Metasurface absorber with ultra-thin thickness designed for a terahertz focal plane array detector.

Focal Plane Array (FPA) detector has characteristics of low cost, operating at room temperature, compatibility with the silicon CMOS technology, and high detecting performance, therefore it becomes a hot spot in infrared (IR) or terahertz (THz) detect field recently. However, the tradition structure of micro-bolometer has the conflict of the pixel size and thermal performance. In order to improve the detecting performance of small pixel size bolometer, high fill factor and low thermal conductance design should be considered. In IR detecting, double layers structure is an efficient method to improve the absorption of micro-bolometer and reduce thermal conductance. The three-dimension model of small size micro-bolometer was built in this article. The thermal and mechanical characters of those models were simulated and optimized, and finally the double layer structure micro-bolometer was fabricated with multifarious semiconductor recipes on the readout integrated chip wafer. For THz detecting, to improve the detecting performance, different dimension THz detectors based on micro-bridge structure were designed and fabricated to get optimizing micro-bolometer parameters from the test results of membrane deformation. A nanostructured titanium thin film absorber is integrated in the micro-bridge structure of the VOx micro-bolometer to enhance the absorption of THz radiation. Continuous-wave THz detection and imaging are demonstrated with a 2.52 THz far infrared CO2 laser and fabricated 320×240 vanadium oxide micro-bolometer focal plane array with optimized cell structure. With this detecting system, THz imaging of metal concealed in wiping cloth and envelope is demonstrated.

There are some difficulties in the development of uncooled focal plane array (FPA) detectors due to the absence of full simulation model which reflects the characterization of FPA detectors by variations of various parameters. In this paper we propose the simulator for the both readout integrated circuit (ROIC) and bolometer FPA which is based on a thermal equivalence equation of bolometer and mathematical modeling of optical and electrical part in infrared sensor system. The simulator shows the characteristics and the behaviors of individual components of infrared sensor system in the transient-state and steady-state. We present here the simulation results for output characteristics of detectors owing to variations of parameters induced non-uniformity in FPA detectors and find the dominant parameter to be the leading source non-uniformity in FPA detectors. We also present the simulation results for some typical ROICs to cancel the bias-heating which wastes most of the dynamic range of infrared sensor system. These show the effectiveness of compensation for the bias-heating according to variations of parameters. Using the proposed simulator we can expect the quantitative amount of non-uniformity due to the statistical variations in various processing steps and design of ROIC components. It can be used for the systematic design of infrared sensor system which cannot be performed in fabrication procedure.

Terahertz (THz) imaging technology has a great potential application owing to the unique properties of THz wave that has both permeability and feature of straight [1]. Especially for real-time THz imaging detectors, field-effect transistor (FET)-based plasmonic THz detectors [2] are now being intensively developed in multi-pixel array configuration by exploiting the silicon (Si) CMOS technology advantages of low-cost and high integration density. In terms of the circuit design approach, by utilizing resistive self-mixing in the FET channel, a 0.65 THz focal plane array (FPA) detector was reported [3] and more recently, a 1 k-pixel camera has been demonstrated for a real-time THz imaging by 65-nm CMOS technology [4]. In this work, we experimentally demonstrate the real-time terahertz (THz) imaging of moving object on the conveyer belt by implementing asymmetric FET-based plasmonic 2×200 array scanner in 65-nm CMOS technology. Based on the enhanced detecting performance from our previous works [5][6], fast and uniform detection results are presented by novel device and circuit design for real-time THz imaging.

FTIR spectroscopy is one of the most powerful methods for material characterization. However, the sensitivity of this analytical tool is often very limited especially for materials with weak infrared absorption or when spectral bands of the targeted trace material overlap with the spectral bands of major components. Fortunately, for heterogeneous samples, there is an opportunity to improve the sensitivity of detection by using an imaging approach. This paper explores the opportunity of enhancing the sensitivity of FTIR spectroscopy to detect trace amounts of materials using the FTIR imaging approach based on a focal plane array (FPA) detector. Model sample tablets of ibuprofen in hydroxypropyl methylcellulose (HPMC) have been used to exemplify the detection limits of FTIR spectroscopy using: (a) a conventional mercury cadmium telluride (MCT) detector and (b) a FPA detector. The sensitivity level was compared and it has been found that for this particular set of samples, the lowest concentration of ibuprofen in HPMC that can be detected using attenuated total reflection (ATR) measuring mode with the single element MCT detector was 0.35 wt% while using the FPA detector, the presence of drug has been detected in a sample that contains as little as 0.075 wt% of drug. The application of using this enhanced sensitivity offered by the multi-channel detector to probe trace amounts of drug particles left on the surface of a finger after handling a small amount of the drug has also been demonstrated. These results have broad implications for forensic, biomedical and pharmaceutical research.

To investigate the damage threshold and mechanism of a mid-infrared HgCdTe focal plane array (FPA) detector, relevant experimental and theoretical studies were conducted. The line damage threshold of a HgCdTe FPA detector may be within the range of 0.59 Jcm−2 to 0.71 Jcm−2. The full frame damage threshold of the detector may be in the range of 0.86 Jcm−2 to 1.17 Jcm−2. Experimental results showed that when the energy density reaches 1.17 Jcm−2, the detector exhibits irreversible full frame damage and is completely unable to image. Based on the finite element method, a three-dimensional model of HgCdTe FPAs detector was established to study the heat transfer mechanism, internal stress, and damage sequence. When HgCdTe melts, we think that the detector is damaged. Under these conditions, the theoretical damage threshold calculated using the detector model is 0.55 Jcm−2. The difference between theoretical and experimental values was analyzed. The relationship between damage threshold and pulse width was also studied. It was found that when the pulse width is less than 1000 ns, the damage threshold characterized by peak power density is inversely proportional to pulse width. This relationship can help us predict the experimental damage threshold of an FPA detector. This model is reasonable and convenient for studying the damage of FPA detectors with a mid-infrared pulse laser. The research content in this article has important reference significance for the damage and protection of HgCdTe FPA detectors.

Over the last decade Fourier transform infrared (FT-IR) and near-infrared (NIR) spectroscopic imaging with focal plane array (FPA) detectors have proved powerful techniques for the rapid visualization of samples by a combination of spectroscopic and spatial information. Using these methods, selected sample areas can be analyzed with reference to the identification and localization of chemical species by FT-IR spectroscopy in the transmission or attenuated total reflection (ATR) mode and by NIR spectroscopy in diffuse reflection with a lateral resolution in the micrometer range. The present communication focuses on the quantitative determination of the active ingredient composition of a solid drug formulation by NIR spectroscopic imaging with a focal plane array detector and the results obtained are compared to the quantitative data obtained by conservative light-fiber NIR spectroscopic diffuse reflection measurements with a single-element detector. The communication also addresses the issue of penetration depth of NIR radiation into the investigated solid material.

A macro sampling chamber equipped with mid-infrared (IR) focal plane array (FPA) detector was used to examine chemical treatments of cotton fabrics. Conventional IR methods typically examine individual points in a sample, while the FPA detector provides spatially resolved spectra that can corroborate chemical treatment and its distribution on the cotton fabric. Characterizations of three distinct treatments are presented: non-durable treatments of N, N-diethyl-3-methylbenzamide (DEET), an active ingredient in commercially available insect repellents, a phosphazine-based fire retardant, and fabric treated with deuterated water. All chemical treatments examined in this study exhibited distinct vibrational bands that could be used as markers of the fabric treatments. Our results suggest that the mid-IR FPA detector can be used to characterize fabric treatment uniformity and provide qualitative confirmation of fabric chemical treatment.

In this paper, we present the design, simulation, fabrication and initial characterization results of terahertz (THz) focal plane arrays (FPAs) employing Sb-based heterostructure backward diodes (HBDs) integrated with lens-coupled folded-dipole antennas (FDAs). For single array element design, FDAs with high embedding-impedances have been designed for impedance matching to HBDs without additional matching network. Under impedance matching conditions, a maximum detector responsivity of ~21,000 V/W could be obtained for single array pixel at 200 GHz. In order to expand the single element design into full 2-D THz FPAs, the off-axis radiation patterns of the FDA mounted on an extended hemispherical silicon lens have been analyzed using the ray tracing technique. In addition, mutual coupling between two adjacent FDAs has been studied using full-wave simulation. The above results along with initial array fabrication and device characterization results have demonstrated the potential to achieve room-temperature, high-performance and large-scale FPAs for THz imaging applications.

During the past decade, Fourier transform infrared (FT-IR) microspectroscopy has been used successfully in many studies to differentiate normal and diseased tissue samples obtained from a variety of organs, including colon, cervix, prostate, and breast [1-4]. In most cases, spectra were collected for a select area of a tissue or IR images were constructed by collecting spectra point-by-point using a mapping stage on an FTIR microscope equipped with a single-element detector. Recently, a new technique has been developed for performing vibrational spectroscopic imaging microscopy using a liquid-nitrogen-cooled focal-plane array (FPA) detector and a step-scanning FT-IR spectrometer coupled to a refractive microscope [5]. With this configuration, equipped with a new 64-pixel x 64-pixel Mercury-Cadmium-Telluride (MCT) FPA detector, it has now become possible to image an 800-um × 800-um area of a specimen without moving the sample.

Most terahertz (THz) time-domain (pulsed) imaging experiments that have been performed by raster scanning the object relative to a focused THz beam require minutes or even hours to acquire a complete image. This slow image acquisition is a major limiting factor for real-time applications. Other systems using focal plane detector arrays can acquire images in real-time, but they are too expensive or are limited by low sensitivity in the THz range. More importantly, such systems cannot provide spectroscopic information of the sample. To develop faster and more efficient THz time-domain (pulsed) imaging systems, this research used random projection approach to reconstruct THz images from the synthetic and real-world THz data based on the concept of compressed/compressive sensing/sampling (CS). Compared with conventional THz time-domain (pulsed) imaging, no raster scanning of the object is required. The simulation results demonstrated that CS has great potential for real-time THz imaging systems because its use can dramatically reduce the number of measurements in such systems. We then implemented two different CS-THz systems based on the random projection method. One is a compressive THz time-domain (pulsed) spectroscopic imaging system using a set of independent optimized masks. A single-point THz detector, together with a set of 40 optimized two-dimensional binary masks, was used to measure the THz waveforms transmitted through a sample. THz time- and frequency-domain images of the sample comprising 20×20 pixels were subsequently reconstructed. This demonstrated that both the spatial distribution and the spectral characteristics of a sample can be obtained by this means. Compared with conventional THz time-domain (pulsed) imaging, ten times fewer THz spectra need to be taken. In order to further speed up the image acquisition and reconstruction process, another hardware implementation - a single rotating mask (i.e., the spinning disk) with random binary patterns - was utilized to spatially modulate a collimated THz. After propagating through the sample, the THz beam was measured using a single detector, and a THz image was subsequently reconstructed using the CS approach. This demonstrated that a 32×32 pixel image could be obtained from 160 to 240 measurements. This spinning disk configuration allows the use of an electric motor to rotate the spinning disk, thus enabling the experiment to be performed automatically and continuously. To the best of our knowledge, this is the first experimental implementation of a spinning disk configuration for high speed compressive image acquisition. A three-dimensional (3D) joint reconstruction approach was developed to reconstruct THz images from random/incomplete subsets of THz data. Such a random sampling method provides a fast THz imaging acquisition and also simplifies the current THz imaging hardware implementation. The core idea is extended in image inpainting to the case of 3D data. Our main objective is to exploit both spatial and spectral/temporal information for recovering the missing samples. It has been shown that this approach has superiority over the case where the spectral/temporal images are treated independently. We first proposed to learn a spatio-spectral/temporal dictionary from a subset of available training data. Using this dictionary, the THz images can then be jointly recovered from an incomplete set of observations. The simulation results using the measured THz image data confirm that this 3D joint reconstruction approach also provides a significant improvement over the existing THz imaging methods.

Summary form only given. Terahertz (THz) imaging technology attracts a lot of attentions for security and biochemical applications because many biochemical molecules have characteristic absorption spectra in the THz frequency region. THz camera, which can take real-time THz images is one of the most important detection devices for the development of THz imaging technology. NEC has developed a THz camera called IRV-T0830 consisting of uncooled microbolometer THz focal plane arrays (THz-FPAs) [1]. Although the operation of real-time imaging system with this camera was demonstrated recently [2], improvement of performance is still required. In this study, we have improved structure of the THz-FPAs to increase the sensitivity in the THz frequency region, and evaluated frequency-dependent sensitivity of the improved THz camera systematically.IRV-T0830 detects THz waves by sensing resistance change of the microbolometers. The resistance change is originated from the temperature increase by the absorption of incident THz waves. In addition, reflection layer is formed at the bottom of air gap located under the microbolometer layer in order to increase the THz absorption efficiency. Optical cavity is formed between reflection layer and THz absorption layer and it was found that the thickness of air gap is closely related to sensitivity of the THz camera [1]. Here, we have developed a new THz camera which air-gap thickness is near to wave length of THz frequency region to increase the sensitivity. THz beam used in sensitivity-measurement experiment was generated by tilted-pulse-front excitation of lithium niobate (LN) crystal. Spectrum of generated THz beam was obtained by electro-optic (EO) sampling method. The spectrum extended to 2 THz. We have measured frequency dependence of sensitivity of THz camera by putting THz band-pass filters in front of THz camera, and measured incident polarization direction angle dependence by rotating THz camera. It was found that sensitivities of both the new THz camera and IRV-T0830 depend on polarization of incident THz beam. Therefore, we estimated frequency dependence of sensitivities by averaging count numbers of each THz camera throughout all polarization angles. Frequency dependence of sensitivity for each THz camera is shown in Fig.1. The sensitivity of the new THz camera is three or four times larger than IRV-T0830 in frequency range of 1-2 THz. THz beam-spot images obtained by two cameras are shown in Fig.2. A band-pass filter which has transmission peak at 1.0 THz and full width at half maximum of 0.24 THz was used, and the power of incident THz beam penetrating the band-pass filter is about 100 μW. The image obtained by the new THz camera is much clearer than IRV-T0830. These results indicate that the structural modification of FPA successfully improved the performance of the THz camera, which should become a critical tool for the development of THz imaging technology.

During the past decade, Fourier transform infrared (FT-IR) microspectroscopy has been used successfully in many studies to differentiate normal and diseased tissue samples obtained from a variety of organs, including colon, cervix, prostate, and breast. IR images were constructed by collecting spectra point-by-point using a mapping stage on a FT-IR microscope equipped with a single-element detector. Five years ago, in collaboration with NIH scientists Dr. Neil Lewis and Dr. Ira Levin, Procter and Gamble researchers developed a technique for performing vibrational spectroscopic imaging microscopy using a liquid-nitrogen-cooled focal-plane array (FPA) detector and a step-scanning FT-IR spectrometer coupled to a refractive microscope. With this configuration, equipped with a new 64-pixel x 64-pixel Mercury-Cadmium-Telluride (MCT) FPA detector, we are able to image an 800-μm x 800-μm area of a specimen without moving the sample.For all of these IR imaging studies, the tissue samples were prepared in a manner that was special and different from that commonly used in a pathology laboratory, i.e.,the histopathology sample was not identical to the spectroscopic sample.

Hughes Aircraft Company has successfully developed and demonstrated MWIR staring focal plane array (FPA) technology using Schottky barrier detectors with arrays consisting of 30 micron pixel spacings in 256 x 256 array format. The hybrid Schottky barrier FPA is a key technology which will permit implementation of a simple, low-cost, single field-of-view IR sensor design. The use of Schottky barrier detectors in FPAs is an outgrowth of earlier work conducted by RADC. A 128 x 128 hybrid Schottky barrier FPA was demonstrated in September 1984, with good detection and recognition performance and image quality against tactical targets. The Schottky barrier 256 x 256 MWIR hybrid focal plane array is a result of an on-going developmental process which has evolved from a 62 x 58 FPA, through a 128 x 128 FPA. Evolution of these arrays has included both improvements in the detector arrays as well as the readout or signal processing structure. The readout has been redesigned to reduce the number of clocks and biases necessary for operation. Reported is the requirement, design, fabrication, and test results of this high density hybrid FPA based upon platinum silicide infrared detector technology. The hybrid approach has advantages of ease of fabrication, high optical fill factor, compatibility with existing multiplexer technology, and excellent imaging performance. We review past Schottky FPA development at Hughes Aircraft and discuss the technical trade-offs of our approach. Discussed is the design, fabrication, and test results of our most recent Schottky FPA.

Metasurfaces are engineered thin surfaces comprising two‐dimensional (2D) arrays of sub‐wavelength‐spaced and sub‐wavelength‐sized resonators. Metasurfaces can locally manipulate the amplitude, phase, and polarization of light with high spatial resolution. In this paper, we report numerical and experimental results of a vortex‐beam‐generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consists of a 2D array of dielectric cross‐shaped resonators with spatially varying length, thereby providing the desired spatially varying phase shift to the incident light. The metasurface was found to be insensitive to the polarization of the incident light. The dimensions of the cross‐resonators were calculated using rigorous finite‐difference time‐domain analysis. The spectral scalability via physical scaling of the meta‐resonators is demonstrated using two vortex‐generating optical elements operating at 8.8 μm (IR) and 0.78 THz. The vortex beam generated in the mid‐IR spectral range was imaged using a Fourier transform IR (FTIR) imaging miscroscope equipped with a focal plane array detector. This design could be used for efficient wavefront shaping and various optical imaging applications in the mid‐IR spectral range, where polarization insensitivity is desired.

Noise equivalent temperature difference performance of an IR detector in a hybrid focal plane array