Fig. 2: Local pressure modulates the refractive index.
From: Ultrasonically sculpted virtual relay lens for in situ microimaging

a Example of a standing pressure wave profile of a resonance mode of an ultrasonic cavity with a diameter of 38 mm formed by a piezoelectric cylinder with a wall thickness of 3 mm, filled with water. The inset shows a 2D cross-section of the pressure. b Cross-section of the induced refractive index profile in the medium as a result of the ultrasonic standing wave