Pulsed Propagation of Polariton Luminescence

Abstract

Pulsed luminescence is observed in time-and frequency-resolved measurements of the Z3 exciton luminescence of CuCl single crystals under excitation at the highly absorptive region of the Zn exciton resonance. A high-repetition-rate tunable picosecond uv light source was used. The pulse traveling time coincides well with the time calculated from the corresponding polariton group velocity. It is found that the redistribution of polaritons due to intraband relaxation is not important to the temporal behavior of the free-exciton luminescence, but the polariton propagation effect is essential, in contrast to the discussions found in previous work.PACS numbers: 71.36.+C, 42.65.Re, 78.47.+p, 78.55.Hx The optical properties of a dipole-allowed exciton are described well with the exciton-polariton (EP) concept. 1 ' 2 In this description, the radiative recombination process of photocreated excitons is regarded as a combined process of propagation to a sample surface as polaritons and subsequent conversion into photons. Thus the spectral shape of the EP luminescence is determined by the population distribution function of the EP's and the boundary conditions at the sample surface. The EP's are nonequilibrium in nature, since there is no lower energy limit in the EP state. However, Toyozawa 3 studied the population dynamics of EP's taking into account a phonon scattering rate in the sample and an escape rate at the surface, and pointed out that the polaritons can accumulate in the region, named "bottleneck," that is situated just below the energy Cl j of the transverse exciton at k =0. As long as the extrinsic nonradiative recombination rate is small, one may expect formation of a thermal-equilibrium distribution of excitons in the energy region above the bottleneck. This effect is often quoted as the origin of a peak found in EP luminescence spectra. Heim and Wiesner measured a frequencydependent decay of EP luminescence in CdS and observed a prolonged polariton decay time in the region below ftr. 4,5 On the assumption of a spatially homogeneous distribution of polaritons, their results were analyzed and found to be direct evidence of the accumulation of EP's in the bottleneck region. 6 In analyses of polariton dynamics, the spatially homogeneous distribution is often assumed for simplicity. In actual experiments, however, the EP's are inhomogeneously created in space, because the incident photon energy is usually in a highly absorptive region, e.g., in the region of a band-to-band transition. It has already been pointed out that this spatial dependence of the EP distribution causes a forward-backward variation of the line shapes of steady-state EP luminescence from platelet samples with finite thickness. 7,8 If such a spatial inhomogeneity exists, it should also be reflected in the temporal behavior of the EP luminescence. Therefore, it is clear from this argument that one has to take into account the space and time evolution of the polariton distribution in order to understand correctly the temporal profile of the EP l...

Time-resolved investigation of exciton-polariton dephasing in CuCl

“…The frequency dependence of the coupling parameters are in agreement with the theoretical estimations [Eqs. (4)] with, in particular, a faster increase of /PE with (o en . We emphasize here that none of the temperature behavior of the damping rates can be reproduced with solely one mechanism.…”

“…Furthermore, extraction of this information is complicated because of the inherent polariton propagation and transmission at the crystal surfaces which may substantially alter the bulk polariton features. 4 The exciton-polariton dephasing has also been addressed in the time domain using a time-resolved four-wave-mixing technique. This technique proceeds via strong linear polariton excitation within the absorption layer, close to the crystal surface, and, hence, only gives access to the high-excitation regime where the polariton dephasing is dominated by polariton-polariton interactions.…”

Exciton luminescence spectra of CuCl films and their change by photo-irradiation

“…The Z 3 -exciton couples with light field strongly and forms the exciton polariton. Pulsed propagation of the polariton was observed for bulk single crystals [2,3]. Much shorter decay time of the exciton luminescence than that of bulk crystals [4] and the hot exciton luminescence [4,5] were reported for thin films of 3-120 nm thickness.…”

Ultrafast decay of photoluminescence from high-density excitons inAlxGa1−xNmixed crystals: Diffusive propagation of exciton-polaritons

“…The control of light propagation has been widely studied in a variety of semiconductors. [13][14][15][16][17][18][19][20] In disordered materials, multiple photon scattering leads to light diffusion, and light propagation is in connection with weak localization and coherent backscattering of light. [21][22][23][24] In GaN crystals, bound exciton states due to inevitable impurities cause multiple photon scattering.…”