Significance Ultrasound has been widely used in various clinical fields as a safe,low-cost,portable,and fast imaging technology.Currently,piezoelectric transducers are the primary means of generating ultrasound.However,the limited bandwidth of the ultrasound signals generated by the piezoelectric effect restricts high-resolution ultrasound applications.Furthermore,miniaturizing piezoelectric transducers is challenging,and a reduction in their size leads to decreased sensitivity.However,high-precision ultrasound is indispensable for biomedical applications.In the photoacoustic process,light is converted into sound waves.Photoacoustic technology is rapidly being developed for photoacoustic imaging,diagnosis,and sensing in the life sciences.Recently,laser-generated ultrasound technology has emerged as a novel technique distinct from traditional piezoelectric methods.The transducer usually consists of a light absorber and surrounding medium with a high coefficient of thermal expansion.The light absorber can effectively convert light energy into heat energy,and the surrounding medium allows it to produce high-intensity ultrasound.This is a relatively simple method of generating a broadband high-frequency ultrasound signal using a short-pulse laser.This technology combines the excellent imaging depth of ultrasound with the resolution of an optical method,enabling a higher sound pressure and broader bandwidth.It also offers high-resolution ultrasound imaging and therapeutic capabilities with minimal electromagnetic interference.Laser-generated ultrasound transducers have significant advantages in relation to miniaturization,providing a new perspective for the further development of multifunctional ultrasound technology and a novel approach for clinical precision diagnosis and treatment in the future.Progress The composition of an optical ultrasound transducer has transitioned from a single nanoparticle coating or a single metal layer to a composite of different nanoparticles and polymers.With the adoption of composite materials with high optical absorption and expansion coefficients,there are various designs for optical ultrasound transducers.The basic performance of a transducer is determined by the nanoparticles utilized(candle soot,carbon black,graphene,carbon nanotubes,gold nanoparticles),while the rich variety of transducer structures(flat,concave,fiber)provides flexible solutions for different biomedical applications(Figs.1 and 2).Short laser pulses are utilized to provide a broad acoustic bandwidth,resulting in higher resolution,less tissue impact,and minimum electromagnetic interference.Recent studies have used optical ultrasound transducers in new high-resolution functional ultrasound applications.The flexibility of the photoacoustic film allows it to be integrated on the surface of the fiber or within the probe(Figs.4 and 5)to achieve a compact optical ultrasound transducer that can be incorporated into minimally invasive surgical devices used in a variety of microscale biomedical imaging applications(Figs.6 and 7).Simultaneously,because of the self-focusing effect of the concave transducer and high photoacoustic conversion efficiency of the material,an optical transducer can provide a concentrated sound pressure,tight focus,and higher resolution(Fig.3).The high-amplitude ultrasound produced by an optical transducer has been utilized in cavitation therapy,thrombolysis,and other functional areas(Figs.8-10).Moreover,the optical resolution provided by an optical ultrasound transducer offers higher precision for neuromodulation(Fig.12).In summary,optical ultrasound transducers have been extensively applied in the biomedical field because of their unique advantages.Conclusions and Prospects Optical ultrasound transducers are an emerging device that complements piezoelectric transducers.Because of their excellent material properties,optical ultrasound transducers have advantages such as a wide bandwidth,high resolution,electromagnetic immunity,flexibility,and ease of miniaturization.These features make the technology more suitable for applications requiring high-pressure ultrasound,high resolution,and micro-scale precision.As advancements are made in material systems and processing in the future,optical ultrasound transducers will drive progress in clinical applications such as diagnostics and imaging,as well as in precision machining.The performance and functionality of optical ultrasound transducers will further improve in the future,offering groundbreaking opportunities for high-resolution imaging and detection,precise localization,image-guided surgery and therapy,interventional procedures,and micro-scale treatment and modulation.Further research by scientists on optical ultrasound transducers is important for their utilization in clinical technologies.
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