The physics and femtosecond applications of photorefractive multiple quantum wells
Abstract
Photorefractive quantum wells are dynamic holographic devices that operate at low optical intensities. A spatial variation in the incident light creates a corresponding refractive index and absorption pattern. Photocarriers are generated, then transport by diffusion and drift, and trap at deep level defects. The resulting space charge field is converted into a diffraction grating by an electro-optic effect. The thin-film grating is probed in the Raman-Nath regime by degenerate four-wave mixing, two-wave mixing, and nondegenerate four-wave mixing. The material system is Aluminum Gallium Arsenide epilayers and AlGaAs/GaAs multiple quantum wells grown by molecular beam epitaxy. The devices are operated in the transverse or Franz-Keldysh geometry, for which an electric field is applied in the plane of the quantum wells (perpendicular to the growth direction). The transport is parallel to the grating vector, and the electro-optic effect is the field ionization of excitons. The purpose of the first part of this work was to complete the study of transverse geometry photorefractive structures. The main result is conclusive evidence proving that hot-electrons cause the photorefractive phase shift which produces nonreciprocal energy transfer. The electron velocity-field nonlinearity is shown to correspond to the onset of the photorefractive phase shift. The second part demonstrates that these devices are useful for characterizing and controlling ultrashort pulses with a duration of 100 fs using nondegenerate four-wave mixing. Pulses were characterized using photorefractive quantum wells to perform electric-field correlation where the pulses write the grating in the photorefractive quantum wells, and a continuous wave diode laser probes the grating. The shape of a femtosecond pulse diffracted from photorefractive quantum wells is measured when a continuous wave diode laser writes the grating, and the femtosecond pulse probes the grating. The shape of the diffracted pulse is measured with electric-field cross correlation and spectral interferometry. The diffracted pulse is broadened by the reduction of its spectrum from the bandwidth of the diffraction spectrum, but remains nearly transform limited. Therefore, photorefractive quantum wells are suitable for use in a spectral holography pulse shaping apparatus.
Degree
Ph.D.
Advisors
Nolte, Purdue University.
Subject Area
Optics|Condensation|Electrical engineering
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