251 research outputs found
Spin lifetimes and strain-controlled spin precession of drifting electrons in zinc blende type semiconductors
We study the transport of spin polarized electrons in n-GaAs using spatially
resolved continuous wave Faraday rotation. From the measured steady state
distribution, we determine spin relaxation times under drift conditions and, in
the presence of strain, the induced spin splitting from the observed spin
precession. Controlled variation of strain along [110] allows us to deduce the
deformation potential causing this effect, while strain along [100] has no
effect. The electric field dependence of the spin lifetime is explained
quantitatively in terms of an increase of the electron temperature.Comment: 5 pages, 6 figure
On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs
The electronic properties of shallow acceptors in p-doped GaAs{110} are
investigated with scanning tunneling microscopy at low temperature. Shallow
acceptors are known to exhibit distinct triangular contrasts in STM images for
certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to
identify their energetic origin and behavior. A crucial parameter - the STM
tip's work function - is determined experimentally. The voltage dependent
potential configuration and band bending situation is derived. Ways to validate
the calculations with the experiment are discussed. Differential conductivity
maps reveal that the triangular contrasts are only observed with a depletion
layer present under the STM tip. The tunnel process leading to the anisotropic
contrasts calls for electrons to tunnel through vacuum gap and a finite region
in the semiconductor.Comment: 11 pages, 8 figure
Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field
The effect of an external electric field F on the excitonic photoluminescence
(PL) spectra of a symmetric coupled double quantum well (DQW) is investigated
both theoretically and experimentally. We show that the variational method in a
two-particle electron-hole wave function approximation gives a good agreement
with measurements of PL on a narrow DQW in a wide interval of F including
flat-band regime. The experimental data are presented for an MBE-grown DQW
consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide
pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers.
The bias voltage is applied along the growth direction. Spatially direct and
indirect excitonic transitions are identified, and the radius of the exciton
and squeezing of the exciton in the growth direction are evaluated
variationally. The excitonic binding energies, recombination energies,
oscillator strengths, and relative intensities of the transitions as functions
of the applied field are calculated. Our analysis demonstrates that this simple
model is applicable in case of narrow DQWs not just for a qualitative
description of the PL peak positions but also for the estimation of their
individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B
Femtosecond differential transmission measurements on low temperature GaAs metal-semiconductor-metal structures
Intersubband gain in a Bloch oscillator and Quantum cascade laser
The link between the inversion gain of quantum cascade structures and the
Bloch gain in periodic superlattices is presented. The proposed theoretical
model based on the density matrix formalism is able to treat the gain mechanism
of the Bloch oscillator and Quantum cascade laser on the same footing by taking
into account in-plane momentum relaxation. The model predicts a dispersive
contribution in addition to the (usual) population-inversion-dependent
intersubband gain in quantum cascade structures and - in the absence of
inversion - provides the quantum mechanical description for the dispersive gain
in superlattices. It corroborates the predictions of the semi-classical
miniband picture, according to which gain is predicted for photon energies
lower than the Bloch oscillation frequency, whereas net absorption is expected
at higher photon energies, as a description which is valid in the
high-temperature limit. A red-shift of the amplified emission with respect to
the resonant transition energy results from the dispersive gain contribution in
any intersubband transition, for which the population inversion is small.Comment: 10 pages, 6 figure
Ultra-narrow linewidth CW sub-THz generation using GS based OFCG and n-i-pn-i-p superlattice photomixers
A report is presented on the photonic synthesis of ultra-narrow line-width continuous-wave (CW) sub-THz signals using a gain-switching (GS) based optical frequency comb generator (OFCG), selective optical filtering and a n-i-pn-i-p superlattice photomixer. This setup provides continuous tunability with a tuning resolution in the range of 0.1 Hz at 120 GHz and full width at half maximum of the generated signals below the limits of the measurement setup (< 10 Hz). The advantages of this system make it a very good candidate for applications requiring extremely low phase noise and continuous tunability, such as high resolution spectroscopy in the sub-THz and THz range.Work supported by the Spanish Ministry of Science
and Technology through the project TEC2009-14525-C02-02. The work
by A.R. Criado has been supported by the Spanish Ministry of Science
and Technology under the FPI Program, Grant# BES2010-030290.Publicad
Acoustically driven storage of light in a quantum well
The strong piezoelectric fields accompanying a surface acoustic wave on a
semiconductor quantum well structure are employed to dissociate optically
generated excitons and efficiently trap the created electron hole pairs in the
moving lateral potential superlattice of the sound wave. The resulting spatial
separation of the photogenerated ambipolar charges leads to an increase of the
radiative lifetime by orders of magnitude as compared to the unperturbed
excitons. External and deliberate screening of the lateral piezoelectric fields
triggers radiative recombination after very long storage times at a remote
location on the sample.Comment: 4 PostScript figures included, Physical Review Letters, in pres
Continuous wave sub-THz photonic generation with ultra-narrow linewidth, ultra-high resolution, full frequency range coverage and high long-term frequency stability
We report on a photonic system for generation of high quality continuous-wave (CW) sub-THz signals. The system consists on a gain-switching-based optical frequency comb generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using Fabry&-PĂ©rot tunable filters (FPTFs) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth 10 Hz at 120 GHz, complete frequency range coverage (60&-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz ({hbox{10}} -12} of generated frequency), high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, offers a very high performance, especially suited for sub-THz photonic local oscillators and high resolution spectroscopy.This work was supported by the Spanish Ministry of Science and Technology through the Project TEC2009-14525-C02-02. The
work of Ă. R. Criado has been supported by the Spanish Ministry of Science and Technology under the FPI Program, Grant BES2010-030290
Polarization dependence of the electroabsorption in low-temperature grown GaAs for above band-gap energies
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