Time-resolved photoluminescence quenching measurements in InAs/GaAs quantum dots using terahertz laser pulses

Carrier dynamics and relaxation processes in self assembled quantum dots (QDs) are of fundamental interest due to their influences on the efficiency and performances of optoelectronic devices [1]. The intersublevel relaxation mechanisms influence the temporal response of the photoluminescence (PL) [2]. It is therefore interesting to study the carrier relaxation in a series of QD samples where the intersublevel separation varies, resulting in different relaxation times. In this paper we present our work on time-resolved PL quenching measurements on QD ensembles using terahertz pulses, to study the effect of carrier redistribution on PL. Interband quasi-resonant excitation was done by a Ti-Sapphire laser and the PL emission was measured using a spectrometer coupled to a streak camera. Terahertz pulses were obtained from a Free Electron Laser (FEL) synchronized to the Ti:Sapphire laser. The FEL wavelengths were tuned to excite intersublevel transitions in the QDs (ranging from 23 meV to 15 meV) which caused partial depletion of the electronic ground state resulting in quenching of the interband PL. The samples studied consisted of a series of self-assembled InAs/GaAs QDs for which the intersublevel relaxation times varied from few ps to ns, as a result of thermal annealing [3]. Simultaneous time and wavelength resolved measurements enabled us to study the carrier redistribution by the terahertz pulse and their dynamics. PL measurements were done with and without FEL pulses. Figure 1 shows the calculated difference of two such streak camera images measured for a typical QD sample. The blue regions show the PL quenching dip caused by the FEL excitation for two interband transition energies corresponding to the QD ensemble. The data was fitted using exponential functions convoluted with a Gaussian system response. From the fitting parameters the PL quenching depth and recovery times were extracted. The recovery times were found to be significantly shorter than the intersublevel relaxation times suggesting that other mechanisms like interdot transfer and multiphoton excitations were also involved. We performed measurements for different FEL excitation powers, which showed an eventual saturation of the PL quenching. We observed an increase in the PL signal for the lower energy transition after the recovery of the PL, as shown by the orange region in Figure 1. However the carrier redistribution was found to depend on the intersublevel properties of the samples. In this work we will present a comparative analysis for different QD samples, with emphasis on the effect of intersublevel relaxation times on the carrier dynamics.