Focal Plane Array Detector Research Articles

Development of a High Performance 1280×1024 InGaAs SWIR FPA Detector at Room Temperature

A 1280 × 1,024 In0.53Ga0.47As short wave infrared (SWIR) focal plane array (FPA) detector with a planar-type back-illuminated process has been fabricated. With indium bump flip-chip bonding techniques, the InGaAs photodiode arrays were hybrid-integrated to the CMOS readout integrated circuit (ROIC) with correlated double sampling (CDS). The response spectrum is 0.9–1.7 μm. The test results show that the dark current density is 2.25 nA/cm2 at 25 °C, the detectivity D* is up to 1.1 × 1013 cm · Hz1/2/W, the noise electron is as low as 48 e− under correlated double sampling mode, the quantum efficiency is 88% at 1550 nm, and the operability is more than 99.9%. Moreover, the dark current and noise electron have been studied theoretically in depth. The results indicate that the diffusion current is the main contribution of the dark current, and the readout integrated circuit noise electron is the main source of FPA noise.

Improving the quality of active millimeter wave standoff imaging by incorporating the cross-polarized scattering wave.

In this paper, we study the feasibility of incorporating the cross-polarized scattered wave in active standoff millimeter-wave imaging in order to improve the edge detection and background suppression for metallic objects. By analyzing the scattering from a perfectly conducting (PEC) patch of a simple geometrical shape we show that the edge diffraction is the major source of cross-polarized scattering. A similar scattering behavior is also observed for a PEC patch placed on a dielectric medium. Hence, the cross-polarized scattered field conveys valuable information about the edges of the object. In addition, the cross-polarized scattering can be utilized to resolve the object from an unstructured reflective background. To put these ideas to the test, a standoff imaging system composed of a continuous-wave (CW) semiconductor source, a focal plane array detector (camera), and collimating and objective lenses at 95 GHz is utilized to image the co- and cross-polarized reflection from metallic patches both in the presence and in the absence of a background medium. In agreement with theory, the experiments reveal that the edges of the object can be enhanced and reflections from a smooth background medium can be suppressed by using the cross-polarized scattering. In this regard, the conducted experiments on the metallic patches placed on the human body also yield promising results.

Compressed sensing in the far-field of the spatial light modulator in high noise conditions

Single-pixel imaging techniques as an alternative to focal-plane detector arrays are being widely investigated. The interest in these single-pixel techniques is partly their compatibility with compressed sensing but also their applicability to spectral regions where focal planes arrays are simply not obtainable. Here, we show how a phased-array modulator source can be used to create Hadamard intensity patterns in the far-field, thereby enabling single-pixel imaging. Further, we successfully illustrate an implementation of compressed sensing for image reconstruction in conditions of high noise. In combination, this robust technique could be applied to any spectral region where spatial light phase modulators or phased-array sources are available.

Functional assessment of biodegradable cotton nonwoven substrates permeated with spatial insect repellants for disposable applications

Arboviral diseases and malaria transmitted by mosquitoes are major health concerns worldwide responsible for millions of deaths annually. Spatial repellents have been used as preventative measures via direct application to the skin or disposable devices worn outside of clothing. Naturally derived, plant-based repellents are safe, effective alternatives to synthetic repellents and may be applied to disposable substrates including adhesive patches. Many disposables are composed of synthetic petrochemical-based polyester and polypropylene nonwoven fabrics, contributing to growing concern surrounding microplastic pollution. Sustainable and biodegradable substrates including those made from cotton are cost effective and environmentally friendly. Therefore, we explored the utilization of cotton-based substrates for the application of natural spatial and synthetic contact insect repellents. Cotton fibers used in the study were commercially available greige cotton, scoured and bleached greige cotton and reginned greige cotton motes, a value-added byproduct of the ginning process. Synthetic polyester and polypropylene were used for comparison. Thermogravimetric/derivative thermogravimetric analysis (TG/DTG) in combination with modulated differential scanning calorimetry (mDSC) were used to discern vaporization properties of repellents from the substrates. Retention times of repellents on the substrates were analyzed using a Fourier-transform infrared spectroscopy–focal plane array detector and compared with the TG/DTG and mDSC data. Mosquito landing assays confirmed the feasibility of using cotton-based substrates treated with plant-based spatial repellents as an effective and environmentally friendly alternative to synthetic materials.

Research of high-performance uncooled infrared focal plane array detector with common-leg structure

A novel micro bridge structure for uncooled vanadium oxide infrared focal plane array (IRFPA) detector was developed. This micro bridge used common-leg structure, which was shared by column adjacent pixels. Common-leg structure greatly increased the leg length, reducing the thermal conductivity, which could effectively improve the response rate and reduced the negative electron-transfer dissociation (NETD). Furthermore, the micro bridge adopted double-layer process to increase the area of the deck and the vanadium oxide, which could improve the filling rate and the detector performance. The detector array was 384×288 with 12 μm pixels. The readout integrated circuit (ROIC) adopted rolling-shutter mode for integration and a pixel-by-pixel mode for readout. The high reliability metal vacuum package was used for the device. The results showed that the NETD of the detector was less than 15 mK, and the response rate was greater than 44 mV/K. Its performance index could meet the requirements of civil and military applications.

Examination of Cotton Maturity and Maturity Distribution Using an Infrared Focal Plane Array Imaging System

Cotton maturity describes the thickness of the cotton secondary cell wall. There is a need for developing non-destructive methods for measuring maturity that also examine distribution. The current study seeks to expand reported infrared (IR)-based maturity determinations using an IR imaging system with a focal-plane array (FPA) detector. Adapted equations were used to examine the maturity of cotton standards and of a larger set of upland cotton varieties (30 total). Maturity values obtained with a Cottonscope and from IR determinations showed strong linearity, R2 = 0.95, while contour plots provided a visual representation of the maturity distribution in the samples. These results validate the use of IR measurement for examining cotton maturity and establish the use of the FPA IR system for examining and imaging cotton maturity distributions.

Monitoring the effects of chemical stimuli on live cells with metasurface-enhanced infrared reflection spectroscopy.

Infrared spectroscopy has found wide applications in the analysis of biological materials. A more recent development is the use of engineered nanostructures - plasmonic metasurfaces - as substrates for metasurface-enhanced infrared reflection spectroscopy (MEIRS). Here, we demonstrate that strong field enhancement from plasmonic metasurfaces enables the use of MEIRS as a highly informative analytic technique for real-time monitoring of cells. By exposing live cells cultured on a plasmonic metasurface to chemical compounds, we show that MEIRS can be used as a label-free phenotypic assay for detecting multiple cellular responses to external stimuli: changes in cell morphology, adhesion, and lipid composition of the cellular membrane, as well as intracellular signaling. Using a focal plane array detection system, we show that MEIRS also enables spectro-chemical imaging at the single-cell level. The described metasurface-based all-optical sensor opens the way to a scalable, high-throughput spectroscopic assay for live cells.

Characterization of silicon drift detectors with electrons for the TRISTAN project

Sterile neutrinos are a minimal extension of the standard model of particle physics. A promising model-independent way to search for sterile neutrinos is via high-precision β-spectroscopy. The Karlsruhe tritium neutrino (KATRIN) experiment, equipped with a novel multi-pixel silicon drift detector focal plane array and read-out system, named the TRISTAN detector, has the potential to supersede the sensitivity of previous laboratory-based searches. In this work we present the characterization of the first silicon drift detector prototypes with electrons and we investigate the impact of uncertainties of the detector’s response to electrons on the final sterile neutrino sensitivity.

(Invited) FTIR Spectroscopical Imaging of the Substrate / Polymer Interface upon Exposure to Corrosive Environments

Organic coating is an efficient method to protect metal surfaces from corrosive atmospheres but is dependent on the presence of a stable interface between the substrate and the polymer coating. However, coatings based on polymers have mostly only limited barrier properties for water and oxygen. Transport of water to the interfacial region is for most coatings inevitable upon prolonged exposure in aqueous environments. Ingress of water molecules can destabilize the substrate / polymer interface and lead to delamination of the polymer coating and initiation of corrosion processes. In this work was spectrochemical imaging of the substrate / polymer interfacial region using a FTIR-microscope with a focal plane array (FPA) detector is performed in order to follow the water up-take at the substrate / polymer interfacial region with micrometer sized lateral resolution. Chemical imaging of the interfacial region between substrates, such as Ge and Ge with thin Zn metallic layers, and organic coatings was performed during exposure to 0.1 M NaCl and humid air (95 %RH). The water uptake and water induced alterations in the substrate / polymer interfacial region on the microscale was followed. Simultaneous electrochemical impedance measurements allowed correlation of electrochemical information from the coated metal with the chemical imaging of the interfacial region.

A study on the surface correction of large format infrared detectors

Large format hybrid infrared focal array detectors, which require a wide range and high resolution, are of significant technical importance for earth observation systems and scientific space programmes. A typical large hybrid infrared focal plane array (IRFPA) detector formed by a detector chip is directly hybridized to a silicon readout integrated circuit (Si-ROIC) via indium bumps. However, for a large format IRFPA detector, the connection rate of the indium bumps strongly depends on the warpage of the Si-ROIC and the detector chip, which usually results from technological processes. In this paper, a surface correction structure is proposed to optimize the warpage of the Si-ROIC and the detector chip before flip-chip. The warpage of the Si-ROIC is analysed by the finite element method. The simulation results are in good agreement with the experiment. After optimization, the warpage of a 2048 × 2048 Si-ROIC decreased from 13 μm to less than 3 μm. Thus, it is beneficial to improve the IRFPA bump connection rate. Compared with traditional growth stress films, this method is more flexible and has easy accessibility. Eventually, it could enhance the stability of the large format IRFPA detector.

Orientation Matters: Polarization Dependent IR Spectroscopy of Collagen from Intact Tendon Down to the Single Fibril Level.

Infrared (IR) spectroscopy has been used for decades to study collagen in mammalian tissues. While many changes in the spectral profiles appear under polarized IR light, the absorption bands are naturally broad because of tissue heterogeneity. A better understanding of the spectra of ordered collagen will aid in the evaluation of disorder in damaged collagen and in scar tissue. To that end, collagen spectra have been acquired with polarized far-field (FF) Fourier Transform Infrared (FTIR) imaging with a Focal Plane Array detector, with the relatively new method of FF optical photothermal IR (O-PTIR), and with nano-FTIR spectroscopy based on scattering-type scanning near-field optical microscopy (s-SNOM). The FF methods were applied to sections of intact tendon with fibers aligned parallel and perpendicular to the polarized light. The O-PTIR and nano-FTIR methods were applied to individual fibrils of 100–500 nm diameter, yielding the first confirmatory and complementary results on a biopolymer. We observed that the Amide I and II bands from the fibrils were narrower than those from the intact tendon, and that both relative intensities and band shapes were altered. These spectra represent reliable profiles for normal collagen type I fibrils of this dimension, under polarized IR light, and can serve as a benchmark for the study of collagenous tissues.

High spatial resolution analysis of H2O in silicate glass using attenuated total reflection FTIR spectroscopy coupled with a focal plane array detector

High resolution spectroscopic techniques that are used to determine volatile concentration in geological glasses are on the forefront for determining small scale degassing or hydration processes. Several spectroscopic methods are used for this purpose but they mostly require complicated sample preparation. Here, these methods are extended by the use of attenuated total reflectance (ATR) FTIR spectroscopy coupled with a focal plane array detector (FPA). ATR-FTIR spectroscopy requires only singly polished samples. The coupling to an FPA detector, which is novel for the analysis of volatile bearing glasses, enables spatial resolution on a micrometer scale. An ATR-FPA calibration to quantify H2O concentration in peralkaline rhyolitic glass is presented, which is comparable to conventional ATR calibrations using a single element detector. The results confirm that calibrations for different glass compositions are comparable after correction for glass density as suggested by a previous study. A method is developed to compare calibrations obtained with ATR objectives that have different angles of incidence. Despite the larger errors that come with the use of an FPA detector, the method allows the detection of H2O concentration gradients on a micrometer scale.The ATR-FPA calibration was applied to experimentally vesiculated melts that were quenched to glasses. It was possible to resolve only several μm-sized H2O resorption halos around vesicles that formed during isobaric cooling. Besides vesiculated glasses, the new method may enable the analysis of small melt inclusions and partially crystallized glasses and may allow the spatially resolved analysis of CO2 dissolved in silicate glasses.

Space Application Requirement Breakdown and Sensor Concept Implementation for MCT-Based LWIR and VLWIR 2D High-Performance Focal Plane Detector Arrays at AIM

Future demanding scientific space and earth observation missions such as for exoplanet atmospheric spectral analysis or earth climate monitoring will define new benchmarks in technological feasibility and come along with increasingly stringent requirements on instrument subsystems. The requirements imposed on infrared detectors are dependent on the type of optical instrument, the chosen observation approach and the spectral range to be covered. With advanced detector architectures on hand and suitable␣technology approaches in mind, various electrical and electro-optical quantities have to be traded off against each other and possibly be prioritized, reflecting the specific instruments’ needs. From such considerations, implications on the detector material, design and processing arise. In this methodological paper, we will illustrate specific trade-off considerations for focal plane detector arrays (FPAs) based on mercury cadmium telluride (MCT) as detector material, e.g. with respect to modulation transfer function (MTF), detection efficiency (QE), noise level, defect density, etc. As an illustration, we will discuss simulation and implementation approaches and solutions found for a presently ongoing cornerstone pre-development activity in the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR). In particular, we will consider the thermal infrared (TIR) channels of the European Space Agency (ESA) Land and Surface Temperature Monitoring Mission (LSTM) observation instrument, for which we will present a photodiode array design approach balanced for key electro-optical FPA performance parameters.

Terahertz Synthetic Aperture Imaging with a Light Field Imaging System

In terahertz imaging systems based on Gaussian beam active illumination and focal plane array detectors, severe image distortion has been observed, which significantly reduces the resolving power of the imaging system. To solve this problem, a novel computational method, Light Field Imaging (LFI), has been introduced for terahertz imaging. A conventional transmission-type terahertz imaging system based on a gas-pumped terahertz source and terahertz Focal Plane Array Detectors (FPA) arrays is established to analyze the problem of image distortion. An experimental virtual camera array terahertz LFI system is also established. With the acquisition and reconstruction of synthetic aperture terahertz light fields, the improvement on resolving power and SNR performance have been validated.

A mapping approach for the investigation of Ti–OH relationships in metamorphic garnet

Garnet is a nominally anhydrous mineral that can incorporate several hundreds of ppm H2O in the form of OH groups, where H+ substitutes for cations in the garnet structure. To understand the effect of such small amounts of H2O on the physical and chemical properties of garnet, it is essential to determine where and how the OH groups are incorporated into the mineral structure. We investigated correlations between major and minor element maps acquired with the electron probe micro-analyser and H2O maps measured with Fourier transform infrared spectroscopy coupled to a focal plane array detector at the microscale to determine possible coupled substitutions. A set of algorithms was developed to match the maps pixel by pixel. They allow the computation of the garnet structural formula taking the H2O content into account and the calculation of correlations between H2O and other elements on the basis of 10,000 s of points. This new approach was applied to two hydrous garnet samples both showing H2O and chemical zoning. The first sample consists of a grossular-rich garnet from a high-pressure metarodingite ranging from 200 to 900 ppm H2O. The second sample contains a Ti-rich andradite garnet ranging from 200 to 8500 ppm H2O. For the grossular-rich garnet, a 1:1 correlation between Ti and H has been observed suggesting that H occurs as tetrahedral (2H)2+ point defect, charge compensated by 2 Ti4+ on the octahedral site. Based on this, a new hydrous garnet endmember with the formula Ca3Ti2H2Si2O12 is proposed. This 2TiVI$$\leftrightarrow$$ (2H)IV exchange mechanism is mainly responsible for the high amounts of TiO2 (up to 11 wt%) in the investigated Ti-andradite. The incorporation of (2H)2+ instead of (4H)4+ on the tetrahedral site has important consequences for the normalisation of the garnet and hence on the determination of Fe2+/Fe3+ based on stoichiometry. In the garnet from the metarodingite, a small-scale zoning in H2O contents of less than 100 µm can be resolved, indicating that the Ti–H defect is stable up to eclogite facies conditions and not modified by diffusion even at timescales of millions of years.

Transmission Fourier Transform Infrared Spectroscopic Imaging, Mapping, and Synchrotron Scanning Microscopy with Zinc Sulfide Hemispheres on Living Mammalian Cells at Sub-Cellular Resolution.

Fourier transform infrared (FT-IR) spectroscopic imaging and microscopy of single living cells are established label-free technique for the study of cell biology. The constant driver to improve the spatial resolution of the technique is due to the diffraction limit given by infrared (IR) wavelength making subcellular study challenging. Recently, we have reported, with the use of a prototype zinc sulfide (ZnS) transmission cell made of two hemispheres, that the spatial resolution is improved by the factor of the refractive index of ZnS, achieving a λ/2.7 spatial resolution using the synchrotron-IR microscopy with a 36× objective with numerical aperture of 0.5. To refine and to demonstrate that the ZnS hemisphere transmission device can be translated to standard bench-top FT-IR imaging systems, we have, in this work, modified the device to achieve a more precise path length, which has improved the spectral quality of the living cells, and showed for the first time that the device can be applied to study live cells with three different bench-top FT-IR imaging systems. We applied focal plane array (FPA) imaging, linear array, and a synchrotron radiation single-point scanning method and demonstrated that in all cases, subcellular details of individual living cells can be obtained. Results have shown that imaging with the FPA detector can measure the largest area in a given time, while measurements from the scanning methods produced a smoother image. Synchrotron radiation single-point mapping produced the best quality image and has the flexibility to introduce over sampling to produce images of cells with great details, but it is time consuming in scanning mode. In summary, this work has demonstrated that the ZnS hemispheres can be applied in all three spectroscopic approaches to improve the spatial resolution without any modification to the existing microscopes.

Constraints and performance trade-offs in Auger-suppressed HgCdTe focal plane arrays.

Majority carrier depletion has been proposed as a method to suppress the dark current originating from quasi-neutral regions in HgCdTe infrared focal plane array detectors. However, a very low doping level is usually required for the absorber layer, a task quite difficult to achieve in realizations. In order to address this point, we performed combined electromagnetic and electric simulations of a planar $ 5 \times 5 $5×5 pixel miniarray with 5 µm wide square pixels, assessing the effect of the absorber thickness, its doping level in the interval $ {N_D}{ = [10^{14}}{,10^{15}}] \;{{\rm cm}^{ - 3}} $ND=[1014,1015]cm-3, and temperature in the interval 140 K-230 K, both in the dark and under illumination. Looking for a trade-off, we found that the path towards high-temperature operation has quite stringent requirements on the residual doping, whereas a reduction of the absorber thickness helps only moderately to reduce the dark current. Under illumination, interpixel cross talk is only slightly cut down by a decrease of temperature or absorber doping in the considered intervals, whereas it gets more effectively reduced by thinning the absorber.

Development of a spaceborne pulse tube cooler operating at 170K

The Infrared Focal Plane Array (IRFPA) detector needs quite low dark noise for image detection. It can be cooled to demanding low temperature by pulse tube cooler (PTC) because of low vibration and low electromagnetic interference (EMI) at cold end. A high-capacity pulse tube cooler driven by a moving-magnet linear compressor is presented in this paper. The regenerator and pulse tube are arranged in coaxial. The inertance tube and reservoir are used as passive phase shifter of the PTC. A numerical thermodynamic model is established to design and optimize overall performance of the pulse tube cold finger aimed for the best efficiency. Based on the principle of electric-mechanical-acoustic coupling field, a transient co-simulation of the PTC is proposed. The mass of the PTC is less than 12 kg without electronic controller. The oscillating linear compressor has a pair of opposite pistons to eliminate vibration and the input electric input power is 400 W at maximum. A typical cooling performance of 50 W at 170 K has been achieved with 228 W input power at reject temperature of 293 K, provided by water cooling. The specified Carnot efficiency is 15.8%. Additionally, overall cooling performances of the PTC at 150K-200 K are investigated by experiment, Not only could this PTC be used for space mission but also a promising alternative to the domestic low temperature applications.

Baseline characterisation of microlitter in the sediment of torrents and the sea bottom in the Gulf of Tigullio (NW Italy)

Microlitter (<5 mm) distribution was investigated in the bottom sediments of the Entella Torrent and its tributaries as well as in the sea area in front of its mouth (Gulf of Tigullio; NW Italy). Microlitter was extracted from sediment using a concentrated hypersaline solution (1.2 g cm−3), filtered with 0.45-μm porosity and black grid membranes and examined using a binocular microscope. Items were counted, dimensionally measured and classified according to type, shape, colour and appearance. A total of 56 torrent and sea sediment samples revealed the presence of 4,302 items. The mean concentrations were 1.5 items cm−3± 1.3 standard deviation (SD) and 1.6 items cm−3± 1.3 SD for torrent samples and sea sediment samples, respectively. Transparent fibres predominated, followed by fragments. The most common colour was white-cream. In total, 1.8% of items were analysed with Fourier transform infrared spectroscopy (FT-IR) 2D imaging, using a focal plane array (FPA) detector to identify their composition. Nylon was the most common plastic polymer type; however, approximately 50% of the analysed samples consisted of cotton transparent fibres. The influences of human activities (cities, port and discharges) on microlitter composition were highlighted and torrents were confirmed as a vector of microlitter from land to sea.

Effect of point spread function position bias in wavefront coding infrared imaging system

By an experimental finding, for a given radiant point source and a given infrared detector sampling, an optical point spread function (PSF) falling on different positions of an infrared detector focal plane array, will result in different measured PSFs. Those measured PSFs known as decoding kernels will further produce different decoded images. Therefore, this work explores the effect of PSF position bias on decoded images in a wavefront coding infrared imaging system. This paper theoretically analyzes the effect of PSF position bias on decoded images. A simulation is conducted for qualitatively evaluating the effect of PSF position bias on decoded images by mean structural similarity (MSSIM) index. Simulated results prove that the probability for MSSIM index greater than 0.75 is 91.38%, and that for less than 0.5 is only 1.22% but this minority seriously degrades the decoded images. We construct an experimental setup for capturing the raw PSF image at a random position and proposes a procedure to reduce the noise of a raw PSF image. The experimental result demonstrates the feasibility for measuring a PSF to achieve a decoded image with good quality.