The high sensitivity, rapid response, and label-free characteristics provided by whispering gallery mode (WGM) resonators enable many biochemical applications, such as early diagnostics and prognostics, food and water quality inspection, chemical threat sensing, and early detection of hazardous gases. The WGM microtoroid resonator has been shown to be one of the most sensitive biochemical sensors in existence, capable of detecting single molecules. The primary barrier to commercializing this sensor for field work is the tapered optical fiber used to evanescently couple light into the resonator. The fiber must be precisely aligned with the microtoroid for phase matching and efficient energy; however, it is fragile and susceptible to mechanical vibrations due to fluid flow and air currents. Research led by professor Judith Su at the University of Arizona has yielded a design that eliminates the need for a tapered optical fiber altogether. The team designed a free-space coupling system for microtoroid resonators that uses a single objective lens with a digital micromirror device (DMD) for light injection, scattered light collection, and imaging the microtoroid. The DMD, which filters out some of the stray light, can be used to select a region of interest. The use of a single objective lens allows for a more compact system, a cheaper design, and easier alignment. (a) An objective lens is used to couple free space light (red beam) into the microtoroid. The resonant scatter light is collected at the opposite edge, as indicated by the orange beam. (b) A scanning electron micrograph of a microtoroid resonator is shown. (c) Microtoroid resonance wavelength at different temperatures are displayed. (d) Resonance wavelength shift vs temperature. A linear fit is shown as a solid black line. Courtesy of Sartanee Suebka, Euan McLeod, and Judith Su. The DMD reflectivity is larger than 95% from 700 nm to 2500 nm, so the system’s optical components are designed for use in the near-infrared (NIR) waveband. However, the system design is compatible with other WGM mode resonators and wavelengths, although at other wavelengths, some components may need to be replaced to fit the application. Using 100-μm diameter microtoroids, the researchers achieved quality factors as high as 1.6 x 108. Electromagnetically induced transparency-like and Fano resonances were observed in a single cavity, which they attribute to indirect coupling in free space. These resonances enhance the sensitivity of the resonator. The large coupling area — about 10 μm in diameter for a numerical aperture equal to 0.14 — removes the need for precise positioning. To verify the strong sensing performance of the microtoroid resonator, the researchers combined the system with the frequency locked whispering evanescent resonator (FLOWER) approach and performed temperature sensing experiments. They tracked the resonance through FLOWER while adjusting the input power. They observed a thermal nonlinear optical effect, as the refractive index changed when the intracavity optical power changed. The system demonstrated far-field excitation with a signal-to-noise ratio of more than 26 decibels. The team showed that it was possible to enhance the far-field coupling efficiency of the microtoroid resonator by using a highly divergent laser beam. Also, by scanning the far-field beam, it was possible for the team to study the electric field profile inside the resonator. The realization of a far-field excitation system could make on-chip, microtoroid resonator sensing platforms feasible for field use. The researchers previously demonstrated that microtoroid resonators could detect hazardous gases at low parts-per-trillion. Their current work brings the development of a hazardous gas early detection system using microtoroid resonators within reach. The team is working on adapting the system for biosensing detection in aqueous environments and also multiplexing the sensors to enable simultaneous, multitarget detection. The work from the University of Arizona team could help expand the use of WGM microtoroid resonators in real-world applications. “We believe that this far-field coupling system can be used for spectroscopy and biosensing, and is the foundation of a fully on-chip, microtoroid resonator sensing platform,” researcher Sartanee Suebka said. “This approach has made our experiments a lot easier. We aim to miniaturize our system to make it more convenient for practical use.” The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-024-01418-0).
