Origin of Gouy Phase Shift Identified by Laser-Generated Focused Ultrasound

Abstract

The Gouy phase shift, the phase change of a converging wave passing through a focus, is a general characteristic of waves of any kind, including light and sound. Direct observation of the phase shift has been typically limited to high-frequency short pulses, e.g., terahertz light pulses and picosecond sound pulses. We demonstrate the Gouy phase shift of relatively low-frequency, megahertz ultrasound, widely implemented for ultrasound imaging and sensing. This demonstration is experimentally enabled with monopolar ultrasound pulses generated by nanosecond laser excitation of a light-absorbing concave film, which undergoes polarity reversal from before to after focusing. Moreover, our simulation results for various input pressure profiles show that the Gouy phase shift is observed for beams with significant diffractions. The physical origin of the phase anomaly is intuitively explained based on Huygens’ principle and the acoustic interference of secondary waves, providing further understanding of the Gouy phase shift in optics as well.

Frequency-Resolved High-Frequency Broadband Measurement of Acoustic Longitudinal Waves by Laser-Based Excitation and Detection

“…In the far field, sound energy decreases with distance z by [ 50 ], while it remains constant in the near field. In addition, the ultrasonic wave experiences a phase shift of in the transition between near and far field [ 51 , 52 ].…”

Laser-induced shock-wave-expanded nanobubbles in spherical geometry

“…The initially spherically expanding compressional shock wave, which soon after its emission propagates sonically as a monopolar pressure pulse [2] , [3] , is partially reflected from the concave surface and refocused at a location, different from the breakdown spot [4] . In the process of the acoustic focusing, a nearly spherically converging compression is transformed into a diverging rarefaction with a large negative peak pressure [5] , [6] . This transient tensile stress causes the existing nanobubbles in the liquid to expand, in the regions where the transient tensile stress is sufficiently large.…”

Bohm approach to the Gouy phase shift

“…The Gouy phase has been used to generate arbitrary cylindrical vector beams [7], to shape vectorially structured light with custom propagationevolution properties [6] and to give a new Bateman-Hillion solution to the Dirac equation for a relativistic Gaussian electron beam [8], among other applications. Moreover, its origin has been investigated over the years [3,9,10].…”