We have fabricated low-dark-current InGaAs photodetectors utilizing an epitaxial structure incorporating an InAlGaAs passivation layer and a simple mesa isolation process, and requiring no implant or diffusion steps. At 295 K, areal and perimeter dark current contributions are 15 nA/cm<sup>2</sup> and 9 pA/cm, respectively, in devices with large aspect ratios biased at -0.1 V. High responsivity was achieved even at zero bias in these devices. Devices were modeled using a commercial drift-diffusion simulator. Good fits to reverse dark current-voltage measurements were obtained using a model that included both bulk and interfacial generation mechanisms. Assuming similar electron and hole Shockley-Read-Hall lifetimes, dark current under small reverse bias are consistent with generation at the interface between the absorber and underlying layers. With increasing negative bias a large increase in dark current is associated with depletion near the InAlGaAs/absorber interface, while small increases in current at large reverse bias suggest long Shockley-Read-Hall lifetimes in the absorber. Forward biasing of these devices results in efficient injection of minority carrier holes into the absorber region, mimicking photogeneration and providing a method to predict the performance of illuminated detector arrays.
The quantum cascade laser (QCL) is currently the only solid-state source of coherent THz radiation capable of
delivering more than 1 mW of average power at frequencies above
~ 2 THz. This power level combined with
very good intrinsic frequency definition characteristics make QCLs an extremely appealing solid-state solution
as compact sources for THz applications. I will present results on integrating QCLs with passive rectangular
waveguides for guiding and controlling the radiation emitted by the QCLs and on the performance of a THz
integrated circuit combining a THz QCL with a Schottky diode mixer to form a heterodyne receiver/transceiver.
We describe a monolithically integrated THz transceiver consisting of a Schottky diode embedded into a THz
quantum cascade laser (QCL) waveguide. Besides functioning as a heterodyne receiver for externally incident
radiation, the device is a useful tool for characterizing the performance and dynamics of the QCL. Here we
present an overview of the device, demonstrate receiver operation, and present laser dynamics measurements
especially related to feedback of the QCL's emission due to retroreflections.
In this paper, we describe our efforts to control the thermal emission from a surface utilizing structured surfaces with
metal/dielectric interfaces. The goal was not to eliminate the emission, but to control the output direction and spectrum.
We focus on methods that lead to high emissivity at grazing angles, with low emission near normal. We describe the
fabrication and measurement of large passive devices (15×15 mm) and arrays of smaller chips for thermal emission
control in the longwave infrared (8 to 12 micron) spectral region. All the devices consist of a metal base layer covered
with dielectric/metal posts or lines, 0.5 microns tall. The posts (0.9×0.9 micron) and lines (0.3 micron wide) are subwavelength.
One-dimensional and two-dimensional devices with a 3 micron pitch will be shown. The devices are
measured with both a hemispherical directional reflectometer and a variable angle directional emissometer. Both
simulated and experimental results show the thermal emission effectively limited to a small spectral region and grazing
angles from the surface (≥ 80°) in stark contrast to the typical Lambertian radiation seen from unstructured material.
Finally, the effect of this thermal emission control is illustrated using an infrared camera.
We have fabricated mid-wave infrared photodetectors containing InAsSb absorber regions and AlAsSb barriers in
n-barrier-n (nBn) and n-barrier-p (nBp) configurations, and characterized them by current-voltage, photocurrent, and
capacitance-voltage measurements in the 100-200 K temperature range. Efficient collection of photocurrent in the nBn
structure requires application of a small reverse bias resulting in a minimum dark current, while the nBp devices have
high responsivity at zero bias. When biasing both types of devices for equal dark currents, the nBn structure exhibits a
differential resistance significantly higher than the nBp, although the nBp device may be biased for arbitrarily low dark
current at the expense of much lower dynamic resistance. Capacitance-voltage measurements allow determination of the
electron concentration in the unintentionally-doped absorber material, and demonstrate the existence of an electron
accumulation layer at the absorber/barrier interface in the nBn device. Numerical simulations of idealized nBn devices
demonstrate that photocurrent collection is possible under conditions of minimal absorber region depletion, thereby
strongly suppressing depletion region Shockley-Read-Hall generation.
Integration of THz quantum cascade lasers (QCLs) with single-mode 75 μm x 37 μm rectangular waveguide components, including horn antennas, couplers, and bends, for operation at 3 THz has been designed and fabricated using thick gold micromachining. Measurements on the isolated waveguide components exhibit fairly low loss and integration with THz QCLs has been demonstrated. This technology offers the potential for realizing miniature integrated systems operating in the 3 THz frequency range.
Firing systems typically incorporate isolation-based architectures that are established by the safety themes of particular weapon systems. Robust electrical diversion barriers are implemented to isolate energy from detonation-critical components until the event of intended use of the system. An optical trigger assembly is being developed to enhance the safety of new firing systems. It couples a fast trigger signal through an exclusion region barrier without compromising the integrity of the barrier in abnormal environment situations. A laser diode generates an optical pulse that is coupled through a sapphire stub to a photoconductive semiconductor switch (PCSS). The PCSS drives a vacuum switch tube to complete the triggering chain in the firing system. A general discussion and comparison of triggering technology options, and the design characteristics and performance parameters of the specific optical trigger point design are presented in this paper.