354 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
A Unified Elementary Approach to the Dyson, Morris, Aomoto, and Forrester Constant Term Identities
We introduce an elementary method to give unified proofs of the Dyson,
Morris, and Aomoto identities for constant terms of Laurent polynomials. These
identities can be expressed as equalities of polynomials and thus can be proved
by verifying them for sufficiently many values, usually at negative integers
where they vanish. Our method also proves some special cases of the Forrester
conjecture.Comment: 20 page
Polarized positron beams via intense two-color laser pulses
Generation of ultrarelativistic polarized positrons during interaction of an
ultrarelativistic electron beam with a counterpropagating two-color petawatt
laser pulse is investigated theoretically. Our Monte Carlo simulation based on
a semi-classical model, incorporates photon emissions and pair productions,
using spin-resolved quantum probabilities in the local constant field
approximation, and describes the polarization of electrons and positrons for
the pair production and photon emission processes, as well as the classical
spin precession in-between. The main reason of the polarization is shown to be
the spin-asymmetry of the pair production process in strong external fields,
combined with the asymmetry of the two-color laser field. Employing a feasible
scenario, we show that highly polarized positron beams, with a polarization
degree of , can be produced in a femtosecond time scale,
with a small angular divergence, mrad, and high density cm. The laser-driven positron source, along with laser
wakefield acceleration, may pave the way to small scale facilities for high
energy physics studies
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
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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