Ultra-low threshold pH sensor based on a whispering gallery mode microbubble resonator

Abstract: Laser sensing has a wide range of applications. In this paper, we propose a pH sensing laser with an ultra-low threshold and low sample consumption based on a whispering-gallery-mode microbubble resonator. Rhodamine 6G aqueous solutions with different pH values are used as the lasing gain media, which are injected through the microfluidic channel and interact with the high-quality-factor microbubble resonator to achieve lasing. Subtle pH changes of the aqueous solution lead to changes in lasing intensity in real time and the threshold reaches a minimum of 0.091 uJ/mm2. The low pump energy density effectively avoids the self-aggregation and photobleaching effects of dye molecules present in high-concentration rhodamine 6G solutions. The lasing characteristics under different pH conditions were determined experimentally and theoretically, and the results are in good agreement. Due to the deprotonation of amino groups in highly alkaline environments, the lasing threshold is highly dependent on the pH of rhodamine 6G aqueous solutions. In the pH range of 10.16-13.14, the lasing intensity changes considerably with the increasing pH. The proposed pH-sensing laser exhibits a fast response time, low toxicity, and a high signal-to-noise ratio, making it promising for highly sensitive alkaline detection in biological applications.