I. Lahiri

Laser-based ultrasound detection using photorefractive quantum wells

We demonstrate a laser-based adaptive ultrasonic homodyne receiver using dynamic holography in AlGaAs/GaAs photorefractive multiple quantum wells. The dynamic hologram acts as an adaptive beamsplitter that compensates wavefront distortions in the presence of speckle and requires no path-length stabilization. The photorefractive quantum wells have the unique ability to achieve maximum linear homodyne detection regardless of the value of the photorefractive phase shift by tuning the excitonic spectral phase. We achieve a root mean square noise-equivalent surface displacement of 6.7×10−7 A(W/Hz)1/2.

Photorefractive p‐i‐n diode quantum well spatial light modulators

We demonstrate the performance of all‐semiconductor photorefractive p‐i‐n diodes operating in the longitudinal quantum‐confined Stark geometry. Low‐temperature‐grown shallow quantum wells provide high‐mobility vertical transport, and potential steps incorporated into the semiconductor buffer layers increase the transit time across the buffer and therefore increase the quantum efficiency for trapping of charge before it is swept out to the doped p‐type and n‐type contacts. The buffer design and the doped contacts both make all‐semiconductor photorefractive devices possible, with peak transient output diffraction efficiencies approaching 3%, but without the need for dielectric insulating layers. We also redefine device speed by making a distinction between transient rise times and frequency response, showing that in these p‐i‐n devices the update rate is an order of magnitude slower than the inverse rise time.

High‐efficiency Stark‐geometry photorefractive quantum wells with intrinsic cladding layers

Output diffraction efficiencies approaching 40% and input diffraction efficiencies approaching 3% have been achieved in photorefractive p‐i‐n quantum well diodes operating in the longitudinal Stark geometry. The device structure consists of a low‐temperature‐grown multiple quantum well isolated from the doped contacts by an intrinsic standoff layer of Al0.5Ga0.5As. All charge trapping and screening occurs within the quantum wells without the need for trapping in specialized buffer layers used in previous designs. This new design operates at lower voltages (18 V) and lower fields (6 V/μm) than previously demonstrated.

Ultrafast‐lifetime quantum wells with sharp exciton spectra

Sharp quantum‐confined excitons in semi‐insulating low‐temperature‐growth AlAs/GaAs quantum wells with 15 ps carrier lifetimes are demonstrated. High‐quality well‐barrier interfaces can be grown at low substrate temperatures and annealed up to temperatures of 700 °C, beyond which interface mixing broadens the exciton transitions. Electroabsorption from the quantum‐confined Stark effect in as‐grown modulators approaches 10 000 cm−1, which is comparable to traditional high‐temperature growth quantum wells. The low‐temperature growth quantum well structures eliminate the need for postgrowth processing, such as ion implantation for photorefractive quantum wells, ultrafast saturable absorption, or electro‐optic sampling applications.

THE ROLE OF EXCESS ARSENIC IN INTERFACE MIXING IN LOW-TEMPERATURE-GROWN ALAS/GAAS SUPERLATTICES

Undoped low‐temperature‐grown AlAs/GaAs superlattices experience pronounced interface intermixing with increasing anneal temperatures up to 900 °C. Quantum confinement shifts caused by intermixing of low‐temperature‐grown and standard‐temperature‐grown superlattices were studied using electromodulation spectroscopy. The effective activation energy for intermixing in the low‐temperature‐grown superlattices during 30 s isochronal postgrowth anneals was found to be (0.32±0.04) eV, anomalously smaller than for standard‐temperature‐grown superlattices. Roughening of the interfaces caused by arsenic precipitates accompanies the intermixing. Qualitative features of the intermixing have been confirmed using high resolution transmission electron microscopy and studies on x‐ray rocking curves.

Enhanced diffusion in nonstoichiometric quantum wells and the decay of supersaturated vacancy concentrations

Enhanced superlattice disordering in nonstoichiometric AlAs/GaAs quantum wells exhibits weak temperature dependence because of the decay of the supersaturated concentration of group‐III vacancies. We present a formalism for transient enhanced diffusion in nonstoichiometric materials with which we can extract migration enthalpies Hm by assuming that the vacancy decay is thermally activated with an enthalpy Ha. By analyzing the electroabsorption from the quantum‐confined Stark effect for a set of isochronal and isothermal anneals, we extract a migration enthalpy Hm=(1.8±0.2) eV for group‐III vacancies, as well as an activation enthalpy Ha=(0.7±0.2) eV for vacancy annihilation.

Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well diodes

Oscillatory mode coupling between two coherent laser beams is produced when an interference pattern moves against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200cm(-1)during two-wave mixing using moving gratings.

Photorefractive Stark-geometry quantum wells: diffraction nonlinearities and displacement currents

Abstract An equivalent electronic circuit is formulated to illustrate the importance of displacement current and diffraction nonlinearities in the time-evolution of diffracted signals from Stark-geometry photorefractive quantum wells. Quadratic electro-optic effects and quadratic diffractin efficiencies mix the spatial and temporal harmonics of space-charge electric fields generated during dynamic holography. The electro-optic and diffraction nonlinearities invalidate superposition, but small-signal analysis remains useful. Lateral transport that limits device spatial resolution can be included in an equivalent circuit with lateral circuit elements and transverse electric fields.

Investigation of interface intermixing and roughening in low‐temperature‐grown AlAs/GaAs multiple quantum wells during thermal annealing by chemical lattice imaging and x‐ray diffraction

The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well (MQW) structures grown at a low substrate temperature (310 °C) by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x‐ray diffraction. The low‐temperature‐grown MQW is of high crystalline quality comparable to the standard‐temperature‐grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700 °C. The effective activation energy for interdiffusion is estimated as 0.24±0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low‐temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga.

Characterization of the photo-EMF response for laser-based ultrasonic sensing under simulated industrial conditions

Summary form only given as follows. There is a desire in manufacturing environments to nondestructively evaluate components and control processes in real time. Laser-based ultrasound has the potential to be a robust diagnostic for many applications. A simple and inexpensive sensor based on the photoinduced-electromotive effect has been demonstrated to be functional under a variety of manufacturing conditions. To optimize the detector performance, a parametric study was undertaken in which we measured the bandwidth, linearity, and sensitivity of our device as a function of various sensor material properties and optical architectures, as well as under simulated (yet quantifiable) industrial conditions. To quantify the sensitivity of the detector, an interferometric apparatus was set up with a calibrated phase signal introduced by an electro-optic (EO) modulator in one of the legs. To simulate different amplitudes of ultrasonic surface displacements, the detector output was monitored as a function of the EO drive voltage, at a fixed-drive frequency. The data show a high degree of linearity in the displacement range of interest for many commercial applications.