Recent advances in optical materials have enabled the development of a wide range of integrated photonic devices from high speed modulators to frequency combs. With low optical loss over a wide wavelength range and environmental stability in ambient environments for several weeks, silicon oxynitride (SiO<sub>x</sub>N<sub>y</sub>) shows potential in many of these applications. However, unlike many classic optical materials, the thermo-optic response (dn/dT) in both the visible and near-IR is poorly characterized, limiting researcher’s ability to accurately model device performance. Here, we leverage the intrinsic thermal response of resonant cavities to measure the dn/dT of SiO<sub>x</sub>N<sub>y</sub> with a 12.7:1 and 4:1 oxygen to nitrogen ratio based on EDX measurements. The thermo-optic coefficient is measured in the visible and near-IR and compared with SiO<sub>2</sub>. The refractive indices of the silicon oxynitride films were also measured using spectroscopic ellipsometry. Based on an analysis of the O:N ratio and a comparison with both SiO<sub>2</sub> and Si<sub>3</sub>N<sub>4</sub>, an expression for the dependence of the dn/dT on the stoichiometric ratio is developed.
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.
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. <p> </p> 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.
Ultra high quality optical resonators have enabled accumulation of exceptionally high intensities of light from low input powers. This feature opens new horizons in low power observation of physical phenomena such as lasing, sensing and radiation pressure driven oscillations. Radiation pressure instability facilitates transfer of energy from photons to mechanical degree of freedom in optical resonators. In high quality toroidal micro cavities, radiation pressure is demonstrated in the form of "dynamic back action" and results mechanical oscillations with sub-Hz linewidth. Since the toroidal cavities are symmetrical in nature, the exerted radiation pressure can mainly excite radially symmetric modes such as the first cantilever mode and the radially breathing mode. Study of these modes reveals important information about interaction of light and mechanical mode as well as intrinsic properties of the resonator as a mechanical oscillator. However, there are some unexcited mechanical modes that in some cases have even higher mechanical quality factors compared to the usually excited ones. Most of the properties of these mechanical modes remain unknown because the radially symmetric force does not provide a component to excite them. In this research, we have developed a novel method to fabricate asymmetric toroidal resonators (minor and major diameters), which enables us to regeneratively excite unobserved asymmetric modes. One key feature is that the optical quality factor is relatively high despite the asymmetry. As a result, we are able to excite the asymmetric modes with sub-mW threshold powers. Complementary modeling is also performed, confirming the experimental findings.
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.
Ultra-high quality factor (UHQ) resonant cavities are able to store light for long periods of time, resulting in high circulating intensities. As a result, numerous nonlinear optical phenomena appear, such as radiation pressure oscillations and lasing. However, deleterious behaviors also occur, such as optothermal broadening of the resonant linewidth. The degree of distortion is directly related to the circulating power in the cavity, the material absorption, and the thermo-optic coefficient of the cavity material. Specifically, a portion of the circulating power is absorbed by the material and converted to heat. This thermal energy is able to induce a refractive index change in the cavity which is experimentally observed as a resonant wavelength change. This behavior has been observed in numerous cavities, but one interesting case is the toroidal cavity, as it has a particularly complex geometry providing multiple thermal transport pathways. To accurately capture this complex behavior, we have developed a COMSOL Multiphysics model which combines the thermal and optical components. The model uses the non-uniform optical mode profile as the heat source. As such, changes in device geometry and wavelength are inherently captured. To verify the modeling, we characterize the optothermal threshold for a series of toroidal cavities across a range of wavelengths and device geometries. Additionally, the thermal time constant of the structure is explored. Of note, the membrane thickness is shown to play a critical role in the optothermal behaviors.
Optical splitters are one of the fundamental and most necessary components in modern photonic devices. They are used
for splitting, coupling and monitoring photonic systems. So far, most splitters were fabricated based on evanescent field
coupling or Y-branch dividers for which high precision lithography alignment is needed. In this work, we have proposed
a new type of splitter which works based on multimode bend loss and mode mismatch coupling. Since this device has a
novel suspended multimode region, it could be functionalized to detect any type of molecule binding to the surface. We
have developed a 3D FDTD model to simulate the device’s behavior. One of the most interesting features of this device
is that they have a broadband response and the coupling ratio doesn’t change by wavelength and is constant. The main
idea behind coupling mechanism in this splitter is the existence of a protrusion region in which the mode conversion
High optical field intensities build up inside microtoroids owing to its ultra-high quality factor, making them an ideal platform for plasmonic-photonic interactions with noble metals and a suitable pump source for microlaser. In this work, a microlaser based on hybrid silica microtoroids coated with gold nanorods is theoretically modeled and experimentally demonstrated. Theoretically, we used 3-D Comsol Multiphysics and modeled the interaction between the optical mode of the microtoroids and the surface plasmonic resonance of gold nanorods, both on and off resonance. To thoroughly study the role that the polymer layer plays in the plasmonic laser system, we perform a series of finite element method simulations in which the polymer layer thickness and refractive index is varied, and its effect on the plasmonic resonance is quantified. Experimentally, we demonstrated a visible laser at 575nm from hybrid microtoroids with a 30μWthreshold and an approximately 1nm linewidth. We have also varied the gold nanorod concentration on the microtoroids surface, and studied its effect on the Quality factor and the threshold power in order to get the optimum concentration for lasing.