All-optical generation and coherent control of ultrafast shift and injection currents in GaAs quantum well structures
Final Report Abstract
Photocurrents being induced in different GaAs structures by optical femtosecond laser excitation have been investigated theoretically and experimentally. This work has been performed according to our original and renewal proposals and has led to significant new insight into light-matter interaction, in particular, on the generation of electronic transport via optical excitation. Some of the main results obtained in our work linked to the renewal proposal are briefly highlighted in the following. Our microscopic theoretical approach of using multisubband Bloch equations based on k.p band structure calculations has been extended to the analysis of shift currents and rectification currents. Thus we have developed a unified Bloch-equation-based framework to describe injection, shift, and rectification currents in semiconductors and semiconductor nanostructures which is capable of following the dynamics of the photoexcited material excitation and includes the light-matter interaction non-perturbatively. For bulk GaAs we analyzed the excitation intensity dependence of shift currents originating from phase-space filling and Rabi flopping. The appearance of shift currents could be related to symmetry properties and to specific parameters of the k.p approach. Furthermore, we computed rectification currents and currents which only exist in the presence of magnetic fields. For GaAs quantum wells we obtained shift currents and analyzed their symmetry properties and intensity dependence. Due to saturation originating from phase-space-filling a perturbative approach may overestimate the shift current for high intensities and Rabi flopping may lead to oscillations in the shift current transient. The computed frequency dependence of the shift current is in good agreement with experiment for several quantum well widths. We have demonstrated that the non-instantaneous response of the optically induced coherent polarization tremendously influences the real-space shift of electronic charges in semiconductors and leads to new types of shift currents. For resonant excitation of excitons with an optical pulse whose linear polarization is slowly rotating versus time an antisymmetric shift current linked to a second-order nonlinear tensor that is antisymmetric in its last two Cartesian indices has been observed. This new current requires a phase mismatch between two orthogonal transition dipole moments. Moreover, for non-resonant excitation of excitons with an elliptically polarized optical pulse whose major and minor axis slowly decreases and increases in time and become minor and major axis, respectively, we detected an additional symmetric shift current being associated with a second-order nonlinear tensor that is symmetric in its last two Cartesian indices. Both of these currents vanish for continuouswave excitation and, thus, differ from previously known shift currents. These observations, which can even be made at room temperature and are expected to occur in a variety of semiconductor crystal classes, contribute to a better understanding of light-matter interaction involving degenerate bands. Thus, they are expected to prove important for future studies of coherent and nonlinear optical effects in semiconductors. We have induced ultrafast magneto-photocurrents in a GaAs crystal employing interband excitation with femtosecond laser pulses at room temperature and non-invasively separated surface and bulk contributions to the overall current response. The separation between the different symmetry contributions was achieved by measuring the simultaneously emitted terahertz radiation for different sample orientations. Excitation intensity and photon energy dependences of the magneto-photocurrents for linearly and circularly polarized excitations revealed an involvement of different microscopic origins, one of which is the inverse Spin-Hall effect. These studies are important for a better understanding of the complex momentum-space carrier dynamics in magnetic fields. Moreover, they offer the possibility to study very different relaxation processes such as polar versus anisotropic momentum relaxation under the same experimental conditions. Apart from the scientific issues, new collaborations could be established and several invited talks were given. The successful collaboration between the Braunschweig and Paderborn groups will continue.
Publications
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“Terahertz spectroscopy of shift currents resulting from asymmetric 110- oriented GaAs/AlGaAs quantum wells”, Appl. Phys. Lett. 95, 151110 (2009)
S. Priyadarshi, M. Leidinger, K. Pierz, A.M. Racu, U. Siegner, M. Bieler, and P. Dawson
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“Microscopic analysis of charge and spin photocurrents injected by circularly polarized one-color laser pulses in GaAs quantum wells,” Phys. Rev. B 82, 115316 (2010)
H. T. Duc, J. Förstner, und T. Meier
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“Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells“, Phys. Rev. Lett. 104, 217401 (2010)
S. Priyadarshi, A. M. Racu, K. Pierz, U. Siegner, M. Bieler, H. T. Duc, J. Förstner, and T. Meier
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“All-optical generation of coherent in-plane charge oscillations in GaAs quantum wells,” Phys. Rev. B (R) 83, 121307 (2011)
S. Priyadarshi, K. Pierz, U. Siegner, P. Dawson, and M. Bieler
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“Intensity-dependent ultrafast dynamics of injection currents in unbiased GaAs quantum wells”, phys. stat. sol. (RRL), 5, 119 (2011)
M. Pochwala, H. T. Duc, J. Förstner, and T. Meier
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"All-Optically Induced Ultrafast Photocurrents: Beyond the Instantaneous Coherent Response," Phys. Rev. Lett. 109, 216601 (2012)
S. Priyadarshi, K. Pierz, and M. Bieler
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"All-optically induced currents resulting from frequency-modulated coherent polarization," Appl. Phys. Lett. 102, 112102 (2013)
S. Priyadarshi, K. Pierz, and M. Bieler
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"Femtosecond Quantum Interference Control of Electrical Currents in GaAs: Signatures beyond the Perturbative chi-(3)-Limit," Phys. Rev. B 88, 165204 (2013)
E. Sternemann, T. Jostmeier, C. Ruppert, H. T. Duc, T. Meier, and M. Betz
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"Time-domain calculations of shift currents in bulk GaAs," Proc SPIE 9361, 93611V (2015)
R. Podzimski, H. T. Duc, and T. Meier
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"Ultrafast magnetophotocurrents in GaAs: Separation of surface and bulk contributions," Appl. Phys. Lett. 106, 142108 (2015)
C. B. Schmidt, S. Priyadarshi, S. A. Tarasenko, and M. Bieler
