[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultrafast Dynamics in Quantum Dot Doped Nanocomposites at Low Temperatures: Study by Means of Site-Selective Spectroscopy

摘要： Study of the dynamics and luminescence properties of nanocomposite materials based on semiconductor quantum dots is an important and urgent trend of modern physics and materials science. Semiconductor quantum dots have a number of unusual photophysical and spectral properties, which are determined by their structure, size and microscopic features of interaction with the local environment. Considerable efforts are aimed to search for and study new luminescence labels [1] and suitable matrices for functional quantum-dot-based materials, e.g., liquid crystals [2]. In this paper we discuss the new data on ultrafast dynamics in nanocomposites doped with semiconductor colloidal nanocrystals (quantum dots, QDs) revealed by two types of site-selective spectroscopy techniques. Optical dephasing in a thin film of double-coated CdSe/CdS/ZnS QDs was studied in a wide range of cryogenic temperatures using a method of the photon echo. Experiments were performed using unique incoherent photon echo spectrometer (see e.g. [3]). We measured the decay curves of 2-pulse incoherent photon echo signals in ensemble of QDs and obtained very short relaxation times (about hundreds of femtoseconds) in a temperature range from 4.5 K to 50 K [4]. These results are extremely different from the data obtained in the ensembles of impurity organic dye molecules for which the relaxation times are about nanoseconds at this very temperature. Possible reasons for such ultrafast optical dephasing can be associated: (1) with structural inhomogeneities of QDs themselves; (2) with the features of internal dynamics of the emitting core (e.g., blinking of QDs); (3) with the surface states in a QDs shells. Furthermore, the size variance and strongly inhomogeneous local environment (local fields in a solid matrix) can lead to fast relaxation in the QDs ensemble. In order to separate the contributions to the optical dephasing of the processes of interaction of quantum dots with the matrix from the processes inside the QDs themselves, we measured the temperature dependences of the luminescence spectra of nanocomposites. Generally, with the temperature decrease, the maxima of the exciton bands in a luminescence spectra shift to the UV spectral range. Such phenomenon can be described taking into account the electron-phonon (exciton-phonon) interaction. The temperature shift of the exciton luminescence spectrum can be described using the modified Varshni equation [5]. Theoretical model that takes into account the electron-phonon interaction made it possible to quantitatively describe the temperature dependences of the exciton luminescence spectra of QDs, as well as to determine the values of the Huang-Rhys factor and the average energy of phonons in nanocrystals. In our experiments no noticeable temperature changes in the exciton maxima widths have been observed, which is related, on the one hand, to the resolution of used experimental technique and, on the other hand, to the occurrence of significant inhomogeneous broadening of the spectra caused by a large dispersion of QD sizes. Describing the temperature behavior of the position and width of the luminescence spectra of QDs one should take into account an interaction of the electronic transition of the impurity with vibrational excitations, e.g. with local or quasi-local phonons in a solid matrix. In this connection, we have studied the influence of the matrix on luminescence properties of QD-doped nanocomposites. The combination of the photon echo and luminescence spectroscopy is a powerful method to study the ultrafast processes of interaction of the impurity ensemble of QDs with a solid matrix as well as the intrinsic dynamics of quantum dots themselves.