1,074 research outputs found
Ultrafast topological phenomena in gapped graphene
In the model of gapped graphene, we have shown how the recently predicted
topological resonances are solely related to the presence of an energy band gap
at the and points of the Brillouin zone. In the field of a
strong single-oscillation chiral (circularly-polarized) optical pulse, the
topological resonance causes the valley-selective population of the conduction
band. This population distribution represents a chiral texture in the
reciprocal space that is structured with respect to the pulse separatrix as has
earlier been predicted for transition metal dichalcogenides. As the band gap is
switched off, this chirality gradually disappears replaced by an achiral
distribution characteristic of graphene.Comment: 10 pages, 11 figure
Femtosecond valley polarization and topological resonances in transition metal dichalcogenides
We theoretically introduce the fundamentally fastest induction of a
significant population and valley polarization in a monolayer of a transition
metal dichalcogenide (i.e., and ). This may be
extended to other two-dimensional materials with the same symmetry. This valley
polarization can be written and read-out by a pulse consisting of just a single
optical oscillation with a duration of a few femtoseconds and an amplitude of
. Under these conditions, we predict a new
effect of {\em topological resonance}, which is due to Bloch motion of
electrons in the reciprocal space where electron population textures are formed
defined by non-Abelian Berry curvature. The predicted phenomena can be applied
for information storage and processing in PHz-band optoelectronics.Comment: 9 pages, 7 figure
Ultrafast strong-field absorption in gapped graphene
We study theoretically the strong-field absorption of an ultrafast optical
pulse by a gapped graphene monolayer. At low field amplitudes, the absorbance
in the pristine graphene is equal to the universal value of percent.
Although the ultrafast optical absorption for low field amplitudes is
independent of the polarization, linear or circular, of the applied optical
pulse, for high field amplitudes, the absorption strongly depends on the pulse
polarization. For a linearly polarized pulse, the optical absorbance is
saturated at the value of percent for the pulse's amplitude of
, but no such saturation is observed for a circularly
polarized pulse. For the gapped graphene, the absorption of a linearly
polarized pulse has a weak dependence on the bandgap, while for a circularly
polarized pulse, the absorption is very sensitive to the bandgap. %Opening a
bandgap in graphene by placing in on, for example, SiC substrate strongly
modify the ultrafast absorption at small field amplitudes
Femtosecond currents in transition metal dichalcogenides monolayers
We theoretically study the interaction of an ultrafast intense linearly
polarized optical pulse with monolayers of transition metal dichalcogenides
(TMDCs). Such a strong pulse redistributes electrons between the bands and
generates femtosecond currents during the pulse. Due to the large bandwidth of
the incident pulse, this process is completely off-resonant. While in TMDCs the
time-reversal symmetry is conserved, the inversion symmetry is broken and these
monolayers have the axial symmetry along armchair direction but not along the
zigzag one. Therefore, the pulse polarized along the asymmetric direction of
TMDC monolayer generates both longitudinal, i.e., along the direction of
polarization, and transverse, i.e., in the perpendicular direction, currents.
Such currents result in charge transfer through the system. We study different
TMDC materials and show how the femtosecond transport in TMDC monolayers depend
on their parameters, such as lattice constant and bandgap
Anomalous Ultrafast All-Optical Hall Effect in Gapped Graphene
We propose an ultrafast all-optical anomalous Hall effect in two-dimensional
(2D) semiconductors of hexagonal symmetry such as gapped graphene (GG),
transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (h-BN).
To induce such an effect, the material is subjected to a sequence of two
strong-field single-optical-cycle pulses: a chiral pump pulse followed within a
few femtoseconds by a probe pulse linearly polarized in the armchair direction
of the 2D lattice. Due to the effect of topological resonance, the first (pump)
pulse induces a large chirality (valley polarization) in the system, while the
second pulse generates a femtosecond pulse of the anomalous Hall current. The
proposed effect is the fundamentally the fastest all-optical anomalous Hall
effect possible in nature. It can be applied to ultrafast all-optical storage
and processing of information, both classical and quantum
Laser pulse waveform control of Dirac fermions in graphene
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 . 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
Topological resonance and single-optical-cycle valley polarization in gapped graphene
For gapped graphene, we predict that an intense ultrashort
(single-oscillation) circularly-polarized optical pulse can induce a large
population of the conduction band and a large valley polarization. With an
increase in the bandgap, the magnitude of the valley polarization gradually
increases from zero (for the native gapless graphene) to a value on the order
of unity. The energy bandwidth of the electrons excited into the conduction
band can be very large ( eV for a reasonable pulse amplitude of
). These phenomena are due to the effect of
topological resonance: the matching of the topological (geometric) phase and
the dynamic phase. Gapped graphene with tunable bandgap can be used as a
convenient generic model of two-dimensional semiconductors with honeycomb
generic lattice structures and broken inversion symmetry, such as transition
metal dichalcogenides
Fundamentally fastest optical processes at the surface of a topological insulator
We predict that a single oscillation of a strong optical pulse can
significantly populate the surface conduction band of a three-dimensional
topological insulator, Bi2Se3. Both linearly- and circularly-polarized pulses
generate chiral textures of interference fringes of population in the surface
Brillouin zone. These fringes constitute a self-referenced electron hologram
carrying information on the topology of the surface Bloch bands, in particular,
on the effect of the warping term of the low-energy Hamiltonian. These
electron-interference phenomena are in a sharp contrast to graphene where there
are no chiral textures for a linearly-polarized pulse and no interference
fringes for circularly-polarized pulse. These predicted reciprocal space
electron-population textures can be measured experimentally by time resolved
angle resolved photoelectron spectroscopy (TR-ARPES) to gain direct access to
non-Abelian Berry curvature at topological insulator surfaces.Comment: 10 pages, 10 figure
Topological resonance in Weyl semimetals in circularly-polarized optical pulse
We study theoretically the ultrafast electron dynamics of three-dimensional
Weyl semimetals in the field of a laser pulse. For a circularly-polarized
pulse, such dynamics is governed by topological resonance, which manifests
itself as a specific conduction band population distribution in the vicinity of
the Weyl points. The topological resonance is determined by the competition
between the topological phase and the dynamic phase and depends on the
handedness of a circularly polarized pulse. Also, we show that the conduction
band population induced by a circularly-polarized pulse that consists of two
oscillations with opposite handedness is highly chiral, which represents the
intrinsic chirality of the Weyl points
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