Whispering gallery mode microresonators significantly enhance light-matter interactions, making them ideal platforms for a wide range of applications, including lasers, nonlinear converters, modulators, and sensors. Recently, the integration of sensitive materials such as graphene within optical microcavities has overcome the inert nature of the traditional optical microresonators, paving the way for highly sensitive biochemical detection. However, challenges such as Q factor deterioration, complex mode analysis, and demanding operation processes remain, resulting in intricate experimental setups, high excitation thresholds, and issues with device reliability and portability. Besides, the selectivity in the sensing process is also a challenge which relates to the material property. In this work, we present a gas sensor by combining functionalized graphene with a microrod resonator, addressing these challenges with the low threshold, simple structure, easy operation, high sensitivity, and switchable selectivity. By monitoring the shift of the resonant mode caused by the adsorption of gas molecules, we achieve the 1.1 ppb level detection of NH₃ and CO₂ in the P-doped graphene based microresonator and demonstrate 4 ppb level detection of NO₂ with high selectivity by changing the doping state of graphene from P to N. Our approach showcases the advantages of low cost, high sensitivity, and switchable selectivity, providing a promising solution for flexible and high-performance chemical sensing systems.

Whispering gallery mode (WGM) microresonators emerged as a promising platform for highly sensitive sensing applications due to their high-quality factors and small mode volumes. They offer the advantages of the ultrahigh sensitivity and compact size, rendering them suitable across multiple fields. A stable encapsulation process is essential for practical applications to establish a reliable coupling system between the microcavity and its waveguide coupler, especially for the microtoroidal resonator and tapered fiber coupler. However, adjusting the coupling coefficient after the packaging process poses challenges, thereby compromising coupling accuracy and limiting its range of applications. It is imperative to provide a platform of tunable coupling for packaged WGM resonators. Here, we provide an approach for leveraging the magnetostrictive effect to dynamically regulate the fiber-cavity coupling, enabling the measurement of the magnetic field as an example. Moreover, we show the fine-tuning of coupling within the packaged WGM microresonator, allowing the precision control of the optomechanical effect. Through this method, a tunable coupling platform in a packaged system is realized, opening up new dimensions of research in various fields.