Ultrafast physics behind the nonradiative relaxation process of chromium ions in forsterite crystals.
Item
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Title
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Ultrafast physics behind the nonradiative relaxation process of chromium ions in forsterite crystals.
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Identifier
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AAI9405518
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identifier
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9405518
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Creator
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Demos, Stavros Gregorios.
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Contributor
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Adviser: Robert R. Alfano
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Date
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1993
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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, Condensed Matter
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Abstract
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The nonradiative relaxation following photoexcitation has been studied in Cr{dollar}\sp{lcub}4+{rcub}{dollar}-doped forsterite (Mg{dollar}\sb2{dollar}SiO{dollar}\sb4{dollar}) using picosecond laser excitation and ultrasensitive photon counting detection. The experimental techniques utilized were time resolved antiStokes Raman scattering and up-converted hot and ordinary luminescence.;The up-converted hot luminescence technique allowed the investigation of the upper state nonradiative relaxation of the excited state manifold of Cr{dollar}\sp{lcub}4+{rcub}{dollar}-doped forsterite. The excitation involves the absorption of two photons per photoexcited ion in a two-step absorption. Discrete peaks are observed in the hot up-converted luminescence spectrum and are attributed to the population of nonequilibrium vibronic levels during the deexcitation of the ions by phonon emission. This work reveals that the phonon modes participating in the initial steps of the nonradiative relaxation of the photoexcited ions have energies 218 {dollar}\pm{dollar} 20, 325 {dollar}\pm{dollar} 20, 365 {dollar}\pm{dollar} 20 and 513 {dollar}\pm{dollar} 12 cm{dollar}\sp{lcub}-1{rcub}{dollar}. The shape of the luminescence spectral envelope suggests two electronic bottlenecks at {dollar}\approx{dollar}2.1 and {dollar}\approx{dollar}2.45 eV associated with slower rates of vibrational relaxation at different parts of the excited state manifold. Time resolved measurements indicated that the average time for phonon emission is of the order of hundreds of fs.;Information on the nonequilibrium phonon dynamics of the 225, 335 and 370 cm{dollar}\sp{lcub}-1{rcub}{dollar} modes of forsterite has been obtained using time resolved Raman scattering. Laser pulses of 450 fs in duration and 590 nm in wavelength were used to excite the Cr ions 2.1 eV above the ground state. The probe pulses (obtained from the same laser) are monitoring the nonequilibrium phonon population through the intensity of the antiStokes Raman lines at various pump-probe delay times. Experiments were performed at room and liquid nitrogen temperatures. The observed nonequilibrium phonon populations are associated with the overall complex nonradiative decay following the excitation of the impurity Cr{dollar}\sp{lcub}4+{rcub}{dollar} ions. Using rate equations to describe the electron-lattice system, the nonradiative relaxation time and the phonon lifetimes were estimated by fitting to the experimental data. The nonradiative relaxation time is estimated to be in the order of few ps while the phonon lifetimes are of the order of 10 ps. Best fit suggests the presence of an electronic bottleneck immediately after photoexcitation with an estimated lifetime of 3 ps at room temperature.
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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.
