Ultrasonically induced microscopic refractive index gradient and the relationship with high-frequency ultrasonic cavitation

The refractive index of a medium can be modulated by external stimuli such as pressure, temperature, or electromagnetic forces. This principle enables fast, precise, and reversible optical control and has catalyzed the development of optical devices and optical measurement technology. Here, we report the relationship between the refractive index change induced by high-frequency ultrasonic irradiation and ultrasonic cavitation. The cavitation nanobubbles generated by ultrasonic irradiation were measured using dynamic light scattering to be approximately 100 nm in diameter. The apparent volume fraction of the nanobubbles induced near the surface of the ultrasonic transducer was calculated using an effective medium approximation. The apparent volume fraction was 0.12 at the position where the refractive index change was maximal (Δn = 0.04: value from the fitting function). The technique to control light propagation in a local (microscale) region with ultrasonic irradiation has a wide range of applications from optofluidic devices for lab-on-chip devices to variable-focus lenses for industrial metrology.