William A. Beck

Current status of quantum well focal plane arrays

The state of the art in quantum well infrared photodetectors (QWIPs) is reviewed, with emphasis on the current status of QWIP focal plane arrays (FPAs). These FPAs are progressing rapidly in size from 256 by 256 to 640 by 480 pixels, and typically have temporal and spatial noise below 20 mK when operated up to 77 K in an f/1.7 camera system. Frames from videotapes recorded with recent FPAs are presented.

Device physics and focal plane array applications of QWIP and MCT

Infrared sensor technology is critical to many commercial and military defense applications. Traditionally, cooled infrared material systems such as indium antimonide, platinum silicide, mercury cadmium telluride, and arsenic doped silicon (Si:As) have dominated infrared detection. Improvement in surveillance sensors and interceptor seekers requires large size, highly uniform, and multicolor IR focal plane arrays involving medium wave, long wave, and very long wave IR regions. Among the competing technologies are the quantum well infrared photodetectors based on lattice matched or strained III-V material systems. This paper discusses cooled IR technology with emphasis on QWIP and MCT. Details will be given concerning device physics, material growth, device fabrication, device performance, and cost effectiveness for LWIR, VLWIR, and multicolor focal plane array applications.

Laboratory and field imaging test results on single-color and dual-band QWIP focal plane arrays

Abstract We report on recent laboratory and field measurements on quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). The results of laboratory measurements of imaging performance such as noise-equivalent temperature difference (NEΔT), minimum resolvable temperature, conversion efficiency, uniformity of response and dark current and their dependence on operating temperature are presented on large format (640×480 pixels) single-color long-wavelength infrared (LWIR) and dual-band (256×256 pixels) mid-wavelength infrared (MWIR)/LWIR arrays. We found that under optimum operating conditions (65 K and 1 V bias), the NEΔT of the LWIR FPA was approximately 30 mK and was limited by the optics and noise from the closed-cycle cooler. We will show field imagery of military targets taken with this FPA. In addition, field imagery taken with the LWIR QWIP camera of a large-transient event will be compared with that taken with an InSb MWIR camera. We will also show imagery of a recent total eclipse of the moon. We will also present laboratory results on a simultaneously integrating, pixel-registered dual-band FPA showing excellent operability and response uniformity with NEΔT of approximately 30 mK in both bands (T=60 K). The dual-band FPA was also taken to the field and we will present imagery of various targets acquired with this FPA.

Generalized analysis of stability and numerical dispersion in the discrete-convolution FDTD method

A simple technique is described for determining the stability and numerical dispersion of finite-difference time-domain (FDTD) calculations that are linear, second-order in space and time, and include dispersion by the discrete convolution method. The technique is applicable to anisotropic materials. Numerical examples demonstrate the accuracy of the technique for several anisotropic and/or dispersive materials.

Microstrip antenna coupling for quantum-well infrared photodetectors

Abstract We computed the expected performance of microstrip antennas as optical couplers in quantum-well infrared photodetectors (QWIPs) by the finite-difference time-domain technique. The microstrip antenna consisted of a metal square that is placed on one side of a thin QW stack, across from a continuous ground plane on the other side of the stack. The size of the square was one-half wavelength in the QW material, or ∼1.2 μm for a long-wavelength GaAs/AlGaAs QWIP. An array of these antennas can be used to couple to an arbitrary-size pixel. Unlike the elements of a diffraction grating, each antenna in the array acts as a nearly independent coupler whose peak wavelength is determined by the dimensions of the antenna, not by the period of the antenna array. Because of this local coupling, the array of microstrip antennas maintains absorption in small pixels much more effectively than do grating couplers. Furthermore, the microstrip antenna yields moderately high absorption over a useful spectral bandwidth with a very thin QW stack. Such a stack has a high photoconductive gain, which is useful for many applications.

Uncooled infrared detectors and focal plane arrays

Over the past several years, uncooled IR detectors and focal plane arrays have been rapidly developed. Impressive progress has been made in both resistive microbolometers and pyroelectric thin-film detectors with noise equivalent temperature differences projected to be 10 to 20 mK with F/1 optics for such structures. Noise equivalent temperature of 50 mK bulk pyroelectric detectors and thin film resistive microbolometers are already demonstrated and in production. Other novel schemes, such as bimaterial capacitors, are also promising for uncooled IR detection. The US Army Research Laboratory is involved in developing ferroelectric materials to take advantage of the pyroelectric properties. The goal is to develop crystal oriented thin films to further improve detector performance. In this presentation, the operating principle of resistive microbolometers and pyroelectric detectors, and recent progress of uncooled RI focal plane arrays are discussed. In addition, the uncooled RI detector program at the Army Research Laboratory, that includes research facilities for and research efforts toward uncooled detectors and focal plane arrays is presented.

Optical absorption modeling of thermal infrared detectors by use of the finite-difference time-domain method.

The optical absorption of thin-film thermal infrared detectors was calculated as a function of wavelength, pixel size, and area fill factor by use of the finite-difference time-domain (FDTD) method. The results indicate that smaller pixels absorb a significantly higher percentage of incident energy than larger pixels with the same fill factor. A polynomial approximation to the FDTD results was derived for use in system models.

Enhanced Infrared Absorption by Ferroelectric-Conducting Oxide Thin-Film Structures

Infrared absorbing free-standing bridge structures are anticipated to be used in the next generation of ferroelectric uncooled infrared detectors. These structures will consist of resonant cavities with an absorbing bridge element, which contains the ferroelectric detector and the contact electrodes. An oxide electrode is preferable to a metallic electrode, because the metallic electrode reflects too much of the incoming radiation at reasonable thickness. For this paper, spectral transmission and reflection of lanthanum nickelate (LNO), barium ruthenate (BRO) and strontium ruthenate (SRO) films were measured, and it was found that the properties could be described as a simple two-dimensional conducting sheet. The optical absorption of several free-standing bridge structures was modeled, and optimized dimensions and electrode conductivity are presented which yield greater than 80% absorption over the full 8-14 w m band.

Passive Infrared Sensing Using Plasmonic Resonant Dust Particles

We present computational and experimental results of dust particles that can be tuned to preferentially reflect or emit IR radiation within the 8–14 μm band. The particles consist of thin metallic subwavelength gratings patterned on the surface of a simple quarter wavelength cavity. This design creates distinct IR absorption resonances by combining the plasmonic resonance of the grating with the natural resonance of the cavity. We show that the resonance peaks are easily tuned by varying either the geometry of the grating or the thickness of the cavity. Here, we present a computational design algorithm along with experimental results that validate the design methodology.

Passive infrared sensing using plasmonic resonant dust particles

In this paper we present computational and experimental results of dust particles that can be tuned to preferentially reflect or emit IR radiation within the 8-14 μm band. The particles consist of thin metallic subwavelength gratings patterned on the surface of a simple quarter wavelength cavity. This design creates distinct IR absorption resonances by combining the plasmonic resonance of the grating with the natural resonance of the cavity. We will show that the resonance peaks are easily tuned by varying either the geometry of the grating or the thickness of the cavity. Here, we present a computational design algorithm along with experimental results that validate the design methodology.