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Ultrafast Phase-Locked Spectroscopy of Excitons in Quantum Nanostructures

Kazuhiro Komori, Gary R. Hayes, B. Deveaud, X.L. Wang, Mutsuo Ogura, M. Watanabe

Published Apr 1, 2002• 5 citations
Publisher

Abstract

We have investigated the ultrafast coherent dynamics of exciton-polaritons and excitons in semiconductor quantum nanostructures, including InGaAs/AlGaAs multi quantum well Bragg structures and AlGaAs/GaAs crescent shaped quantum wire samples, by using a phase-locked spectroscopic technique. In the multi quantum well Bragg sample, a two-stage decay due to the superradiant exciton-polariton mode is clearly observed at the resonant condition. In the crescent shaped quantum wire sample, the highly sensitive coherent control measurement enables us to observe the dephasing characteristics and a very fast dephasing time T-2 of 200 to 300 Is with power dependence was observed.

DOI
10.1002/1521-396x(200204)190:3<855::aid-pssa855>3.0.co;2-#

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We have investigated the ultrafast coherent dynamics of exciton-polaritons and excitons in semiconductor quantum nanostructures, including InGaAs/AlGaAs multi quantum well Bragg structures and AlGaAs/GaAs crescent shaped quantum wire samples, by using a phase-locked spectroscopic technique. In the multi quantum well Bragg sample, a two-stage decay due to the superradiant exciton-polariton mode is clearly observed at the resonant condition. In the crescent shaped quantum wire sample, the highly sensitive coherent control measurement enables us to observe the dephasing characteristics and a very fast dephasing time T-2 of 200 to 300 Is with power dependence was observed.
We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconductor quantum nanostructures. An InGaAs/AlGaAs multi-quantum-well Bragg structure and an AlGaAs/GaAs crescent-shaped quantum wire structure were used as samples for the control of the exciton-polariton and excitons in the nanostructure. In the multi-quantum-well Bragg sample, a two-stage decay due to the super-radiant exciton-polariton mode is clearly observed under the resonance conditions. Highly sensitive coherent control measurement enables us to observe the dephasing characteristics in the crescent-shaped quantum wire sample, and a very fast dephasing time T 2 of between 200 fs and 300 fs with power dependence was observed.
The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.
Summary form only given. Ultrafast coherent control of excitons in semiconductors have been reported using Michelson interferometer with precise control of the pulse interval in time domain. In order to improve the phase controllability and realize arbitrary manipulation, the pulse shaping technique is a more attractive candidate. In this paper, we report very fine phase control of coherent, excitons in GaAs/AlGaAs single quantum wells using phase-locked pulses prepared by pulse shaping technique.
In this paper, we report on the ultrafast coherent control of excitons in quantum wells using a phase-locked pulse sequence generated by pulse-shaping techniques. The pulse-shaping system with a double liquid-crystal spatial light modulator (SLM), and a Ti3+-sapphire laser is used to generate phase-locked pulses. The ultrafast coherent control of the exciton population and polarization is demonstrated by the observation of the reflectivity change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements. In single quantum wells, good coherent control characteristics with 87% coherent carrier destruction are demonstrated at the low excitation power of 0.15 mW (∼1.1×1010 photons/cm2). In addition, the preliminary manipulation of the exciton population is demonstrated by the phase-locked triple pulses. In the case of coupled quantum wells, the modulation of the SLM is carefully controlled and the coherent control of both the exciton population and the polarization is demonstrated in the two different coupled quantum-well samples.
Picosecond relaxation processes of excitons, including dephasing processes, are systematically studied in CuCl, CdSe and GaAs/AlAs MQW (multi-quantum well structures) by observing time-resolved induced absorption, luminescence and non-degenerate four-wave mixing. By studying the transient induced absorption from the exciton (polariton) state to the excitonic molecule state, time-resolved energy distribution of excitons in CuCl is determined (Fig.1).1) Another technique, i.e. time-resolved luminescence measurement, is applied to excitons in CdSe and GaAs/AlAs MQW (Figs.2 and 3).2,3) By analyzing the above-mentioned experiments the distribution of excitons in the two-dimensional energy-time coordinates is successfully determined. Thus, energy relaxation processes of excitons are clearly visualized. Prior to energy relaxation, coherently generated excitons are considered to be dephased. Phase relaxation of excitons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved non-degenerate four-wave mixing (Fig.4).4) These new techniques will be demonstrated and the dephasing as well as energy relaxation mechanism of excitons and excitonic polaritons will be discussed. In the above experiments characteristics of a picosecond tunable optical parametric oscillator pumped by a mode-locked Nd3+:YAG laser as well as a picosecond tunable dye laser synchronously pumped by a mode-locked ion laser are complementarily utilized.
Summary form only given. Ultrafast dephasing of excitons in semiconductor quantum wells is important because it presents insight into fundamental interactions and possible coherent control of optical properties. Standard measurement techniques include time- and frequency-domain four-wave mixing, resonant Rayleigh scattering, and Faraday rotation. We report on a new time-resolved technique based on reflective probing of pump-induced transient linear birefringence. This has various advantages, including sensitivity, applicability to opaque as-grown samples, ease of implementation, and freedom from superimposed signals.