211 research outputs found
Ultrafast non-linear optical signal from a single quantum dot: exciton and biexciton effects
We present results on both the intensity and phase-dynamics of the transient
non-linear optical response of a single quantum dot (SQD).
The time evolution of the Four Wave Mixing (FWM) signal on a subpicosecond
time scale is dominated by biexciton effects. In particular, for the
cross-polarized excitation case a biexciton bound state is found. In this
latter case, mean-field results are shown to give a poor description of the
non-linear optical signal at small times. By properly treating exciton-exciton
effects in a SQD, coherent oscillations in the FWM signal are clearly
demonstrated. These oscillations, with a period corresponding to the inverse of
the biexciton binding energy, are correlated with the phase dynamics of the
system's polarization giving clear signatures of non-Markovian effects in the
ultrafast regime.Comment: 10 pages, 3 figure
Tight-Binding model for semiconductor nanostructures
An empirical tight-binding (TB) model is applied to the
investigation of electronic states in semiconductor quantum dots. A basis set
of three -orbitals at the anions and one -orbital at the cations is
chosen. Matrix elements up to the second nearest neighbors and the spin-orbit
coupling are included in our TB-model. The parametrization is chosen so that
the effective masses, the spin-orbit-splitting and the gap energy of the bulk
CdSe and ZnSe are reproduced. Within this reduced TB-basis the
valence (p-) bands are excellently reproduced and the conduction (s-) band is
well reproduced close to the -point, i.e. near to the band gap. In
terms of this model much larger systems can be described than within a (more
realistic) -basis. The quantum dot is modelled by using the (bulk)
TB-parameters for the particular material at those sites occupied by atoms of
this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots
embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals).
Strain-effects are included by using an appropriate model strain field. Within
the TB-model, the strain-effects can be artifically switched off to investigate
the infuence of strain on the bound electronic states and, in particular, their
spatial orientation. The theoretical results for spherical nanocrystals are
compared with data from tunneling spectroscopy and optical experiments.
Furthermore the influence of the spin-orbit coupling is investigated
Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation
The impact of ground state amplification on the laser emission of In(Ga)As
quantum dot excited state lasers is studied in time-resolved experiments. We
find that a depopulation of the quantum dot ground state is followed by a drop
in excited state lasing intensity. The magnitude of the drop is strongly
dependent on the wavelength of the depletion pulse and the applied injection
current. Numerical simulations based on laser rate equations reproduce the
experimental results and explain the wavelength dependence by the different
dynamics in lasing and non-lasing sub-ensembles within the inhomogeneously
broadened quantum dots. At high injection levels, the observed response even
upon perturbation of the lasing sub-ensemble is small and followed by a fast
recovery, thus supporting the capacity of fast modulation in dual-state
devices
Exciton spin dynamics in spherical CdS quantum dots
Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in
detail experimentally and theoretically. Exciton states are calculated using
the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger
parameters, when the wurtzite lattice crystal field splitting and Coulomb
interaction between the electron-hole pair are taken into account exactly, both
the electron and hole wavefunction in the lowest exciton state are of S-type.
This rules out the spatial-symmetry-induced origin of the dark exciton in CdS
quantum dots. The exciton bleaching dynamics is studied using time- and
polarization-resolved transient absorption technique of ultrafast laser
spectroscopy. Several samples with a different mean size of CdS quantum dots in
different glass matrices were investigated. This enabled the separation of
effects that are typical for one particular sample from those that are general
for this type of material. The experimentally determined dependence of the
electron spin relaxation rate on the radius of quantum dots agrees well with
that computed theoretically.Comment: 24 pages, 10 figure
New Mechanism for Electronic Energy Relaxation in Nanocrystals
The low-frequency vibrational spectrum of an isolated nanometer-scale solid
differs dramatically from that of a bulk crystal, causing the decay of a
localized electronic state by phonon emission to be inhibited. We show,
however, that an electron can also interact with the rigid translational motion
of a nanocrystal. The form of the coupling is dictated by the equivalence
principle and is independent of the ordinary electron-phonon interaction. We
calculate the rate of nonradiative energy relaxation provided by this mechanism
and establish its experimental observability.Comment: 4 pages, Submitted to Physical Review
Equilibrium shapes and energies of coherent strained InP islands
The equilibrium shapes and energies of coherent strained InP islands grown on
GaP have been investigated with a hybrid approach that has been previously
applied to InAs islands on GaAs. This combines calculations of the surface
energies by density functional theory and the bulk deformation energies by
continuum elasticity theory. The calculated equilibrium shapes for different
chemical environments exhibit the {101}, {111}, {\=1\=1\=1} facets and a (001)
top surface. They compare quite well with recent atomic-force microscopy data.
Thus in the InP/GaInP-system a considerable equilibration of the individual
islands with respect to their shapes can be achieved. We discuss the
implications of our results for the Ostwald ripening of the coherent InP
islands. In addition we compare strain fields in uncapped and capped islands.Comment: 10 pages including 6 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Skin tolerant inactivation of multiresistant pathogens using far-UVC LEDs
Multiresistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause serious postoperative infections. A skin tolerant far-UVC (< 240 nm) irradiation system for their inactivation is presented here. It uses UVC LEDs in combination with a spectral filter and provides a peak wavelength of 233 nm, with a full width at half maximum of 12 nm, and an irradiance of 44 µW/cm2. MRSA bacteria in different concentrations on blood agar plates were inactivated with irradiation doses in the range of 15–40 mJ/cm2. Porcine skin irradiated with a dose of 40 mJ/cm2 at 233 nm showed only 3.7% CPD and 2.3% 6-4PP DNA damage. Corresponding irradiation at 254 nm caused 11–14 times higher damage. Thus, the skin damage caused by the disinfectant doses is so small that it can be expected to be compensated by the skin's natural repair mechanisms. LED-based far-UVC lamps could therefore soon be used in everyday clinical practice to eradicate multiresistant pathogens directly on humans
Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells
Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm(-2), sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities
Broadband semiconductor light sources operating at 1060 nm based on InAs:Sb/GaAs submonolayer quantum dots
In this paper, we investigate the potential of submonolayer-grown InAs:Sb/GaAs quantum dots as active medium for opto-electronic devices emitting in the 1060 nm spectral range. Grown as multiple sheets of InAs in a GaAs matrix, submonolayer quantum dots yield light-emitting devices with large material gain and fast recovery dynamics. Alloying these structures with antimony enhances the carrier localization and red shifts the emission, whereas dramatically broadening the optical bandwidth. In a combined experimental and numerical study, we trace this effect to an Sb-induced bimodal distribution of localized and delocalized exciton states. While the former do not participate in the lasing process, they give rise to a bandwidth broadening at superluminescence operation and optical amplification. Above threshold laser properties like gain and slope efficiency are mainly determined by the delocalized fraction of carriers
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