arXiv subjects

A. V. Kavokin

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

At least 73 records · Page 4Linked to original sources

arXiv2015

Giant absorption of light by molecular vibrations on a chip

Vibrational overtone spectroscopy of molecules is a powerful tool for drawing information on molecular structure and dynamics. It relies on absorption of near infrared radiation (NIR) by molecular vibrations. Here we show the experimental evidence of giant enhancement of the absorption of light in solutions of organic molecules due to the switch from ballistic to diffusive propagation of light through a channel silicate glass waveguide. We also experimentally address a dynamics of absorption as a function of time of adsorption of the organic molecules on a waveguide. The observed enhancement in diffusion regime is by a factor of 300 in N-Methylaniline and by factor of 80 in Aniline compared to the expected values in the ballistic propagation of light in a waveguide. Our results underscore the importance of a guide surface modification and the disordered6 molecular nano-layer in enhancement of absorption by amines on engineered integrated system.

physics.optics↗

arXiv2015

Quantization of entropy in a quasi-two-dimensional electron gas

We demonstrate that the partial entropy of a two-dimensional electron gas (2DEG) exhibits quantized peaks at resonances between the chemical potential and electron levels of size quantization. In the limit of no scattering, the peaks depend only on the subband quantization number and are independent on material parameters, shape of the confining potential, electron effective mass and temperature. The quantization of partial entropy is a signature of a topological phase transition in a 2DEG. In the presence of stationary disorder, the magnitude of peaks decreases. Its deviation from the quantized values is a direct measure of the disorder induced smearing of the electronic density of states.

cond-mat.mes-hall↗

arXiv2015

Dynamics of the Energy Relaxation in a Parabolic Quantum Well Laser

We explore two parabolic quantum well (PQW) samples, with and without Bragg mirrors, in order to optimise the building blocks of a Bosonic Cascade Laser. The photoluminescence spectra of a PQW microcavity sample is compared against that of a conventional microcavity with embedded quantum wells (QWs) to demonstrate that the weak coupling lasing in a PQW sample can be achieved. The relaxation dynamics in a conventional QW microcavity and in the PQW microcavity was studied by a non-resonant pump-pump excitation method. Strong difference in the relaxation characteristics between the two samples was found. The semi-classical Boltzmann equations were adapted to reproduce the evolution of excitonic populations within the PQW as a function of the pump power and the output intensity evolution as a function of the pump-pump pulse delay. Fitting the PQW data confirms the anticipated cascade relaxation, paving the way for such a system to produce terahertz radiation.

cond-mat.mes-hall↗

arXiv2015

Spin-noise-based magnetometry of an $n$-doped GaAs microcavity in the field of elliptically polarized light

Recently reported optical nuclear orientation in the $n$-doped GaAs microcavity under pumping in nominal transparency region of the crystal [Appl. Phys. Lett. $\mathbf{106}$, 242405 (2015)] has arisen a number of questions, the main of them concerning mechanisms of angular momentum transfer from the light to the nuclear spin system and the nature of the light-related magnetic fields accompanying the optical nuclear polarization. In this paper, we use the spin noise spectroscopy for magnetometric purposes, particularly, to study effective fields acting upon electron spin system of an $n$-GaAs layer inside a high-Q microcavity in the presence of elliptically polarized probe beam. In addition to the external magnetic field applied to the sample in the Voigt geometry and the Overhauser field created by optically oriented nuclei, the spin noise spectrum reveals an additional effective, "optical," magnetic field produced by elliptically polarized probe itself. This field is directed along the light propagation axis, with its sign being determined by the sign of the probe helicity and its magnitude depending on degree of circular polarization and intensity of the probe beam. We analyze properties of this optical magnetic field and suggest that it results from the optical Stark effect in the field of the elliptically polarized electromagnetic wave.

cond-mat.mes-hall↗

arXiv2015

Multiple quantum beats of quantum confined exciton states in InGaAs/GaAs quantum well

Multiple quantum beats of a system of the coherently excited quantum confined exciton states in a high-quality heterostructure with a wide InGaAs/GaAs quantum well are experimentally detected by the spectrally resolved pump-probe method for the first time. The beat signal is observed as at positive as at negative delays between the pump and probe pulses. A theoretical model is developed, which allows one to attribute the QBs at negative delay to the four-wave mixing (FWM) signal detected at the non-standard direction. The beat signal is strongly enhanced by the interference of the FWM wave with the polarization created by the probe pulse. At positive delay, the QBs are due to the mutual interference of the quantum confined exciton states. Several QB frequencies are observed in the experiments, which coincide with the interlevel spacings in the exciton system. The decay time for QBs is of order of several picoseconds at both the positive and negative delays. They are close to the relaxation time of exciton population that allows one to consider the exciton depopulation as the main mechanism of the coherence relaxation in the system under study.

cond-mat.mes-hall↗

arXiv2015

Quantum statistics of bosonic cascades

Bosonic cascades formed by lattices of equidistant energy levels sustaining radiative transitions between nearest layers are promising for the generation of coherent terahertz radiation. We show how, also for the light emitted by the condensates in the visible range, they introduce new regimes of emission. Namely, the quantum statistics of bosonic cascades exhibit super-bunching plateaus. This demonstrates further potentialities of bosonic cascade lasers for the engineering of quantum properties of light useful for imaging applications.

cond-mat.quant-gas↗

arXiv2015

Spin waves in semiconductor microcavities

We show theoretically that a weakly interacting gas of spin-polarized exciton-polaritons in a semiconductor microcavity supports propagation of spin waves. The spin waves are characterised by a parabolic dispersion at small wavevectors which is governed by the polariton-polariton interaction constant. Due to spin-anisotropy of polariton-polariton interactions the dispersion of spin waves depends on the orientation of the total polariton spin. For the same reason, the frequency of homogeneous spin precession/polariton spin resonance depends on their polarization degree.

cond-mat.mes-hall↗

arXiv2015

Single-shot measurement of transient nuclear magnetization with spin-noise spectroscopy in n-GaAs microcavities

We exploit spin noise spectroscopy (SNS) to directly observe build-up of dynamic nuclear polarization and relaxation of a perturbed nuclear spin-system to its equilibrium state in a single-shot experiment. The SNS experiments were performed on a layer of bulk $n$-type GaAs embedded into a high-finesse microcavity with negative detuning. The dynamic nuclear spin polarization is observed as a shift of the peak in the electron spin noise spectrum due to the build-up of the Overhauser field acting on the electron spin. The relaxation dynamics of nuclear spin system was revealed in the time-resolved SNS experiments where the exponential decay of the Overhauser field with characteristic timescale of hundreds of seconds was detected. We show that elliptically polarized laser beam tuned in resonance with the cavity mode, whose energy corresponds to nominal transparency of the semiconductor, can nevertheless produce a sizable nuclear polarization.

cond-mat.mes-hall↗

arXiv2015

Permanent Rabi oscillations in coupled exciton-photon systems with PT-symmetry

We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on incoherent excitonic reservoir engineering. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time (PT) symmetry of the coupled exciton-photon system realised in a specific regime of pumping to the exciton state and depletion of the reservoir. For effective non-zero exciton-photon detuning, permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realisation of integrated circuits based on exciton-polariton condensates.

cond-mat.mes-hall↗

arXiv2015

Nontrivial relaxation dynamics of excitons in high-quality InGaAs/GaAs quantum wells

Photoluminescence (PL) and reflectivity spectra of a high-quality InGaAs/GaAs quantum well structure reveal a series of ultra-narrow peaks attributed to the quantum confined exciton states. The intensity of these peaks decreases as a function of temperature, while the linewidths demonstrate a complex and peculiar behavior. At low pumping the widths of all peaks remain quite narrow ($< 0.1$ meV) in the whole temperature range studied, $4 - 30K$. At the stronger pumping, the linewidth first increases and than drops down with the temperature rise. Pump-probe experiments show two characteristic time scales in the exciton decay, $< 10$ps and $15 - 45ns$, respectively. We interpret all these data by an interplay between the exciton recombination within the light cone, the exciton relaxation from a non-radiative reservoir to the light cone, and the thermal dissociation of the non-radiative excitons. The broadening of the low energy exciton lines is governed by the radiative recombination and scattering with reservoir excitons while for the higher energy states the linewidths are also dependent on the acoustic phonon relaxation processes.

cond-mat.mes-hall↗

arXiv2015

Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs

Comparative studies of the resonant transmission of light in the vicinity of exciton resonances, measured for 15 few-micron GaAs crystal slabs with different impurity concentrations $N$, reveal an unexpected tendency. While $N$ spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as $N^{1/6}$. We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in the semiconductor.

cond-mat.mes-hall↗

arXiv2014

Exciton-photon correlations in bosonic condensates of exciton-polaritons

Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the "hidden variable" which characterizes the rate of exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.

cond-mat.mes-hall↗

arXiv2014

Spanning the full Poincaré sphere with polariton Rabi oscillations

We propose theoretically and demonstrate experimentally a generation of light pulses whose polarization varies temporally to cover selected areas of the Poincaré sphere with tunable swirling speed and total duration (1 ps and 10 ps respectively in our implementation). The effect relies on the Rabi oscillations of two polarized fields in the strong coupling regime, excited by two counter-polarized and delayed pulses. The interferences of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle on the sphere of controllable radius to a single point at long times. The positioning of the initial and final states allows to engineer the type of polarization spanning, including a full sweeping of the Poincaré sphere. The universality and simplicity of the scheme should allow for the deployment of time varying polarization fields at a technologically exploitable level.

cond-mat.quant-gas↗

arXiv2014

Controllable structuring of exciton-polariton condensates in cylindrical pillar microcavities

We observe condensation of exciton polaritons in quantum states composed of concentric rings when exciting cylindrical pillar GaAs/AlGaAs microcavities non-resonantly by a focused laser beam normally incident at the center of the pillar. The number of rings depends on the pumping intensity and the pillar size, and may achieve 5 in the pillar of 40 mkm diameter. Breaking the axial symmetry when moving the excitation spot away from the pillar center leads to transformation of the rings into a number of bright lobes corresponding to quantum states with nonzero angular momenta. The number of lobes, their shape and location are dependent on the spot position. We describe the out-of-equilibrium condensation of polaritons in the states with different principal quantum numbers and angular momenta with a formalism based on Boltzmann-Gross-Pitaevskii equations accounting for repulsion of polaritons from the exciton reservoir formed at the excitation spot and their spatial confinement by the pillar boundary.

cond-mat.mes-hall↗

arXiv2014

Superconductivity in Semiconductor Structures: the Excitonic Mechanism

We study theoretically the effect of the fermion and boson densities on the superconductivity transition critical temperature $(T_c)$ of a two dimensional electron gas (2DEG), where superconductivity is mediated by a Bose-Einstein condensate of exciton-polaritons. The critical temperature is found to increase with the boson density, but surprisingly it decreases with the 2DEG density increase. This makes doped semiconductor structures with shallow Fermi energies better adapted for observation of the exciton-induced superconductivity than metallic layers. For the realistic GaAs-based microcavities containing-doped and neutral quantum wells we estimate $T_c$ as close to 50K. Superconductivity is suppressed by magnetic fields of the order of 4T due to the Fermi surface renormalisation.

cond-mat.supr-con↗

arXiv2014

Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities

Photoinduced Kerr rotation by more than $π/2$ radians is demonstrated in planar quantum well microcavity in the strong coupling regime. This result is close to the predicted theoretical maximum of $π$. It is achieved by engineering microcavity parameters such that the optical impedance matching condition is reached at the smallest negative detuning between exciton resonance and the cavity mode. This ensures the optimum combination of the exciton induced optical non-linearity and the enhancement of the Kerr angle by the cavity. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.

cond-mat.mes-hall↗

arXiv2014

Nonlinear optical spectroscopy of indirect excitons in biased coupled quantum wells

Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.

cond-mat.mes-hall↗

arXiv2014

Exciton-Polariton Oscillations in Real Space

We introduce and model spin-Rabi oscillations based on exciton-polaritons in semiconductor microcavities. The phase and polarization of oscillations can be controlled by resonant coherent pulses and the propagation of oscillating domains gives rise to phase-dependent interference patterns in real space. We show that interbranch polariton-polariton scattering controls the propagation of oscillating domains, which can be used to realize logic gates based on an analogue variable phase.

cond-mat.mes-hall↗