Optical gain in nanocrystalline silicon superlattices

This work has focused on the optical properties of Si nanocrystals, especially whether optical gain could be achieved. Several experimental methods have been used to explore the possibility of optical gain in several Si nanocrystal structures in order to provide a sound basis towards realizing the first Si-based injection laser.; The main structures used in this study are silicon nanocrystal superlattices. The initial structures consisted a-Si/a-SiO2 superlattices grown on a Si substrate by radio frequency magnetron sputtering and plasma oxidation. The a-Si layers were crystallized by an annealing procedure that converts the entire a Si layers into an array of densely packed nanocrystals with a narrow size distribution. Waveguides have been grown to perform gain studies using the variable stripe length method. Gain experiments have been performed under both continuous wave pumping and pulsed laser pumping. No gain was observed with continuous wave pumping. However under intense pulse pumping, a strong fast component in the nanosecond range emerged on top of the usual slow (microseconds) decay. The fast component disappeared completely as the pumping length was reduced below a critical interaction length where amplified spontaneous emission could no longer build up. A clear threshold was measured for the fast emission component as a function of the pumping rate. Maximum gain values in the order of 100 cm-1 have been measured and tuning from optical loss to gain has been obtained by simply increasing the pumping intensity on the sample. This result was reproduced in two different laboratories using different pumping sources to eliminate the possibility of experimental artifacts. Similar experiments were performed on ion implanted Si nanocrystal samples and on SiO/SiO2 superlattices. No optical gain was observed. Based on those results a simple microscopic picture was established.; Pump probe measurements have also been performed on several samples. A model consisting of 3 components was used to fit the experimental results. No optical gain was observed from this type of measurement. Some possible reasons were given to explain the absence of optical gain in this type of measurement.