Masahiro Matsuoka

Nonlinear dynamical relaxation processes in semiconductor-doped glasses at liquid-nitrogen temperature

Excitation power dependences of photoluminescence spectra and radiative lifetimes were examined in semiconductor (CdSxSe1−x) -doped glasses at liquid-nitrogen temperature. When we considered a three-level system, the nonlinear power dependences were explained well in terms of a change of carrier-recombination routes that was due to population saturation of a low-lying trapping state. Experiments with superposed, two mode-locked pulse trains consisting of strong and weak excitation pulses clearly confirmed the population dynamics of the trapping state. Seemingly contradictory experimental results reported thus far for the nonlinear-optical response are explained in light of our nonlinear photoluminescence experiments.

Ultrafast pump-probe measurement using intensity correlation of incoherent light

A new type of the pump–probe technique with temporally incoherent light is proposed for use in the subpicosecond and femtosecond time regions in the wide-wavelength range. This method permits measurement of the longitudinal relaxation time T1 with the time resolution determined by the characteristic time of the intensity correlation of the incident light independently of its pulse width. For a demonstration of this technique, absorption recovery time T1 = 180 psec of a dye solution was measured with Q-switched laser pulses of 150-nsec duration. Differences between this method and the corresponding one used to measure the transverse relaxation time T2 with incoherent light is discussed.

Laser-induced irreversible change of the carrier-recombination process in CdSXSe 1– X-doped glasses

Laser-induced irreversible changes of luminescence intensity and lifetime have been systematically investigated in CdSxSe1–x-doped glass in order to clarify the laser-induced carrier-recombination process. These changes proceed at different rates, depending on the laser’s irradiation time, intensity, and sample temperature. A new phenomenological model for this laser-induced process is proposed in which a new recombination route for carriers is created in the trapping state by activation of a distributed potential barrier of this state.

Theory of Cooperative Cascade Emission. I. : Linear Stochastic Theory

The time evolution of the cascade omission from an assembly of the three-level atoms excited by two-photon process is investigated under a linear approximation. The propagation effect and the stochastic nature of the quantum noise are fully taken into account. It is found that the time evolution of the emitted radiation shows different behaviors in the three time stages: In the earliest stage a negative feedback mechanism due to the two-photon coherence is important, and the emitted radiation exhibits a characteristic growing behavior peculiar to the cascade system; in the intermediate one this mechanism is destroyed by the dephasing, and a superradiant growth begins; and finally in the latest one this growth is suppressed by the dephasing of the active polarization, and a stationary radiation appears.

Angular Dependence of Two Photon Absorption in Thallous Chloride

A measurement on the dependence of two photon absorption coefficient on the polarization angle of light in thallous chloride was performed. Two polarized light beams, one from Nd-glass laser and another from Xe-flash lamp, were made incident on the sample from the antiparallel directions, and the directions of both polarizations were rotated. The angular dependence was found to belong mostly to A 1 g representation in octahedral group from the band edge (∼3.56 eV) to as high as 4.2 eV. But the angular dependence was found to change at higher energy, and N g component seemed to mix with A 1 g . Two photon absorption spectra at liquid nitrogen temperature confirmed the assumption that the two photon transition spectrum was of the second class. A discussion about the method of analysis for this type of transition is presented.

Measurement of ultrafast optical pulses with two-photon interference.

We introduce a new two-photon interference scheme for measurement of the coherence time and pulse width of ultrafast pulses. Experiments were performed with the output pulses of a cw mode-locked dye laser, with results in good agreement with the conventional second-harmonic autocorrelation measurement.

Optical Second Harmonic Generation in the Forbidden 2 2 S 1⁄2 –3 2 S 1⁄2 Transition of Atomic Lithium Vapor

Second harmonic generation (SHG) in atomic Li vapor is studied when the 2 2 S 1/2 -3 2 S 1/2 two-photon transition is resonantly excited. This is an SHG in a medium with inversion symmetry, with no externally apptied electric or magnetic field, and with no apparent multipole moments associated with the resonant excitation. The SH polarization is found to be parallel to the intensity gradient of the incident laser beam when a cylindrical lens focusing is applied. The mechanism of this highly forbidden SHG is explained by the radial electric field induced by free expansion of photo-electrons around the laser beam axis.

Phonon-Induced Transverse Relaxation in LaF3:Pr3+ Measured by Photon Echoes

Temperature dependences of backward photon echo intensity have been measured from 5°K to 13°K on the 3P0–3H4 (4778 A) and 1D2–3H4 (5925 A) transitions of LaF3:Pr3+. Over six orders of magnitude (on the 3P0–3H4 transition) and three orders of magnitude (on the 1D2–3H4 transition) of the photon echo intensity, the experimental results are explained by the one-phonon process (the direct process) taking into account the energy differences between the echo levels and their adjacent levels. Preliminary measurements have also been made on the 4G7/2–4I9/2 (5221 A) and 4G5/2–4I9/2 (5775 A) transitions of LaF3:Nd3+.

Radiative and non-radiative decay processes of the excited state of the colored form of photochromic furylfulgide

Abstract The time evolution of the luminescence of the colored form of a furylfulgide dispersed at various concentrations in a poly(methyl methacrylate) film was measured as a function of the luminescence photon energy. The observed decay time of the luminescence is about 1–2 ns and one order of magnitude shorter than the radiative lifetime (14 ns) estimated from the absorption intensity. The decay time is independent of temperature below 77 K. These results suggest that the non-radiative tunneling process from the excited state to the ground state is responsible for the decay.

Obsevation of Coherence in Two-Photon Excited Rb Vapor

A coherent superposition state of 5S and 5D levels of Rb vapor, when it was excited by two-photon resonant transition between these levels, was observed by applying an interrogation pulse which was non-resonant to the system. The emphasis is laid on this coherence in the atomic excitation, instead of just the population inversion, in explaining the origin of various strong collimated emissions observed by many authors. Saturation of the off diagonal density matrix element between 5S and 5D levels was observed at higher excitation level due to the population change. An analysis of the saturation is given using a simple model for the coherence of the dye laser beam.