153 research outputs found
Structure- and laser-gauges for the semiconductor Bloch equations in high-harmonic generation in solids
The semiconductor Bloch equations (SBEs) are routinely used for simulations
of strong-field laser-matter interactions in condensed matter. In systems
without inversion or time-reversal symmetries, the Berry connections and
transition dipole phases (TDPs) must be included in the SBEs, which in turn
requires the construction of a smooth and periodic structure gauge for the
Bloch states. Here, we illustrate a general approach for such a structure-gauge
construction for topologically trivial systems. Furthermore, we investigate the
SBEs in the length and velocity gauges, and discuss their respective advantages
and shortcomings for the high-harmonic generation (HHG) process. We find that
in cases where we require dephasing or separation of the currents into
interband and intraband contributions, the length gauge SBEs are
computationally more efficient. In calculations without dephasing and where
only the total current is needed, the velocity gauge SBEs are structure-gauge
independent and are computationally more efficient. We employ two systems as
numerical examples to highlight our findings: an 1D model of ZnO and the 2D
monolayer hexagonal boron nitride (h-BN). The omittance of Berry connections or
TDPs in the SBEs for h-BN results in nonphysical HHG spectra. The structure-
and laser-gauge considerations in the current work are not restricted to the
HHG process, and are applicable to all strong-field matter simulations with
SBEs
HHG in solids: Multi-band couplings leading to multiple plateaus
We discuss the theory of HHG in solids, relevant to recent experimental observations of multiple plateaus in HHG from solid argon. A multi-band model explains the cutoffenergies and relative strengths of the different plateaus
Time-frequency representations of high order harmonics
We calculate time-frequency representations (TFRs) of high-order short pulse harmonics generated in the interaction between neon atoms and an intense laser field, including macroscopic effects of propagation and phase matching in the non-linear medium. The phase structure of the harmonics is often complicated and the TFR can help to resolve the different components of this structure. The harmonic pulses exhibit an overall negative chirp, which can be attributed in part to the intensity dependence of the harmonic dipole phase. In some cases, the harmonic field separates in the time-frequency domain and clearly exhibits two different chirps. We also compute an experimental realization of a TFR (using Frequency Resolved Optical Gating, FROG) for a high harmonic. Due to the complicated time structure of the harmonics, the FROG trace is visually complex. © 2001 Optical Society of America
Imperfect Recollisions in High-Harmonic Generation in Solids
We theoretically investigate high-harmonic generation in hexagonal boron
nitride with linearly polarized laser pulses. We show that imperfect
recollisions between electron-hole pairs in the crystal give rise to an
electron-hole-pair polarization energy that leads to a double-peak structure in
the subcycle emission profiles. An extended recollision model (ERM) is
developed that allows for such imperfect recollisions, as well as effects
related to Berry connections, Berry curvatures, and transition-dipole phases.
The ERM illuminates the distinct spectrotemporal characteristics of harmonics
emitted parallel and perpendicularly to the laser polarization direction.
Imperfect recollisions are a general phenomenon and a manifestation of the
spatially delocalized nature of the real-space wave packet, they arise
naturally in systems with large Berry curvatures, or in any system driven by
elliptically polarized light
Space-time considerations in the phase locking of high harmonics
The combination of several high order harmonics can produce an attosecond pulse train, provided that the harmonics are locked in phase to each other. We present calculations that evaluate the degree of phase locking that is achieved in argon and neon gases interacting with an intense, 50 fs laser pulse, for a range of macroscopic conditions. We find that phase locking depends on both the temporal and the spatial phase behavior of the harmonics, as determined by the interplay between the intrinsic dipole phase and the phase matching in the nonlinear medium. We show that, as a consequence of this, it is not possible to compensate for a lack of phase locking by purely temporal phase manipulation
Laser-induced bound-state phases in high-order harmonic generation
We present single-molecule and macroscopic calculations showing that
laser-induced Stark shifts contribute significantly to the phase of high-order
harmonics from polar molecules. This is important for orbital tomography, where
phases of field-free dipole matrix elements are needed in order to reconstruct
molecular orbitals. We derive an analytical expression that allows the
first-order Stark phase to be subtracted from experimental measurements
Quantum path distributions for high-order harmonics in rare gas atoms
We present quantum path distributions of high-order harmonics produced by rare gas atoms interacting with an intense 810 nm laser field, obtained by numerical integration of the time-dependent Schrodinger equation within the single active electron approximation. We find that the distributions are sensitive to the atomic potentials, and differ from the distributions predicted by the strong field approximation. We demonstrate that these differences can lead to significant differences in the time-frequency behavior of the harmonics produced in a macroscopic nonlinear medium
Characterizing Anomalous High-Harmonic Generation in Solids
Anomalous high-harmonic generation (HHG) arises in certain solids when
irradiated by an intense laser field, as the result of a nonlinear
perpendicular current akin to a Hall current. Here, we theoretically
characterize the anomalous HHG mechanism, via development of an ab-initio
methodology for strong-field laser-solid interaction that allows a rigorous
decomposition of the total current. We identify two key characteristics of the
anomalous harmonic yields: an overall increase with laser wavelength; and
pronounced minima at certain intensities or wavelengths around which the
emission time profiles drastically change. Such signatures can be exploited to
disentangle the anomalous harmonics from the competing interband harmonics, and
thus pave way for the experimental identification and time-domain control of
pure anomalous harmonics.Comment: 7 pages, 4 figure
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