Tsuneyoshi Mita

Radiative and Two-Photon Resonance Raman Processes Associated with Excitonic Molecules in CuCl

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.

Exciton spatial dispersion determined through the two-photon Raman scattering via excitonic molecule state at large wave vectors in CuCl

Abstract With simultaneous excitation of two dye laser beams the Raman scattering is studied in which the intermediate state is the excitonic molecule and the final states are the longitudinal and transverse excitons. The L-T splitting shows a large k-dependence. The effective masses are determined to be (2.3 ± 0.1)m0 and (3.1 ± 0.1)m0 for the transverse and longitudinal excitons, respectively.

Light mixing induced by the giant two-photon absorption of excitonic molecules in CuCl

Abstract Three-wave mixing of light has been observed in connection with a giant two-photon absorption generating excitonic molecules. The efficiency of mixing of light for energies 2Ω 1 - Ω 2 and 2Ω 2 - Ω 1 is enhanced remarkably when Ω 1 and/or Ω 2 is varied across the giant two-photon absorption band-region. Moreover, the mixed lights with energies 3Ω 1 - 2Ω 2 and 4Ω 1 - 3Ω 2 have also been detected.

Generation of excitonic molecules by giant two-photon absorption in CuCl and CuBr and their Bose condensation

Abstract Theoretical expectation that excitonic molecules are generated directly by the two-photon absorption, has been confirmed experimentally in CuCl and CuBr crystals. The luminescence of the radiative decay of the excitonic molecule thus created directly is found to show a very sharp emission band which is attributable to the Bose condensation at K ≈ 0. In connection with the sharp emission, the two-photon resonance Raman scattering is studied.

A Comment on the Broadening of the Excitonic Molecule Band in CuCl

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.

Resonance Raman scattering and luminescence under two-photon excitation

Abstract Two-photon forward Raman scattering in CuCl is the four polariton process; two incident polaritons ħω o (k o ) are scattered into a pair of lower branch polaritons having different energies via excitonic molecule state at k=2k o . Scattering into lower and upper branch polaritons can take place via excitonic molecule with small k o . At resonant excitation, the secondary emission consists of Raman and luminescence components. From the line shapes of luminescence bands, the relaxation of resonantly created excitonic molecules is discussed.

Distribution of Excitonic Molecules under the Giant Two-Photon Excitation in CuCl

Line shapes of the luminescence, coexisting with the Raman scattering, of excitonic molecules (EM) have been studied in the case of the resonant excitation. The EMs are resonantly created with wave vectors, such as k =0, k =2 k 0 and k =2.3 ∼12.2 k 0 ( k 0 =4.44 ×10 5 cm -1 ), by the two-photon excitation technique. By the use of model calculations based on the Gaussian distribution in the k-space, the EMs are inferred to re-distribute anisotropically in the range | k -2 k 0 | ≤2.5 k 0 and | k | ≤1.5 k 0 for the excitation into k =2 k 0 and k =0, respectively. EMs created at large k re-distribute isotropically, and the emitted light is shown to be due to the hot luminescence during the cooling process.

Resonant Two-Photon Excitation of Excitonic Molecule in CuCl With Using Circularly Polarized Light

The \(\varGamma_{1}\)-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.

Relaxation of Excitonic Molecules Generated by the Giant Two-Photon Laser Excitation in CuCl

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.

Luminescence of Excitonic Molecules Distributed around k =0 in CuCl

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.