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Quantum Electronics and Laser Science Conference

Coherent control as a probe of scattering mechanisms in semiconductor quantum structures

X. Marie, Th. Amand, P. Renucci, M. Paillard, P. Le Jeune, J. Barrau

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. The coherence decay of optically excited electronic systems provides one of the most powerful tools to investigate interaction processes of excited states. In semiconductors, the optical coherence phenomena are usually investigated by nonlinear techniques such as four wave mixing (FWM), photon echo, or reflectivity experiments. Exciton-phonon, exciton-exciton, or exciton-free carriers scattering processes have been explored in recent years with these techniques. As demonstrated recently, a direct measurement of the optical dephasing time can be obtained through the temporal coherent control of electronic excitations. In these experiments, the emission following the excitation by an ultrashort optical pulse sequence, is detected in a non-specular direction (secondary emission). This provides an alternative approach to FWM techniques, which allows us to analyse the scattering processes either with phonons or due to mutual collisions. Using the coherent control technique monitored through the secondary emission (SE), we report on the coherent dynamics of excitons in bare quantum wells (QW) and compare it to that of exciton-polaritons in microcavity (MC) embedded quantum wells.

DOI
10.1109/qels.1999.807368

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Summary form only given. The coherence decay of optically excited electronic systems provides one of the most powerful tools to investigate interaction processes of excited states. In semiconductors, the optical coherence phenomena are usually investigated by nonlinear techniques such as four wave mixing (FWM), photon echo, or reflectivity experiments. Exciton-phonon, exciton-exciton, or exciton-free carriers scattering processes have been explored in recent years with these techniques. As demonstrated recently, a direct measurement of the optical dephasing time can be obtained through the temporal coherent control of electronic excitations. In these experiments, the emission following the excitation by an ultrashort optical pulse sequence, is detected in a non-specular direction (secondary emission). This provides an alternative approach to FWM techniques, which allows us to analyse the scattering processes either with phonons or due to mutual collisions. Using the coherent control technique monitored through the secondary emission (SE), we report on the coherent dynamics of excitons in bare quantum wells (QW) and compare it to that of exciton-polaritons in microcavity (MC) embedded quantum wells.
Temporal coherent control experiment have been performed on semiconductor quantum well structures, using a sequence of polarized excitation pulses. The oscillatory behaviour of the populations, spin and alignment of the exciton states are observed through the time resolved secondary emission when the time separation between the two excitation pulses is varied. We show that, in specular detection direction, for bare quantum wells at 10 K and low exciton densities, the oscillations amplitude decay is determined by the exciton inhomogeneous broadening. In non-specular detection directions however, when the SE is dominated by resonant Rayleigh scattering, the decay is longer and becomes temperature dependent, which strongly supports that it is now determined by the exciton dephasing time.
Summary form only given. Optical interactions in semiconductor microcavities embedded with quantum wells enter a nonperturbative regime when the dipole coupling rate between the exciton and the cavity mode exceeds relevant decay rates. In this regime, coherent energy exchange between the exciton and the cavity mode, i.e. normal mode oscillation (NMO), takes place when the cavity mode is excited impulsively by a short laser pulse. Similar coherent oscillations have also been observed in transient four-wave mixing (FWM) responses. In this paper, we present experimental and theoretical studies on the nature of oscillations in these transient FWM responses. We show that, in contrast to earlier beliefs, oscillations in time-integrated transient FWM do not correspond to NMO of the exciton or photon populations. Theoretical studies further reveal that FWM arising from different physical mechanisms including band filling, excitation-induced dephasing, and renormalized electric fields is characterized by highly distinct coherent dynamics. Oscillations in transient FWM reflect interference between these coherent optical processes and can be used as a unique probe for exciton many-body interactions in the nonperturbative regime.
Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.
Summary form only given. The optical properties of semiconductor quantum wells (QW) and, in particular, the coherent dynamics of excitons following resonant excitation with ultrafast laser pulses have attracted much attention in recent years. It is generally accepted that there is a transfer of coherence between the optical field and the QW that disappears in a characteristic time T/sub 2/ (picoseconds for GaAs) after the laser is turned off. However, the questions as to how the coherence is actually induced and that of the nature of the coherent state of the solid are poorly understood. In this work we address these points by re-examining the longstanding problem of the (classical vs. quantum) nature of the ubiquitous beats associated with the light-hole (LX) heavy-hole (HX) excitons, which are observed in transient optical experiments on QW. To this end, we consider the coherent behavior of excitons using the simplest albeit non-trivial model where they are treated as non-interacting bosons. Our results bear directly on studies for which nonlinear effects are not important and are not aimed directly at explaining the fourwave-mixing (FWM) experiments that dominate the field. Nevertheless, since nonlinear effects are, typically, weak compared with harmonic contributions, the free boson picture provides in all cases the correct lowest order wavefunction of the photoexcited solid.
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.
Summary form only given. The optical coherent control of quantum mechanical systems enables the direct manipulation of quantum states, and has been demonstrated on excitonic transitions in semiconductors. We have demonstrated the coherent control of the nonlinear optical response of quantum-well excitons in a resonant high-Q microcavity. There are two significant advantages to using excitons in microcavities instead of bare quantum wells: first, the multiple reflections which occur in the cavity strongly enhance the optical nonlinearity, enabling large-signal coherent control, and second, 2-color coherent control may be implemented, where a pump pulse tuned to one of the normal modes of the coupled exciton-microcavity system can control the response of a probe tuned to the other normal mode. The nonlinear response of the normal modes is due to the nonlinearity of the underlying exciton transition.