Static refractive index change induced by high-frequency, high-intensity ultrasound

The propagation of optical waves is influenced by sound waves propagating in media, resulting in changes to light propagation, such as diffraction and refraction. This paper discusses change in light propagation in water induced by high-frequency (100 MHz range), high-intensity (on the order of MPa) ultrasound. The high-frequency, high-intensity ultrasonic waves can be radiated by an ultrasound transducer fabricated using a KNbO3 piezoelectric film grown by the hydrothermal method. A 160-MHz ultrasound induced a giant static refractive gradient in front of the transducer face in water, resulting in the optical refraction. The refractive index of water changed from its initial value toward that of air. Optical methods using a fiber-optic sensor or Schlieren photography can profile the refractive index distribution under ultrasonication. The propagation path of laser beams that penetrate the acoustic field can be predicted using a simulation model of the refractive-index gradient based on the experimental data and the Euler–Lagrange equation. The ultrasonic cavitation bubbles generated in water were analyzed using dynamic light scattering. The findings implied that the cavitation nano- and microbubble cloud generated by ultrasound primarily induced the refractive-index gradient in water. This phenomenon has the potential for application to future optical devices.