Fig. 1. Schematic diagram of picosecond ultrasonic principle

Fig. 2. Schematic diagram of optoacoustic signal

Fig. 3. Schematic diagrams of optical distortion method. (a) No deformation of the sample surface when Z₀>0; (b) no deformation of the sample surface when Z₀<0; (c) deformation caused by stress on the sample surface when Z₀>0; (d) deformation caused by stress on the sample surface when Z₀<0

Fig. 4. Schematic diagram of optoacoustic signal in Au film

Fig. 5. Schematic diagram of experimental setup for picosecond ultrasonic based on dual-comb ASOPS

Fig. 6. Autocorrelation curves and spectra of dual-comb, illustrations are spectra of dual-comb. (a) Pump laser; (b) probe laser

Fig. 7. Thickness measurement results of Au film. Thickness direct measurement results of Au film with standard thickness of (a) 43.67 nm, (c) 81.00 nm, and (e) 165.33 nm; fitting results of the oscillation parts with standard thickness of (b) 43.67 nm, (d) 81.00 nm, and (f) 165.33 nm