretically and were designed to enhance the performance of wavelength-selective uncooled IR sensors. All components of the MPMAs are based on thin layers of metals such as Au without oxide insulators for increased absorption. The absorption properties of the MPMAs were investigated by rigorous coupled-wave analysis. Strong wavelength-selective absorption is realized over a wide range of MWIR and LWIR wavelengths by the plasmonic resonance of the micropatch and the narrow-gap resonance, without disturbance from the intrinsic absorption of oxide insulators. The absorption wavelength is defined mainly by the micropatch size and is longer than its period. The metal post width has less impact on the absorption properties and can maintain single-mode operation. Through-holes can be formed on the plate area to reduce the thermal mass. A small pixel size with reduced thermal mass and wideband single-mode operation can be realized using all-metal-based MPMAs.

적외선영상의 파장대역변환을 위한 추정온도 오차 보정

The modern infrared (IR) imaging systems use mainly one or more wavelength bands among short wavelength IR (SWIR), middle wavelength IR (MWIR), and long wavelength IR (LWIR) bands. In the process of IR image synthesis and modeling, IR image wavelength-band conversion which transforms arbitrary band image to other band one is required. The wavelength-band conversion procedure includes a temperature estimation process of an object surface. However, in this procedure, an approximated Planck's radiation equation causes errors in estimated temperature. In this paper, we propose an estimation temperature error attenuation method in IR image band conversion procedure. The estimated temperature is corrected with a slope information of radiance according to it. The corrected temperature is used for generation of the other band IR image. The verification of proposed method is demonstrated through the simulation.

Selective CdTe Nanoheteroepitaxial Growth on Si(100) Substrates Using the Close-Spaced Sublimation Technique Without the Use of a Mask

The development of HgCdTe detectors requires high sensitivity, small pixel size, low defect density, long-term thermal-cycling reliability, and large-area substrates. CdTe bulk substrates were initially used for epitaxial growth of HgCdTe films. However, CdTe has a lattice mismatch with long-wavelength infrared (LWIR) and middle-wavelength infrared (MWIR) HgCdTe that results in detrimental dislocation densities above mid-106 cm−2. This work explores the use of CdTe/Si as a possible substrate for HgCdTe detectors. Although there is a 19% lattice mismatch between CdTe and Si, the nanoheteroepitaxy (NHE) technique makes it possible to grow CdTe on Si substrates with fewer defects at the CdTe/Si interface. In this work, Si(100) was patterned using photolithography and dry etching to create 500-nm to 1-μm pillars. CdTe was selectively deposited on the pillar surfaces using the close-spaced sublimation (CSS) technique. Scanning electron microscopy (SEM) was used to characterize the CdTe selective growth and grain morphology, and transmission electron microscopy (TEM) was used to analyze the structure and quality of the grains. CdTe selectivity was achieved for most of the substrate and source temperatures used in this study. The ability to selectively deposit CdTe on patterned Si(100) substrates without the use of a mask or seed layer has not been observed before using the CSS technique. The results from this study confirm that CSS has the potential to be an effective and low-cost technique for selective nanoheteroepitaxial growth of CdTe films on Si(100) substrates for infrared detector applications.

EVALUATION OF DUAL-SPECTRUM IR SPECTROGRAM SYSTEM ON INVASIVE DUCTAL CARCINOMA (IDC) BREAST CANCER

Invasive Ductal Carcinoma (IDC) is one of the most frequently diagnosed breast cancers. IDC accounts for about 8 out of 10 of all invasive breast cancers. While early detection of breast cancer is essential for the reduction of death rate, there may be already more than 107 cells in a breast cancer when it can be observed by X-ray mammogram. In contrast, the passive IR spectrogram proposed by Szu et al. was shown to be promising in detecting breast cancers several months ahead of mammogram. With energy readings from two IR cameras, middle wavelength IR (MIR, 3–5 μm) and long wavelength IR (LIR, 8–12 μm), dual-spectrum IR (DS-IR) spectrogram may be computed by using the deterministic neighborhood-based blind source separation algorithm developed by Szu et al..4–7 To evaluate the performance of the DS-IR spectrogram on detection of IDC, a DS-IR spectrogram hardware system is built and a sub-pixel super-resolution registration is developed to implement the deterministic neighborhood-based blind source separation algorithm. Clinical tests have been carried out with the approval of Institutional Review Board of National Taiwan University Hospital. From August 2007 to June 2008, 35 patients aged between 30–66 (average age 49) with IDC breast cancers were recruited in this project. The results demonstrate that 62.86% of success rate for IDC detection may be achieved with the cross-sectional data. Longitudinal study shows that breast cancers may be detected more accurately by cross-referencing s1 maps of multiple time-points.

Different approximations of carrier statistics in middle-wavelength infrared HgCdTe photovoltaic devices with nonparabolic band

The optical bandgap and photoresponse characteristics of middle-wavelength infrared (MWIR) mercury-cadmium-telluride (HgCdTe) photodiodes have been performed based on a self-consistent solution of the Poisson's equation, the electron/hole continuity equations, and three-generation-recombination processes as Auger, Shockley-Read-Hall and optical generation recombination. Three different carrier-density approximations: (i) parabolic conduction-band approximation, (ii) Bebb's nonparabolic expression, and (iii) Harman's nonparabolic approximation, are proposed to calculate the optical bandgap and photoresponse of MWIR HgCdTe photovoltaic devices by considering the carrier degeneracy and the nonparabolic conduction band. It is found that omitting nonparabolic effect can lead to an enormous deviation in the simulation result, especially for heavily doped HgCdTe devices. On the basis of the calculated results of photoresponse, the parabolic conduction-band and Harman's nonparabolic approximations can lead to the response peak shift to short and long wavelengths, respectively.

Software lens compensation applied to athermalization of infrared imaging systems

A different approach, aiming to achieve the constant blur status of point-spread function (PSF) at a certain defocused plane, is described. The correlation between the two PSF is used to control the PSF blur similarity, and simultaneously the Strehl ratio is also used to control the PSF blur minimization. By designing the PSF so that it is significantly insensitive to defocus or related defocus quantity, for example, due to temperature change, all the constantly blurred images can be accurately de-blurred by a simple inverse restoration filter for an adequate range of defocus. We refer to that as “software lens compensation” and apply a design method to solve the athermalization of middle wavelength infrared (MWIR) imaging systems. The resultant PSF is almost invariant in the temperature range from −10 to 50°C at the same focal plane. Consequently, the constant blur spot can be removed by a simple digital signal processing. Thus, clear and sharp de-blurred images at different temperatures are obtained.