## 4 Experimental Study  
The schematic of the experimental setup is illustrated in Figure 8. A burst of nanosecond laser pulses was used to illuminate the phantom. A 1310 nm continuous-wave interrogation beam was focused on the phantom. The reflected interrogation content from the phantom was then collected. The exciting beam was focused onto the phantom, using a focusing mirror with a focal length of 100 mm. The probe beam was passed through a polarized beam splitter, followed by a \(\lambda/4\) zero-order wave plate. Then, the probe beam was focused onto the phantom using a 0.4 NA objective lens. The back-reflected probe beam returned through the \(\lambda/4\) zero-order wave plate and was redirected through the polarized beam splitter and focused onto the photodiode for detection. The excitation beam was 266 nm, with a 5 ns pulsed laser, and laterally exposed to the particles. To verify the accuracy, experiments were carried out for different material interfaces with different material pairs. We conducted the experiments on the interface of the agarose gel and silicon. The results are shown in Figure 9. The rapidly changing part of the curve is consistent with the results reported in the literature [19], which is related to the perturbation of photoacoustic signals. There is a slow decline at the back end of the curve, which is caused by the photothermal phenomenon. As shown in Figure 9, the pulse width of the pulsed laser was 5.8 ns. The maximum amplitude occurred at 4.16 ns from the time-domain signal of agarose gel and silicon interface and returned to a stable state at about 60 \(\upmu\)s.  
The same experimental results were also revealed in the interface of other material pairs, such as copper, aluminum, and carbon fiber. The thickness of agar covered above the material was the same, while the thicknesses of the rectangular copper sheet, rectangular aluminum sheet, and carbon fiber used in the experiments were 1 mm, 1 mm, and 7 \(\upmu\)m, respectively. Figure 10 shows the thermal cooling times with different delay times for the same pulse delay, which are all in the range of \(\upmu\)s to ms. The cooling time of each interface was about 57 \(\upmu\)s, 500 \(\upmu\)s, and 20 ms for the interfaces between the agarose gel and copper, aluminum, or carbon fiber, respectively. As different materials have different thermal diffusion coefficients and thermal conductivity coefficients (Table 1), different materials showed different thermal diffusion times, and the thicknesses of the materials are different, leading to different cooling times. Figure 8: The schematic of the experiment setup. Figure 9: A representative experimental time-domain signal for the agarose gel and silicon interface. Under the irradiation of a nanosecond pulse laser, the generation time of the photoacoustic signal is generally within a nanosecond range, and the change in refractive index generated via the photoacoustic signal should also be within the nanosecond scale. However, due to the thermal cooling time, which reached a scale of a microsecond to a millisecond after laser-induced heating, the corresponding induced refractive index would also be in the corresponding order. Even if the thermally induced refractive index change was smaller than that induced by pressure, it could not be ignored in the heating zone. Furthermore, different amounts of incident pulse laser energy excite different heat, pressure, and photoacoustic signal; therefore, the corresponding refractive index changes were different. As evident in Figure 11, the incident pulse energy in (B) was half that of (A), and the time to reach a static steady state of (B) was about 3.1 ms, faster than that of 5 ms in (A). The corresponding experimental results correspond to the simulation results, which showed that the refractive index change caused by heat cannot be ignored in the range of the pulse laser spot area. Figure 11: Temporal experimental time-domain results between agarose gel and silicon under different pulse laser intensities.