  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.
