Anomalous currents refer to electronic currents that flow perpendicularly to the direction of the accelerating electric field. Such anomalous currents can be generated when Terahertz fields are applied after an optical interband excitation of GaAs quantum wells. The underlying processes are investigated by numerical solutions of the semiconductor Bloch equations in the length gauge. Excitonic effects are included by treating the manybody Coulomb interaction in time-dependent Hartree-Fock approximation and additionally also carrier-phonon scattering processes are considered. The band structure and matrix elements are obtained from a 14-band k · p model within the envelope function approximation. The random phase factors of the matrix elements that appear due to the separate numerical diagonalization at each k-point are treated by applying a smooth gauge transformation. We present the macroscopic Berry curvature and anomalous current transients with and without excitonic effects. It is demonstrated that the resonant optical excitation of excitonic resonances can significantly enhance the Berry curvature and the anomalous currents.
All envisaged practical implementations of cryogenic processors, including quantum computers and classical processors based on single flux quantum (SFQ) signals, require massive data transfer from and to classical high performance computers (HPCs). Cryogenic computing has recently become a very hot topic, including superconducting quantum computers (QCs), and classical processors based on single flux quantum (SFQ) signals. All envisaged practical implementations of cryogenic processors require massive data transfer from and to classical HPCs. The project aCryComm aims to develop building blocks for cryogenic photonics interconnects and eventually enable this challenging data transfer. The long-term goal is the development of an open-access platform to integrate classical optical interfaces based on low-loss silicon photonics, plasmonics, and nano light sources together with superconducting photonic and electronic devices, including SFQ-based co-processors for HPCs and for QCs.
The anomalous velocity causes spin-polarized carriers to move perpendicular to an electric or parallel to a magnetic driving field. It is at the origin of the spin Hall and anomalous Hall effects. Here, we employ time-domain THz spectroscopy to study the anomalous velocity and the anomalous Hall effect, both of which are optically induced in GaAs, on a sub-picosecond time scale. Our results not only enable a distinction between intrinsic (Berry curvature) and extrinsic (scattering) effects but also demonstrate an inversion of anomalous Hall currents versus excitation photon energy.
Using a microscopic theory that combines k.p band structure calculations with multisubband semiconductor Bloch equations we are capable of computing coherent optically-induced rectification, injection, and shift currents in semiconductors and semiconductor nanostructures. A 14-band k.p theory has been employed to obtain electron states in non-centrosymmetric semiconductor systems. Numerical solutions of the multisubband Bloch equations provide a detailed and transparent description of the dynamics of the material excitations in terms of interband and intersubband polarizations/coherences and occupations. Our approach allows us to calculate and analyze photocurrents in the time and the frequency domains for bulk as well as quantum well and quantum wire systems with various growth directions. As examples, we present theoretical results on the rectification and shift currents in bulk GaAs and GaAs-based quantum wells. Moreover, we compare our results with experiments on shift currents. In the experiments the terahertz radiation emitted from the transient currents is detected via electro-optic sampling. This comparison is important from two perspectives. First, it helps to validate the theoretical model. Second, it allows us to investigate the microscopic origins of interesting features observed in the experiments.
We report spatially resolved measurements of frequency, phase, and amplitude of GHz and THz emitters employing
an unstabilized THz frequency comb. Different optoelectronic detection methods are compared regarding accuracy
and invasiveness. The measurement setup allows for high-precision measurements of the frequency (accuracy 9·10-14), relative amplitude (standard deviation of the mean 0.1 %) and phase (standard deviation of the mean 0.2 °). By
simultaneously measuring the emission of a 30 GHz emitter as well as the emission of a CO2 laser at 28 THz, a large
spectral coverage is demonstrated.
Keywords:
Femtosecond optoelectronic techniques are routinely employed for the generation and detection of ultrashort voltage
pulses. However, so far, not much effort has been spend to determine the exact shape of such voltage pulses over a
very broad frequency range. For this purpose, i.e., for the realization of a broadband voltage pulse standard, it is
essential to (i) know the transfer function of the detection technique and (ii) be able to separate forward and backward
propagating signals from each other. Here we report the realization of a voltage pulse standard with frequency
components exceeding 500 GHz and a 500 MHz frequency spacing.
The excitation of exciton resonances in semiconductors with certain polarization-shaped optical pulses reveals the existence of new photocurrents. For resonant excitation of excitons with an optical pulse whose linear polarization is slowly rotating versus time we observe an antisymmetric shift current linked to a second-order nonlinear tensor that is antisymmetric in its last two Cartesian indices. 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 observe 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 continuous-wave excitation and, thus, differ from previously known shift currents.
We report on the observation of ultrafast in-plane charge oscillations resulting from simultaneous optical excitation of
heavy- and light-hole excitons in (110)-oriented GaAs quantum wells. These charge oscillations arise from a
displacement between heavy and light-hole states in the plane of the quantum well, and, thus, differ significantly from
previously observed charge oscillations directed along the growth direction of a quantum well. Our observations are
evidence for the existence of a strong far-infrared inter-subband transition dipole moment between heavy- and light-hole
subbands for in-plane wave vectors which we estimate to be ~0.5 eA for the quantum wells under investigation. We
attribute this in-plane transition dipole moment to strong band mixing in the (110)-oriented structures.
We experimentally and theoretically investigate injection currents generated by femtosecond single-color circularly-polarized
laser pulses in (110)-oriented GaAs quantum wells. The current measurements are performed by
detecting the emitted Terahertz radiation at room temperature. The microscopic theory is based on a 14 x 14
k • p band-structure calculation in combination with the multi-subband semiconductor Bloch equations. For
symmetric GaAs quantum wells grown in (110) direction, an oscillatory dependence of the injection currents on
the exciting photon energy is obtained. The results of the microscopic theory are in good agreement with the
measurements.
We have experimentally investigated injection currents generated by all-optical excitation of GaAs/AlGaAs quantum
wells excited with 130 fs optical pulses. The currents have been detected via free-space THz experiments at room
temperature. Our experiments prove that Coulomb effects strongly influence injection currents. This becomes most
prominently visible when exciting light-hole exciton transitions. At this photon energy we observe a pronounced phase
shift of the current transients which is due to oppositely oriented heavy-hole and light-hole type contributions. We are
currently developing a microscopic theory based on a 14×14 k.p model in combination with the semiconductor Bloch
equations to describe the observed features quantitatively. The combined theoretical and experimental approach will
allow us to analyze the influence of the bandstructure and interaction effects on the injection current amplitude and
current dynamics.
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