Spatiotemporal modulation of refractive index in an optical path medium allows for control of light propagation. This refractive index change (Δn) can be achieved by external stimulation such as temperature or density change, but there is a limit to the extent to which Δn can be controlled by external stimulation acting on a single medium. Here, we demonstrate a technique to form a giant refractive index gradient (Δn = 0.06) in a small region of water (< 10 mm) using a high-frequency, high-intensity (in the 100-megahertz-range, on the order of megapascals) ultrasonic wave. Ultrasonic radiation in water can statically modulate the refractive index in water from the initial value (n = 1.33) toward that of air.

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