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High-Speed All-Optical Switching in Semiconductor Microcavities via Coherent Control of Excitons

Theodore B. Norris

Published Apr 1, 2001• 0 citations
OpenAlex

Abstract

We have carried out an extensive series of experiments on coherent control of semiconductor quantum microcavities. In these experiments, a phase-locked femtosecond optical pulse pair is used to excite the lower normal mode (cavity polariton), thereby controlling the reflection of a signal pulse tuned to the upper normal mode. Such a controlled reflection may be used as an all-optical switch with picosecond switching time. We have demonstrated that the effect of the microcavity is to provide a huge enhancement of the coherently controlled response compared to bare quantum wells. We have investigated in detail the microscopic physical processes responsible for the nonlinear optical response; the measured response fits well with theories based on the electron-hole Hamiltonian in the coherent-X(5) and 2nd-Born approximations. The physical limits to the coherent control are determined by excitation-induced dephasing; this limits the applicability of the switch in practical terms. We have discovered that the use of phase-locked excitation pulses is that one can generate novel intraband coherences in the quantum wells, arising from true quantum correlations between the cavity field and the carrier populations. The discovery of this effect was perhaps the first genuinely quantum optical effect observed in semiconductor microcavities.

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Quantum Electronics and Laser Science Conference•Jan 20, 2003•0 citations
We have carried out an extensive series of experiments on coherent control of semiconductor quantum microcavities. In these experiments, a phase-locked femtosecond optical pulse pair is used to excite the lower normal mode (cavity polariton), thereby controlling the reflection of a signal pulse tuned to the upper normal mode. Such a controlled reflection may be used as an all-optical switch with picosecond switching time. We have demonstrated that the effect of the microcavity is to provide a huge enhancement of the coherently controlled response compared to bare quantum wells. We have investigated in detail the microscopic physical processes responsible for the nonlinear optical response; the measured response fits well with theories based on the electron-hole Hamiltonian in the coherent-X(5) and 2nd-Born approximations. The physical limits to the coherent control are determined by excitation-induced dephasing; this limits the applicability of the switch in practical terms. We have discovered that the use of phase-locked excitation pulses is that one can generate novel intraband coherences in the quantum wells, arising from true quantum correlations between the cavity field and the carrier populations. The discovery of this effect was perhaps the first genuinely quantum optical effect observed in semiconductor microcavities.
The normal modes in a nonperturbatively coupled quantum-well semiconductor microcavity are linear superpositions of the QW exciton and cavity mode when the QW exciton transition is resonant with the cavity mode. When the lower normal mode is excited by a phase-locked pair of optical pulses, the nonlinear response of a probe pulse tuned to the upper mode is controlled. Thus the normal modes are coupled in their nonlinear optical response due to the nonlinearity of the exciton underlying the two normal modes. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses produces a differential reflectivity of up to 10%. Besides the coherent control of normal modes which is explainable with in the frame of semiclassical models, we observe a purely quantum mechanical phenomenon in our system. The quantum correlations between the field and carrier density lead to intraband coherences which live much longer than the interband dephasing time.
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.
A fully quantized theory of optical interactions with semiconductors reveals quantum correlations between the field and carrier density. These quantum correlations lead to intraband coherences which live much longer than the interband dephasing time; the correlations are observed as oscillations in the time-resolved reflectivity in a pump-probe experiment on a normal-mode microcavity. Coherent control of the microcavity normal modes using phase-locked pump pulses enables considerable enhancement of the intraband oscillations due to quantum interference; the detailed phase dependence is well reproduced by the theory.
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.
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.