Fig. 4.

Temporal focusing of ultrasonic pulses. a A schematic showing the pulse focusing caused by the GVD effect when injecting a chirped Gaussian pulse. b Temporal pulse width as a function of propagation distance when exciting a 2.5 μs chirped pulse with a center frequency of 5.8 MHz and various chirp parameters. The GVD coefficient k₂ at this frequency is −0.28 ns² m⁻¹ in Fig. 2c, indicating an anomalous dispersion regime. c, d Temporal evolution of the ultrasonic wave when exciting a chirped Gaussian pulse with a chirp parameter C = ±3 and 9.7 and measuring it at the right edge. The observed wave packets except for the first are associated waves composed of the incident and reflected waves at the WG edge, which increase the peak amplitude nearly twofold while maintaining the pulse width