We theoretically study the Dirac fermion dynamics in a graphene monolayer in the presence of an applied ultrafast laser pulse. The pulse has the duration of a few femtoseconds and the amplitude of 0.1 - 0.5 V/Å. The waveform of the pulse is described by Hermit Gaussian polynomials with varying carrier-envelope phase. We show that the ultrafast dynamics of Dirac fermions strongly depends on the carrier-envelope phase and the frequency of the applied pulse. The ultrafast pulse generates an electric current which results in a finite transferred charge. The ultrafast field-driven current and the corresponding net transferred charge depend on the waveform of the applied pulse. Our results pave the way for the development of ultrafast information processing in the terahertz domain.
We theoretically study the interaction of three dimensional topological Weyl semimetals with an ultrafast circularly polarized optical pulse. We use the lower-energy approximation of the full Hamiltonian of the system in the reciprocal space near the Weyl points. We present the results for TaAs, which has two pairs of Weyl points. The ultrafast pulse causes a finite electron conduction band population both during and after the pulse. We show that the electron dynamics for such materials is coherent and highly irreversible, i.e., the residual conduction band population is comparable to the maximum conduction band population during the pulse. For a pulse propagating in the z direction, the large population of electrons is located near the Weyl points and along the separatrix which is defined as a set of the initial points for which electron trajectories in the reciprocal space pass precisely through the (k<sub>x</sub>, k<sub>y</sub>) = (0, 0) point for different values of k<sub>z</sub>. For small k<sub>z</sub>, the system behaves similar to graphene and, the interband dipole matrix elements are highly localized near (k<sub>x</sub>, k<sub>y</sub>) = (0, 0) point and the conduction band population has sharp maximum along the separatrix. However, for large k<sub>z</sub>, the system behaves as a gapped graphene with delocalized interband dipole matrix and the transfer of electrons between the valence band and the conduction bands are not confined within a narrow region. We also show that the optical pulse causes electrical current and net charge transfer through the system during the pulse.
We discuss the topological properties of graphene superlattices excited by ultrafast circularly-polarized laser pulses with strong electric field amplitude, aiming to directly observe of the Berry phase, a geometric quantum phase encoded in the graphene’s electronic wave function. As a continuing research on our recent paper, Phys. Rev. B 96, 075409, we aim to show that the broken symmetry system of graphene superlattice and the Bragg reflection of electrons creates diffraction and “which way” interference in the reciprocal space reducing the geometrical phase shift and making it directly observable in the electron interferograms. Such a topological phase shift acquired by a carrier moving along a closed path of crystallographic wave vector is predictably observable via time and angle resolved photoemission spectroscopy (tr-ARPES). We believe that our result is an essential step in control and observation of ultrafast electron dynamics in topological solids and may open up a route to all-optical switching, ultrafast memories, and petahertz-scale information processing technologies.
We propose an attosecond strong optical field interferometry in graphene which reveals the chirality of graphene without employing a magnetic field. A circularly polarized optical pulse with strong amplitude and femtosecond time scale causes the electron to circle in the reciprocal space through which it accumulates the dynamic phase along the closed trajectory as well as the nontrivial geometric phase known as Berry’s phase. The resulting interference fringes carry rich information about the electronic spectra and interband dynamics in graphene near the Dirac points. Our findings hold promises for the attosecond control and measurement of electron dynamics in condensed matters as well as understanding the topological nature of the two-dimensional Dirac materials.
This paper investigates the interaction of buckled Dirac materials (silicene and germanene) with ultrashort and ultrastrong optical pulses. Highly intensive few-cycle pulses strongly modify the electronic and optical properties of these two dimensional materials. Electron dynamics in such a short optical pulse is coherent and can be robustly controlled by altering the propagation direction, as well as the polarization angle of the pulse. The strong nonlinearity of the system for fields applied (~ V/Å) causes the violation of the charge (C) and parity (P) symmetries, effectively reducing the system’s symmetry from hexagonal to triangular. Such symmetry violations are related to the electron transfer between the sublattices caused by the normal field component and result in nonreciprocity, optical rectification and the appearance of a cross current.
We report on our study of the effects of external magnetic field on the intersubband optical transitions in
quantum cascade systems. We address the properties of two types of cascade structures: terahertz quantum
cascade lasers and quantum dot infrared photodetectors. In both cases we study the optical properties of active
regions of the systems: optical emission in the case of quantum cascade lasers and optical absorption in the case
of photodetectors. The new features and new peaks in the optical spectra appear in the tilted magnetic field for
quantum cascade lasers and in the parallel magnetic field for quantum dot photodetectors. In the relation to the
possible spintronic application of cascade structures we study interplay of spin-orbit and magnetic field effects in
cascade systems. If spin-orbit coupling is strong enough then at finite parallel magnetic field the optical emission
spectra of quantum cascade lasers have two-peak structure.
Recent discovery of the coherent lasing from various disordered materials adds a new dimension to the conventional physics of light propagation in multiply scattering media. It suggests that in the situation, when the propagation of light is diffusive on average, the coherent feedback can be provided by the sparse disorder configurations that efficiently trap a photon, and thus, serve as random resonators. This scenario of random resonators has been
substantiated experimentally by the ensemble averaging of the power Fourier transforms of the random emission spectra. In this paper the current status of the experiment and theory of coherent random lasing is reviewed.
General properties of emission spectra of an interacting two-dimensional (2D) electron-hole (e- h) system in a strong magnetic field are discussed. The procedures are proposed for determining some parameters of magnetorotons, elementary excitations of an incompressible 2D liquid, from emission spectra. A manifestation of fractional charges in optical spectra is also discussed. the violation of a hidden symmetry inherent in the magnetospectroscopy of 2D systems in the framework of the standard model is discussed as applied to different spectroscopic experiments.