Reflection and time-reversal of ultrasonic waves in the vicinity of the Rayleigh angle at a fluid-solid interface

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When sending a plane ultrasonic wave toward a fluid-solid interface, the reflected wave is affected, depending on the incident angle. Around the Rayleigh angle the reflection coefficient has a strong and rapidly varying imaginary part. This has the effect of distorting the reflected wave front. If this reflected wave is time-reversed and sent back toward the interface, the reflected wave of this time-reversed wave should not present any distortion, as the time-reversal process restores the original phases. A theoretical and experimental study of these phenomena has been done. The distortion of the reflected waves around the Rayleigh angle is observed and as expected this distortion is canceled by the time-reversal process. However a significant loss of energy in the time-reversed signal is observed for incident angles around the Rayleigh angle, as part of the energy contained in the Rayleigh wave escapes the time-reversal mirror and is lost for the time-reversal process. In a second part, it is shown by simulations and experiments how this signal distortion by reflection around Rayleigh angle influences spatial focusing of waves by time-reversal or simple time-delay methods.