Titanium enhanced Raman microcavity laser

摘要

Due to their long photon lifetimes, ultra high quality factor (Q) silica microcavities form an ideal platform for microlaser development. Previous work verified that these devices exhibit Raman lasing, because the high Q compensates for the low Raman gain of silica. However, only devices with Q＞1E8 are able to achieve microwatt thresholds, limiting the application space. One approach for overcoming this barrier is to increase the inherent Raman gain of the material without degrading the optical performance of the device. To address this challenge, we synthesize a series of titanium (Ti) doped silica sol-gels with different concentrations of Ti, including a null. The refractive indices of the coatings are characterized using spectroscopic ellipsometry and increase linearly with the concentration of Ti from 1.44 to 1.51. We apply the sol-gel as a conformal coating on silica toroidal microcavities and characterize the basic cavity properties (Q) and the lasing behavior, including the lasing threshold and the slope efficiency. All measurements are performed in ambient conditions. Although the cavity Q's are modest (5E6), comparable lasing thresholds (microwatt) to higher Q silica devices are achieved because of the reduction in mode volume and the increase in Raman gain due to the presence of the Ti. Additionally, the efficiency of the laser increases with increasing Ti concentration.