Yasuaki Masumoto

Pico-second spectroscopy of the secondary radiation from resonantly excited excitonic molecules in CuCl

Abstract Pico-second time-resolved measurements of the two-photon resonant Raman scattering via excitonic molecules in CuCl were carried out for the first time. The Raman scattering leaving transverse excitons decays as fast as the laser light. When the energy of the incident light falls in the vicinity of the two-photon resonant absorption, both Raman and luminescence lines are simultaneously observed. In case of just resonant excitation, the transient response indicates that the secondary radiation can be decomposed into the Raman and the luminescence components as regards its temporal and spectral behavior.

Two-dimensional shrinkage of the exciton wavefunction in quantum wells probed by optical absorption

Abstract The oscillator strength of the lowest 1s excitons in GaAsAlAs quantum wells is studied as a function of well layer thickness Lz by means of optical absorption. It is enhanced more than the 1 L z dependence with the decrease of Lz. The results present the first full experimental observation of two-dimensional shrinkage of the exciton wavefunction in quantum wells.

High intensity effect on two-photon resonant, coherent Raman scattering via excitonic molecules in CuCl and CuBr

Abstract Two-phonon resonant, coherent Raman scattering (four-photon degenerate coherent scattering) via excitonic molecules has been studied as a function of intensity of incident light in CuCl and CuBr. Observed line-shaped of coherent scattering spectra broadens as excitation intensity is raised. This broadening is found to be closely correlated with that of the two-photon absorption. These results are qualitatively explained by taking account of the excitation intensity-dependence of χ(3), but cannot be explained by the autoinization model.

Polarization memory of luminescence from photo-generated carriers in aSixC1−x:H (x∼0.2)

Abstract Photoluminescence of aSi x C 1−x :H (x∼0.2) is studied with a particular interest on the localization of carriers. Taking the polarization memory of luminescence as a probe, we clarify the localized character of photo-generated carriers as functions of excitation photon energy, emitted photon energy, temperature and time. The picosecond time-resolved measurement of the polarization memory is shown useful to study dynamical relaxation processes of carriers in disordered systems.

Dynamical Aspects of Self-Trapping of 1s Excitons in RbI and KI

Previously direct measurements of the self-trapping time of 1 s excitons in KI was reported. In this paper, measurements for RbI are described. Picosecond pulses of the fifth harmonic (212.8 nm) of a Nd 3+ : YAG laser, which can directly generate the 1 s excitons are used as the excitation source. The rise time of the E x luminescence is measured at 4.2 K. From the observed rise time, the self-trapping time is estimated to be 2.0±0.5 ns. The height of the adiabatic potential barrier separating the free and self-trapped exciton states is measured to be 18 and 30 meV for RbI and KI, respectively. The self-trapping time and the barrier height in RbI and KI are compared, and their changes with changing material are discussed on the basis of the theoretical calculation.

Superbroadened distribution of excitonic polaritons in CuCl

Abstract Superbroadened distribution of excitonic polaritons injected by intense laser beam is observed around the two-photon resonance of excitonic molecules in CuCl. By measuring the induced absorption from the injected excitonic polariton state to the excitonic molecule state, the energy distribution of excitonic polaritons is derived. The origin of this broadening is attributed to four-polariton parametric scattering. This phenomenon gives clear explanation to the excitation intensity-dependent broadening of two-photon absorption of excitonic molecules in CuCl.

Ultrafast relaxation of excitons in GaAs-AlAs multi-quantum-well structures studied by resonant Rayleigh-type optical mixing

Abstract Ultrafast relaxation of high-density (∼ 1012−1013 cm−2 per layer) excitons in GaAs-AlAs multi-quantum-well structures is studied for the first time by using the method of resonant Rayleigh-type optical mixing (resonant non-degenerate four-wave-mixing in the frequency domain). The result shows that both dephasing relaxation and cross relaxation are ascribed to a common origin which is the energy-changing mutual collision of excitons. The obtained collision time of high-density excitons lies in the order of subpicoseconds.

Picosecond relaxation processes of photo-generated carriers in a-SixC1−x:H (x∼0.2)

Abstract Photoluminescence of a-Si x C 1−x :H (x∼0.2) prepared by glow discharge of tetramethylsilane is studied systematically in the picosecond time region by using tunable picosecond light pulses and a streak camera. The sample, temperature and energy dependences are studied together with linear polarization memory. The experimental results indicate that the radiative recombination of photo-generated electron-hole pairs is of the geminate (exciton-like) type.

Dynamics of excitonic polaritons in CuCl studied by picosecond induced absorption

Abstract Dynamical relaxation processes of excitonic polaritons have been studied by observing the transient induced absorption from the excitonic polariton to excitonic molecule state in CuCl. Frequent mutual collision quickly establishes the quasi-Maxwell-Boltzmann distribution. Energy- dependent relaxation time due to LA phonon scattering and radiative lifetime dependent on sample thickness are well explained by the existing theory based on the polariton concept.

Picosecond spectroscopy of two-dimensional excitons in GaAs- and AlxGa1−xAs-AlAs multi-quantum-well structures

Abstract Picosecond energy relaxation processes of excitons in the well layers of GaAs- and Al x Ga 1− x As-AlAs multi-quantum-well structures have been studied. The energy-loss rate of excitons is almost proportional to the exciton linewidth of the absorption spectra regardless of the material of the well layers and the well layer thickness. The energy relaxation processes of excitons are discussed on the basis of the localized exciton transfer model.