Picosecond Dynamics of Excitonic Polaritons in the Bottleneck Region

Abstract

Ultrashort light pulses have made possible the direct observation of complicated relaxation processes of material excitations in solids. The dynamics of this relaxation for phot on-coupled excitations is reflected in the transient resonant secondary emission (RSE) spectrum. Strong wavelength dependence of RSE time evolution in the resonant region makes it necessary to use simultaneous spectral and temporal resolution. In the pioneering work of Toyozawa¹ it was pointed out that energy relaxation of excitons coupled to photons (polaritons) is not restricted to the exciton band, but can continue below the band bottom through the so called “polariton bottleneck.” It was also shown that this bottleneck plays a crucial role in the formation of the RSE spectrum, as the relaxation is slowed down considerably near the bottleneck. The formation of the RSE spectrum in the polariton framework is the result of the relaxation of initially excited polaritons through multiple scattering by phonons and other defects of an ideal lattice (surfaces, impurities, etc.). The polariton emission has several characteristics features: i) the lineshape of the polariton emission reflects the quasistationary distribution (QSD) of polaritons in the sample (if the exciton lifetime is long compared to the characteristic timescale of relaxation processes), ii) two maxima appear in the spectrum due to the upper and lower polariton branches, iii) the spectral shape of the emission band depends on the crystal size and temperature.