Due to their high quality factors, which result in large circulating optical intensities, microcavities are an attractive platform for creating frequency combs. Over the past decade, in an attempt to achieve both a high Q and a high third order susceptibility, many different material systems have been explored including silica, silicon, silicon nitride, and fluorides. However, these devices are ultimately limited by the material’s fundamental performance. In contrast, entirely new physical phenomena have been realized with nanomaterials. One strategy to leverage these emerging nanomaterials to enhance frequency comb generation is to create hybrid optical cavities in which novel nanomaterials are coated on or attached to the surface of a microresonator.
In the present work, we demonstrate a hybrid platform consisting of a gold nanoparticle coated whispering gallery mode silica microsphere. The hybrid device supports Q factors above 10 million at 1550nm, indicating that the nanoparticles are interacting with the optical field. Additionally, we demonstrate that the nanoparticles enhance the optical field in comparison to a plain silica optical cavity-based frequency comb, further reducing the comb threshold and increasing the comb span. The effect is studied over a range of gold nanoparticle concentrations. The mechanism and enhancement is further elucidated with finite element method modeling.
Optical cavities are able to confine and store specific wavelengths of light, acting as optical amplifiers at those wavelengths. Because the amount of amplification is directly related to the cavity quality factor (Q) (or the cavity finesse), frequency comb research has focused on high-Q and ultra-high Q microcavities fabricated from a range of materials using a variety of methods. In all cases, the comb generation relies on a nonlinear process known as parametric frequency conversion which is based on a third order nonlinear interaction and which results in four wave mixing (FWM). Clearly, this approach requires significant optical power, which was the original motivation for using ultra-high-Q cavities. In fact, the majority of research to date has focused on pursuing increasingly high Q factors. However, another strategy is to improve the nonlinearity of the resonator through intelligently designing materials for this application. In the present work, a suite of nanomaterials (organic and inorganic) have been intelligently designed with the explicit purpose to enhance the nonlinearity of the resonator and reducing the threshold for frequency comb generation in the near-IR. The nanomaterials do not change the structure of the comb and only act to reduce the comb threshold. The silica microcavity is used as a testbed for initial demonstration and verification purposes. However, the fundamental strategy is translatable to other whispering gallery mode cavities.
Responsive or reactive materials offer the possibility for the development of low-power diagnostics for preventative healthcare. We have synthesized and characterized a functional polymeric material which irreversibly cleaves upon exposure to UV light. Because this cleavage is selective to UV wavelength, it could form the foundation of a UV indicator strip, allowing patients healthy and unhealthy populations to monitor their exposure. In a complementary project, we developed an all-fiber polarimetric elastography system for characterizing the mechanical properties of visco-elastic materials, such a tissue, and for correlating this signal with cellular/molecular-level markers.
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.