K. V. Kavokin

Fast control of nuclear spin polarization in an optically pumped single quantum dot.

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin, or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 10(5) optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-μK nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.

Measurements of nuclear spin dynamics by spin-noise spectroscopy

We exploit the potential of the spin noise spectroscopy (SNS) for studies of nuclear spin dynamics in n-GaAs. The SNS experiments were performed on bulk n-type GaAs layers embedded into a high-finesse microcavity at negative detuning. In our experiments, nuclear spin polarisation initially prepared by optical pumping is monitored in real time via a shift of the peak position in the electron spin noise spectrum. We demonstrate that this shift is a direct measure of the Overhauser field acting on the electron spin. The dynamics of nuclear spin is shown to be strongly dependent on the electron concentration.

Spin noise explores local magnetic fields in a semiconductor.

Rapid development of spin noise spectroscopy of the last decade has led to a number of remarkable achievements in the fields of both magnetic resonance and optical spectroscopy. In this report, we demonstrate a new – magnetometric – potential of the spin noise spectroscopy and use it to study magnetic fields acting upon electron spin-system of an n-GaAs layer in a high-Q microcavity probed by elliptically polarized light. Along with the external magnetic field, applied to the sample, the spin noise spectrum revealed the Overhauser field created by optically oriented nuclei and an additional, previously unobserved, field arising in the presence of circularly polarized light. This “optical field” is directed along the light propagation axis, with its sign determined by sign of the light helicity. We show that this field results from the optical Stark effect in the field of the elliptically polarized light. This conclusion is supported by theoretical estimates.I. I. Ryzhov, G. G. Kozlov, D. S. Smirnov, M. M. Glazov, 1 Yu. P. Efimov, S. A. Eliseev, V. A. Lovtcius, V. V. Petrov, K. V. Kavokin, 1 A. V. Kavokin, 1 and V. S. Zapasskii Spin Optics Laboratory, St.-Petersburg State University, 1 Ul’anovskaya, Peterhof, St.-Petersburg 198504, Russia Ioffe Institute of the RAS, 26 Polytekhnicheskaya, St.-Petersburg 194021, Russia Resource Center “Nanophotonics”, St.-Petersburg State University, 1 Ul’anovskaya, Peterhof, St.-Petersburg 198504, Russia Department of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom

Electronic control of the polarization of light emitted by polariton lasers

We propose a mechanism of electronic control of the polarization of the light emitted by polariton lasers. An electric field applied along the growth axis of the microcavity splits the ground polariton state into [110] and [1–1¯0] polarized components. We perform kinetic simulations which show that above a pumping threshold, a condensate of exciton polaritons is formed at the lowest-energy state. In this regime, the emission of the polariton laser is either [110] or [1–1¯0] linearly polarized depending on the direction of the applied field.

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.

Exciton spin decay modified by strong electron-hole exchange interaction

We study exciton spin decay in the regime of strong electron-hole exchange interaction, which occurs in a wide variety of semiconductor nanostructures. In this regime the electron spin precession is restricted within a sector formed by the external magnetic field and the effective exchange fields triggered by random spin flips of the hole. Using Hanle effect measurements, we demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te quantum wells. We present calculations that provide a consistent description of the experimental results, which is supported by independent measurements of the parameters entering the model.

Optics of spin-noise-induced gyrotropy of an asymmetric microcavity

S. V. Poltavtsev,1 I. I. Ryzhov,1 R. V. Cherbunin,1 A. V. Mikhailov,1 N. E. Kopteva,1 G. G. Kozlov,1 K. V. Kavokin,1, 2 V. S. Zapasskii,1 P. V. Lagoudakis,3 and A. V. Kavokin1, 3 1Spin-Optics laboratory, St. Petersburg State University, 198504 St. Petersburg, Russia 2A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia 3Department of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom (Dated: February 19, 2014)

Electron charge and spin delocalization revealed in the optically probed longitudinal and transverse spin dynamics in n -GaAs

The evolution of the electron spin dynamics as consequence of carrier delocalization in

Nuclear spin relaxation inn-GaAs: From insulating to metallic regime

n

Nuclear spin warm up in bulk n-GaAs

-type GaAs is investigated by the recently developed extended pump-probe Kerr/Faraday rotation spectroscopy. We find that isolated electrons localized on donors demonstrate a prominent difference between the longitudinal and transverse spin relaxation rates in magnetic field, which is almost absent in the metallic phase. The inhomogeneous transverse dephasing time