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Photonic integrated circuits fabricated on silicon-on-insulator platforms offer convenient foundations to implement highly sensitive, compact, robust, and low-cost technology in sensing applications. The potential of this technology in hydrogen gas sensing is discussed in this study. A single-slot hybrid microring-resonator-(MRR) based hydrogen gas sensor utilizing a coaxial palladium (Pd) microdisk is demonstrated. Detection is based on expansion of Pd upon hydrogen exposure toward the slot between the outer radius of the Pd microdisk and the inner radius of the MRR and the subsequent shift of the whispering gallery modes (WGMs) propagating in the MRR. Finite-difference time-domain simulations indicate a sensitivity as high as 11.038 nm/% hydrogen, provided that optimum geometrical design parameters are chosen. This sensitivity value is ∼23 times higher than other existing WGM-based hydrogen sensor demonstrations.

Optical whispering gallery modes (WGMs) were observed in elastic scattering spectra recorded from oil-in-water emulsion droplets in a microfluidic channel. Droplets with diameters ranging between 15 and 50 μm were trapped by optical tweezers near the tip of a single mode fiber that enabled the excitation of the WGMs using a tunable laser. Quality factors of the WGMs were observed to increase with droplet size. WGMs with quality factors of more than 10 4 were observed for droplets with diameters around 45 μm. In some cases, recorded WGMs drifted monotonically to the blue end of the spectrum due to droplet dissolution in the host liquid. Fluctuating spectral shifts to both blue and red ends of the spectrum were also observed. These were attributed to the presence of randomly diffusing particulate contaminants in the droplet liquid, indicating the potential of optically trapped droplet resonators for optical sensing applications.