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Applied Physics Letters

Study of exciton-polariton spin dynamics

HAMID RAHIMPOUR SOLEIMANI, S. Cronenberger, Olivier Crégut, J.-P. Likforman, Mathieu Gallart, T. Ostatnický, P. Gilliot, B. Hönerlage

Published Nov 29, 2004• 22 citations
Publisher

Abstract

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.

DOI
10.1063/1.1830080

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•Jan 1, 1992•0 citations
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.
We demonstrate that the spin coherence relaxation of excitons can be measured in a three-beam four-wave mixing experiment in the ${\ensuremath{\chi}}^{(3)}$ regime, using a sequence of contracircularly polarized pulses. In contrast to other techniques like Faraday rotation measurement, our method allows one to access spin coherence without applying an external magnetic field. The superposition of opposite spin states (exciton-polariton spin coherence) is then probed in a nondegenerate configuration at the biexciton-exciton transition. Measurements are performed on a bulk CuCl platelet. The polariton effect is taken into account by modeling the propagation of the pulses. The exciton-polariton spin coherence time is demonstrated to be mainly determined by their spin lifetime.
Microcavity polariton spin dynamics is investigated in high-finesse microcavities under resonant excitation using polarization- and time-resolved secondary emission spectroscopy. Using an appropriate calibration procedure, the instantaneous polariton occupation numbers of the modes with $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ wave vectors can be determined. When the phase-matching conditions are achieved, we show that Coulomb scattering dominates the other processes, and that stimulation occurs when the occupation of the $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ final state with a given mode polarization approaches unity. Under elliptically polarized light, we show that the stimulated polariton scattering leads to an increase with time of the circular polarization of the $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ states, while the linear polarization dynamics is characterized first by a fast drop to negative or zero values immediately after the excitation pulse, followed by weak-amplitude nonperiodic oscillations. The latter are interpreted as the manifestation of the pseudospin precession of the pumped polariton modes around the self-induced exchange effective field.
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.
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.