Cavity optomechanics on a microfluidic resonator with water and viscous liquids

Currently, optical or mechanical resonances are commonly used in microfluidic research. However, optomechanical oscillations by light pressure were not shown with liquids. This is because replacing the surrounding air with water inherently increases the acoustical impedance and hence, the associated acoustical radiation losses. Here, we bridge between microfluidics and optomechanics by fabricating a hollow-bubble resonator with liquid inside and optically exciting vibrations with 100 MHz rates using only mW optical-input power. This constitutes the first time that any microfluidic system is optomechanically actuated. We further prove the feasibility of microfluidic optomechanics on liquids by demonstrating vibrations on organic fluids with viscous dissipation higher than blood viscosity while measuring density changes in the liquid via the vibration frequency shift. Our device will enable using cavity optomechanics for studying non-solid phases of matter, while light is easily coupled from the outer dry side of the capillary and fluid is provided using a standard syringe pump. Scientists in the USA have successfully used optomechanics to probe a microfluidic system. Kyu Hyun Kim and co-workers at the University of Michigan and the University of Illinois at Urbana-Champaign fabricated a bulbous hollow-glass capillary and filled it with sugar-water solution. They then used a tapered optical fibre to couple light evanescently in the perimeter of the capillary and thus excite optical whispering gallery modes. This optical mode excites a vibrational mode of the liquid-filled glass structure via the centrifugal radiation pressure that light applies while circumferentially circulating, which in turn modulated the light at a particular frequency. The frequency of the induced vibrations varied with the sugar concentration of the liquid, thus indicating the potential of the structure to function as a sensor. The researchers found that input optical powers as small as 1 mW were sufficient to induce vibrations.