Quantum Beats and Exciton Coherence in Time‐Resolved Resonant Light Scattering

Abstract

The time-resolved resonant light scattering spectrum following short pulse excitation provides information on the various inelastic (energy relaxation) and elastic (phase relaxation) relaxation processes of an exciton system. Taking properly the implications of the measurement process into account this can be exploited to discriminate between Raman scattering processes, proceeding without loss of coherence, and hot luminescence, occurring after phase relaxation. The coherent nature of the states can be directly revealed by quantum beats in the time-dependent scattering spectrum if the resonant states are energetically split. Such experiments are presented for bound excitons in CdS and the lowest exciton-polariton in Cu,O. These measurements allow to deduce feefined magneto-optical parameters due to the high energy resolution of this method. The exciton dephasing times found are surprisingly long (up to 1 ns) and determined by energy relaxation due to phonon scattering. Pure dephasing due to elastic scattering gives only a minor contribution.

Propagation Hanle effect of quadrupole polaritons in Cu2O

“…This fact was observed experimentally in Cu 2 O crystal, where the propagation quantum beats of quadrupole polaritons were revealed for the first time. [11][12][13][14][15] The group velocity along the polariton branches changes in a wide interval of values. For example, on the LPB in the range of wave numbers k smaller than the wave number k 0 of the intersection of noninteracting exciton and photon branches the group velocities are large and comparable with the light velocity, whereas in the range kϾk 0 they are small or practically zero, if the spatial dispersion effects can be neglected.…”

“…The total energy relaxation time or the population lifetime is usually measured by time-resolved luminescence spectroscopy. [11][12][13][14][15] As was emphasized in Ref. 13 the quantum beat method allows one to distinguish between coherent Raman scattering and incoherent hot luminescence.…”

“…The energy splitting of the states determines the beating frequency of the emitted light. This splitting can be determined with a high resolution, being independent of the inhomogeneous broadening, 15 whereas the phase relaxation times of the excited states can be determined from the damping of the beats. 13 The coherence time, which is also known as the dephasing time, has been measured by intervals of time during which the excited states lose their initial phases.…”

“…These phenomena became known as propagation quantum beats. [11][12][13][14][15]18 Timeintegrated propagation quantum beats will give rise to the propagation Hanle effect.…”

Transversal-Longitudinal Splittings of Quadrupole-Active Exciton Level in Cu2O Crystal

“…In this case, however, polariton propagation effects do not lead to an additional beat structure. This is due to the fact that in the Raman process polariton states with different wave vectors interact with different phonons and do not interfere [35,36]. It has to be mentioned that in the last decade the exchange splitting of dipole-active quantum well excitons was considered [37,38].…”