175 research outputs found
Large quantum dots with small oscillator strength
We have measured the oscillator strength and quantum efficiency of excitons
confined in large InGaAs quantum dots by recording the spontaneous emission
decay rate while systematically varying the distance between the quantum dots
and a semiconductor-air interface. The size of the quantum dots is measured by
in-plane transmission electron microscopy and we find average in-plane
diameters of 40 nm. We have calculated the oscillator strength of excitons of
that size and predict a very large oscillator strength due to Coulomb effects.
This is in stark contrast to the measured oscillator strength, which turns out
to be much below the upper limit imposed by the strong confinement model. We
attribute these findings to exciton localization in local potential minima
arising from alloy intermixing inside the quantum dots.Comment: 4 pages, 3 figures, submitte
Coherent spin dynamics of an interwell excitonic gas in GaAs/AlGaAs coupled quantum wells
The spin dynamics of an interwell excitons gas has been investigated in n-i-n
GaAs/AlGaAs coupled quantum wells (CQWs). In these heterostructures the
electron and the hole are spatially separated in neighboring quantum wells by a
narrow AlAs barrier, when an electric field is applied. The time evolution
kinetics of the interwell exciton photoluminescence has been measured under
resonant excitation of the 1sHH intrawell exciton, using a pulsed tunable
laser. The formation of a collective exciton phase in time and the temperature
dependence of its spin relaxation rate have been studied. The spin relaxation
rate of the interwell excitons is strongly reduced in the collective phase.
This observation provides evidence for the coherence of the indirect excitons
collective phase at temperatures below a critical .Comment: 8 pages, 9 figure
Size-Dependence of the Wavefunction of Self-Assembled Quantum Dots
The radiative and non-radiative decay rates of InAs quantum dots are measured
by controlling the local density of optical states near an interface. From
time-resolved measurements we extract the oscillator strength and the quantum
efficiency and their dependence on emission energy. From our results and a
theoretical model we determine the striking dependence of the overlap of the
electron and hole wavefunctions on the quantum dot size. We conclude that the
optical quality is best for large quantum dots, which is important in order to
optimally tailor quantum dot emitters for, e.g., quantum electrodynamics
experiments.Comment: 5 pages, 3 figure
Phonon-induced polariton superlattices
We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion
Magnetophonon resonance in photoluminescence excitation spectra of magnetoexcitons in GaAs/Al0.3Ga0.7As superlattice
Strong increase in the intensity of the peaks of excited magneto-exciton (ME)
states in the photoluminescence excitation (PLE) spectra recorded for the
ground heavy-hole magneto-excitons (of the 1sHH type) has been found in a
GaAs/AlGaAs superlattice in strong magnetic field B applied normal to the
sample layers. While varying B the intensities of the PLE peaks have been
measured as functions of energy separation between excited ME peaks
and the ground state of the system. The resonance profiles have been found to
have maxima at close to the energy of the GaAs LO-phonon.
However, the value of depends on quantum numbers of the
excited ME state. The revealed very low quantum efficiency of the investigated
sample allows us to ascribe the observed resonance to the enhancement of the
non-radiative magneto-exciton relaxation rate arising due to LO-phonon
emission. The presented theoretical model, being in a good agreement with
experimental observations, provides a method to extract 1sHH magneto-exciton
``in-plane" dispersion from the dependence of on the
excited ME state quantum numbers.Comment: 9 pages, 6 figure
Self-phase modulation of a single-cycle THz pulse
We demonstrate self-phase modulation (SPM) of a single-cycle THz pulse in a semiconductor, using bulk n-GaAs as a model system. The SPM arises from the heating of free electrons in the electric field of the THz pulse. Electron heating leads to an ultrafast reduction of the plasma frequency, which results in a strong modification of the THz-range dielectric function of the material. THz SPM is observed directly in the time domain as a characteristic reshaping of single-cycle THz pulse. In the frequency domain, it corresponds to a strong frequency-dependent refractive index nonlinearity of n-GaAs, which is found to be both positive and negative within the broad spectrum of the THz pulse. The spectral position of zero nonlinearity is defined by the electron momentum relaxation rate. Nonlinear spectral broadening and compression of the single-cycle THz pulse was also observed
Electron microscopic and optical investigations of the indium distribution GaAs capped InxGa1-xAs islands
Results from a structural and optical analysis of buried InxGa1-xAs islands carried out after the process of GaAs overgrowth are presented. It is found that during the growth process, the indium concentration profile changes and the thickness of the wetting layer emanating from a Stranski-Krastanow growth mode grows significantly. Quantum dots are formed due to strong gradients in the indium concentration, which is demonstrated by photoluminescence and excitation spectroscopy of the buried InxGa1-xAs islands. (C) 1997 American Institute of Physics
- âŚ