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CRC Press eBooks

Coherent control and switching

Jeremy J. Baumberg

Published Dec 5, 2020• 0 citations
Publisher

Abstract

This chapter discusses some experiments on coherent control of carrier densities, angular momenta, electron spin and exciton polarisations. The use of direct gap semiconductors as the active medium in all-optical switching devices is favoured by their large optical nonlinearities. The presence of spin polarised electrons in the sample can also be seen in the longer timescale dynamics after coherent carrier control. Selection of angular momentum is achieved by using shorter optical pulses which have a broad enough spectral content to excite both the heavy and light hole exciton states. Selected coherent pulse shaping techniques are introduced and used to demonstrate novel phenomena produced by coherent control in semiconductor heterostructures. New features in the interaction of light with semiconductor heterostructures are resolved, highlighting nonlinear mechanisms that can be of future use, as well as significantly modifying the picture of how light interacts with such samples.

DOI
10.1201/9781003072829-3

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This chapter discusses some experiments on coherent control of carrier densities, angular momenta, electron spin and exciton polarisations. The use of direct gap semiconductors as the active medium in all-optical switching devices is favoured by their large optical nonlinearities. The presence of spin polarised electrons in the sample can also be seen in the longer timescale dynamics after coherent carrier control. Selection of angular momentum is achieved by using shorter optical pulses which have a broad enough spectral content to excite both the heavy and light hole exciton states. Selected coherent pulse shaping techniques are introduced and used to demonstrate novel phenomena produced by coherent control in semiconductor heterostructures. New features in the interaction of light with semiconductor heterostructures are resolved, highlighting nonlinear mechanisms that can be of future use, as well as significantly modifying the picture of how light interacts with such samples.
Systematic investigation of the spin dephasing times in GaAs/(Al,Ga)As quantum wells and its depending on parameters like temperature, magnetic fields and doping will be shown in Chapter 3.In Chapter 4 are presented results of investigation of the nuclear spin systems in quantum well heterostructures.The chapter 5 presents the results of studying the carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum wells by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 mil-liKelvin.In Chapter 6 is devoted to experimental and theoretically studying of optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization.For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state.on the eigenstates |s z ; ↑ and |s z ; ↓ yields the states themselves independent of time.Along the field axis (longitudinal direction) these quantum-mechanical spin states are thus stationary.Any loss of coherence is induced by coupling of those states with the environment.Applying the time-evolution operator on the spin states perpendicular
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.
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.
We demonstrate the coherent destruction of photogenerated excitons in semiconductor quantum wells within a few hundred femtoseconds of their excitation. Coherent control of carrier dynamics is achieved with phase-locked pairs of 100 fs infrared pulses. The technique induces an optical response which is faster than the inverse of the exciton linewidth superseding Fourier limits for a single pulse. Energy selectivity enables the coherent transfer of angular momentum between hole states. Such phase-tailored pulse trains can be utilized to investigate the generation process and intermediate virtual states in quantum structures.
Summary from only given. Recently we have demonstrated an optically-addressed coherent polarization switch that is based on the near-resonant excitation of a spin-polarized population of virtual excitons in MQWs. By taking advantage of the nonlinearities associated with a virtual carrier population, this near resonant device exhibits a pulse-width-limited response; by operating near resonance it is capable of producing relatively large contrast ratios in thin samples; and finally, by generating a spin-polarized carrier population, the device produces a rotation of the signal polarization in unstrained MQWs. Here, we experimentally analyze the switching mechanisms by systematically performing spectrally and temporally resolved differential transmission measurements and by fully determining the polarization state of the transmitted signal as a function of time delay.
Schemes for coherent control of carrier dynamics in semiconductors are investigated theoretically. We consider coherent control of carrier generation and charge oscillations by quantum interference of one- and three-photon absorption, as well as one- and two-photon absorption. It is shown that, for coherent light pulses shorter than characteristic phase breaking times, the relative phase between the two excitation routes can be used to control the extent of charge oscillations in semiconductor double wells. Furthermore, we show that optical gain and absorption can be controlled coherently by two coherent light fields. In particular, optical gain can be shifted from one intersubband transition to another.