Photorefractive quantum wells: Materials, devices and systems

Indrajit Lahiri, Purdue University

Abstract

Photorefractive quantum wells are dynamic holographic materials that combine the advantages of large excitonic electroabsorption with large carrier mobilities to produce high-sensitivity holographic devices operable at extremely low optical intensities compatible with image processing applications. They are finding a niche as versatile optical devices in systems such as joint image correlators, femtosecond autocorrelators, laser-based ultrasound detection, and time-to-space mapping. These devices contain high defect densities which trap and store photogenerated carriers. The trapped space-charge results in a spatially varying space-charge field that matches the periodicity of the incident intensity pattern, which alters the optical properties of the device. The principal objective of the present study was to develop a new material system (low-temperature-grown AlGaAs/GaAs quantum wells) for photorefractive applications. We have achieved ultrafast electron recombination lifetimes in combination with sharp excitonic features, a goal which was considered mutually exclusive, by engineering low-temperature-grown AlAs/GaAs quantum wells. This material system was used to develop photorefractive p-i-n quantum well diodes operating in the quantum-confined Stark geometry for holographic applications. These photorefractive diodes operate with photocarrier transport perpendicular to the grating vector, unlike traditional photorefractive materials. They operate under transient electrical excitation resulting in a rich variety of spatio-temporal dynamics. We have optimized these devices to achieve record output diffraction efficiencies and shown that the design of these devices is relatively flexible as long as the principal functions of photoconductivity, electro-optics, and charge trapping are supplied by one or several functional layers. Nonreciprocal energy transfer, normally forbidden in Stark-effect devices, has been achieved using moving gratings. This led to the development of these photorefractive diodes to measure Doppler shifts by using electrically strobed gratings. Related to this work on longitudinal geometries, transverse-field photorefractive quantum wells were developed as adaptive beamsplitters to interrogate surface vibrations for laser-based ultrasound detection systems. These devices are tuned to quadrature for true linear detection by tailoring the excitonic spectral phase, a new contribution that is unique to the photorefractive quantum wells. This work has evolved from the physics of semi-insulating materials, to optimizing devices, to building complete systems using photorefractive quantum wells.

Degree

Ph.D.

Advisors

Nolte, Purdue University.

Subject Area

Condensation|Optics|Materials science

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