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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Gateable spin memory in InP quantum dots

I. A. Yugova, I. V. Ignatĭev, S. Yu. Verbin, I. Ya. Gerlovin, Vladimir K. Kalevich, A. Yu. Shiryaev, Kirill V. Kavokin, Yasuaki Masumoto

Published Jun 5, 2002• 0 citations
Publisher

Abstract

In photoluminescence kinetics of the InP quantum dots, we have found a long-lived component of the degree of circular polarization, showing practically no decay during the lifetime of excitation. It is shown that this effect is observed only in the charged quantum dots. The amplitude of the long-lived PL polarization can be controlled by an applied electric field and may be as high as 50% in the case of quasi resonance excitation. The existence of the long-lived component of the polarization is the clear evidence of large lifetime of the electron spin polarization in quantum dots.

DOI
10.1117/12.514458

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In photoluminescence kinetics of the InP quantum dots, we have found a long-lived component of the degree of circular polarization, showing practically no decay during the lifetime of excitation. It is shown that this effect is observed only in the charged quantum dots. The amplitude of the long-lived PL polarization can be controlled by an applied electric field and may be as high as 50% in the case of quasi resonance excitation. The existence of the long-lived component of the polarization is the clear evidence of large lifetime of the electron spin polarization in quantum dots.
Single InP-based quantum dots emitting in the third telecom window are probed quasi-resonantly in polarization-resolved microphotoluminescence experiments. For charged quantum dots we observe negative circular polarization being a fingerprint of the optical spin writing of the carriers within the quantum dots. The investigated quantum dots have a very dense ladder of excited states providing relatively easy quasi-resonant optical excitation, and together with telecom wavelengths emission they bring quantum gates and memories closer to compatibility with fiber-optic communication.
Abstract We have studied the influence of an external electric field on the optical spin polarization of initially negatively charged InAs/GaAs quantum dots (QDs). The spin polarization of the carriers was controlled by measuring the photoluminescence (PL) circular polarization. We have found that the PL polarization of the QD is negative under an excitation above the GaAs barrier, and that it becomes positive under an excitation below the barrier. An applied negative electric field destroys both positive and negative polariza‐tion. The time decay of the negative polarization was found from time‐resolved measurements to exceed strongly the QD radiative lifetime. This fact indicates a long‐living spin memory in the dots under study.
The optically-induced coherent spin dynamics in a charged quantum dot (QD) is studied theoretically using a new dynamical model for rigorous description of circularly polarized ultrashort optical pulse resonant interactions with the electron-trion system. Generalized pseudospin master equation is derived for description of the time evolution of spin coherences and spin populations in terms of the real state pseudospin (coherence) vector including dissipation in the system through spin relaxation processes. The equation is solved in the time domain self-consistently with the vector Maxwell equations for the optical wave propagation coupled to it via macroscopic medium polarization. Using the model the long-lived electron spin coherence left behind a single resonant ultrashort optical excitation of the electron-trion transition in a charged QD is simulated in the lowand high-intensity Rabi oscillations regime. Signatures of the polarized photoluminescence (PL) resulting from the numerical simulations, such as the appearance of a second echo pulse after the excitation and a characteristic PL trace shape, specific for initial spin-up orientation, are discussed for realization of high-fidelity schemes for coherent readout of a single spin polarization state.
Chiral-induced spin selectivity (CISS) enables spin selectivity of charge carriers in chiral molecular systems without magnetic materials. While spin selectivity has been widely investigated, its quantum coherence has not yet been explored. Here, we investigate spin-dependent photoluminescence (PL) dynamics in multilayer quantum-dot (QD) assemblies coupled by chiral linkers. Using circularly polarized excitation in the presence of an external magnetic field, we observe a pronounced modulation of the PL lifetime that depends on the magnetic field magnitude and geometry. The lifetime difference between left- and right-circularly polarized excitations exhibits a field-angle dependence, consistent with spin precession driven by the transverse magnetic-field component relative to the chiral axis. A model incorporating coupled spin precession and decay processes reproduces the experimental trends. These results establish chiral QD assemblies as a room-temperature platform for probing quantum coherent manifestations of the CISS effect, with implications for spintronic and quantum technologies.
We have experimentally studied the spin dynamics of excitons, electrons, and trions in charge-tunable InP/InGaP quantum dots (QDs) excited by picosecond resonant laser pulses by observing the time-resolved Kerr rotation. In singly charged QDs, inversion of the spin polarization direction of doped electrons is found to be caused simply by variation in the pulse intensity, which is accompanied by an abrupt change of the spin coherence time. This phenomenon is reproduced by density-matrix calculations allowing for the reaction on the QD electron-trion four-level system during its coherent radiation emission. This result means that the optical coherence is another critical factor affecting electron spin coherence.
We report quantum beat phenomena between different polarization states in InAs quantum dots, observed in time-resolved photoluminescence and transient dichroism measurements. The polarization of the emitted light can be changed from linear to circular by applying a small magnetic field. Following quasiresonant excitation the electron and hole spin states remain stable during the exciton lifetime, independent of the applied magnetic field. We show that the exciton spin coherence is partially preserved in the energy relaxation process. These experiments enable us to measure the anisotropic exchange energy $\ensuremath{\Delta}{E}_{XY}$ and the exciton ggg factor. We find $\ensuremath{\Delta}{E}_{XY}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ and $g\ensuremath{\simeq}2.5$ for the quantum dot excitons.