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Journal of the Physical Society of Japan

Broadening of Excitonic Molecule State under the Giant Two-Photon Excitation in CuCl

Tadashi Itoh, 隆 上遠野

Published Mar 1, 1982• 37 citations
Publisher

Abstract

Origin of the broadening and asymmetry of the giant two-photon absorption (GTA) band for the direct generation of excitonic molecules (EM) is investigated through the measurement of the induced circular dichroism and depolarization of a transmitted laser light resonated around the GTA peak. The plausible origin is suggested to be ascribed to the broadening of the EM state caused by the collision between EM and excitonic polariton both created resonantly with the GTA.

DOI
10.1143/jpsj.51.707

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physica status solidi (b)•Dec 1, 1988•2 citations
Origin of the broadening and asymmetry of the giant two-photon absorption (GTA) band for the direct generation of excitonic molecules (EM) is investigated through the measurement of the induced circular dichroism and depolarization of a transmitted laser light resonated around the GTA peak. The plausible origin is suggested to be ascribed to the broadening of the EM state caused by the collision between EM and excitonic polariton both created resonantly with the GTA.
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.
The broadening of the excitonic molecule band in CuCl is studied from the giant two-photon absorption (GTA) measurements by a weak linearly polarized probe light beam under the simultaneous excitation around the same GTA by another strong pumping light beam of the orthogonal polarization to the probe beam. The width of the GTA line monitored by the probe light remains almost constant less than 0.3 meV under the strong irradiation of the pumping light. It is pointed out that the broadened width of the GTA band which have been observed by the strong one monochromatic beam experiments is not associated with the mutual collision between the really produced excitonic molecules.
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.
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.
Through the studies of excitation spectra of emissions in CuCl crystals by a frequency tunable dye laser, excitonic molecules have been found to be generated directly by the giant two-photon absorption at the photon energy of 3.187 eV, as suggested by Hanamura. Two-photon excitation of crystals at 1.6 K enhanced very narrow emission bands at 3.1647 eV and at 3.1709 eV. These photon energies coincide with those of the high energy edges of the M L and M T bands due to the radiative annihilation of excitonic molecules having K =0. From these facts the Bose condensation of excitonic molecules at K ∼0 has been suggested.
Abstract The optical nonlinearity associated with the giant two‐photon absorption, GTA, of excitonic molecules in CuCl is discussed. The metamorphoses of the four‐wave mixing spectra due to the generation of the exciton grating are demonstrated by the observation of their polarization characteristics. The propagation characteristics of the excitation around the GTA resonance are observed by micro‐Raman spectroscopy. The two‐photon Raman scattering light is used as a probe to monitor the spatial character of the excitation. The propagation characteristics show a drastic change in the very narrow spectral range around the GTA resonance energy.