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We applied a simple method to estimate the Minimum Resolvable Temperature Difference (MRTD) of an LWIR and an MWIR camera. A so-called Siemens star, in our case a thin, black aluminum plate framing a circle that is missing (cut out) every other spoke, is mounted in front of a black body whose temperature is relatively close to room temperature. From short recordings of the black body and Siemens star both the Noise Equivalent Temperature Difference (NETD) and the Modulation Transfer Function (MTF) are extracted and a simple estimate of MRTD = NETD/MTF is obtained. The imaged Siemens star almost completely covers the focal plane array; hence, an MRTD curve for the whole array is obtained. We investigated the effect of Non-Uniformity Correction (NUC) and Bad-Pixel Removal (BPX), two often applied pre-processing techniques, on the MRTD estimate. We find that (1) BPX has only limited effect on the result; (2) NUC is required to obtain a good MTF; and (3) NUC is not a prerequisite to obtain a good NETD estimate, but this is contingent on having a proper segmentation tool or template available. Without a segmentation algorithm, NUC together with simple intensity thresholding provides a sufficiently good segmentation and accordingly a good estimate of NETD.

A fiber-coupled Terahertz time domain spectroscopy (THz-TDS) system based on photoconductive antennas, pumped by a 100-fs fiber laser, has been used to characterize materials in transmission and reflection mode. THz images are acquired by mounting the samples under investigation on an x-y stage, which is stepped through the beam while the transmitted or reflected THz waveform is captured. The samples include a carbon fiber epoxy composite and a sandwich-structured composite panel with an aramid fiber honeycomb core in between two skin layers of fiberglass reinforced plastic. The former has an artificially induced void, and from a comparison of recorded reflected time-domain signals, with and without the void, a simple model for the structure of the composite is proposed that describes the time-domain signals reasonably well. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only. Copyright 2016 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic electronic or print reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

False alarm rates must be kept sufficiently low if a method to detect and identify objects or substances is to be implemented in real life applications. This is also true when trying to detect and identify dangerous substances such as explosives and drugs that are concealed in packaging materials. THz technology may be suited to detect these substances, especially when imaging and spectroscopy are combined. To achieve reasonable throughput, the detection and identification process must be automated and this implies reliance on algorithms to perform this task, rather than human beings. The identification part of the algorithm must compare spectral features of the unknown substance with those in a library of features and determining the distance, in some sense, between these features. If the distance is less than some defined threshold a match is declared. In this paper we consider two types of spectral characteristic that are derived from measured time-domain signals measured in the THz regime: the absorbance and its derivative. Also, we consider two schemes to measure the distance between the unknown and library characteristics: Spectral Angle Mapping (SAM) and Principal Component Analysis (PCA). Finally, the effect of windowing of the measured time-domain signal on the performance of the algorithms is studied, by varying the Blackman-Harris (B-H) window width. Algorithm performance is quantified by studying the receiver-operating characteristics (ROC). For the data considered in this study we conclude that the best performance is obtained when the derivative of the absorbance is used in combination with a narrow B-H window and SAM. SAM is a more straight-forward method and requires no large training data sets and tweaking.

HgTe nanowires nucleated by Au particles have been grown on Si and GaAs substrates by molecular beam epitaxy. The wires are polycrystalline. They evolve from crooked to straight during growth and have rounded to rectangular cross-sections. The widths are in the range 20–500 nm, with lengths up to 4 μm. The height of the nanowires is typically less than the width. The nanowires have been characterized by scanning electron microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy. The effects of substrate material, substrate preparation and growth conditions have been investigated.

Measurements are presented of the spectral response of MBE (molecular beam epitaxy)-grown, ion-milled, lateral photodiodes fabricated in the Hg1−xCdxTe material system with x≈0.31. The optical collection length is extracted from measurements taken from diodes of variable diameter. From the temperature dependence of the optical collection length, it is suggested that radiative recombination and Auger recombination processes determine the lifetime of the minority charge carriers.

CdxHg1−xTe layers with bandgap in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions were grown by molecular beam epitaxy, and one-dimensional (1D) and two-dimensional (2D) arrays of planar photodiodes were fabricated by ion milling of vacancy-doped layers. The grown layers have varying densities of needle-shaped structures on the surface. The needles are not associated with twins or dislocations in the layers, but could instead be due to (111) facets being reinforced by a preferential Te diffusion direction over steps on the surface. The needles do not seem to affect diode quality. 64 element 1D arrays of 26×26 μm2 or 26×56 μm2 diodes were processed, and zero-bias resistance-times-area values (R0A) at 77 K of 4×106 Ω cm2 at cutoff wavelength λCO=4.5 μm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball-bonding to the 1D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The median measured noise equivalent temperature difference (NETD) on a LWIR array was 14 mK for the 42 operable diodes. 2D arrays showed reasonably good uniformity of R0A and zero-bias current (I0) values. The first 64×64 element 2D array of 16×16 μm2 MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK. Images from both a 1D and a 2D array are shown.

Imaging one-dimensional (1-D) and two-dimensional (2-D) arrays of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) planar photodiodes were fabricated by ion milling of vacancy-doped molecular beam epitaxy CdxHg1−xTe layers. Sixty-four-element 1-D arrays of 26×26 µm2 or 26×56 µm2 diodes were processed. Zero-bias resistance-area values (R0A) at 77 K of 4×106 Θcm2 at cutoff wavelength λCO=4.5 µm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball bonding to the 1-D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The best measured noise equivalent temperature difference (NETD) on a LWIR array was 8 mK, with a median of 14 mK for the 42 operable diodes. The best measured NETD on a MWIR array was 18 mK. Two-D arrays showed reasonably good uniformity of R0A and zero-bias current (I0) values. The first 64×64 element 2-D array of 16×16 µm2 MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK.

Terahertz images containing spectral information in each pixel are recorded in transmission mode using a fibercoupled time-domain spectroscopy system. The images are acquired by mounting a sample holder on an x–y stage, which is stepped across the beam in the two transverse directions, while the transmitted THz waveform is captured. The materials under investigation consist of uncovered and hidden samples of an explosive (RDX) and simulants (lactose and tartaric acid). Spectral angle mapping is used to identify the materials in the Terahertz images by comparing the spectrum in each pixel with a library of reference spectra for the different materials. We test the performance of several spectral characteristics derived from the measured transmission spectra. Robustness is studied by investigating the Receiver-Operating-Characteristics (ROCs). The ROCs are used to find which of the spectral characteristics is most robust to different sample preparation conditions, without the need for extensive pre-treatment of the data, such as baseline correction. Simple theoretical considerations are used to support the experimental results. This is the authors accepted and refereed manuscript to the article. © 2015 IEEE.

Planar mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) photodiodes were fabricated by ion milling molecular beam epitaxy (MBE) CdxHg1-xTe (CMT) layers with and without compositional grading in the layer. Linear arrays with 32 and 64 diodes, as well as test diodes of varying size, were fabricated. Good quantum efficiencies were measured, and MWIR diodes, with cutoff wavelength λco = 4.5 µm, had zero-bias resistance-area values (RoA) in excess of 1 × 107Ωcm2, whereas LWIR diodes with λco = 8.9-9.3 µm had RoA = 3 × 102Ωcm2 at 77 K. Comparison between a limited number of layers indicates that in layers with a gradient the RA values are a factor of ∼10 larger, and possibly more uniform, than in layers without a gradient.

Date of Conference: 5-9 May 2014 Conference name: SPIE Sensing Technology + Applications, 2014 - Proceedings - Terahertz Physics, Devices, and Systems VIII: Advanced Applications in Industry and Defense We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection. © 2014 SPIE.

In this paper, we study the electrical noise of commercially available laser diodes: a group of index guided AlGaInP diodes lasing at 635 nm (SDL3038-11) and a group of InGaAlP-multi-quantum-well diodes lasing at 670 nm (SVL71B). In particular, the effect of stress (high current and high temperature) on the noise is investigated. Measurements of the magnitude of the 1/f noise as a function of the operating current (10 nA to 10 mA) revealed an anomaly. After increasing proportionally with the current at small currents (10 nA to 10 μA), the 1/f noise tends to saturate with increasing currents in the range from 10 to 100 μA. For larger operating currents, the 1/f noise increases again with the current, proportional to I2. This anomaly is even more pronounced after the devices have been stressed. We conclude that measurements of the 1/f noise at low bias currents are sensitive to the degradation of the active region of the laser diodes, while measurements of the 1/f noise at high bias currents can predict failure related to the quality of the crystal layers of the laser diodes.

The electrical noise of commercially available laser diodes, an index guided AlGaInP diode lasing at 635 nm (SDL3038-11) and an InGaAlP-multiquantum well diode lasing at 670 nm (SVL71B), has been investigated over a wide current range of six orders of magnitude. After increasing proportionally with current at small currents (10 nA to 10 μA), the 1/ f noise tends to saturate with increasing current in the range from 10 to 100 μA. For larger operating currents, the 1/ f noise increases again, and with the current proportional to I 2 . Different noise sources were discovered below the lasing threshold current. The electrical noise at lower currents must be measured to assess the degradation of the active region of the laser diode.

Low-frequency noise of hydrogenated-amorphous-silicon (α-Si:H) thin-film transistors (TFTs) with a thin active layer and an inverted staggered device structure operating in the conducting mode has been investigated. Pure 1/f-noise spectra were observed. The results show that the physical location of the noise in α-Si:H TFTs is different from that in crystalline metal–oxide–semiconductor field-effect transistors. The noise contributions from the channel and interface have been determined for the device operating in different modes. The 1/f noise of α-Si:H TFTs stems from the channel when the device is operated in the linear region at high gate voltages. However, the 1/f noise of α-Si:H TFTs generated at the interface becomes significant when the device is operated in the saturation region. The interface noise can be explained by the number fluctuation model (ΔN model). The channel noise can be explained by either the ΔN model or the mobility fluctuation model (Δμ model).

We describe experiments where different explosives were hidden under common barrier materials, and THz radiation was used to detect and identify these explosives. Our THz system, a time-domain spectroscopy (TDS) system, is based on a femtosecond laser whose radiation is converted into THz radiation by a low-temperature grown GaAs photoconductive switch. A similar switch detects the reflected signal. The advantage of using a TDS system is that pulses reflected from the barrier and the actual explosive, arrive at different instances at the detector. This simplifies the separation of the barrier signature from the explosive signature, compared to a frequency domain system. However, partial temporal overlap between the two pulses makes it challenging to completely separate the spectral characteristics of the explosive from the characteristics of the barrier. Also, in addition to attenuating the THz-pulses, transmission through barrier materials may add spectral features to the reflected signal, hampering recognition of the explosive. On top of that, the explosive may have a rough surface, which reduces the strength of the reflected signal. In this contribution we shall address these issues and discuss strategies that may be used to face these challenges.

We report measurements of the low-frequency noise of HFETs built on low temperature grown (LTG) GaAs buffer and superlattice buffer layers. A distinct generation-recombination noise source with activation energy of 0.70 eV was observed in devices built on the LTG GaAs buffer. In addition, both structures exhibited a trap whose activation energy depended on the gate-to-source voltage. This dependence is believed to be due to interactions between electrons in the channel and traps in the AlGaAs supply layer. We also report measurements of the low-frequency noise and phase noise of MESFETs with an LTG GaAs passivation layer and of conventional MESFETs without the passivation. We found the phase noise of the LTG GaAs MESFETs to be significantly lower at small offset frequencies and to have a weaker frequency dependence. A similar reduction in the low-frequency noise was observed. This supports the idea that the LTG GaAs passivation reduces the number of traps with large activation energies.

A ship's exhaust gas contains both hot gas molecules, which emit infrared radiation at specific wavelengths (line emitters), and soot particles which emit broad-banded, like a black body. Our modeling shows that the observed radiance from these emissions falls at different rates with distance. The attenuation of intensity is caused by absorption and scattering of the emitted radiation in the atmosphere. The hottest part of the exhaust plume is spatially confined to a relative small volume. Usually, a ship's hull and its superstructure have a higher temperature than the sky or sea background. The temperature difference is generally not very large. However, the ship has a spatial extent that is much larger than the plume's. In this work we study how both the emitted radiation from the plume and the ship's total signature decrease with increasing distance. This study is based on experimental data that was collected during a measurement campaign at the southwest coast of Norway. Shore-based digital IR cameras, both LWIR and MWIR, recorded image sequences of ships as they sailed away from close to shore (~ 1 km) in a zigzag pattern out to about 10 km. We used a statistical method to identify the gas cloud pixels and used their integrated radiance as a measure for the plume intensity. The ship signature is defined here as the integrated radiance over all the ship's pixels in the imagery. From infrared spectroscopic data, collected using a Fourier Transform Infrared spectrometer aimed at the ship's plume when the ship is close to shore, a model is obtained for the composition of the exhaust gas. This model was used to perform FASCODE simulations to study numerically the attenuation with distance of the plume radiance. Our work shows that this approach may be well suited to explain the observed signal decay rate with distance.

In order to detect an object, the object has to be distinguished from its background. Often a contrast number is defined, the difference between the signals from the object and its background. Hence, detailed knowledge of both is required. Background measurements made during two measurement campaigns are compared with results from ShipIR modeling efforts. Specifically, background radiance profiles, extracted from infrared camera recordings and spectrometer recordings of the sea and sky, and spectral features are highlighted.

Low‐frequency electrical current noise measurements are reported on double‐heterojunction AlGaAs/GaAs laser diodes fabricated on GaAs and on Si substrates. The noise spectra show a frequency dependence proportional with f−γ with γ close to unity. The spectral intensity is proportional to the current for smaller currents ( 1 mA). The diodes built on the GaAs substrate are 50 times less noisy than the ones built on the Si substrate. This effect is attributed to the fact that the density of dislocations at the Si interface is much larger than at the GaAs substrate/device interface.

We present results from infrared imaging experiments, performed under hot and humid conditions at Chesapeake Bay, Maryland, USA in the summer of 2006. Specifically, the objective was to study the intensity of the exhaust gases from a ship at different distances. In particular there is an interest to quantify the intensity decrease of the plume with distance and correlate this with simulations of atmospheric transmission. For this purpose the ship ran a predetermined course making broad-side passes at predetermined distances from the shore-based IR camera as part of the course. The distances were 1.6, 2.4, 3.2, 4, 6, and 8 km. The cameras are sensitive in the 3 - 5 μm and 8 - 12 μm wavelength ranges. Digital recordings were made during the ship broad-side passes. It is challenging to identify gas cloud pixels against a background because the pixels are not necessarily clustered. We present a statistical method to identify the gas cloud pixels, calculate their average intensity, and determine the contrast between the gas pixels and the background pixels as a function of distance. The contrast versus distance data are then compared with simulations using standard atmospheric transmission software.

We report a measurement technique that would allow the mapping of the low-frequency noise characteristics of devices across a wafer. We show that we can reliably measure the noise by using a probe station. We also show that the sensitivity of the setup is not limited by the probes. >

We report measurements of the low-frequency noise and phase noise of conventional unpassivated GaAs metal semiconductor field-effect transistors (MESFETs) and of MESFETs fabricated using an overlapping-gate structure and the low-temperature grown (LTG) GaAs as a passivation layer. The noise of the LTG-GaAs passivated MESFET was found to behave quite differently from that of a conventional MESFET and to be significantly reduced at low offset frequencies. These observations are explained in terms of the surface passivating effect of the LTG-GaAs. Low-frequency noise measurements seem to support the idea that the LTG-GaAs passivation reduces the number of active traps, in particular traps with large activation emergies. These results indicate that LTG-GaAs passivation can substantially reduce the near-carrier phase noise of MESFET-based oscillators.

Measurements of low-frequency electrical noise (LFN) in commercially available laser diodes, an index guided AlGaInP diode lasing at 635 nm (SDL3038-11) and an InGaAlP-multiquantum well diode lasing at 670 nm (SVL71B), have been conducted. In particular the effect of stress (high current and high temperature) on the noise is investigated. We compared the noise spectra, light output power and current-voltage dependence measured before and after stress step. Our results give rise to a question: can low-frequency noise measurement be a practical diagnostic tool for assessing reliability of laser diodes?

The spectral noise characteristics of Aerojet GaAs n-JFETs have been investigated down to liquid helium temperatures. Voltage noise characterization was performed with the FET in 1) the floating gate mode, 2) the grounded gate mode to determine the lowest noise readings possible and 3) with an extrinsic silicon photodetector at various detector bias voltages, to determine optimum operating conditions. Current noise characterization was measured at the drain in the temperature range 300 to 77 K. Device design and MBE processing are described. Static I-V characterization is done at 300, 77 and 6 K. The measurements indicate that the Aerojet GaAs n-JFET is a quiet and stable device at liquid helium temperatures. Hence, it can be considered as a readout line driver or infrared detector preamplifier as well as a host of other cryogenic applications. Its noise performance is superior to that of Si MOSFETs operating at liquid helium temperatures, and is equal to the best Si n-JFETs operating at 300 K

Etude du bruit dans le rendement quantique (η) des photodiodes, cause par la variation dans la surface de generation-recombinaison des porteurs. Dans les photodiodes modernes p + -i-n, la recombinaison-generation se passe generalement dans le volume

Noise in Si p+-i-n power diodes was measured. At low frequencies the noise was of the 1 ƒ type and at higher frequencies we observed shot noise. In the 1 ƒ noise regime, and at back bias, S I d (ƒ)/||I d | was found to be independent of the back bias; this can be explained with the help of the Hooge equation. The noise measurements then yield αH/τ, where αH is the Hooge parameter and τ the time constant associated with the hole-electron recombination process. Evaluating τ from admittance data then gives αH = (4.0 ± 0.8) × 10−3, in good agreement with the fundamental value of 4.6 × 10−3. A discussion is given of finding τ from the measurements of the (complex) h.f. device admittance.

A systematic study of the 1/f noise dependence on the channel length in p‐ and n‐channel silicon MOSFETs (metal‐oxide‐semiconductor field‐effect transistors) is presented. Devices made by the same procedure on the same chip were used. In this way, the nonuniformity of noise sources, which strongly depend on the fabrication procedure (like 1/f noise produced by surface trapping), was avoided. Hooge’s parameter αH was used as a measure of the magnitude of the 1/f noise in the device. The αH was found to vary as the square of the channel length in p MOSFETs and in the most of the n MOSFETs. Existing theories and known noise mechanisms do not explain this dependence. The incorporation of a recent theory involving the acceleration 1/f noise in semiconductors, developed by van der Ziel, explains the experimental data very well. The αH was found to be independent upon the electric field and the extraction of an effective time constant was used for comparison between theory and experiment.

1/f noise in n+–p and n–i–p Hg1−xCdxTe photodiodes is discussed. The n+–p diodes have coherent‐state 1/f noise or umklapp 1/f noise. The n–i–p diodes have much lower values for the Hooge parameter αH and their noise is probably due to generation–recombination‐type (trapping) 1/f noise.

Measurements of the low-frequency spectral intensity of the current fluctuations in p-channel GaAs/AlGaAs heterostructure insulated-gate field-effect transistors are discussed. The measurements were performed at 77 K and a drain current of 1 mu A. The spectra of two types of devices are compared, one grown directly on the substrate and the other embedded in an n-well. The latter type produced markedly less noise, its spectrum being almost perfect 1/f noise. The former type exhibited, in addition to the 1/f noise, a significant generation-recombination noise component in the spectrum. >