Fumio Sasaki

Ultrafast optical nonlinearities in type-II AlGaAs/AlAs multiple-quantum-well structures and their device applicability

We have studied the ultrafast optical nonlinearities in type-II AlGaAs/AlAs multiple-quantum-well (MQW) structures. In the type-II AlGaAs/AlAs system, only the holes are confined in the well layer and the photo-generated electrons are scattered into the barrier layer within several picoseconds. Thus the optical nonlinearities of the type-II system are quite different from those of the type-I quantum-well system due to this real-space electron transfer. The remarkable difference is the induced absorption observed at the low-energy side of the heavy-hole exciton peak, which provides a new mechanism for ultrafast switching devices.

Experimental and theoretical investigation of femtosecond carrier relaxation in CdSe

Abstract A femtosecond spectral hole burning technique is employed to study the relaxation of nonequilibrium carriers via carrier-carrier and carrier-LO-phonon scattering. Excitation by 70-fs laser pulses several LO phonon energies above the exciton resonance at 10 K in CdSe results in a transient spectral hole that disappears in less than 100 fs. The experiments are compared with a theory that involves numerical evaluation of the semiconductor Bloch equations in the presence of carrier-carrier and carrier-LO phonon scattering.

Intraband energy relaxation of hot carriers in CdSe

Abstract Intraband energy relaxation processes of hot carriers in CdSe were investigated by using the ultrafast pump-and-probe spectroscopy. Under the irradiation of the intense pump pulses of 2.12 eV, both excitons and bands in the absorption spectra are bleached. The bleached structure shows a non-thermalized hot carrier distribution when the pump is turned on, but shows thermalized hot carrier distribution immediately after the pump is turned off. The cooling of the thermalized hot carrier distribution was observed in the picosecond time domain.

Interlayer Γ-X scattering in staggered-alignment AlGaAs-AlAs ternary alloy multi-quantum-well structures

Abstract The interlayer Γ-X scattering rate of photoexcited electrons was measured in staggered-alignment type II AlGaAs-AlAs ternary alloy multi-quantum-well structures. The scattering process was directly probed by the femtosecond pump-and-probe spectroscopy. The mean Γ-X scattering time of electrons across the interface at 4.2 K was determined to be 1.0–1.2 ps, which is longer than that observed recently in GaAs-AlAs short period superlattices and is 20 times longer than that observed in bulk GaAs. The slowing mechanism is ascribed to the small penetration of the evanescent Γ electrons into the AlAs barrier layers.

Non-equilibrium distribution of hot carriers in a CdSe thin film

Femtosecond pump-and-probe spectroscopy was adopted to study the hot carrier distribution in a CdSe thin film. In the experiment, a nonthermalized hot carrier distribution was observed when the pump was turned on. The nonthermalized distribution was observed at the 200 meV broad energy range extending from the pump energy to the low energy side. The low energy tail of the nonthermalized distribution indicates that the carrier-LO phonon scattering competes with the carrier-carrier scattering in the thermalization process. In this sense, this observation is a unique example of the nonthermalized hot carrier distribution in semiconductors. From the ratio of the nonthermalized distribution to the total one, the thermalization time was estimated to be 16-40 fs. In the wake of the thermalization, cooling of photogenerated carriers was observed. With an increase of the excitation density up to 1019 cm-3, the carrier cooling rate was slower. The observed cooling rate was slower than the theoretical calculation, taking account of the screening of the carrier-phonon interaction. This is ascribed to the hot phonon effects of the LO phonon and the TO phonon.

Femtosecond optical nonlinearities under resonant excitation of excitons in CdSe

Abstract Femtosecond pump-and-probe spectroscopy was utilized to study the optical nonlinearities of excitons in CdSe. Under resonant excitation of the A-exciton, a 42 meV energy broadening of the B-exciton structure was observed. This broadening is caused by collisions between the A-exciton and the B-exciton. This is a rather unique observation of the scattering process between different kinds of excitons. Analysis indicates that the A-exciton-B-exciton scattering time is 31 fs. This time is well explained by a simple theory. Under resonant excitation of the B-exciton, a fast recovery of the bleaching was observed at the B-exciton. The recovery time of 0.9 ps is explained as the transformation time of B-excitons into A-excitons emitting LO phonons. The observed time constant agrees with the calculated scattering time based on the Frohlich interaction within an order of magnitude.

Degenerate Four Wave Mixing in Staggered Type-II AlxGa1−xAs/AlAs Multiple Quantum Well Structures

Recently much attention has been paid to carrier dynamics in staggered type-II multiple quantum well structures. In these systems, the energy level of the lowest Γ conduction electron state in a well layer is higher than that of the Γ-electron state in a barrier layer. Thus the photoexcited F-electrons in the well layer are easily scattered into the X-state in the barrier layer. The electrons scattered into the X-state in the barrier layer are expected to form X-Γ indirect excitons with holes in the well layer. We and several authors have measured these interlayer scattering times using femtosecond pump-and-probe technique with white light continuum [1,2,3]. However, the dephasing and momentum relaxation processes of the carriers in staggered type-II systems have not been understood. To clarify these relaxation processes, we have performed the time-resolved degenerate-four-wave mixing experiments in staggered type-II multiple quantum well structures.

Distinct two-step absorption saturation of excitons in GaAs multi-quantum-well structures under high-density excitation

Abstract Distinct two-step absorption saturation of the first-, second- and third-subband excitons in GaAs-AlAs multi-quantum-wells was observed in the picosecond time-domain at 4.2 K under the high-density band-to-band excitation. The absorption bleaching of the first-subband heavy exciton rises initially with a time constant of 1.4 ps and then rises again after an onset time of 4 ps. The initial rise of the bleaching takes place simultaneously with a dynamical red shift of the electron-hole plasma luminescence. The later rise is accompanied by a blue shift of the exciton. The bleaching of the second-subband and the third-subband excitons shows two-component decays. The two-step rise and decay are well explained by phase space filling and exchange effects of both excitons and the electron-hole plasma.

Tunneling spectroscopy in photo-excited semiconductor superlattices

Abstract Tunneling processes of photo-excited carriers in GaAs-AlGaAs semiconductor superlattices under the electric field are discussed on the basis of static and dynamic experiments.

Dynamical tunneling spectroscopy in photo-excited semiconductor superlattices

Abstract Population mixing and time-resolution techniques are applied to study dynamical carrier processes in GaAs-AlGaAs superlatices in a perpendicular electric field. Competitive nature of ultrafast processes of photo-excited electron-hole pairs, exciton formation and pair ionization followed by tunneling through barriers, are clarified under the various electric fields.