Michael J. Cich

A spatial light modulator for terahertz beams

We design and implement a multipixel spatial modulator for terahertz beams using active terahertz metamaterials. Our first-generation device consists of a 4×4 pixel array, where each pixel is an array of subwavelength-sized split-ring resonator elements fabricated on a semiconductor substrate, and is independently controlled by applying an external voltage. Through terahertz transmission experiments, we show that the spatial modulator has a uniform modulation depth of around 40% across all pixels, and negligible crosstalk, at the resonant frequency. This device can operate under small voltage levels, at room temperature, with low power consumption and reasonably high switching speed.

Fiber-Optically Controlled Pulsed Power Switches

The development and testing of fiber-optically controlled trigger generators (TGs) based on high gain photoconductive semiconductor switches (PCSSs), constructed from high resistivity GaAs, are described in this paper. The TGs are optimized to trigger the high voltage switches (HVSs) in pulsed power systems, where they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future pulsed power systems are even more dependent on triggering, as they consist of many more HVS and, in some cases, produce shaped pulses by independent timing of the HVS. The goal of the PCSS TG is to improve timing precision and replace high voltage trigger cables or line-of-sight optics with fiber-optic trigger control. The PCSS trigger has independent EMP-free timing control via 200-mum-diameter optical fibers. This design is simpler than other TG because optical isolation allows PCSS triggers to be remotely located near the HVS at any voltage. PCSS can improve the performance of prime power HVS, diverters, and diagnostics by supplying trigger pulses with subnanosecond jitter and rise time that are more precise and easily adjusted than the conventional TG. For pulse-charged HVS, the PCSS TG can generally derive their trigger energy from the stray fields of the HVS. High gain PCSS capabilities for producing pulsed power TG have been demonstrated previously (not all simultaneously): 220 kV, 8 kA, 350-ps rise time, 100-ns pulsewidth, 50-ps rms jitter, and 10-kHz repetition rate. Furthermore, PCSS has previously triggered a 300-kV trigatron with 100-ps rms jitter.

Comparison of nBn and nBp mid-wave barrier infrared photodetectors

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.

Crystallographic dependence of the lateral undercut wet etching rate of InGaP in HCl

The crystallographic dependence of the lateral etch rate in 12 M HCl of InGaP lattice matched to GaAs has been measured. The etch rate at 20 °C is found to have twofold rotational symmetry about [100] and varies between <0.01 μm/min for mesas oriented along 〈011〉 directions and ∼0.9 μm/min for mesas 55° and 125° from [011] towards [011]. Etch fronts consist of {111}A planes. The etch rate also depends on the direction of etch step flow, suggesting that reconstruction plays an important role during InGaP wet etching.

Mesa-isolated InGaAs photodetectors with low dark current

We demonstrate InGaAs photodiodes with an epitaxial heterostructure that allows simple mesa isolation of individual devices with low dark current and high responsivity. An undoped InAlAs barrier and passivation layer enables isolation of detectors without exposing the InGaAs active region, while simultaneously reducing electron diffusion current. Photodetectors with mesa sizes as small as 25×25 μm2 exhibit dark current densities of 10 nA/cm2 at 295 K and responsivities of 0.62 A/W at 1550 nm.

Pulsed- and DC-Charged PCSS-Based Trigger Generators

Prior to this research, we have developed high-gain, GaAs, photoconductive semiconductor switches (PCSSs) to trigger 50–300 kV high voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed power switches operating at 50–300kV by locating the trigger generator directly at the HVS. This was demonstrated for two types of DC-charged trigatrons and two types of field distortion mid-plane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute (HCEI) used in the linear transformer driver. The lowest rms jitter obtained from triggering a HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed, fiber-optically controlled, low jitter trigger generators (TGs) for HVSs. TGs are critical sub-systems for reliable, efficient pulsed power facilities because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future facility scale pulsed power systems are even more dependent on triggering, as they consist of many more triggered HVSs and produce shaped-pulses by independent timing of the HVSs. As pulsed power systems become more complex, the complexity of the associated trigger systems also increases. One means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or DC charged pulsed power systems this can be particularly challenging as the DC hold off of the PCSS dramatically declines. This paper presents results seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed charged PCSS based TG. Switch operating conditions as low as 45% of self break were achieved. A DC charged PCSS based TG is also introduced and demonstrated over a 39 kV – 61 kV operating range. DC charged PCSS allow the TG to be directly charged from slow or DC charged pulsed power systems. GaAs PCSSs and neutron irradiated GaAs (n-GaAs) PCSSs were used to investigate the DC charged operation.

Terahertz quantum cascade laser integration with on-chip micromachined rectangular waveguides

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.

Integrated chip-scale THz technology

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.

DC-Charged GaAs PCSSs for Trigger Generators and Other High-Voltage Applications

The demand for greater flexibility and increased energy density in pulsed-power systems is moving highly interactive components closer together. The development of compact technologies for less complex and more robust system designs is critical. A key system component that can impact these goals is the trigger generator (TG). Inexpensive, compact, and fiber-optically controlled TGs that deliver trigger pulses with subnanosecond jitter have been created with photoconductive semiconductor switches (PCSSs). However, high-voltage (HV) GaAs PCSSs are typically pulsed charged for less than 100 s so that they can hold off 60-100 kV/cm without self-triggering into high-gain (lock-on) switching or initiating surface flashover. Since many new pulsed-power system designs are based on dc-charged HV switches, pulse charging the trigger system is an additional complication requiring space, HV switching components, and HV cables. A further improvement in PCSS-based TG is to move from pulsed to dc-charged PCSSs. This paper reports results from dc-charged GaAs PCSSs with 0.25-1.0 cm gaps, extending previously reported results on smaller devices at 3 kV to a new regime of 100 kV. To hold off high fields for longer periods and to extend GaAs PCSSs to dc applications, we have utilized neutron-irradiated GaAs (n-GaAs). Neutron irradiation in GaAs increases the defect density, shortens the carrier recombination time, and (for devices with large insulating regions) reduces the dark current, which improves the dc hold-off strength. PCSS contacts in this research were created using rapid thermal annealing (RTA) to produce high adhesion and low contact resistance. However, this can reduce the defect density near the contacts by annealing some of the n-induced defects. Hence, a range of RTA temperatures and neutron doses was studied to understand the tradeoff space for contact adhesion and dc hold-off. This paper presents results from I-V characterization and dc hold-off on irradiated and nonirradiated GaAs PCSSs. These PCSS devices were demonstrated to hold off fields of 39-61 kVDC/cm, respectively. Irradiation doses over a range of 3×1013 - 1×1015 (1 MeV Si equivalent) were explored in search of the optimal performance. Additionally, the impact of the fabrication processes on the benefits of irradiation is explored, and the observation of unusual low-frequency oscillations during GaAs I-V testing is discussed.

Pulsed and DC charged PCSS based trigger generators

Prior to this research, we have developed high-gain, GaAs, photoconductive semiconductor switches (PCSSs) to trigger 50–300 kV high voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed power switches operating at 50–300kV by locating the trigger generator directly at the HVS. This was demonstrated for two types of DC-charged trigatrons and two types of field distortion mid-plane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute (HCEI) used in the linear transformer driver. The lowest rms jitter obtained from triggering a HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed, fiber-optically controlled, low jitter trigger generators (TGs) for HVSs. TGs are critical sub-systems for reliable, efficient pulsed power facilities because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future facility scale pulsed power systems are even more dependent on triggering, as they consist of many more triggered HVSs and produce shaped-pulses by independent timing of the HVSs. As pulsed power systems become more complex, the complexity of the associated trigger systems also increases. One means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or DC charged pulsed power systems this can be particularly challenging as the DC hold off of the PCSS dramatically declines. This paper presents results seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed charged PCSS based TG. Switch operating conditions as low as 45% of self break were achieved. A DC charged PCSS based TG is also introduced and demonstrated over a 39 kV – 61 kV operating range. DC charged PCSS allow the TG to be directly charged from slow or DC charged pulsed power systems. GaAs PCSSs and neutron irradiated GaAs (n-GaAs) PCSSs were used to investigate the DC charged operation.