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arXiv:cond-mat/0504368 · Physical Review B

Optical pumping of the electron spin polarization in bulk CuCl

W. Ungier, R. Buczko

Published Oct 17, 2005• 3 citations
arXivHTMLPDF

Abstract

In CuCl bulk crystal negatively charged excitons (trions ${X}^{\ensuremath{-}}$) can be induced by the resonant optical excitation of extra electrons in conduction band minimum. In the case of circularly polarized light and due to the top valence band structure only the electrons with one spin orientation (for example, antiparallel to the direction of the light propagation in the case of ${\ensuremath{\sigma}}^{+}$ polarization) contribute to the formation of trions ${X}^{\ensuremath{-}}$. At the same time spontaneous decay of ${X}^{\ensuremath{-}}$ populates both parallel and antiparallel electron spin states, thus producing an enhancement of one spin orientation. We propose to use this mechanism for optical pumping of electronic spin polarization. We estimate the momentum matrix elements of electron-trion transitions which are the main factors determining pumping rate. The pumping dynamics is described by the density matrix evolution equations which couple to the Maxwell wave equation for the coherent pumping laser pulse propagating through the sample. The results of our simple model calculations suggest that spin polarization close to 100% can be achieved in CuCl in the time scale of the order of 100 ps.

DOI
10.1103/physrevb.72.165320

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In CuCl bulk crystal negatively charged excitons (trions X\ensuremath−{X}^{\ensuremath{-}}X\ensuremath−) can be induced by the resonant optical excitation of extra electrons in conduction band minimum. In the case of circularly polarized light and due to the top valence band structure only the electrons with one spin orientation (for example, antiparallel to the direction of the light propagation in the case of \ensuremathσ+{\ensuremath{\sigma}}^{+}\ensuremathσ+ polarization) contribute to the formation of trions X\ensuremath−{X}^{\ensuremath{-}}X\ensuremath−. At the same time spontaneous decay of X\ensuremath−{X}^{\ensuremath{-}}X\ensuremath− populates both parallel and antiparallel electron spin states, thus producing an enhancement of one spin orientation. We propose to use this mechanism for optical pumping of electronic spin polarization. We estimate the momentum matrix elements of electron-trion transitions which are the main factors determining pumping rate. The pumping dynamics is described by the density matrix evolution equations which couple to the Maxwell wave equation for the coherent pumping laser pulse propagating through the sample. The results of our simple model calculations suggest that spin polarization close to 100% can be achieved in CuCl in the time scale of the order of 100 ps.
Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a nnn-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and nonadditive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion.
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.
In this work, we study the resonant nonlinear optical response obtained when optical fields are used to excite charged semiconductor quantum dots (QDs). The basic physics of charged excitons (trions) created by optical excitation of charged QDs is of interest from the point of view of optically driven spin based quantum computing (QC). Stimulated Raman excitation, resonantly enhanced by the optical dipole coupling to the trion state, was used to optically generate electron spin coherence in the ground state of charged QDs. The evolution of the spin coherence was monitored through quantum beats in the phase-sensitively detected four wave mixing signal. The decay of the beats is a measure of the spin coherence time, which was found to be at least an order of magnitude greater than either the trion dipole coherence, or the Raman coherence time between excitons in a single neutral QD. A fascinating outcome of the experiment was the first observation of a contribution to the spin coherence induced by the vacuum field, which is also responsible for spontaneous emission from the trion state, known as spontaneously generated coherence (SGC). QC demands that coherent optical manipulations should be performed within the spin coherence time. We performed coherent optical control experiments, with a pair of phase-locked pump pulses, that showed the quantum nature of the coherence through interference of different quantum mechanical pathways. We showed both in experiment and theory that we can manipulate the spin coherence on the time scale of the Larmor frequency, as well as on an ultrafast femtosecond time scale, corresponding to the optical frequency of the trion state. Finally, resonant optical excitation of both the bright and nominally forbidden transitions from a single electron spin in a QD to the trion state were demonstrated. This allows for direct optical access to all the transitions required for optically driven QC with QD electron spins. We used frequency-domain nonlinear spectroscopy measurements on single QD trions to obtain the trion dipole decoherence and decay rates. Further, we demonstrated that the single electron spin coherence could be generated and detected optically, and found that the spin coherence time was significantly greater than in ensemble measurements at the same magnetic field.
We study the relaxation dynamics of spin-polarized exciton-polaritons using a technique in a pump-probe configuration: According to their total angular momentum, exciton-polaritons can be excited by circularly polarized light of either σ+ or σ− helicities. In addition, a bound state of two excitons may exist to which the transition is only allowed when using two photons of opposite helicities. After excitation by a pump pulse of σ+ helicity, additional polaritons from a probe pulse of σ− helicity may be absorbed to excite biexcitons, while absorption of those with σ+ helicity becomes possible only after a spin-flip process. It is characterized by a time-constant τs, which can be determined by a time-resolved study of the induced transmission changes at the exciton–biexciton transition. We demonstrate this method using bulk CuCl, in which the biexciton binding energy is large and in which effects due to polariton propagation can be nicely evidenced. We thus identify a spectral region where the spin-flip process is dominating, which is situated just below the polariton bottleneck.
The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped 14nm14\phantom{\rule{0.3em}{0ex}}\mathrm{nm}14nm GaAs quantum well in the frequency range of optical transitions to the heavy hole (HH) trion and to the light-hole (LH) trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.
Within the framework of light-semiconductor interaction and the Lutting-Kohn model for the semiconductor band structure, we theoretically studied the mechanism of the polarization switch with coherent transient features. The result shows, in the coherent limitation, when the pump pulse Ep and signal pulse Es are orthogonal polarized, the third order polarization P(3), with first order of Es and second order of |Ep|, approach to zero, due to the destructive quantum interference, which results in an ultrafast pump-signal relative polarization dependent P(3), therewith a polarization switch. The study also shows that, when the light interacts with semiconductors coherently, the polarization state of the light is transferred to the relative phase of the electron spin, rather than the electron spin itself. The population of the polarization eigenstate excited by the pump pulse, do not origin from the Coulomb force of electrons, and should not thought phenomenologically as the exciton characterized by light polarization. This population will dephase to real carriers population, i.e. electron with exact spin |↑> or |↓>, by losing phase coefficients of the polarization eigenstates, in an ultrashort quantum dephase time.