Fig. 1. Schematic diagram of different types of laser-generated ultrasound transducers^[10]

Fig. 2. Different materials constructing laser-generated ultrasound transducer. (a) CNF-PDMS^[45]; (b) CNT- PDMS^[52]; (c) CS- PDMS^[39]; (d) CB-PDMS^[56]

Fig. 3. Concave laser-generated ultrasound transducers composed of different materials. (a) MWCNT-PMMA^[44]; (b) CNT-PDMS^[43]; (c) CB-PDMS^[48]; (d) CSNP-PDMS^[67]

Fig. 4. Structure of optical fiber transducers composed of different materials. (a) ZnO-graphite-epoxy^[38]; (b) gold nanocomposite-PDMS^[34]; (c) CB-PDMS^[69]; (d) CNT-PDMS^[73]; (e) reduced graphene oxide-PDMS^[36]; (f) CNT-PDMS^[74]

Fig. 5. Structure schematics of different types of optical fiber transducers. (a) Fiber transducer composed of carbon-polymer film and planoconcave lens^[70]; (b) schematic of the optical fiber HIFU device and the picture of the fabricated HIFU probe^[68]; (c) schematic of the focused optical ultrasound transducer^[76]; (d) structure schematic of angle-focused optical fiber transducer^[75]

Fig. 6. Schematic of tissue imaging with optical ultrasound transducer^[82]. (a) Imaging schematic; (b) pulse-echo ultrasound images of the cerebrum and cerebellum longitudinal sections obtained by tissue imaging of a piece of brain tissue using a probe consisting of a CSNP-PDMS optical transducer with a laser of 1064 nm and an FP sensor; (c) microscopy photos of the imaged brain tissue

Fig. 7. Schematic diagrams of all-optical ultrasound imaging. (a) All-optical ultrasound cross-sections of swine aorta using the probe consisting of MWCNT- PDMS coating and FP cavity^[72]; (b) in vivo all-optical ultrasound imaging with a all-optical ultrasound imaging probe placed directly on the neck skin of a human volunteer^[84]; (c) schematic of the side-viewing optical ultrasound transmitter and rotational optical ultrasound images of an ex vivo swine carotid artery^[15]; (d) concept of ultrawide-bandwidth AO-IVUS and 3D ultrasound image of arterial wall by rotational pullback scan of AO-IVUS^[44]

Fig. 8. Experimental setup and principle of ultrasonic transducer based on perforated photoacoustic lens and PVDF hydrophone^[85].

Fig. 9. Schematics of optical ultrasound drug delivery. (a) Schematic of the laser-generated-focused ultrasound-mediated drug delivery^[49] (Once the carbon black-PDMS LGFU transducer is excited by the pulsed laser, the absorbed laser energy causes rapid heat transfer and simultaneous thermal expansion of the LGFU lens. The laser-generated pulsed waves can promote the drug release efficiency by cavitation effects and oscillating the microgels' shells); (b) potential design of a photoacoustic image-guided HIFU probe^[68]

Fig. 10. Schematic diagrams of thrombolysis based on an optically focused ultrasound transducer. (a) Schematic of laser-generated-ultrasound thrombolysis technique: microbubble-mediated approach and dual-excitation approach^[48]; (b) schematic diagram of a focused high-intensity ultrasound thrombolysis experiment^[96]

Fig. 11. Cavitation application of LGFU. (a) Targeted cultured ovarian cancer cells (SKOV3) removal by the LGFU, where the white arrows indicate the single cell to be detached by the LGFU, and the black arrows indicate that the cellular interconnection was severed when the LGFU spot moves to the black-dotted region^[43]; (b) demonstration of cell cluster fractionation by LGFU: under LGFU, the cluster is fractionated primarily at the focal center and the prolonged exposure of LGFU enlarges the fractionated zone over the periphery^[64]; (c) simulation of various geometries of the LGFU transducer to determine the achievable pressure at the focal point, and surface plots of peak negative pressures on the geometric focus of transducers for two cases^[98]

Fig. 12. Schematic diagrams of cavitation-based acoustic wave cutting experimental setup and cavitation-based acoustic cutting of complex shapes of tissue gels^[99]

Fig. 13. Optical ultrasound neuromodulation. (a) Schematic diagram of ultrasound stimulation of rat brains using a fCNT sensor composed of MWCNT-PDMS and EEG signals of three rat brains before and after ultrasound stimulation^[111]; (b) non-invasive brain stimulation via a concave photoacoustic transducer composed of CSNP-PDMS^[47]; (c) schematic diagrams of fiber photoacoustic emitters structure and high-precision brain stimulation^[113]; (d) schematic diagrams of the structure of tapered fiber optoacoustic emitters and stimulation of a single neuron^[114]; (e) neural stimulation and regeneration via biocompatible PA films^[116]; (f) schematic diagrams of the structure of CSFOE and dual site neuron stimulation by CSFOE^[115]

Fig. 14. Applications of optical ultrasound in other fields. (a) Schematic diagrams of the laser ultrasonic transducer and the damage detection by the transducer^[119]; (b) structural sketch of the ultrasonic transducer based on CNTs-PDMS composite film and the experimental setup schematic for magneto-acousto-electrical non-destructive testing based on CNTs-PDMS composite film^[120]