Figure 1 shows a schematic of the experimental system. A custom built broadband dye laser pumped by a frequency-doubled Q-switched Nd:YAG (neodymium-doped yttrium aluminum garnet) laser (SPOT-10-100-532, Elforlight Ltd, UK: 532 nm; ; 2 ns pulse duration; 30 kHz pulse repetition rate) was used as the illumination source. The output light has a center wavelength of 580 nm and a bandwidth of 20 nm [Fig. 2(a)]. The currently achievable PRR of the dye laser is 5 kHz. The output light was coupled into a single mode optical fiber (SMF). After exiting the SMF the light was collimated and directed into the source arm of a free-space Michelson interferometer. A beam-splitter cube split the input light into the sample and reference arms. In the reference arm, a BK7 glass plate was used to compensate for the group-velocity dispersion mismatch between the two interfering arms. In the meantime, the reflected light from the front surface of the glass plate was detected by a photodiode, the output signal of which was used to trigger the data acquisition of the ultrasonic signal. In the sample arm, the light was scanned by an galvanometer scanner and was focused on the sample by an achromatic lens (). The combined reflected light from the sample and reference arms was first coupled into a SMF and then detected by a spectrometer. The spectrometer consisted of a transmission grating (, Edmund), an imaging lens (), and a line scan CCD camera (Aviiva-SM2-CL-2010, 2048 pixels operating in 12-bit mode, e2V). The sample clock of an analogue output board, which controlled the galvanometer scanner, triggered a digital delay generator (DG645, Stanford Research Systems). Two outputs of the digital delay generator were used to trigger the CCD camera and laser, respectively.