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Journal of the Physical Society of Japan

Luminescence of Excitonic Molecules Distributed aroundk=0 in CuCl

N. Nagasawa, Tsuneyoshi Mita, Masayasu Ueta

Published Sep 15, 1979• 6 citations
Publisher

Abstract

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.

DOI
10.1143/jpsj.47.909

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Journal of the Physical Society of Japan•Feb 1, 1984•17 citations
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.
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 Γ1\varGamma_{1}Γ1​-symmetry of the lowest excitonic molecule state of CuCl has been confirmed experimentally from the measurements of the giant two-photon absorption and resonance Raman scattering by using circularly polarized dye laser light. The giant two-photon transition is found to be forbidden for single beam of circularly polarized light while it is allowed for two beams of circularly polarized lights impinging in crystals from directly opposite directions. It has been verified that the lowest excitonic molecule state is composed of two-electron- and two-hole-states both of the composite spin angular momentum zero.
Line shapes of the giant two-photon absorption (GTA) band for the direct generation of excitonic molecules (EM) and the subsequent secondary emission of the EM resonantly generated have been studied in CuCl at various pump powers with the use of a high resolution spectroscopic method. The mutual collision of the EM, the rate of which is nearly proportional to their density, is found to be the main origin for causing the broadening of the GTA band, the decrease of the Raman yield and the rapid increase of the EM luminescence. The collision rate increases up to 10 12 /sec for the pump power of ∼1 MW/cm 2 . With the use of pump and probe method further evidence is obtained for the effectiveness of the mutual collision of the EM.
Under the off-resonant excitation at the slightly higher energy side of the giant two-photon absorption band (GTA) for the direct generation of excitonic molecules (EM) in CuCl, new narrow emission bands designated as X T and X L have been found in the energy region of the M T and M L broad bands, respectively. Their photon energies increase with the decrease of the pump photon energy from the higher-energy side of the resonance and finally, at the on-resonant excitation, their bands merge into M T 0 and M L 0 bands, respectively, previously reported by Mita et al. Based on the detailed studies on these bands, it is found that there exists a certain kind of transverse relaxation process which acts on the EM just after their generation by the GTA and brings about the X emission as a hot luminescence.