70 research outputs found
Inversion of the excitonic spectrum in semiconductor microcavities
We show that coupling of odd photonic modes in microcavities with odd excitied exciton states in wide quantum wells (WQWs) situated at the nodes of the electric field of the cavity mode may lead to the inversion of the excitonic spectrum: an odd exciton-polariton state (X2) may lie at lower energy than even (X1) exciton state. The kinetics of exciton-polariton relaxation is expected to be very peculiar in this case: the excitons are accumulated in the optically inactive even state at the beginning, and then scatter to the upper and lower odd polariton modes. This regime is favourable for Bose-Einstein condensation of X1 excitons whose life-time becomes much longer because of their decoupling from the cavity mode. Experimentally, the evidence for condensation of “dark” X1 excitons would come from the coherent emission of light from the ground odd exciton-polariton state. If the WQW is shifted from the node of the field, X1 exctions get coupled to light, and the anticrossing between even and odd polaritons can be distinguished in reflection spectra.We show that coupling of odd photonic modes in microcavities with odd excitied exciton states in wide quantum wells (WQWs) situated at the nodes of the electric field of the cavity mode may lead to the inversion of the excitonic spectrum: an odd exciton-polariton state (X2) may lie at lower energy than even (X1) exciton state. The kinetics of exciton-polariton relaxation is expected to be very peculiar in this case: the excitons are accumulated in the optically inactive even state at the beginning, and then scatter to the upper and lower odd polariton modes. This regime is favourable for Bose-Einstein condensation of X1 excitons whose life-time becomes much longer because of their decoupling from the cavity mode. Experimentally, the evidence for condensation of “dark” X1 excitons would come from the coherent emission of light from the ground odd exciton-polariton state. If the WQW is shifted from the node of the field, X1 exctions get coupled to light, and the anticrossing between even and odd polaritons can be distinguished in reflection spectra
Injection of Orbital Angular Momentum and Storage of Quantized Vortices in Polariton Superfluids
We report the experimental investigation and theoretical modeling of a rotating polariton superfluid relying on an innovative method for the injection of angular momentum. This novel, multipump injection method uses four coherent lasers arranged in a square, resonantly creating four polariton populations propagating inwards. The control available over the direction of propagation of the superflows allows injecting a controllable nonquantized amount of optical angular momentum. When the density at the center is low enough to neglect polariton-polariton interactions, optical singularities, associated with an interference pattern, are visible in the phase. In the superfluid regime resulting from the strong nonlinear polariton-polariton interaction, the interference pattern disappears and only vortices with the same sign are persisting in the system. Remarkably, the number of vortices inside the superfluid region can be controlled by controlling the angular momentum injected by the pumps
Optical Anisotropy and Pinning of the Linear Polarization of Light in Semiconductor Microcavities
We report a strong experimental evidence of the optical anisotropy in a
CdTe-based microcavity: the polarization of light is pinned to one of the
crystallographic axes independently on the polarization of the excitation. The
polarization degree depends strongly on the excitation power, reaching almost
100 % in the stimulated regime. The relaxation time of the polarization is
about 1 ns. We argue that all this is an effect of a splitting of the polariton
doublet at k=0. We consider different sources for the splitting and conclude
that the most likely one is optical birefringence in the mirrors and/or the
cavity.Comment: 15 pages, 2 figures, accepted in Solid State Communication
Effects of inhomogeneous broadening on reflection spectra of Bragg multiple quantum well structures with a defect
The reflection spectrum of a multiple quantum well structure with an inserted
defect well is considered. The defect is characterized by the exciton frequency
different from that of the host's wells. It is shown that for relatively short
structures, the defect produces significant modifications of the reflection
spectrum, which can be useful for optoelectronic applications. Inhomogeneous
broadening is shown to affect the spectrum in a non-trivial way, which cannot
be described by the standard linear dispersion theory. A method of measuring
parameters of both homogeneous and inhomogeneous broadenings of the defect well
from a single CW reflection spectrum is suggested.Comment: 27 pages, 6 eps figures; RevTe
Low-temperature spin relaxation in n-type GaAs
Low-temperature electron spin relaxation is studied by the optical
orientation method in bulk n-GaAs with donor concentrations from 10^14 cm^{-3}
to 5x10^17 cm^{-3}.
A peculiarity related to the metal-to-insulator transition (MIT) is observed
in the dependence of the spin lifetime on doping near n_D = 2x10^16 cm^{-3}. In
the metallic phase, spin relaxation is governed by the Dyakonov-Perel
mechanism, while in the insulator phase it is due to anisotropic exchange
interaction and hyperfine interactio
Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature
Funding: the State of Bavaria and the ERC (unlimit-2D), the DFG via SFB689, GRK 1570 and KO3612/1-1.In this Rapid Communication, we address the chiral properties of valley exciton-polaritons in a monolayer of WS2 in the regime of strong light-matter coupling with a Tamm-plasmon resonance. We observe that the effect of valley polarization, which manifests in the circular polarization of the emitted photoluminescence as the sample is driven by a circularly polarized laser, is strongly enhanced in comparison to bare WS2 monolayers and can even be observed under strongly nonresonant excitation at ambient conditions. In order to explain this effect in more detail, we study the relaxation and decay dynamics of exciton-polaritons in our device, elaborate the role of the dark state, and present a microscopic model to explain the wave-vector-dependent valley depolarization by the linear polarization splitting inherent to the microcavity. We believe that our findings are crucial for designing novel polariton-valleytronic devices which can be operated at room temperature.PostprintPeer reviewe
Decay rate and renormalized frequency shift of a quantum wire Wannier exciton in a planar microcavity
The superradiant decay rate and frequency shift of a Wannier exciton in a
one-dimensional quantum wire are studied. It is shown that the dark mode
exciton can be examined experimentally when the quantum wire is embedded in a
planar microcavity. It is also found that the decay rate is greatly enhanced as
the cavity length is equal to the multiple wavelength of the emitted
photon. Similar to its decay rate counterpart, the frequency shift also shows
discontinuities at resonant modes.Comment: 12 pages, 2 figures. To appear in P. R. B. September 200
Anisotropic exchange interaction of localized conduction-band electrons in semiconductor structures
The spin-orbit interaction in semiconductors is shown to result in an
anisotropic contribution into the exchange Hamiltonian of a pair of localized
conduction-band electrons. The anisotropic exchange interaction exists in
semiconductor structures which are not symmetric with respect to spatial
inversion, for instance in bulk zinc-blend semiconductors. The interaction has
both symmetric and antisymmetric parts with respect to permutation of spin
components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the
strongest one. It contributes significantly into spin relaxation of localized
electrons; in particular, it governs low-temperature spin relaxation in n-GaAs
with the donor concentration near 10^16cm-3. The interaction must be allowed
for in designing spintronic devices, especially spin-based quantum computers,
where it may be a major source of decoherence and errors
Spin domains in one-dimensional conservative polariton solitons
We report stable orthogonally polarized domains in high-density polariton solitons propagating in a semiconductor microcavity wire. This effect arises from spin-dependent polariton–polariton interactions and pump-induced imbalance of polariton spin populations. The interactions result in an effective magnetic field acting on polariton spin across the soliton profile, leading to the formation of polarization domains. Our experimental findings are in excellent agreement with theoretical modeling taking into account these effects
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