Observation of Time Evolution from Resonant Raman Scattering to Excitonic-Polariton Luminescence in ZnTe

Abstract

We report the evolution from resonant Raman scattering (RS) due to longitudinal-optical and acoustic phonons to excitonic-polariton luminescence in ZnTe at low temperature by picosecond time-resolved spectroscopy. The resonant RS lines deform with decay time of 300 ps into the ordinary polariton luminescence. The finite decay time of the RS lines shows evidence of a real transition on the polariton branch in the resonant RS process.PACS 78.30.Gt, 78.55.Hx The relation between light scattering and luminescence under resonant optical excitation has been discussed by many authors. 1 " 4 A unified picture of secondary emission processes developed by Toyozawa leads us to the understanding of the processes as follows 2 : The secondary photon which is emitted by the electron after interaction with a few optical (acoustic) phonons in the resonant intermediate state is characterized by light scattering. Such photons are normally investigated in Raman (Brillouin) spectroscopy. The photon arising after full thermalization of the electron system by phonons in the intermediate state has the character of ordinary luminescence (OL). The correlation between the excitation photon and the secondary photon is completely lost in this case. The secondary photon emitted from the electron before full thermalization in the intermediate state is denoted as hot luminescence (HL). In these three emission processes, RS, HL, and OL, the difference is only the degree of the relaxation by phonons (either the longitudinal or the dephasing relaxation) in the intermediate states. In RS, when the relaxation of the intermediate electron takes place it is possible to identify which phonon causes the relaxation because the relevant phonons are limited. However, in HL and OL processes a number of phonons are related with the relaxation so that we cannot distinguish individual phonons in the relaxation. In this case the phonon system plays the role of a thermal bath for the electrons. From the above argument we see that these three secondary emission processes cannot be separated from each other with clear boundaries and only the phonon relaxation rate determines the character of the emitted photons.The secondary emission process under resonant excitation therefore transforms from RS type to OL through HL, which takes place usually in a time range of less than 10 ~1 2 s in most solids even at low temperature. Thus the time-resolving measurement in the picosecond time domain could not reveal this transformation in solids.The excitonic-polariton state in direct semiconductors, however, has the possibility of displaying the above evolution of the emission process because the polariton populated around the knee region in the lower branch at low temperature interacts with the acoustic and optical phonons with a much lower rate than an electron in the ordinary electronic state because of the k dependence of the deformation-potential and Frohlich interactions: This is the bottleneck of the polariton. 5 Recent studies on the resonant Brillouin and Raman sc...

Photon–carrier–spin coupling in a one-dimensional Ni(II)-doped ZnTe nanostructure

“…Additionally, the small iconicity factor is quite useful for understanding the correlation between electron localizability in deep valence shells. At RT, the reduction in activation energy and non-radiative transfer of electronic excitation energy to the host matrix shows strong exciton-phonon interaction [47]. The excitons-polaritons with distinct energy of lower polariton branch (LPB) and upper polariton branch (UPB) energies along with a middle polariton are responsible for photon-like excitons-polaritons in polycrystalline ZnTe at 15 k [48,49].…”