Ultrasonic subwavelength phase conjugated mirror with a layer of bubbles

A single layer of gas bubbles in a yield-stress fluid is experimentally shown to behave as a phase-conjugated (PC) mirror with a thickness 250 times smaller than the wavelength (0.14 mm-diameter bubbles for phase-conjugation at 40 kHz). A high amplitude pump wave at frequency 80 kHz interacts with a lower amplitude probe wave centered at 40 kHz. A PC-reflection coefficient of 0.15 is obtained for a 50 kPa pump. A perturbative second-order theory is shown to quantitatively describe the experimental observations.