I. V. Ignatiev

Millisecond-Range Electron Spin Memory in Singly-Charged InP Quantum Dots

We report millisecond-range spin memory of resident electrons in an ensemble of InP quantum dots (QDs) under a small magnetic field of 0.1 T applied along the optical excitation axis at temperatures up to about 5 K. A pump-probe photoluminescence (PL) technique is used for optical-orientation of electron spins by the pump pulses and for study of spin relaxation over the long time scale by measuring the degree of circular polarization of the probe PL as a function of pump-probe delay. Dependence of spin decay rate on temperature and magnetic field suggests two-phonon processes as the dominant spin relaxation mechanism in these QDs at low temperatures.

Quantum Beats in Photoluminescence of InP Quantum Dots in Electric Field

The photoluminescence (PL) kinetics of heterostructures with InP self-assembled quantum dots are studied under quasi-resonant pulse excitation in the presence of an external electric field. An oscillatory behavior of the PL kinetics is shown to arise due to quantum beats of the radiative states. A model of the coherent excitation of nonresonant PL is proposed.

Nuclear spin effects in negatively charged InP quantum dots

Effects of both the dynamic nuclear polarization (DNP) created by circularly polarized light and the fluctuations of average nuclear spin in a quantum dot (QD) on the electron spin orientation are studied for singly negatively charged InP QDs. From the dependence of the negative circular polarization of photoluminescence on the applied longitudinal magnetic field, the hyperfine field BN of a few mT appearing due to DNP and the effective magnetic field Bf of a few tens of mT arising from nuclear spin fluctuations (NSF) are estimated. A lifetime of about 1 μs is estimated for NSF.

Carrier relaxation dynamics in self-assembled quantum dots studied by artificial control of nonradiative losses

Abstract : A novel technique to study carrier relaxation dynamics based on the artificial control of nonradiative losses by an external electric field is proposed. A clear evidence of phonon assisted relaxation as the main relaxation mechanism of hot electron-hole pairs in InP self-assembled quantum dots is found by the proposed method. Efficient one step relaxation processes with emission of acoustic and optical phonons are observed. These findings give new and important insight into the interaction of the electron-hole pairs in quantum dots with the phono subsystem.

Microscopic modeling of exciton spectra in asymmetric quantum wells

A theoretical model of reflectance spectra is applied to the analysis of spectra of the two high-quality structures with asymmetric InGaAs/GaAs quantum wells. The analysis allows to quantitatively describe the exciton resonances related to the lowest quantum-confined exciton states. Main parameters of the quantum well potential profiles are obtained in the modeling.

Decrease of heavy-hole exciton mass induced by uniaxial stress in GaAs/AlGaAs quantum well

D. K. Loginov, P. S. Grigoryev, E. V. Ubiyvovk, Yu. P. Efimov, S. A. Eliseev, V. A. Lovtcius, Yu. P. Petrov, and I. V. Ignatiev Spin Optics Laboratory, St. Petersburg State University, 198504 St. Petersburg, Russia Department of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia Resource Center “Nanophotonics”, St. Petersburg State University, 198504 St. Petersburg, Russia (Dated: May 11, 2015)

Electron-spin relaxation by the interaction with nuclear-spins in InP quantum dots

Effects of nuclear-spins on the electron-spin polarization in singly negatively charged InP quantum dots are studied experimentally at 5 K. Pump-probe photoluminescence measurements of electron-spin relaxation in the microsecond time scale are used to estimate the time period TN ~ 1 mus of the Larmor precession of nuclear-spins in the hyperfine field of electrons, under the vanishing external magnetic field (Be,). From the time-integrated measurements of electron-spin polarization as a function of Be applied along the optical excitation axis, we find that the Overhauser field BN arising from the dynamic nuclear polarization increases linearly with the excitation power (PX). We obtain BN =6 mT at PX = 50 mW. The effective magnetic field of the frozen fluctuations of nuclear- spins is found to be BF ap 15 mT, independent of PX .

EFFECT OF NUCLEAR SPINS ON THE ELECTRON SPIN DYNAMICS IN NEGATIVELY CHARGED InP QUANTUM DOTS

Kinetics of polarized photoluminescence of the negatively charged InP quantum dots in weak magnetic field is studied experimentally. Effect of both the nuclear spin fluctuations and the dynamical nuclear polarization on the electron spin orientation is observed.

SPIN RELAXATION IN MAGNETIC FIELD FOR InP QUANTUM DOTS

Effect of external magnetic field on the optical orientation of electron spins in negatively charged InP quantum dots is studied experimentally using the PL pump–probe method.

Spin dynamics of the neutral and charged InP self-assembled quantum dots

We have studied the kinetics of the polarized photoluminescence of the InP quantum dots in the magnetic field as a function of their charge state. In zero magnetic field, the degree of circular polarization of electrically neutral quantum dots rapidly decays with time constant about 13 ps. In longitudinal magnetic field, a slow component appears in the kinetics of degree of the circular polarization. Decay time of the slow component is about 300 ps. In the kinetics of the negatively charged quantum dots we have found a long-lived component of the degree of circular polarization, showing practically no decay during the lifetime of excitation. Spin relaxation processes responsible for depolarization of photoluminescence are discussed.