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Progress of Theoretical Physics Supplement

Resonance Raman Scattering and Luminescence from Highly Excited States

T. Takagahara

Published May 14, 2013• 0 citations
Publisher

Abstract

Resonance Raman scattering and luminescence under an intense excitation field are studied for both stationary and transient excitation regimes. By means of the Green function theory of resonance Raman scattering and luminescence we explain the characteristic features of the recent experiments on two-photon resonance Raman scattering via excitonic molecule in CuCl where the resonance dip and the asymmetry of the Raman yield around the resonance were observed. We ascribe the origin of the resonance dip of the Raman yield to the resonance enhancement of collisional damping of excitonic molecule and exciton states. The asymmetry of the Raman yield can be naturally understood in terms of the frequency and wave-vector dependent relaxation constants. Furthermore an approach is proposed which incorporates the coherence and correlation among excitonic molecules and excitons in their non-equilibrium state and clarifies their effects on the optical spectra.

DOI
10.1143/ptp.69.420

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Journal of the Physical Society of Japan•Sep 15, 1979•6 citations
Resonance Raman scattering and luminescence under an intense excitation field are studied for both stationary and transient excitation regimes. By means of the Green function theory of resonance Raman scattering and luminescence we explain the characteristic features of the recent experiments on two-photon resonance Raman scattering via excitonic molecule in CuCl where the resonance dip and the asymmetry of the Raman yield around the resonance were observed. We ascribe the origin of the resonance dip of the Raman yield to the resonance enhancement of collisional damping of excitonic molecule and exciton states. The asymmetry of the Raman yield can be naturally understood in terms of the frequency and wave-vector dependent relaxation constants. Furthermore an approach is proposed which incorporates the coherence and correlation among excitonic molecules and excitons in their non-equilibrium state and clarifies their effects on the optical spectra.
Competitive behavior of two channels of resonant Raman scattering and luminescence is discussed for the giant two-photon excitation of excitonic molecules in CuCl. The stochastic theory of intermediate state interaction is applied to describe the time-integrated and time-resolved emission spectra of the excitonic molecule excited resonantly by a short light pulse. Particularly as for the time-resolved spectrum, the frequency-time uncertainty due to the finite observation time is incorporated in the present theory. Our theory explains the observed characteristics of the emission spectra, and the relaxation constants of the excitonic molecule and the exciton in the final state are determined from comparison between the observed and calculated spectra.
Forward Raman scattering of excitonic molecule in CuCl has been studied at the giant two-photon excitation with using laser light having a wave vector K 0 but three different half band-widths, 0.2, 1.4 and 2.5 meV. In cases of laser excitation with broader half band-widths, the Raman and luminescence processes coexist and give emission lines called (LEP) R and (LEP) B , respectively. Both lines show the similar dependence of energy change upon the scattering angle. The (LEP) R and (LEP) B lines show red and blue shift, respectively, with the increase of laser excitation intensity. The excitonic molecule-state generated at K =2 K 0 is responsible for the (LEP) R line, while the (LEP) B line is considered to arise from a state having a certain K value smaller than 2 K 0 . Excitonic molecules generated coherently by the giant two-photon excitation are assumed to relax towards K =0 with keeping their individual momentum at almost the same value.
Various optical spectra such as time-resolved luminescence spectra are studied for excitonic molecules in CuCl under two-photon resonance excitation by using picosecond and nanosecond frequency-tunable laser pulses. Two sharp emission lines observed under resonance excitation are ascribed to the radiative decay of “cold” excitonic molecules created directly at k ∼2 k 0 , while the broad emission band on the low energy side of the sharp line to the luminescence of “hot” molecules created through single excitons. The stimulated emission process gives a considerable influence on both the sharp luminescence lines and the hot molecule band, and further on two-photon resonant excitonic Raman lines as well. Discussions are made on the Bose condensation of the excitonic molecules and also on whether the emission under just-resonance excitation is due to Raman scattering or luminescence.
Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width, ΔΩ1\varDelta\varOmega_{1}ΔΩ1​, of excitation light. In the case of ΔΩ1=0.25\varDelta\varOmega_{1}{=}0.25ΔΩ1​=0.25 meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of ΔΩ1=2.3\varDelta\varOmega_{1}{=}2.3ΔΩ1​=2.3 meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.
The three-wave mixing process has been studied as functions of incident photon energy and also of the geometrical configuration when incident photon energy lies around the two-photon resonance of excitonic molecule. Experiments indicate that the intensity of the third order nonlinear light beam is enhanced when the phase matching condition derived from the dispersion of excitonic polariton holds or when incident photon energy is equal to the two-photon resonance of excitonic molecule. The relation between this three-wave mixing process and two-photon resonant Raman scattering so far observed is discussed. It is shown that this three-wave mixing process is regarded as the two-photon resonant, coherent Raman scattering via excitonic molecules.
Excitonic molecule state in CuCl has been resonantly excited by the combination of two circularly polarized laser beams of slightly different energies and of the opposite propagation vectors. The secondary emissions associated with the excitonic molecule state of a quite small momentum have been investigated by changing the spectral width of the incident laser light. With the increase of the band width of excitation laser, the luminescence becomes dominant and the Raman component is suppressed. An emission line at 3.1869 eV is assigned to the luminescence due to the decomposition of excitonic molecules, distributed in a region of ±0.022 k 0 around 0.050 k 0 , into a pair of polaritons, where k 0 is the wave vector of the photon of 3.1861 eV. The relaxation of the excitonic molecule system really generated at the small momentum region has been discussed.