620 research outputs found
Transient response under ultrafast interband excitation of an intrinsic graphene
The transient evolution of carriers in an intrinsic graphene under ultrafast
excitation, which is caused by the collisionless interband transitions, is
studied theoretically. The energy relaxation due to the quasielastic acoustic
phonon scattering and the interband generation-recombination transitions due to
thermal radiation are analyzed. The distributions of carriers are obtained for
the limiting cases when carrier-carrier scattering is negligible and when the
intercarrier scattering imposes the quasiequilibrium distribution. The
transient optical response (differential reflectivity and transmissivity) on a
probe radiation and transient photoconductivity (response on a weak dc field)
appears to be strongly dependent on the relaxation and recombination dynamics
of carriers.Comment: 9 pages, 8 figure
Gain without inversion in a biased superlattice
Intersubband transitions in a superlattice under homogeneous electric field
is studied within the tight-binding approximation. Since the levels are
equi-populated, the non-zero response appears beyond the Born approximation.
Calculations are performed in the resonant approximation with scattering
processes exactly taken into account. The absorption coefficient is equal zero
for the resonant excitation while a negative absorption (gain without
inversion) takes place below the resonance. A detectable gain in the THz
spectral region is obtained for the low-doped -based superlattice and
spectral dependencies are analyzed taking into account the interplay between
homogeneous and inhomogeneous mechanisms of broadening.Comment: 6 pages, 4 figure
Depletion of carriers and negative differential conductivity in an intrinsic graphene under a dc electric field
The heating of carriers in an intrinsic graphene under an abrupt switching
off a dc electric field is examined taking into account both the energy
relaxation via acoustic and optic phonons and the interband
generation-recombination processes. The later are caused by the interband
transitions due to optical phonon modes and thermal radiation. Description of
the temporal and steady-state responses, including the nonequilibrium
concentration and energy as well as the current-voltage characteristics, is
performed. At room temperature, a nearly-linear current-voltage characteristic
and a slowly-varied concentration take place for fields up to -- 20 kV/cm.
Since a predominant recombination of high-energy carriers due to optical phonon
emission at low temperatures, a depletion of concentration takes place below --
250 K. For lower temperatures the current tends to be saturated and a negative
differential conductivity appears below -- 170 K in the region of fields -- 10
V/cm.Comment: 8 pages, 10 figures, extended versio
Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubband excitation
Ultrafast intersubband excitation of electrons in tunnell-coupled wells is
studied depending on the structure parameters, the duration of the infrared
pump and the detuning frequency. The temporal dependencies of the photoinduced
concentration and dipole moment are obtained for two cases of transitions: from
the single ground state to the tunnel-coupled excited states and from the
tunnel-coupled states to the single excited state. The peculiarities of
dephasing and population relaxation processes are also taken into account. The
nonlinear regime of the response is also considered when the splitting energy
between the tunnel-coupled levels is renormalized by the photoexcited electron
concentration. The dependencies of the period and the amplitude of oscillations
on the excitation pulse are presented with a description of the nonlinear
oscillations damping.Comment: 8 pages, 12 figure
Thermal-radiation-induced nonequilibrium carriers in an intrinsic graphene
We examine an intrinsic graphene connected to the phonon thermostat at
temperature T under irradiation of thermal photons with temperature T_r, other
than T. The distribution of nonequilibrium electron-hole pairs was obtained for
the cases of low and high concentration of carriers. For the case when the
interparticle scattering is unessential, the distribution function is
determined by the interplay of intraband relaxation of energy due to acoustic
phonons and interband radiative transitions caused by the thermal radiation.
When the Coulomb scattering dominates, then the quasi-equilibrium distribution
with effective temperature and non-equilibrium concentration, determined
through balance equations, is realized. Due to the effect of thermal radiation
with temperature concentration and conductivity of carriers in
graphene modify essentially. It is demonstrated, that at the negative
interband absorption, caused by the inversion of carriers distribution, can
occur, i.e. graphene can be unstable under thermal irradiation.Comment: 5 pages, 4 figure
Trion dynamics in coupled double quantum wells. Electron density effects
We have studied the coherent dynamics of injected electrons when they are
either free or bounded both in excitons and in trions (charged excitons). We
have considered a remotely doped asymmetric double quantum well where an excess
of free electrons and the direct created excitons generate trions. We have used
the matrix density formalism to analyze the electron dynamics for different
concentration of the three species. Calculations show a significant
modification of the free electron inter-sublevel oscillations cWe have studied
the coherent dynamics of injected electrons when they are aused by electrons
bound in excitons and trions. Based on the present calculations we propose a
method to detect trions through the emitted electromagnetic radiation or the
current density.Comment: 14 pages, 13 figure
Frequency dependence of induced spin polarization and spin current in quantum wells
Dynamic response of two-dimensional electron systems with spin-orbit
interaction is studied theoretically on the basis of quantum kinetic equation,
taking into account elastic scattering of electrons. The spin polarization and
spin current induced by the applied electric field are calculated for the whole
class of electron systems described by p-linear spin-orbit Hamiltonians. The
absence of nonequilibrium intrinsic static spin currents is confirmed for these
systems with arbitrary (nonparabolic) electron energy spectrum. Relations
between the spin polarization, spin current, and electric current are
established. The general results are applied to the quantum wells grown in
[001] and [110] crystallographic directions, with both Rashba and Dresselhaus
types of spin-orbit coupling. It is shown that the existence of the fixed
(momentum-independent) precession axes in [001]-grown wells with equal Rashba
and Dresselhaus spin velocities or in symmetric [110]-grown wells leads to
vanishing spin polarizability at arbitrary frequency of the applied electric
field. This property is explained by the absence of Dyakonov-Perel-Kachorovskii
spin relaxation for the spins polarized along these precession axes. As a
result, a considerable frequency dispersion of spin polarization at very low
frequency in the vicinity of the fixed precession axes is predicted. Possible
effects of extrinsic spin-orbit coupling on the obtained results are discussed.Comment: 14 pages, 6 figures; published with minor corrections in Phys. Rev.
Voltage and temperature dependencies of conductivity in gated graphene
The resistivity of gated graphene is studied taking into account electron and
hole scattering by short- and long-range structural imperfections the
characteristics of disorder were taken from the scanning tunneling microscopy
data and by acoustic phonons. The calculations are based on the quasiclassical
kinetic equation with the normalization condition fixed by surface charge. The
gate-voltage and temperature effects on the resistance peak, which is centered
at the point of intrinsic conductivity, are found to be in agreement with the
transport measurements.Comment: 4 pages, 4 Fig
Rabi oscillations under ultrafast excitation of graphene
We study coherent nonlinear dynamics of carriers under ultrafast interband
excitation of an intrinsic graphene. The Rabi oscillations of response appear
with increasing of pumping intensity. The photoexcited distribution is
calculated versus time and energy taking into account the effects of energy
relaxation and dephasing. Spectral and temporal dependencies of the response on
a probe radiation (transmission and reflection coefficients) are considered for
different pumping intensities and the Rabi oscillations versus time and
intensity are analyzed.Comment: 6 pages, 6 figure
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