  Raman lasers based on integrated silica whispering gallery mode resonant
cavities have enabled numerous applications from telecommunications to
biodetection. To overcome the intrinsically low Raman gain value of silica,
these devices leverage their ultra-high quality factors (Q), allowing sub-mW
stimulated Raman scattering (SRS) lasing thresholds to be achieved. A closely
related nonlinear behavior to SRS is stimulated anti-Stokes Raman scattering
(SARS). This nonlinear optical process combines the pump photon with the SRS
photon to generate an upconverted photon. Therefore, in order to achieve SARS,
the efficiency of the SRS process must be high. As a result, achieving SARS in
on-chip resonant cavities has been challenging due to the low lasing
efficiencies of these devices. In the present work, metal-doped ultra-high Q
(Q>107) silica microcavity arrays are fabricated on-chip. The metal-dopant
plays multiple roles in improving the device performance. It increases the
Raman gain of the cavity material, and it decreases the optical mode area, thus
increasing the circulating intensity. As a result, these devices have SRS
lasing efficiencies that are over 10x larger than conventional silica
microcavities while maintaining low lasing thresholds. This combination enables
SARS to be generated with sub-mW input powers and significantly improved
anti-Stokes Raman lasing efficiency.
