David A. Cardimona

Design issues relating to low-temperature dark current in quantum well infrared photodetectors

Quantum well infrared photodetectors (QWIPs) have emerged as a viable contender for many remote sensing applications, even in the space environment where low background fluxes are involved. There are, however, several issues that still need to be resolved in order to achieve optimum performance at low operating temperatures. One important issue is the dark current, which is dominated by Fowler-Nordheim and trap-assisted tunneling in this regime. In order to decrease the dark current of QWIPs at low temperatures, we are investigating these mechanisms to better understand them. Our preliminary investigations have uncovered another possible problem; offsets in the I-V characteristics which could impair the compatibility of a QWIP array with a readout circuit. In this paper we discuss these design issues.

Power-dependent dispersion in far-off-resonant raman scattering

We investigate the saturation effects of power broadening, Stark shifting, and population transfer on Stokes conversion in stimulated Raman scattering. We do not make the usual rotating wave approximation because the detuning from the next electronic state is assumed to be in the optical regime. Retaining the counter-rotating terms allows an exact determination of the pump and Stokes indexes of refraction. Steady-state solutions for the Stokes intensity and phase are obtained and the effects of making the rotating wave approximation (RWA) are discussed. Finally, we examine the behavior of these solutions for Stokes conversion in hydrogen gas when geometric propagation is appropriate.

Experimental investigation of the performance of GaAs/AlGaAs quantum-well infrared photodetectors from the Industrial Microelectronics Ctr., Sweden

In this paper we present an experimental investigation of the performance of GaAs/AlGaAs multiple quantum well photodetectors. The purpose of this exercise was to independently evaluate and verify the responsivity of the GaAs/AlGaAs quantum well infrared photodetectors developed at the Industrial Microelectronics Center in Sweden. These devices use 2D gratings to couple radiation into the detectors and a cladding layer to enhance the coupling of radiation. The devices were of two types: those optimized for high detectivity, and those optimized for high quantum efficiency. The tests performed on these devices included measurement of optical responsivity vs. bias, spectral response, Detectivity (D*), and measurement of cross-talk between pixels. Several interesting observations were made during the investigation and will be reported in the paper.

Effects of charge accumulation and biasing on resonant tunneling energies and tunneling dynamics

We introduce charge accumulation in quantum wells through the use of a nonlinear Schrodinger equation. Looking first at infinite and finite square wells allows us to calculate the new energy spectrum including the separate effects of a biasing electric field and charge accumulation. This gives us insight into the new resonant tunneling energies that arise due to the quasibound states being shifted by either the external field or the reaction field built up through the accumulation of charge. Using a double barrier potential, we calculate the transmission coefficient with and without the external bias field and then with charge accumulation. To study the tunneling dynamics, we first start with a single barrier in an infinite well and discover a fractal-like character to the probability for finding an electron wavepacket in one side of the structure. Finally we numerically integrate the full time- dependent nonlinear Schrodinger equation with various barrier potentials to obtain the dynamics of a wavepacket incident on the structures.