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.

We show the implementation of fiber-pigtailed, evanescent-field-interacting, single-walled carbon nanotube (CNT)-based saturable absorbers (SAs) using standard planar lightwave circuit (PLC) fabrication processes. The implemented PLC-CNT-SA device is employed to realize self-starting, high-repetition-rate, all-fiber ring oscillators at telecommunication wavelength. We demonstrate all-fiber Er ring lasers operating at 303-MHz (soliton regime) and 274-MHz (stretched-pulse regime) repetition-rates. The 303-MHz (274-MHz) laser centered at 1555 nm (1550 nm) provides 7.5 nm (19 nm) spectral bandwidth. After extra-cavity amplilfication, the amplified pulse train of the 303-MHz (274-MHz) laser delivers 209 fs (178 fs) pulses. To our knowledge, this corresponds to the highest repetition-rates achieved for femtosecond lasers employing evanescent-field-interacting SAs. The demonstrated SA fabrication method, which is based on well-established PLC processes, also shows a potential way for mass-producible and lower-cost waveguide-type SA devices suitable for all-fiber and waveguide lasers.

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The biosonar of bottlenose dolphins has been shown to have excellent target discrimination performance in cluttered environments, but how the animals process the flow of information used in their adaptive searching process is still an open question. In this work, we present a physics-based model of the process echolocating dolphins may use during target discrimination tasks and validate the model with experimental results. Assuming that dolphins emit biosonar clicks and continuously compare the return echoes with learned representations of echoes from familiar objects, we define a likelihood parameter, a metric that quantifies this comparison. We show that the dolphins’ adaptive search behavior during echolocation can be interpreted as an effort to maximize the likelihood parameter. Our model is validated experimentally using target discrimination tasks in which a dolphin correctly/incorrectly located a given target in the presence of distraction objects in an open lagoon while it was wearing eyecups. The results show that the dolphin positioned itself where the likelihood parameter was maximized demonstrating that our model explains well the dolphin’s behavior during the tasks. [Work supported by the Department of the Navy Grant No. N00014-18-1-2069.]