Thomas M. Brennan

Radiation response of optically-triggered GaAs thyristors

Gallium arsenide optically triggered thyristors that exhibit tolerance to high X-ray dose rates have been fabricated. These two-terminal epitaxial devices feature breakover voltages of 18 V to 38 V with no radiation. They trigger at less than 2 V with only tenths of a milliwatt of laser light, but they do not trigger at 2*10/sup 9/ rad(Si)/s with a bias level as much as 40% to 60% of the zero-radiation breakover voltage. When these devices are bombarded with neutrons, the reduced carrier lifetimes result in a decreased sensitivity to triggering by light and ionizing radiation. The thyristors show some decrease in optical and X-ray sensitivity upon neutron exposure in excess of 10/sup 13/ n/cm/sup 2/. At 10/sup 14/ n/cm/sup 2/, there are major reductions in photon sensitivity, and some instability is observed. >

Optically-triggered GaAs thyristor switches: Integrated structures for environmental hardening

Opticallytriggered thyristor switches often operate in adverse environments such as high temperature and high dose-rate transient radiation which can result in lowered operating voltage and premature triggering. These effects can be reduced by connecting or monolithically integrating a reverse-biased compensating photodiode or phototransistor into the gate of the optically-triggered thyristor. We have demonstrated the effectiveness of this hardening concept in silicon thyristors packaged with photodiodes and in gallium arsenide optically-triggered thyristors monolithically integrated with compensating phototransistors.

Surface-emitting semiconductor laser spectroscopy and microscopy for characterizing normal and sickled red blood cells

We have developed a new intracavity laser technique that uses living or fixed cells at integral components of a laser. The cells are placed on an AlGaAs/GaAs surface-emitting semiconductor wafer and covered with a glass dielectric mirror to form a laser resonator. In this arrangement, the cells serve as optical waveguides (or lens elements) to confine (or focus) light generated in the resonator by the semiconductor. Because of the high transparency, the cells aid the lasing process to generate laser light. This ultra sensitive laser provides a novel imaging/spectroscopic technique for histologic examination which we demonstrate with normal and sickled human red blood cells. Extremely high contrast microscopic images of the cells are observed near 830-850 nm. These images correspond to electromagnetic modes of cell structures and are sensitive to shape of the cell. Using a high resolution spectrometer, we resolve the light emitted from these images into very narrow spectral peaks associated with the lasing modes. Analysis of the spectra reveals that the distribution of peaks is quite different for normal and sickled red blood cells. This technique, in a more developed form, may be useful for the rapid analysis of these and other kinds of normal and abnormal cells.

Optically and electrically addressable surface-emitting laser logic

In the future optoelectronic integrated circuits (OEICs) are destined to evolve into sophisticated functional circuitry upon which a multiplicity of applications will be based, such as: optical communications, optical interconnects, optical computing, optical memory, laser printing and scanning, visual displays, pattern recognition, and neural networks. This evolution of OEICs involves the integration of phototransmitters (semiconductor lasers and light-emitting diodes), photoreceivers (photodetectors and phototransistors), spatial-light modulators, transistors (bipolar and field-effect), diodes, resistors and capacitors, and micro- optic components (e.g., micro lenses). We describe our efforts to date and future directions which are concentrated on the integration of vertical-cavity surface-emitting laser diodes (VCSELs) with transistors, photoreceivers, and micro-optic components. VCSELs, which may be patterned in high densities (over a million in a square cm) and emit light perpendicular to the plane of the substrate, have an ideal light-emitting geometry for the above mentioned applications and for integration with micro-optic components. We describe our efforts to develop monolithic surface-emitting laser logic devices, which we refer to as CELLs, consisting of phototransistors, current controlled bipolar transistors, and voltage-controlled field-effect transistors integrated with a VCSEL to form optically and electrically addressable photonic switching devices having high contrast. We also discuss the integration of micro- optics with VCSELs. Finally, we describe combinations of OEIC components and subassemblies and their applications to several of the above mentioned photonic switching applications.

Spatial patterns in semiconductor lasers

A beam propagating in a planar waveguide exhibits the characteristics of both a fundamental soliton (little change in waist of a 5-/spl mu/m beam; diffraction is compensated by self-focusing nonlinear refraction) and a second-order soliton (lO-/spl mu/m beam focuses to 3 /spl mu/m with side peaks). Thus a semiconductor gain medium in a planar waveguide is almost ideal for the demonstration of fundamental and second-order spatial solitons; it is even a candidate medium for the formation of a light bullet, i.e., a soliton in space and time.<<ETX>>

Semiconductor microcavity lasers

New kinds of semiconductor microcavity lasers are being created by modern semiconductor technologies like molecular beam epitaxy and electron beam lithography. These new microcavities exploit 3D architectures possible with epitaxial layering and surface patterning. The physical properties of these microcavities are intimately related to the geometry imposed on the semiconductor materials. Among these microcavities are surface-emitting structures that have many useful properties for commercial purposes. This paper reviews the basic physics of these microstructured lasers.

Power broadening of coherent energy transfer in semiconductor gain media

Coherent energy transfer gives rise to a new peak and dip in the probe gain spectrum that move proportionally with the intracavity injected power, showing that stimulated emission and absorption significantly speed up the semiconductor response.

Injection-induced instabilities and local gain modification in VCSELs

Injection of a cw narrow-band laser beam into a lasing vertical- cavity surface-emitting laser results in the appearance of new frequencies on the way to injection locking as predicted by our theoretical model. Injection also causes a local asymmetric modification of the lasing line, resulting in a new gain peak at a lower frequency and a dip on the high-frequency side. The peak and dip move out directly as the intracavity injected power, as predicted by our quantum mechanical theory.