121 research outputs found
Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations
Electron-hole recollisions are induced by resonantly injecting excitons with
a near-IR laser at frequency into quantum wells driven by a
~10 kV/cm field oscillating at THz. At K, up to
18 sidebands are observed at frequencies , with . Electrons and holes recollide with
total kinetic energies up to 57 meV, well above the meV
threshold for longitudinal optical (LO) phonon emission. Sidebands with order
up to persist up to room temperature. A simple model shows that LO
phonon scattering suppresses but does not eliminate sidebands associated with
kinetic energies above .Comment: 5 pages, 4 figure
Optical frequency combs from high-order sideband generation
We report on the generation of frequency combs from the recently-discovered
phenomenon of high-order sideband generation (HSG). A near-band gap
continuous-wave (cw) laser with frequency was transmitted
through an epitaxial layer containing GaAs/AlGaAs quantum wells that were
driven by quasi-cw in-plane electric fields between 4 and 50
kV/cm oscillating at frequencies between 240 and 640 GHz.
Frequency combs with teeth at
( even) were produced, with maximum reported , corresponding to a
maximum comb span THz. Comb spectra with the identical product
were found to have similar spans and shapes
in most cases, as expected from the picture of HSG as a scattering-limited
electron-hole recollision phenomenon. The HSG combs were used to measure the
frequency and linewidth of our THz source as a demonstration of potential
applications
Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature
The direct measurement of Berry phases is still a great challenge in
condensed matter systems. The bottleneck has been the ability to adiabatically
drive an electron coherently across a large portion of the Brillouin zone in a
solid where the scattering is strong and complicated. We break through this
bottleneck and show that high-order sideband generation (HSG) in semiconductors
is intimately affected by Berry phases. Electron-hole recollisions and HSG
occur when a near-band gap laser beam excites a semiconductor that is driven by
sufficiently strong terahertz (THz)-frequency electric fields. We carried out
experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum
wells. The observed HSG spectra contain sidebands up to the 90th order, to our
knowledge the highest-order optical nonlinearity observed in solids. The
highest-order sidebands are associated with electron-hole pairs driven
coherently across roughly 10% of the Brillouin zone around the \Gamma point.
The principal experimental claim is a dynamical birefringence: the sidebands,
when the order is high enough (> 20), are usually stronger when the exciting
near-infrared (NIR) and the THz electric fields are polarized perpendicular
than parallel; the sideband intensities depend on the angles between the THz
field and the crystal axes in samples with sufficiently weak quenched disorder;
and the sidebands exhibit significant ellipticity that increases with
increasing sideband order, despite nearly linear excitation and driving fields.
We explain dynamical birefringence by generalizing the three-step model for
high order harmonic generation. The hole accumulates Berry phases due to
variation of its internal state as the quasi-momentum changes under the THz
field. Dynamical birefringence arises from quantum interference between
time-reversed pairs of electron-hole recollision pathways
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
Expanded view of electron-hole recollisions in solid-state high-order harmonic generation: Full-Brillouin-zone tunneling and imperfect recollisions
We theoretically investigate electron-hole recollisions in high-harmonic
generation (HHG) in band-gap solids irradiated by linearly and elliptically
polarized drivers. We find that in many cases the emitted harmonics do not
originate in electron-hole pairs created at the minimum band gap, where the
tunneling probability is maximized, but rather in pairs created across an
extended region of the Brillouin zone (BZ). In these situations, the analogy to
gas-phase HHG in terms of the short- and long-trajectory categorizations is
inadequate. Our analysis methodology comprises three complementary levels of
theory: the numerical solutions to the semiconductor Bloch equations, an
extended semiclassical recollision model, and a quantum wave packet approach.
We apply this methodology to two general material types with representative
band structures: a bulk system and a hexagonal monolayer system. In the bulk,
the interband harmonics generated using elliptically-polarized drivers are
found to originate not from tunneling at the minimum band gap , but
from regions away from it. In the monolayer system driven by linearly-polarized
pulses, tunneling regions near different symmetry points in the BZ lead to
distinct harmonic energies and emission profiles. We show that the imperfect
recollisions, where an electron-hole pair recollide while being spatially
separated, are important in both bulk and monolayer materials. The excellent
agreement between our three levels of theory highlights and characterizes the
complexity behind the HHG emission dynamics in solids, and expands on the
notion of interband HHG as always originating in trajectories tunnelled at the
minimum band gap. Our work furthers the fundamental understanding of HHG in
periodic systems and will benefit the future design of experiments.Comment: 18 pages, 13 figure
Direct evidences for inner-shell electron-excitation by laser induced electron recollision
Extreme ultraviolet (XUV) attosecond pulses, generated by a process known as
laser-induced electron recollision, are a key ingredient for attosecond
metrology, providing a tool to precisely initiate and probe sub-femtosecond
dynamics in the microcosms of atoms, molecules and solids[1]. However, with the
current technology, extending attosecond metrology to scrutinize the dynamics
of the inner-shell electrons is a challenge, that is because of the lower
efficiency in generating the required soft x-ray \hbar\omega>300 eV attosecond
bursts and the lower absorption cross-sections in this spectral range. A way
around this problem is to use the recolliding electron to directly initiate the
desired inner-shell process, instead of using the currently low flux x-ray
attosecond sources.Such an excitation process occurs in a sub-femtosecond
timescale, and may provide the necessary "pump" step in a pump-probe
experiment[2]. Here we used a few cycle infrared \lambda_{0}~1800nm source[3]
and observed direct evidences for inner-shell excitations through the
laser-induced electron recollision process. It is the first step toward
time-resolved core-hole studies in the keV energy range with sub-femtosecond
time resolution.Comment: 6 pages, 4 figure
Dynamics of multiply charged ions in intense laser fields
We numerically investigate the dynamics of multiply charged hydrogenic ions
in near-optical linearly polarized laser fields with intensities of order 10^16
to 10^17 W/cm^2. Depending on the charge state Z of the ion the relation of
strength between laser field and ionic core changes. We find around Z=12
typical multiphoton dynamics and for Z=3 tunneling behaviour, however with
clear relativistic signatures. In first order in v/c the magnetic field
component of the laser field induces a Z-dependent drift in the laser
propagation direction and a substantial Z-dependent angular momentum with
repect to the ionic core. While spin oscillations occur already in first order
in v/c as described by the Pauli equation, spin induced forces via spin orbit
coupling only appear in the parameter regime where (v/c)^2 corrections are
significant. In this regime for Z=12 ions we show strong splittings of resonant
spectral lines due to spin-orbit coupling and substantial corrections to the
conventional Stark shift due to the relativistic mass shift while those to the
Darwin term are shown to be small. For smaller charges or higher laser
intensities, parts of the electronic wavepacket may tunnel through the
potential barrier of the ionic core, and when recombining are shown to give
rise to keV harmonics in the radiation spectrum. Some parts of the wavepacket
do not recombine after ionisation and we find very energetic electrons in the
weakly relativistic regime of above threshold ionization.Comment: submitte
Real-time observation of interfering crystal electrons in high-harmonic generation
Accelerating and colliding particles has been a key strategy to explore the
texture of matter. Strong lightwaves can control and recollide electronic
wavepackets, generating high-harmonic (HH) radiation which encodes the
structure and dynamics of atoms and molecules and lays the foundations of
attosecond science. The recent discovery of HH generation in bulk solids
combines the idea of ultrafast acceleration with complex condensed matter
systems and sparks hope for compact solid-state attosecond sources and
electronics at optical frequencies. Yet the underlying quantum motion has not
been observable in real time. Here, we study HH generation in a bulk solid
directly in the time-domain, revealing a new quality of strong-field
excitations in the crystal. Unlike established atomic sources, our solid emits
HH radiation as a sequence of subcycle bursts which coincide temporally with
the field crests of one polarity of the driving terahertz waveform. We show
that these features hallmark a novel non-perturbative quantum interference
involving electrons from multiple valence bands. The results identify key
mechanisms for future solid-state attosecond sources and next-generation
lightwave electronics. The new quantum interference justifies the hope for
all-optical bandstructure reconstruction and lays the foundation for possible
quantum logic operations at optical clock rates
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