arXiv subjects

A. V. Kavokin

Publications and source records attributed to A. V. Kavokin.

At least 109 records · Page 6Linked to original sources

arXiv2011

Pseudo diamagnetism of four component exciton condensates

We analyze the spin structure of the ground state of four-component exciton condensates in coupled quantum wells as a function of spin-dependent interactions and applied magnetic field. The four components correspond to the degenerate exciton states characterized by $\pm2$ and $\pm1$ spin projections to the axis of the structure. We show that in a wide range of parameters, the chemical potential of the system increases as a function of magnetic field, which manifests a pseudo-diamagnetism of the system. The transitions to polarized two- and one-component condensates can be of the first-order in this case. The predicted effects are caused by energy conserving mixing of $\pm2$ and $\pm1$ excitons.

cond-mat.mes-hall↗

arXiv2011

Spin-to-Orbital Angular Momentum Conversion in Semiconductor Microcavities

We experimentally demonstrate a technique for the generation of optical beams carrying orbital angular momentum using a planar semiconductor microcavity. Despite being isotropic systems, the transverse electric - transverse magnetic (TE-TM) polarization splitting featured by semiconductor microcavities allows for the conversion of the circular polarization of an incoming laser beam into the orbital angular momentum of the transmitted light field. The process implies the formation of topological entities, a pair of optical half-vortices, in the intracavity field.

cond-mat.other↗

arXiv2011

Superconductivity with excitons and polaritons

A Bose--Einstein condensate of exciton polaritons coexisting with a Fermi gas of electrons has been recently proposed as a promising system for realisation of room-temperature superconductivity [Phys. Rev. Lett., 104, 106402 (2010)]. In order to find the optimum conditions for exciton and exciton-polariton mediated superconductivity, we study the attractive mechanism between electrons of a Cooper pair mediated by the exciton and exciton-polariton condensate and analyze the gap equation that follows. We specifically address microcavities with embedded n-doped quantum wells as well as coupled quantum wells hosting a condensate of spatially indirect excitons, put in contact with a two-dimensional electron gas. We show that engineering of the interaction in these peculiar Bose-Fermi mixtures is complex and sometimes counterintuitive, but leaves much freedom for optimization, making promising the realization of high-temperature superconductivity in multilayer semiconductor structures.

cond-mat.supr-con↗

arXiv2011

Probing the Dynamics of Spontaneous Quantum Vortices in Polariton Superfluids

The experimental investigation of spontaneously created vortices is of utmost importance for the understanding of quantum phase transitions towards a superfluid phase, especially for two dimensional systems that are expected to be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time resolved near-field interferometry we track the path of such vortices, created at random locations in an exciton-polariton condensate under pulsed non-resonant excitation, to their final pinning positions imposed by the stationary disorder. We formulate a theoretical model that successfully reproduces the experimental observations.

cond-mat.quant-gas↗

arXiv2010

Stimulated emission of terahertz radiation by semiconductor microcavities

We show that planar semiconductor microcavities in the strong coupling regime can be used as sources of stimulated terahertz (THz) radiation. Emitted THz photons would have a frequency of the spliting of the cavity polariton modes. The optical transition between upper and lower polariton branches is allowed due to mixing of the upper polariton state with one of excited exciton states. This transition is stimulated in the polariton laser regime.

cond-mat.mes-hall↗

arXiv2010

Rotons in a hybrid Bose-Fermi system

We calculate the spectrum of elementary excitations in a two-dimensional exciton superfluid in the vicinity of a two-dimensional electron gas. We show that attraction of excitons due to their scattering with free electrons may lead to formation of a roton minimum. The energy of this minimum may go below the ground state energy which manifests breaking of the superfluidity. The Berezinsky-Kosterlitz-Thouless phase transition temperature decreases due to the exciton-exciton attraction mediated by electrons.

cond-mat.quant-gas↗

arXiv2010

Spin rings in bi-stable planar semiconductor microcavities

A unique feature of exciton-polaritons, inherited from their mixed light-matter origin, is the strongly spin-dependent polariton-polariton interaction, which has been predicted to result in the formation of spin rings in real space [Shelykh et al., Phys. Rev. Lett. 100, 116401 (2008)]. Here we experimentally demonstrate the spin bi-stability of exciton-polaritons in an InGaAs-based semiconductor microcavity under resonant optical pumping. We observe the formation of spin rings whose size can be finely controlled in a spatial scale down to the micrometer range, much smaller than the spot size. We additionally evaluate the sign and magnitude of the antiparallel polariton spin interaction constant.

cond-mat.mes-hall↗

arXiv2009

Observation of Half Quantum Vortices in an Exciton-Polariton Condensate

Singly quantized vortices have been already observed in many systems including the superfluid helium, Bose Einstein condensates of dilute atomic gases, and condensates of exciton polaritons in the solid state. Two dimensional superfluids carrying spin are expected to demonstrate a different type of elementary excitations referred to as half quantum vortices characterized by a pi rotation of the phase and a pi rotation of the polarization vector when circumventing the vortex core. We detect half quantum vortices in an exciton-polariton condensate by means of polarization resolved interferometry, real space spectroscopy and phase imaging. Half quantum vortices coexist with single quantum vortices in our sample.

cond-mat.quant-gas↗

arXiv2009

Polariton-polariton scattering in microcavities: A microscopic theory

We apply the fermion commutation technique for composite bosons to polariton-polariton scattering in semiconductor planar microcavities. Derivations are presented in a simple and physically transparent fashion. A procedure of orthogonolization of the initial and final two-exciton state wavefunctions is used to calculate the effective scattering matrix elements and the scattering rates. We show how the bosonic stimulation of the scattering appears in this full fermionic approach whose equivalence to the bosonization method is thus demonstrated in the regime of low exciton density. We find an additional contribution to polariton-polariton scattering due to the exciton oscillator strength saturation, which we analyze as well. We present a theory of the polariton-polariton scattering with opposite spin orientations and show that this scattering process takes place mainly via dark excitonic states. Analytical estimations of the effective scattering amplitudes are given.

cond-mat.mes-hall↗

arXiv2009

Anisotropic Optical Spin Hall Effect in Semiconductor Microcavities

Propagating, directionally dependent, polarized spin-currents are created in an anisotropic planar semiconductor microcavity, via Rayleigh scattering of optically injected polaritons in the optical spin Hall regime. The influence of anisotropy results in the suppression or enhancement of the pseudospin precession of polaritons scattered into different directions. This is exploited to create intense spin currents by excitation on top of localized defects. A theoretical model considering the influence of the total effective magnetic field on the polariton pseudospin quantitatively reproduces the experimental observations.

cond-mat.mes-hall↗

arXiv2009

Signature of the microcavity exciton-polariton relaxation mechanism in the polarization of emitted light

We have performed real and momentum space spin-dependent spectroscopy of spontaneously formed exciton polariton condensates for a non-resonant pumping scheme. Under linearly polarized pump, our results can be understood in terms of spin-dependent Boltzmann equations in a two-state model. This suggests that relaxation into the ground state occurs after multiple phonon scattering events and only one polariton-polariton scattering. For the circular pumping case, in which only excitons of one spin are injected, a bottleneck effect is observed, implying inefficient relaxation.

cond-mat.mes-hall↗

arXiv2008

Spontaneous Polarisation Build up in a Room Temperature Polariton Laser

We observe the build up of strong (~50%) spontaneous vector polarisation in emission from a GaN-based polariton laser excited by short optical pulses at room temperature. The Stokes vector of emitted light changes its orientation randomly from one excitation pulse to another, so that the time-integrated polarisation remains zero. This behaviour is completely different to any previous laser. We interpret this observation in terms of the spontaneous symmetry breaking in a Bose-Einstein condensate of exciton-polaritons.

cond-mat.mtrl-sci↗

arXiv2008

Stochastic polarization formation in exciton-polariton Bose-Einstein condensates

We demonstrate theoretically the spontaneous formation of a stochastic polarization in exciton-polariton Bose-Einstein condensates in planar microcavities under pulsed excitation. Below the threshold pumping intensity (dependent on the polariton life-time) the average polarization degree is close to zero, whilst above threshold the condensate acquires a polarization described by a (pseudospin) vector with random orientation, in general. We establish the link between second order coherence of the polariton condensate and the distribution function of its polarization. We examine also the mechanisms of polarization dephasing and relaxation.

cond-mat.mes-hall↗

arXiv2008

Magnetic field effect on polarization and dispersion of exciton-polaritons in planar microcavities

The non-local dielectric response theory is extended to describe oblique reflection of light from quantum wells subjected to the magnetic field. This allows us to calculate the dispersion and polarization of the exciton-polariton modes in semiconductor microcavities in the presence of a magnetic field normal to the plane of the structure. We show that due to the interplay between the exciton Zeeman splitting and TE-TM splitting of the photon modes, four polariton dispersion branches are formed with a polarization gradually changing from circular in the exciton-like part to linear in the photon-like part of each branch. Faraday rotation in quantum microcavities is shown to be strongly enhanced as compared with the rotation in quantum wells.

cond-mat.mes-hall↗

arXiv2008

Optical circuits based on Polariton Neurons in Semiconductor Microcavities

By exploiting the polarization multistability of polaritons, we show that polarized signals can be conducted in the plane of a semiconductor microcavity along controlled channels or "neurons". Furthermore due to the interaction of polaritons with opposite spins it is possible to realize binary logic gates operating on the polarization degree of freedom. Multiple gates can be integrated together to form an optical circuit contained in a single semiconductor microcavity.

cond-mat.mes-hall↗

arXiv2008

Suppression of Zeeman splitting and polarization steps in localized exciton-polariton condensates

We show that the condensation of exciton-polaritons in semiconductor microcavities in an applied magnetic field manifests itself in the quenching of the Zeeman splitting of an elliptically polarized condensate. The circular polarization degree of a localized condensate with a finite number of particles increases as a function of the magnetic field with a step-like behavior. The width of each polarization step is fixed by the polariton-polariton interaction constants and the number of steps is fixed by the number of polaritons in the condensate. The magnetic susceptibility of the condensate depends qualitatively on the parity of the number of polaritons.

cond-mat.mes-hall↗

arXiv2006

Polarization multistability of cavity polaritons

New effects of polarization multistability and polarization hysteresis in a coherently driven polariton condensate in a semiconductor microcavity are predicted and theoretically analyzed. The multistability arises due to polarization-dependent polariton-polariton interactions and can be revealed in polarization resolved photoluminescence experiments. The pumping power required to observe this effect is of 4 orders of magnitude lower than the characteristic pumping power in conventional bistable optical systems.

cond-mat.other↗

arXiv2006

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.

cond-mat.mtrl-sci↗