Fig. 1. Photo-induced emission mechanisms of ultrafast acoustic pulses. (a) Thermo-elastic mechanism of photo-induced emission of ultrafast acoustic pulse in metal thin-film; (b) deformation potential mechanism of photo-induced emission of ultrafast acoustic pulse in semiconductor thin-film

Fig. 2. Optical detection mechanisms of ultrafast acoustic pulse. (a) Surface optical detection scheme of acoustic echo in nontransparent media; (b) optical tracking detection scheme of acoustic traveling wave in transparent media

Fig. 3. Featured achievements of pineering ultrafast acoustic pulse studies. (a) In 1984, the first observation of coherent acoustic phonon (CAP) oscillation in a semiconductor thin-film^[32]; (b) in 1986, the first observation of echo wavepacket of ultrafast acoustic pulse in a semiconductor thin-film^[33]

Fig. 4. All-optical excitation and detection of CAP oscillation. (a) First ultrafast spectrum observation of CAP oscillations in multilayer MoS₂^[72]; (b) interlayer energy dissipation mechanism of CAP oscillation in suspended MoSe₂^[74]

Fig. 5. Frequency modulation of CAP oscillation^[75]. (a) Nano-acoustic cavity based on multilayer MoS₂ and its CAP oscillation signal in time domain; (b) layer number dependence of CAP oscillations in nano-acoustic cavity based on multilayer MoS₂

Fig. 6. Interfacial manipulation of CAP oscillation. (a) Mode modulation of CAP oscillation in a MoS₂/nanograting heterostucture^[78]; (b) twist-angle dependence of CAP oscillation in a MoSe₂/WSe₂ heterostructure^[79]

Fig. 7. All-optical emission and detection of ultrafast acoustic pulse^[83]. (a) Femtosecond laser pump-probe of a hydrogel/MoS₂/glass heterostructure; (b) temporal observation of CAP oscillation and ultrafast acoustic pulse, and frequency domain signal-decoupling through transformation

Fig. 8. All-optical emission, detection and manipulation of pulse width in 2D semiconductor/3D dielectric heterostructure^[84]