Abstract

We used new P₂O₅-Na₂O-ZnO-AlF₃-Ga₂O₃ glass system for the formation of PbSe and PbS Semiconductor Quantum Dots (SQD). Thermal treatment of initially colorless glasses at 390-440°C led to their coloring to brown and black depending on the annealing. X-ray diffractometry of the annealed glass samples proved existence of PbSe, PbS SQD with NaCl cubic structure in the glasses Transmission electron microscopy of the samples demonstrated existence of spherical SQD with the size varied between ~2 and 15 nm depending on the duration and the temperature of the heat treatment. The measured spectrum of the secondary X-rays indicated the presence of lead and sulfur in the SQD. The optical absorption spectra of the annealed glass samples with PbS and PbSe SQD were measured in the spectral range 350-3500 nm at several temperatures. Sets of optical absorption peaks corresponded to different quantum transitions were clearly observed even in the spectra measured at room temperature, and the measurements at He temperature indicated narrow (~5-7%) size distribution of the SQD. Theoretical analysis of the energy spectra and optical transitions was carried out in the framework of four-band k•p model with full account of the anisotropy effects. It was demonstrated that strong anisotropy of the energy bands of bulk PbSe, PbS resulted in arising additional optical transitions in the SQD. These anisotropy-induced optical transitions were clearly observed in the measured absorption spectra of the SQD. The calculated energy positions of the optical transitions coincided with the results of the measurements of optical absorption and luminescence spectra. Five optical transitions were distinguished and interpreted, and the calculated dependencies of the energies of these transitions versus the SQD size were in a good agreement with that obtained from the experimental data for 40-110 A SQD diameter range. We also observed that the temperature-coefficient of the first transition energy in PbS and PbSe SQD depended strongly on their size [1]. With decreasing size the temperature-coefficient of the lowest transition energy decreased by more than an order of magnitude from the bulk value, and then changed sign. The measurements of energy relaxation time with pump-and-probe technique [2] showed clear correlation of energy relaxation time with the average SQD radius. Smaller dots were shown to have shorter decay times. Non-linear optical absorption of the SQD-doped phosphate glass was demonstrated in visible and infra-red (1064, 1300, 1540 and 2100 nm) optical range. Q-switch operation of Er:glass laser with PbS SQD (pulses of 110 ns in duration and 0.2 mJ in energy at 1.54 urn) and Ho:YAG laser with PbSe SQD as saturable absorber (pulses of 60 ns in duration and 22 mJ in energy at 2.1 μm) was demonstrated with the Q-switching efficiency ~6% without additional intracavity focusing.

© 2000 IEEE