66 research outputs found
Robust single-parameter quantized charge pumping
This paper investigates a scheme for quantized charge pumping based on
single-parameter modulation. The device was realized in an AlGaAs-GaAs gated
nanowire. We find a remarkable robustness of the quantized regime against
variations in the driving signal, which increases with applied rf power. This
feature together with its simple configuration makes this device a potential
module for a scalable source of quantized current.Comment: Submitted to Appl. Phys. Let
Single-parameter quantized charge pumping in high magnetic fields
We study single-parameter quantized charge pumping via a semiconductor
quantum dot in high magnetic fields. The quantum dot is defined between two top
gates in an AlGaAs/GaAs heterostructure. Application of an oscillating voltage
to one of the gates leads to pumped current plateaus in the gate
characteristic, corresponding to controlled transfer of integer multiples of
electrons per cycle. In a perpendicular-to-plane magnetic field the plateaus
become more pronounced indicating an improved current quantization. Current
quantization is sustained up to magnetic fields where full spin polarization of
the device can be expected.Comment: This article has been submitted to Applied Physics Letter
Single-parameter non-adiabatic quantized charge pumping
Controlled charge pumping in an AlGaAs/GaAs gated nanowire by
single-parameter modulation is studied experimentally and theoretically.
Transfer of integral multiples of the elementary charge per modulation cycle is
clearly demonstrated. A simple theoretical model shows that such a quantized
current can be generated via loading and unloading of a dynamic quasi-bound
state. It demonstrates that non-adiabatic blockade of unwanted tunnel events
can obliterate the requirement of having at least two phase-shifted periodic
signals to realize quantized pumping. The simple configuration without multiple
pumping signals might find wide application in metrological experiments and
quantum electronics.Comment: 4 pages, 4 figure
Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields
We investigate the femtosecond kinetics of an optically excited 2D
magneto-plasma at intermediate and high densities under a strong magnetic field
perpendicular to the quantum well (QW). We assume an additional weak lateral
confinement which lifts the degeneracy of the Landau levels partially. We
calculate the femtosecond dephasing and relaxation kinetics of the laser pulse
excited magneto-plasma due to bare Coulomb potential scattering, because
screening is under these conditions of minor importance. In particular the
time-resolved and time-integrated four-wave mixing (FWM) signals are calculated
by taking into account three Landau subbands in both the valance and the
conduction band assuming an electron-hole symmetry. The FWM signals exhibit
quantum beats mainly with twice the cyclotron frequency. Contrary to general
expectations, we find no pronounced slowing down of the dephasing with
increasing magnetic field. On the contrary, one obtains a decreasing dephasing
time because of the increase of the Coulomb matrix elements and the number of
states in a given Landau subband. In the situation when the loss of scattering
channels exceeds these increasing effects, one gets a slight increase at the
dephasing time. However, details of the strongly modulated scattering kinetics
depend sensitively on the detuning, the plasma density, and the spectral pulse
width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres
Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons
We study theoretically the ultrafast nonlinear optical response of quantum
well excitons in a perpendicular magnetic field. We show that for
magnetoexcitons confined to the lowest Landau levels, the third-order
four-wave-mixing (FWM) polarization is dominated by the exciton-exciton
interaction effects. For repulsive interactions, we identify two regimes in the
time-evolution of the optical polarization characterized by exponential and
{\em power law} decay of the FWM signal. We describe these regimes by deriving
an analytical solution for the memory kernel of the two-exciton wave-function
in strong magnetic field. For strong exciton-exciton interactions, the decay of
the FWM signal is governed by an antibound resonance with an
interaction-dependent decay rate. For weak interactions, the continuum of
exciton-exciton scattering states leads to a long tail of the time-integrated
FWM signal for negative time delays, which is described by the product of a
power law and a logarithmic factor. By combining this analytic solution with
numerical calculations, we study the crossover between the exponential and
non-exponential regimes as a function of magnetic field. For attractive
exciton-exciton interaction, we show that the time-evolution of the FWM signal
is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig
Phonon-induced dephasing of localized optical excitations
The dynamics of strongly localized optical excitations in semiconductors is studied including electron-phonon interaction. The coupled microscopic equations of motion for the interband polarization and the carrier distribution functions contain coherent and incoherent contributions. While the coherent part is solved through direct numerical integration, the incoherent one is treated by means of a generalized Monte Carlo simulation. The approach is illustrated for a simple model system. The temperature and excitation energy dependence of the optical dephasing rate is analyzed and the results are compared to those of alternative approaches
Quantum kinetics and thermalization in a particle bath model
We study the dynamics of relaxation and thermalization in an exactly solvable
model of a particle interacting with a harmonic oscillator bath. Our goal is to
understand the effects of non-Markovian processes on the relaxational dynamics
and to compare the exact evolution of the distribution function with
approximate Markovian and Non-Markovian quantum kinetics. There are two
different cases that are studied in detail: i) a quasiparticle (resonance) when
the renormalized frequency of the particle is above the frequency threshold of
the bath and ii) a stable renormalized `particle' state below this threshold.
The time evolution of the occupation number for the particle is evaluated
exactly using different approaches that yield to complementary insights. The
exact solution allows us to investigate the concept of the formation time of a
quasiparticle and to study the difference between the relaxation of the
distribution of bare particles and that of quasiparticles. We derive a
non-Markovian quantum kinetic equation which resums the perturbative series and
includes off-shell effects. A Markovian approximation that includes off-shell
contributions and the usual Boltzmann equation (energy conserving) are obtained
from the quantum kinetic equation in the limit of wide separation of time
scales upon different coarse-graining assumptions. The relaxational dynamics
predicted by the non-Markovian, Markovian and Boltzmann approximations are
compared to the exact result. The Boltzmann approach is seen to fail in the
case of wide resonances and when threshold and renormalization effects are
important.Comment: 39 pages, RevTex, 14 figures (13 eps figures
Optical microscopy in the nano-world
Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland
Luminescence spectra and kinetics of disordered solid solutions
We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of “radiative” states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the “radiative” localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of CdS(1−c)Sec and ZnSe(1−c)Tec solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated “radiative” states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence
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Ultrafast spectroscopy of GaAs under magnetic field
Surprising and novel results are obtained for both the linear and the nonlinear optical response of GaAs under magnetic field. Using a variety of spectroscopic techniques, we measure field dependent effects due to Coulomb correlation
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