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physica status solidi (b)

Experimental verification of the new model for the two‐photon absorption of biexcitons in CuCl

Masahiro Hasuo, H. Kawano, N. Nagasawa

Published Mar 1, 1995• 6 citations
Publisher

Abstract

Abstract Two different theoretical models of the giant two‐photon absorption due to excitonic molecules (xx) are examined experimentally. The conventional model (Hanamura, 1973) follows the sequence: photon + photon → exciton + photon → xx, while the self‐consistent model (Ivanov and Haug, 1993) gives the other scenario of the two‐photon absorption: photon + photon → exciton + exciton → xx. Our high‐precision measurement of the frequency dependence of the two‐photon xx absorption coefficient in CuCl allows us to discriminate these two models and supports the new self‐consistent model.

DOI
10.1002/pssb.2221880107

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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, \(\varDelta\varOmega_{1}\), of excitation light. In the case of \(\varDelta\varOmega_{1}{=}0.25\) 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 \(\varDelta\varOmega_{1}{=}2.3\) 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.

Journal of the Physical Society of Japan•Sep 15, 1976•96 citations
Abstract Two different theoretical models of the giant two‐photon absorption due to excitonic molecules (xx) are examined experimentally. The conventional model (Hanamura, 1973) follows the sequence: photon + photon → exciton + photon → xx, while the self‐consistent model (Ivanov and Haug, 1993) gives the other scenario of the two‐photon absorption: photon + photon → exciton + exciton → xx. Our high‐precision measurement of the frequency dependence of the two‐photon xx absorption coefficient in CuCl allows us to discriminate these two models and supports the new self‐consistent model.
Abstract LO‐phonon‐assisted two‐photon absorption of the first excited level of the excitonic molecule in CuCl is considered on the basis of the previously derived symmetry properties of this state. A giant cross‐section for this process is predicted mainly due to the “resonant” intermediate state Γ 1/5 of the single exciton. The process under consideration overcomes the difficulties of pure two‐photon absorption (TPA) of p‐type envelope states of the excitonic molecule as stated by Hanamura.
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.
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 induced absorption due to exciton‐biexciton transitions is observed in CuCl with an experimental set‐up using two dye lasers. The shape of the absorption band is studied. The oscillator strength per exciton of this absorption is estimated and compared to theoretical evaluations. The excitation spectrum of this induced absorption is also measured and its features are tentatively explained by the existence of a superficial layer on the samples, where excitons have a very short lifetime.
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.