Nonresonant Optical Excitation Research Articles

Coupled Free-Carrier and Exciton Dynamics in Bulk Wurtzite GaN

The Ensemble Monte Carlo method has been applied to the study of the relaxation dynamics of coupled free-carriers and excitons in bulk Wurtzite GaN, after non-resonant optical excitation. The semiclassical Boltzmann equations for electron and hole populations are completed by an additional equation for the exciton distribution, and are coupled by reaction terms, describing exciton formation and dissociation mediated by phonons. The temporal evolution in the range from 1 to 400 ps show that the exciton formation process and the temporal evolution of the resulting population are sensitive to the excitation energy. The rates for the acoustic piezoelectric interaction have been calculated, including screening effects. Results show that piezo-acoustic rates are bigger in the wurtzite phase, respect to the cubic one, and they are really sensitive to the background doping of the sample.

Monte Carlo Description of Exciton Dynamics in GaN

The energy relaxation of coupled free-carrier and exciton populations in bulk wurzite GaN after nonresonant optical excitation is studied through an Ensemble Monte Carlo method. The set of semiclassical Boltzmann equations for electron and hole populations is complemented by an additional equation for the exciton distribution. The equations are coupled by reaction terms describing phonon-mediated exciton binding and dissociation. All the other relevant scattering mechanisms are also included. Simulations for bulk GaN show that excitonic binding mediated by a LO-phonon is a fast and effective process. Temporal evolution is studied in the range from 1 to 400 ps after photoexcitation. As a consequence of the pronounced energy dependence of the LO-phonon-assisted transition probabilities between free-pair states and excitons, it is found that the efficiency of the exciton-formation process and the temporal evolution of the resulting population are sensitive to the excitation energy.

Experimental investigation of photoluminescence dynamics of cavity polaritons under nonresonant excitation

Low-temperature time-resolved photoluminescence (PL) experiments have been performed on a semiconductor planar microcavity, which contains two sets of three In0.13Ga0.87As/GaAs quantum wells embedded in a 3λ/2 GaAs cavity. The spontaneous emission dynamics of both lower- and upper-branch polaritons is investigated as a function of exciton–cavity detuning under nonresonant optical excitation. It is found that the PL decay times of both branches are independent of cavity detuning while the PL rising kinetics of the lower- and upper-branch polaritons exhibits a significant difference. The rise time of the upper polarition branch shows a strong dependence on cavity detuning, while the rise time of the lower polarition branch is less sensitive to cavity detuning. Our results can be well understood in the framework of the theoretical prediction of Tassone et al.

Resonant excitation of intraband absorption in InAs/GaAs self-assembled quantum dots

We have investigated the infrared absorption between confined levels in the conduction and valence bands of undoped InAs/GaAs self-assembled quantum dots. The intraband absorption, which is measured by photoinduced spectroscopy, is analyzed under resonant and nonresonant optical excitation of the quantum dots. The assignment of electron and hole intraband transitions is achieved on the basis of experimental results obtained with n-and p-doped quantum dots. A careful analysis of the absorption spectra shows that several hole transitions and one electron transition with a large broadening are evidenced in the mid-infrared spectral range. We show that the amplitude of the intraband absorption depends on the pump excitation wavelength and exhibits a maximum when the dots are populated via the wetting layer. The spectral shape of the hole intraband absorption is very weakly dependent on the excitation wavelength. The amplitude of the photoinduced hole intraband absorption exhibits a sublinear behavior with the pump intensity. This feature is explained by the state filling of the quantum dots.

Impurity concentration determination in 6H-SiC

We present a study of ZnCdTe quantum dots (QDs) embedded in Zn-rich ZnCdTe quantum wells (QWs). The Cd-rich ZnCdTe islands are confined along the growth direction by pure ZnTe barriers and laterally by Zn-rich ZnCdTe QWs. Using a time-resolved pump and probe technique, we analyse the phonon emission-cascade in QWs governing, after non-resonant optical excitation in the QWs, the energy relaxation of electron–hole pairs towards self-organized QDs. Since electron–hole pairs thermalize only by emission of an integer number of longitudinal-optical phonons, at the end of the cascade, the QDs become selectively populated. We thus determine the mean value of the optical-phonon emission time. Then, for longer time delays, we observe the population relaxation dynamics of the selected quantum dots.

Spatial and spectral features of polariton fluorescence

The spatial properties of polariton fluorescence are shown to be of crucial importance to determine polariton fluorescence lineshapes. Comparison in GaAs of front and back-side fluorescence under resonant and non-resonant optical excitation of the no-phonon line and its LO-replica gives evidence of this spatial dependence.