Soap bubbles are simple but very fascinating objects for both researchers and the general public. Several of their properties have been studied, including interference colors and the formation of minimal surfaces, but until now they have not been used as optical cavities. In this work, we have demonstrated that dye doped soap bubbles and smectic liquid crystal bubbles emit laser light when pumped with an external laser. Soap bubble lasers operate on the principle of whispering gallery mode resonances, which form when light circulates in a spherical resonator due to multiple total internal reflections. Smectic bubbles are particularly interesting because they have a very thin and uniform wall, typically ⠼100nm thick. For this reason, they support only one optical mode, which results in the laser spectrum consisting of hundreds of regularly spaced peaks, resembling a frequency comb. Using the spectrum of the emitted light captured in time, we were able to measure size changes as small as 10nm of a millimeter-sized bubble. This allows us to use smectic bubbles as very sensitive electric field and pressure sensors. Using the bubbles, we measured the electric field with an exceptional sensitivity with a minimal measurable electric field of 0.35V/mm. They also enable measurement of pressures up to 100 bar with a resolution of 1.5Pa, resulting in a dynamic range of nearly 10^7. Since the bubbles were attached to one end of the capillary, an additional fixed volume allowed for arbitrarily high sensitivity while maintaining excellent dynamic range. Soap and smectic bubble lasers are a very unique type of microresonators as well as one of the best electric field and pressure sensors to date. In the future, they could be used to study thin fluid films and cavity optomechanics.
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