Subpicosecond time-resolved absorption and transient gratings in gallium-arsenide.
Item
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Title
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Subpicosecond time-resolved absorption and transient gratings in gallium-arsenide.
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Identifier
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AAI9000036
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identifier
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9000036
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Creator
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Katz, Alvin I.
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Contributor
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Adviser: Robert R. Alfano
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Date
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1989
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Optics | Physics, Electricity and Magnetism
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Abstract
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Carrier-carrier and intervalley scattering has been studied in submicron thick GaAs under intense photoexcitation by time resolved absorption and four wave mixing techniques using a femtosecond laser and amplifier system which was constructed as part of this thesis.;The laser oscillator is a colliding pulse ring cavity passively modelocked (cpm) Rhodamine 6G dye laser. The saturable absorber is DODCI (3,3{dollar}\sp\prime{dollar}-diethyloxodicarbocyanine iodide). The oscillator produces pulses of 100 femtosecond duration and pulse energies of 50 picojoules at 620 nm and 115 megahertz repetition rate. The oscillator is pumped by a cw argon ion laser operating at a single line (514.5 nm). The amplifier system is a four stage amplifier pumped by a frequency doubled Q-switch Nd Yag laser. The electronics were constructed so as to allow the dye laser to trigger the pockel's cell of the Nd:Yag laser. The amplifier has a gain of 5 {dollar}\times{dollar} 10{dollar}\sp6{dollar}; with an output pulse energy of 500 microjoules and a pulse width of 450 fs at a 20 hertz repetition rate.;Pump and probe absorption techniques were used to measure the time evolution of the photoexcited carrier distribution function in GaAs. These measurements show that the electrons and holes do not achieve a Fermi-Dirac distribution until 800 femtoseconds after excitation.;Time resolved four wave mixing techniques were used to study intervalley scattering in GaAs. Both the dispersive (real part of n) and absorption saturation (imaginary part of n) of the free carrier contributions to the dielectric function is included to correctly explain the data. For energies high above the band gap, the interband (bleaching) contribution is strongly dependent on the distribution of electrons among the different valleys ({dollar}\Gamma{dollar}, L or X) of the conduction band and is opposite in sign from the intraband (dispersive) term. By using a three pulse transient grating experiment under the conditions of high pump intensity and probe energy (2.0 eV) much greater than the band gap (1.424 eV at room temperature), the effective time for intervalley scattering of electrons from the L-valleys back to the {dollar}\Gamma{dollar}-valley is directly measured. From the grating decay time of {dollar}\approx{dollar}9 picoseconds, a L {dollar}\to{dollar} {dollar}\Gamma{dollar} time of 3 ps is estimated which is in good agreement with other recent work using time resolved photoluminescence.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
