Tuning whispering gallery modes using internal aerostatic pressure.

Aerostatic tuning of whispering gallery modes (WGMs) in a microbubble resonator is demonstrated. The optical modes are redshifted over hundreds of gigahertz (GHz) simply by increasing the air pressure (up to 6 bars) inside the microbubble. A description of the pressure tuning properties of the WGMs in microbubbles is given in terms of the corresponding elasto-optical equations of spherical shells and the results are compared to experimental data. Microbubbles as small as 74 μm are tested and the experimental results show excellent agreement with the theory. An estimation method is developed for calculating the wall thicknesses of the microbubbles from the diameters, which are measured via direct microscopy. A geometrical factor χ is defined and a linear relationship between the shift rate (GHz/bar) of the bubbles modes and χ is observed.