This PDF file contains the front matter associated with SPIE Proceedings Volume 8868, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

In this paper, we review a few selected imaging technology development programs at the Defense Advanced Research Projects Agency (DARPA) in the reflective visible to the emissive/thermal long wave infrared (LWIR) spectral bands. For the reflective visible band, results are shown for two different imagers: a gigapixel monocentric multi-scale camera design that solves the scaling issues for a high pixel count, and a wide field of view and a single photon detection camera with a large dynamic range. Also, a camera with broadband capability covering both reflective and thermal bands (0.5 μm to 5.0 μm) with >80% quantum efficiency is discussed. In the emissive/thermal band, data is presented for both uncooled and cryogenically cooled LWIR detectors with pixel pitches approaching the fundamental detection limits. By developing wafer scale manufacturing processes and reducing the pixel size of uncooled thermal imagers, it is shown that an affordable camera on a chip, capable of seeing through obscurants in day or night, is feasible. Also, the fabrication and initial performance of the world’s first 5 μm pixel pitch LWIR camera is discussed. Lastly, we use an initial model to evaluate the signal to noise ratio and noise equivalent differential temperature as a function of well capacity to predict the performance for this thermal imager.

Our group has designed and developed a new SWIR single photon detector called the nano-injection detector that is conceptually designed with biological inspirations taken from the rod cells in human eye. The detector couples a nanoscale sensory region with a large absorption volume to provide avalanche free internal amplification while operating at linear regime with low bias voltages. The low voltage operation makes the detector to be fully compatible with available CMOS technologies. Because there is no photon reemission, detectors can be formed into high-density single-photon detector arrays. As such, the nano injection detectors are viable candidates for SPD and imaging at the short-wave infrared band. Our measurements in 2007 proved a high SNR and a stable excess noise factor of near unity. We are reporting on a high speed version of the detector with 4 orders of magnitude enhancement in speed as well as 2 orders of magnitude reduction in dark current (30nA vs. 10 uA at 1.5V).

A comparison of the ultimate limits of multiple-stage interband cascade infrared photodetectors (ICIPs) with standard single-absorber photovoltaic infrared detectors is presented. In particular, we focus on how the multiple-stage architecture can offer an improvement in sensitivity over single-absorber detectors in situations where the latter detector type is limited by a low photocarrier collection efficiency. A general overview of the ICIP architecture and its benefits in carrier collection improvement, and noise reduction is presented. The developed theory is then applied to compare single- and multiple-stage detectors operating in the strong signal regime. It is found that if ICIPs are designed with photocurrent-matched absorbers, the signal-to-noise ratio depends on the total particle conversion efficiency of the device. The potential benefits of the ICIP architecture are shown to be greatest when the quantum efficiency of singleabsorber detectors is limited by poor photo-carrier extraction.

We present a study of the temperature-dependence of the performance metrics of a set of five GaSb-based MWIR interband cascade infrared photodetectors employing InAs/GaSb superlattice absorbers. The cutoff wavelengths of the detectors varied from 4.3 μm at 78 K to 5.1 μm at 300 K. In this study, the number of stages and absorber thicknesses were varied between the samples. Two of the samples were single-stage devices with long (> 1.0 μm) absorbers, while the other three were multiple-stage detectors with short (< 1.0 μm) absorbers. The detectors were designed so that the incoming signal was traveling in the same direction as the flow of the photo-excited electrons. We experimentally show that multiple-stage detectors with shorter absorbers are able to achieve higher values of R_oA and are have a photoresponse that is less sensitive to temperature. This confirms their potential utility for high-temperature detector operation. For the particular samples in this study, the multiple-stage devices were able to achieve better sensitivities above 250 K than the single-stage samples. It is notable that for most of the samples, a fit of the temperaturedependence of the dark current yielded an activation energy slightly larger than half the zero-temperature bandgap. This suggests that there may be an electric field and depletion region in the absorber and the interband transport in this series of detectors is governed by generation-recombination current, even at high temperature. Also, preliminary results of interband cascade infrared photodetectors at longer wavelengths (> 12 μm) are reported.

RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome the high cost, limited spectral response, and challenges in the reduction in pixel size associated with InGaAs focal plane arrays. The most significant advantage of the CQD technology is ease of fabrication. The devices can be fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. Further, device performance is not expected to degrade significantly with reduced pixel size. We present results for upward-looking detectors fabricated on Si substrates with sensitivity from the UV to ~1.7 μm, compare these results to InGaAs detectors, and present measurements of the CQD detectors temperature dependent dark current.

In order to lessen the strain of cooling requirements on mid-infrared detectors, reducing the volume of the detecting medium is one promising solution. It is necessary to augment the absorption (quantum efficiency) lost when shrinking the detector volume. We present a Quantum Well Infrared Photodetector with a plasmonic structure embedded within and around the detection media. This device has a self-aligned plasmonic-hole array designed for 8μm wavelength and a planar top contact to the array of detector material. This arrangement has an expected field enhancement of an order of magnitude and lends itself to making a Focal Plane Array.

Carbon-based nanostructures including nanotubes (CNTs) and graphene have superior electronic, optoelectronic and mechanical properties, which provide fresh opportunities for designs of novel devices of extraordinary performance in addition to the benefits of low cost, large abundance, and light weight. In this work, a comparative study of two types of uncooled infrared detectors based single-wall as well as multi-wall CNTs and their hybrids with graphene or polymer is presented. One is bolometer in which excitons dissociate via interactions with the phonons on the CNTs. The other implements built-in voltage at the hybrid interface between CNTs and graphene (or polymer) to assist exciton dissociation for photoconductivity. The difference in exciton dissociation has been found to directly affect the device performance such as responsivity and detectivity. This investigation aims at understanding the fundamental physics governing exciton dissociation and charge as well as phonon transport at nanoscales and its impact on the device performance in these CNT-based infrared detectors.

A multi-reflection cell has been employed to increase the sensitivity of the detection of methane gas. However, as the requirements of the detection system need to work on a low frequency range, the influence of the 1/f noise will be considered. This paper deal with this problem by investing the signal processing methods using Fast Fourier Transform (FFT) results have been shown an improvements of about 75% in the methane gas readings.

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO₂, GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process. The structural and optical properties of the grown vertically aligned ZnO NW arrays were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements. The photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Envisioned applications for such sensors/FPAs potentially include threat detection and threat warning.

Electronic properties and sensitivity of metallic bolometers were studied as a function of thin-film thickness in the active area. Our devices are made of platinum and chromium, with an active area of lateral dimensions 1 μm by 300 nm. The thickness of the metallic film was varied between 3.3 nm and 82.3 nm. Temperature coefficient of resistance and resisitvity were characterized, and are respectively increasing and decreasing with the thickness increasing. A threshold thickness of 40 nm is revealed where both parameters reach a constant value. Responsivity and detectivity were evaluated, unveiling the importance of 1/f noise. Responsivity reaches a maximum value of 2×10⁵ V.W^-1 for bolometers with a 7.5 nm thickness. Detectivity keeps a constant value of 1×10⁸ cmHz^1/2/W for samples thicker than 40 nm, before dropping considerably as the thickness is decreased. This loss in detectivity is believed to be due to the prominence of 1/f noise in such thin samples.

In this paper, by comparing the fluorescent spectrum of the multi-alkali cathode with a surface layer of Cs-Sb and that of the multi-alkali cathode without a surface layer of Cs-Sb, we find that the peak wavelength of the fluorescent spectrum of the multi-alkali cathode with a surface layer of Cs-Sb moves towards the SW, and the fluorescent peak is enhanced. This phenomenon shows that after a Cs-Sb surface layer is produced on the multi-alkali cathode Na2KSb basic layer, the work function of multi-alkali cathode decreases; besides, the structure of the Na2KSb basic layer is changed. This means that with the incident light of the same power and the same frequency, the Na2KSb basic layer that has gone through the Cs-Sb surface processing can produce more transition electrons, with higher transition levels, more possibilities of escape from the surface, and greater sensitivity of cathode obtained. Therefore, after the Na2KSbcathode film has experienced the Cs-Sb surface activation, the spectral response increases not only because the work function on the surface decreases, but also because the internal energy band structure of the Na2KSb film is changed. After the Na2KSb cathode film has gone through the Cs-Sb activation, there will be a layer of Cs-Sb film on the surface. Stress existing between the two films result in lattice distortion of the Na2KSb film. As a result, the energy band structure is changed. To further improve the sensitivity of the multi-alkali cathode, we need further reduce the work function of the multi-alkali cathode, and further perfect the performance of the Na2KSb basic layer, so that the incident light with the same power and frequency can generate more transition electrons with higher transition levels, which requires constant process improvement of the process and the Na2KSb material performance.

In this work, we have determined the dielectric and conductance properties of multi-wall carbon nano-tubes (MWCNT) in polyvinylidene fluoride (PVDF) nanocomposite thin films as a function of temperature and frequency. Samples, ranging from 15 - 280 microns in thickness, were measured in the temperature range from 21 to 90°C and in frequencies from 50Hz to 110MHz. The samples were prepared by the solution casting technique. Measures indicate that at constant temperatures, the real dielectric constant decreases at lower frequencies, stays steady at low frequencies but rise at higher frequencies over towards the strong resonance. The dielectric loss, a particular concern as it is inversely related to the conductance, decreases also at lower frequencies but rise at higher frequencies with a steeper slope in each case. Additionally, we have measured the pyroelectric coefficient in the same temperature range, compared the pyroelectric coefficient results with previous measures made on silver nanoparticle in PVDF thin films and provided preliminary evidence of the causative microscopic response mechanism. Our MWCNT:PVDF thin films yield higher figures of merit than that indicated by pure PVDF thin films and results indicate a usage of MWCNT:PVDF thin films in infrared uncooled sensors and vidicon technology.

In recent years, due to the progression of the semiconductor industrial, the uncooled Infrared sensor – microbolometer has opened the opportunity for achieving low cost infrared imaging systems for both military and commercial applications. Therefore, various fabrication processes and different materials based microbolometer have been developed sequentially. The cytochrome c (protein) thin film has be reported high temperature coefficient of resistance (TCR), which is related to the performance of microbolometer directly. Hence the superior TCR value will increase the performance of microbolometer. In this paper, we introduced a novel fabrication process using aluminum which is compatible with the Taiwan Semiconductor Manufacture Company (TSMC) D35 2P4M process as the main structure material, which benefits the device to integrate with readout integrated circuit (ROIC).The aluminum split structure is suspended by sacrificial layer utilizing the standard photolithography technology and chemical etching. The height and thickness of the structure are already considered. Besides, cytochrome c solutions were ink-jetted onto the aluminum structure by using the inkjet printer, applying precise control of the Infrared absorbing layer. In measurement, incident Infrared radiation can be detected and later the heat can be transmitted to adjacent pads to readout the signal. This approach applies an inexpensive and simple fabrication process and makes the device suitable for integration. In addition, the performance can be further improved with low noise readout circuits.

We present a model and a signal-processing algorithm for compensating the nonuniformity (NU) noise and surrounding temperature self-heating e ects on the response of uncooled microbolometer-based infrared cameras. The model for the NU noise considers pixelwise gain and o set parameters. The representation for the self-heating dynamics of the camera is an autoregressive moving average (ARMA) model for camera's internal temperature. The algorithm performs initially a two-point calibration at a known surrounding temperature. Next, without modifying the NU parameters, we dynamically compensate variations in the camera readout using both estimates of the ARMA model and measurements of the surrounding temperature taken by a simple sensor embedded in the camera. Tested on a CEDIP Jade UC33 camera, our system compensates reference black-body images at 30 degrees Celsius, with a peak error below 1.3 and a mean error below 0.3 degrees Celsius, in scenarios where the room temperature varied up to 14 degrees Celsius. Moreover, the regularity and simplicity of the algorithm enables us to implement it on embedded digital hardware, thereby reducing its cost, size, and power consumption. We implemented the algorithm on a Xilinx XC6SLX45 FPGA using xed-point arithmetic. The circuit exhibits an arithmetic error of 0.06 degrees compared to a software double-precision implementation. It compensates 320 × 240-pixel video at up to 1,437 fps and 640 × 480-pixel video at up to 360 fps, using 1% of the logic resources of the FPGA, and less than 1 mW of dynamic power at 110 MHz. Adding Gigabit Ethernet communication, HDMI display, and a pseudocolor map on the chip uses 10% of the resources and consumes 915 mW.

Readout integrated circuits (ROICs) to support space-based infrared detection applications often have severe radiation tolerance requirements. Radiation hardness-by-design (RHBD) significantly enhances the radiation tolerance of commercially available CMOS and custom radiation hardened fabrication techniques are not required. The combination of application specific design techniques, enclosed gate architecture nFETs and intrinsic thin oxide radiation hardness of 180 nm process node commercial CMOS allows realization of high performance mixed signal circuits. Black Forest Engineering has used RHBD techniques to develop ROICs with integrated A/D conversion that operate over a wide range of temperatures (40K-300K) to support infrared detection. ROIC radiation tolerance capability for 256x256 LWIR area arrays and 1x128 thermopile linear arrays is presented. The use of 130 nm CMOS for future ROIC RHBD applications is discussed.

In this work, an application of electrostatically tunable optical infrared filters in a closed-loop control system is presented. The filters are based on a Fabry-Pérot architecture, fabricated in a bulk micromachining process. Compared to surface micromachined devices, this design opens a path to higher optical performance due to the high planarity and low roughness of substrates but also introduces the drawback of acceleration sensitivity because of a moving mass. To overcome this problem, the filter is driven by a closed-loop control system, with feedback from a charge amplifier that measures the capacitance of the control electrodes. A PI-based controller that adapts to nonlinear stiffness and damping by real-time calculation of the deflection dependent controller parameters is developed. Basic system parameters are derived from optical calibration measurements and step-response results. At 5 Hz sinusoidal accelerations of 1 g, the filter wavelength accuracy can be improved from ±35 nm in open-loop case down to ±2 nm in closed-loop operation.

InAs/GaSb type-II superlattices (T2-SLs) are of great interest as they provide a lot of band engineering flexibility. A wide variety of unipolar barrier structures have been investigated with this material system. In this report, we will present our recent work on the development of low noise long-wave infrared (LWIR) InAs/GaSb T2-SLs photodetectors. By adopting a so-called pBiBn design, the dark current of LWIR photodetectors is greatly suppressed. The LWIR pBiBn device has demonstrated a dark current density as low as 1.42×10^-5 A/cm² at -60 mV, and R₀A of 5365 Ωcm² at 76 K. A peak detectivity at 7.8 μm of 7.7×10¹¹ cmHz^1/2W^-1 is obtained at 76 K. Further effort to reduce the operating bias is also reported. By refining the energy-band alignment, a 2-μm-thick LWIR pBiBn device has demonstrated a single pass (no AR coating) quantum efficiency of 20% at 10 μm under zero-bias at 77 K. We have recently extended our efforts to further reduce the dark current by using an interband cascade (IC) photodetector structure. Some further details about the device operation and results will be discussed.

Recently, a new strategy used to achieve high operation temperature (HOT) infrared photodetectors including cascade devices and alternate materials such as type-II superlattices has been observed. Another method to reduce detector’s dark current is reducing volume of detector material via a concept of photon trapping detector.

In this paper, the performance of a novel HOT detector designing so-called interband cascade type-II MWIR InAs/GaSb superlattice detectors is presented. Detailed analysis of the detector’s performance (such as dark current, RA product, current responsivity, and response time) versus bias voltage and operating temperatures (220 – 400 K) is performed pointing out optimal working conditions. At present stage of technology, the experimentally measured R₀A values of interband cascade type-II superlattice detectors at room temperature are higher than those predicted for HgCdTe photodiodes. It is shown that these novel HOT detectors have emerged as competitors of HgCdTe photodetectors.

In this paper we propose an analytical approach of describing the tuning properties of the MG-Y laser by means of analytical description of the wavelength dependence of the three tuning currents. The model is based on the theoretical analysis and it was experimentally verified. The results of experiments are included in the second part of the paper. Finally, we conclude with the discussion on the model precision and applications.

This work presents preliminary results on wavelength sensitivity due to mechanically induced long period fiber grating (LPFG) on both standard single-mode and Er-doped fibers. The work presents and compares results for both types of fibers under different torsion conditions. In order to apply the torsion one of the fiber ends is fixed while torsion is applied on the other end. A LPFG whose period is 503μm is used to press on the fiber after the torsion, this will allow for micro curvatures to be formed on the fiber, which will in turn generate a periodical index perturbation on it. Here, it was noted that the rejection band shifts to shorter wavelengths for Er-doped fibers. It was detected that for torsion of 6 turns applied to 10cm doped fiber the wavelength peaks can shift up to 25nm, which is longer than similar results reported on standard fibers. Therefore, by using Er-doped fibers this technique will give more sensitive and accurate results on the real conditions of the structure under study. These results can be employed for sensing applications, especially for small to medium size structures, being these structures mechanical, civil or aeronautical. Theoretical calculations and simulations are employed for experimental results validation.

Electro-optical/infrared nanosensors are being developed for a variety of defense and commercial systems applications. One of the critical technologies that will enhance EO/IR sensor performance is the development of advanced antireflection coatings with both broadband and omnidirectional characteristics. In this paper, we review our latest work on high quality nanostructure-based antireflection structures, including recent efforts to deposit nanostructured antireflection coatings on large area substrates. Nanostructured antireflection coatings fabricated via oblique angle deposition are shown to enhance the optical transmission through transparent windows by minimizing broadband reflection losses to less than one percent, a substantial improvement over conventional thin-film antireflection coating technologies. Step-graded antireflection structures also exhibit excellent omnidirectional performance, and have recently been demonstrated on 6-inch diameter substrates.

There is a need to assess whether different strategies can be evolved for developing EO/IR systems to support the generation of space-variant multi-domain, signature discriminating imagery without the penalty imposed by the generation and communication of large datasets. In addition it would be beneficial if such strategies could avoid challenges in relation to complexity and the cost of manufacture for both the defence and medical arenas. This paper revisits some of the constructs exploited in biological imaging systems, notably those found in the family of stomatopod crustaceans and assesses the feasibility of pursuing some of the ideas generated from that study using novel comb filter technologies in the context of emerging focal plane and optical system technologies.

Due to the ever increasing complexity of novel semiconductor systems, it is essential to possess design tools and simulation strategies that include in the macroscopic device models the details of the microscopic physics and their dependence on the macroscopic (continuum) variables. Towards this end, we have developed robust multi-scale modeling capabilities that begin with modeling the intrinsic semiconductor properties. The models are fully capable of incorporating effects of substrate driven stress/strain and the material quality (dislocations and defects) on microscopic quantities such as the local transport coefficients and non-radiative recombination rate. Using this modeling approach we have extensively studied UV APD detectors and infrared focal plane arrays. Particular emphasis was placed on HgCdTe and InAsSb arrays incorporating photon trapping structures as well as two-color HgCdTe detectors arrays.

We present a new material platform for uncooled bolometric infrared detection, consisting of a composite membrane of carbon nanotubes and a non-conductive, volume phase-change polymer. Devices using this platform have achieved temperature coefficients of resistance (TCR) in excess of - 40%/K at 300 K, an order of magnitude larger than commercial materials.

High-resolution imaging in ultraviolet (UV) bands has many applications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. In past work, UV avalanche photodiodes (APDs) have been reported as discrete devices demonstrating gain. The next frontier is to develop UVAPD arrays with high gain to demonstrate highresolution imaging. We will discuss a model that can predict sensor performance in the UV band using APDs with various gain and other parameters for a desired UV band of interest. Signal-to-noise ratios (SNRs) can be modeled from illuminated targets at various distances with high resolution under standard atmospheric conditions in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APDs. We will present recent data on the GaN-based APDs for their gain, detector response, dark current noise, and 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APDs in wide-bandgap semiconductors. The paper will also discuss the state of the art in UVAPDs and the future directions for small unit cell size and gain in the APDs.

A system of angle-Doppler resolved reflective tomography imaging Lidar and its algorithm are given. The issue between transverse range resolution and the sampling time of single angle is solved. The condition of far-field diffraction transmission in the laboratory is designed, and the angle-Doppler reflective projections of the target are collected. Filtered back-projection algorithm is used to reconstruct the cross section of the target image. Because of the utilization of coherent detection of coaxial beams, both the imaging signal to noise ratio and the receiving sensitivity are improved. Due to the simplification in configuration and operations without involving signal phase processing, this technique has a great potential for applications in extensive imaging Lidar fields.

Reflective tomography is one of the most promising high-resolution imaging methods for the remote objects. But in practical application, because of the sampling angle error and limited view of projections in signal collecting process, anisotropic resolution is inevitable and the reconstruction image quality of reflective tomography system degrades. In this paper, the theoretical imaging resolution derived from Fourier-Slice theorem is presented, computer simulation and experimental verification are also given. Imaging analysis in this paper will make a complement and perfection of the theory in reflective tomography imaging ladar.