Whispering gallery mode optical resonators integrated on silicon have demonstrated low threshold Raman lasers. One of the primary reasons for their success is their ultra-high quality factors (Q) which result in an amplification of the circulating optical field. Therefore, to date, the key research focus has been on maintaining high Q factors, as that determines the lasing threshold and linewidth. However, equally important criteria are lasing efficiency and wavelength. These parameters are governed by the material, not the cavity Q. Therefore, to fully address this challenge, it is necessary to develop new materials. We have synthesized a suite of metal-doped silica and small molecules to enable the development of higher performance Raman lasers. The efficiencies and thresholds of many of these devices surpass the previous work. Specifically, the silica sol-gel lasers are doped with metal nanoparticles (eg Ti, Zr) and are fabricated using conventional micro/nanofabrication methods. The intercalation of the metal in the silica matrix increases the silica Raman gain coefficient by changing the polarizability of the material. We have also made a new suite of small molecules that intrinsically have increased Raman gain values. By grafting the materials to the device surface, the overall Raman gain of the device is increased. These approaches enable two different strategies of improving the Raman efficiency and threshold of microcavity-based lasers.
High quality whispering gallery mode resonators can greatly enhance the optical field by trapping the light through total internal reflection, which makes these resonators a promising platform for many areas of research, including optical sensing, frequency combs, Raman lasing and cavity QED. Among these resonators, silica microtoroidal resonators are widely used because of their ability to be integrated and to achieve ultrahigh quality factors (above 100 million). However, quality factors of traditional silica toroids gradually decrease over time because there is an intrinsic layer of hydroxyl groups on the silica surface. This layer of hydroxyl groups attracts water molecules in the atmosphere and results in high optical losses. This property of silica degrades the behavior and limits the applications of the integrated silica toroids. In this work, we address this limitation by fabricating integrated microtoroids from silicon oxynitride. The surface of silicon oxynitride has a mixture of hydroxyl groups and fluorine groups. This mixture prevents the formation of a layer of water molecules that causes the optical losses. Our experiments demonstrate that the quality factors of the silicon oxynitride toroids exceed 100 million, and these values are maintained for over two weeks without controlling the storage conditions. As a comparison, quality factors of traditional silica toroids fabricated and stored under same conditions decayed by approximately an order of magnitude over the same duration.
As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology. Because the Q is critical to the device’s utility, an ever-present challenge revolves around maintaining the Q factor over long timescales in ambient environments. The counter-approach is to increase the nonlinear coefficient of relevance to compensate for Q degradation. In the present work, we strive to accomplish both, in parallel. For example, one of the primary routes for Q degradation in silica cavities is the formation of water monolayers. By changing the surface functional groups, we can inhibit this process, thus stabilizing the Q above 100 million in ambient environments. In parallel, using a machine learning strategy, we have intelligently designed, synthesized, and verified the next generation of small molecules to enable ultra-low threshold and high efficiency Raman lasing. The molecules are verified using the silica microcavity as a testbed cavity. However, the fundamental design strategy is translatable to other whispering gallery mode cavities.
High and ultra-quality factor (Q) optical resonators have been used in numerous applications, ranging from biodetection and gyroscopes to nonlinear optics. In the majority of the measurements, the fundamental optical mode is used as it is easy to predict its behavior and subsequent response. However, there are numerous other modes which could give improved performance or offer alternative measurement opportunities. For example, by using a mode located farther from the device surface, the optical field becomes less susceptible to changes in the environment. However, selectively exciting a pre-determined, non-fundamental mode or, alternatively, creating a “designer” mode which has one’s ideal properties is extremely challenging. One approach which will be presented is based on engineering a gradient refractive index (GRIN) cavity. We use a silica ultra-high-Q toroidal cavity as a starting platform device. On top of this structure, we can controllably deposit, layer or grow different materials of different refractive indices, with nm-scale precision, creating resonators with a GRIN region co-located with the optical field. Slight adjustments in the thicknesses or indices of the films result in large changes in the mode which is most easily excited. Even in this architected structure, we have maintained Q>1 million. Using this approach, we have demonstrated the ability to tune the properties of the device. For example, we have changed the thermal response and the UV response of a device by over an order of magnitude.
Silica and silica-doped high quality factor (Q) optical resonators have demonstrated ultra-low threshold lasers based on numerous mechanisms (eg rare earth dopants, Raman). To date, the key focus has been on maintaining a high Q, as that determines the lasing threshold and linewidth. However, equally important criteria are lasing efficiency and wavelength. These parameters are governed by the material, not the cavity Q. Therefore, to fully address this challenge, it is necessary to develop new materials. We have synthesized a suite of silica and polymeric materials with nanoparticle and rare-earth dopants to enable the development of microcavity lasers with emission from the near-IR to the UV. Additionally, the efficiencies and thresholds of many of these devices surpass the previous work. Specifically, the silica sol-gel lasers are co- and tri-doped with metal nanoparticles (eg Ti, Al) and rare-earth materials (eg Yb, Nb, Tm) and are fabricated using conventional micro/nanofabrication methods. The intercalation of the metal in the silica matrix reduces the clustering of the rare-earth ions and reduces the phonon energy of the glass, improving efficiency and overall device performance. Additionally, the silica Raman gain coefficient is enhanced due to the inclusion of the metal nanoparticles, which results in a lower threshold and a higher efficiency silica Raman laser. Finally, we have synthesized several polymer films doped with metal (eg Au, Ag) nanoparticles and deposited them on the surface of our microcavity devices. By pumping on the plasmonic resonant wavelength of the particle, we are able to achieve plasmonic-enhanced upconversion lasing.