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Journal of the Physical Society of Japan

Energy Level Scheme of Excitonic Molecule and Exciton in CuBr

Yoshiko Nasu, Sachio Koizumi, N. Nagasawa, Masayasu Ueta

Published Aug 1, 1976• 31 citations
Publisher

Abstract

Giant two-photon absorption for the direct generation of excitonic molecue is measured. Three absorption peaks are found at 2.9540, 2.9561 and 2.9576 eV, which are responsible for the two-photon transitions to the (\varGamma_{1}(J{=}0)), Γ 5 ( J =2) and (\varGamma_{3}(J{=}2)) states, respectively, of the excitonic molecule.
DOI
10.1143/jpsj.41.715

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Journal of the Physical Society of Japan•Feb 1, 1984•27 citations
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.
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.
The conventional giant oscillator strength model of the two-photon (\ensuremath{\gamma}) generation of an excitonic molecule (m) attributes this process to the x-\ensuremath{\gamma} optical conversion (where x is the exciton) i.e, to the scheme \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+\ensuremath{\gamma}\ensuremath{\rightarrow}m. Recenty [A.L. Ivanov and H. Haug, Phys. Rev. B 48, 1490 (1993)], it was argued that a more adequate description can be done within the bipolariton model which follows the other scenario: \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+x\ensuremath{\rightarrow}m, where the Coulombic resonant coupling of the x components of the two interacting polaritons gives rise to the m formation. In the present work, we develop and analyze for the deuteron potential of an x-x interaction an exactly solvable bipolariton model of the two-\ensuremath{\gamma} m generation. This model treats an m optical creation in terms of the polariton-polariton resonant scattering and includes both the x-y polariton coupling and the x-x attraction beyond a low-order perturbation theory. The bipolariton model and the giant oscillator strength model give different descriptions of the third-order m nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$ and of the two-\ensuremath{\gamma} m absorption. With the high-precision measurements in CuCl of the m radiative width ${\mathrm{\ensuremath{\Gamma}}}^{\mathit{m}}$, ${\mathrm{\ensuremath{\chi}}}^{(3)}$, and the two-\ensuremath{\gamma} m absorption coefficient K(2){\mathit{K}}^{(2)}K(2), we make a systematic comparison between the experiments and the two models, which allows us to unambiguously discriminate both models in favor of the bipolariton one.
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.