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.

Whispering gallery mode microcavities are ideally suited to form microlaser devices because the high circulating intensity within the cavity results in ultralow lasing thresholds. However, to achieve low-threshold Raman lasing in silica devices, it is necessary to have quality factors above 100 million. One approach to circumvent this restriction is to intercalate a sensitizer into the silica, which increases the Raman gain. In the present work, we demonstrate a Raman laser based on a titanium sensitized silica solgel coated toroidal microcavity. By tuning the concentration of the Ti, the Raman efficiency improves over 3× while maintaining sub-mW thresholds.

No abstract

No abstract