Great progress has been made in both micromechanical resonators and microoptical resonators over the past decade, and a new field has recently emerged combining these mechanical and optical systems. In such optomechanical systems, the two resonators are strongly coupled with one influencing the other, and their interaction can yield detectable optical signals that are highly sensitive to the mechanical motion. A particularly high- Q optical system is the whispering gallery mode (WGM) resonator, which has many applications ranging from stable oscillators to inertial sensor devices. There is, however, limited coupling between the optical mode and the resonator’s external environment. In order to overcome this limitation, a novel type of optomechanical sensor has been developed, offering great potential for measurements of displacement, acceleration, and mass sensitivity.
The proposed hybrid device combines the advantages of all-solid optical WGM resonators with high-quality micro-machined cantilevers. For direct access to the WGM inside the resonator, the idea is to radially cut precise gaps into the perimeter, fabricating a mechanical resonator within the WGM. Also, a strategy to reduce losses has been developed with optimized design of the cantilever geometry and positions of gap surfaces.
The cantilever is machined by making fine cuts in a high-Q crystalline WGM resonator using focused ion-beam (FIB) technology. Such cuts can be much smaller than the optical wavelength, which should preserve the quality of the optical resonator. At the same time, reflection from the cantilever surfaces will result in coupling between the degenerate clockwise and counterclockwise propagating WGM. Therefore, a well-established technique of positionsensitive, dual-resonator coupling will be implemented in a novel system with optical and mechanical resonators’ high quality factors. This technique allows for optical cooling, as well as heating, of the mechanical oscillator.
This innovative hybrid system combines the advantages of both WGM and Fabry-Perot (FP) cavity resonators by utilizing the WGM resonator with the aforementioned cuts in the crystal to create an independent micromechanical resonator, residing directly in the middle of the optical WGM as an integral structure of the disk. This feature allows the direct coupling of the mechanical motion to the optical modes, much like a membrane inside an FP cavity. In this configuration, the single- mode optomechanical interaction can be selectively accessed as with a standard WGM resonator, or the coupled optical mode interaction as in that of a membrane-FP cavity.
The challenge of this approach is to maintain the optical finesse in the presence of the air gaps and the corresponding interfaces. The partially reflecting surfaces result in standing waves (SWs) in the resonators, and the mode coupling between them. These interfaces can also introduce scattering and diffraction losses. The estimates and previous WGM experiments suggest that a combination of appropriate microfabrication processes, such as FIB, and strategic use of SW modes, can reduce the losses and yield an optical resonator Q ≈108, higher than any cavity Q of optomechanical systems at the time of this reporting.
This work was done by David C. Aveline, Dmitry V. Strekalov, Nan Yu, and Karl Y. Yee of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47114
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Overview
The document presents a technical overview of Whispering Gallery Mode Resonators (WGMR) developed by NASA's Jet Propulsion Laboratory (JPL). The primary focus is on modified WGMRs that feature a precise notch cut into the perimeter of the resonator discs, allowing for "unlocked" access to the internal optical modes. This innovation enhances the capabilities of optical cavities, enabling researchers to investigate the internal mode structure and optical characteristics more thoroughly than traditional WGMRs, which only provide limited access to the evanescent field around the disc perimeter.
The project, led by Principal Investigator David C. Aveline, outlines the objectives and results achieved during the research. Key accomplishments include the development of precision Focused Ion Beam (FIB) milling techniques for materials such as Calcium Fluoride (CaF2) and Fused Silica, resulting in the fabrication of resonator discs that are 50-75 μm thick with a diameter of 2.4 mm. The team successfully created notches ranging from 10-20 μm wide in the discs' perimeters, which facilitated the observation of WGMR optical modes in the modified structures.
Significant measurements reported include a Quality Factor (Q) of 4.4 x 10^5, a Finesse (F) of 60, and a Free Spectral Range (FSR) of 28 GHz. The research also explored the wavelength dependence of the resonators across various wavelengths, including 850 nm, 960-1000 nm, and 1550 nm.
The document emphasizes the significance of these advancements, highlighting that unlocked WGMRs provide full access to optical modes, which allows for the incorporation of internal structures within the optical cavity. This capability opens up new possibilities for applications in trace gas detection, spectroscopy, microfluidics, and lab-on-a-chip devices. Additionally, the technology is suited for opto-mechanical sensors, precision displacement and inertial sensors, molecular mass detection, and optical modulators, making it relevant for various NASA and JPL technologies, including chip-scale accelerometers and gyroscopes for spacecraft navigation and autonomous robotics.
Overall, the document showcases the innovative research and development efforts at JPL, contributing to advancements in optical resonator technology with potential applications across multiple scientific and commercial fields.
