## 4 Summary and Prospects
The nanoscale dimension of absorbers allows short thermal diffusion time, which is essential for high-frequency ultrasound generation. In addition to the size, high thermal conductivity of the absorber and high thermal diffusivity of the surrounding medium are helpful for optimization of fast heat transition. (iv) LGUS transmitters are better to be made in the form of a film on a transparent substrate or a solid freestanding film, which facilitates laser illumination and their use in water or liquid environment. (v) A tradeoff between the light absorption and film thickness for optimization between the amplitude and bandwidth of generated ultrasound should be considered in designing LGUS transmitters. (vi) For AOUSTs, suitable and elegant integration of LGUS transmitters and ODUS detectors are of great significance in imaging performance and size miniaturization. Despite intense investigation in recent years, LGUS technology still has a lot of room for improvement. The following future directions are of practical significance. (i) The nanocomposites capable of highly-efficient photoacoustic conversion hold great promise for high-amplitude LGUS transmission. Further optimization of these nanocomposites is still a lack of LGUS research. The optimization includes an optimal density or ratio of the absorber in the whole composite, suitable arrangement of the absorber and the surrounding material, enhancement of optical extinction in thin-film transmitters, and robustness against high optical fluence. Such optimization can potentially be used for advanced diagnostic and therapeutic applications. (ii) The future research of fiber-optic LGUS transmitters for interventional imaging and therapy should emphasize their biocompatibility and safety. The toxicity of nanocomposites used as part of medical devices is not fully tested yet. How to reduce side effects and how to ensure a safe delivery of strong laser pulses are key issues in this research. The power of ultrasound should be below the threshold pain level (\(<\)1.5 W/cm\({}^{2}\)) [107], and the fluence of any single laser pulse on the skin surface should follow the ANSI standard (\(<\)20 mJ/cm\({}^{2}\)). (iii) Multi-modality imaging provides complementary information. Besides AOUSI, photoacoustic imaging and other imaging modes, if integrated, will notably improve the acquired medical information about functional physiology and pathology. In addition, development of multi-modality endoscopy is promising by taking advantage of recent progress in optical endoscopic probes [108; 109]. AOUST may share several components of the optical probes, which facilitates the incorporation of ultrasound modality. (iv) The application of LGUS transmitters for real-time ultrasound imaging is rarely investigated. Development of an LGUS-based high-speed imaging system that meets the needs of modern medicine is a valuable research direction for the future. Array implementation of LGUS transmitters, as well as ultrasound receivers, can be one approach to realize real-time ultrasound imaging. (v) The majority of current research focus of LGUS technology still lies in basic research. More clinical and industrial tests should be conducted for its prevalence. LGUS transmitters and their use for AOUSI could provide groundbreaking opportunities for high-resolution imaging and detection, precise localization, image-guided surgery and therapy, interventional procedures, and remote sensing. With advances in LGUS technology, LGUS transmitters and AOUSI systems will continue to improve on their performance and functionality in the coming years, and thus, advanced applications such as high-resolution endoscopic diagnosis and real-time non-destructive assessment becomes possible. Further research on LGUS is of great importance to transform research studies into practical or clinical prevalence. This work was supported by the National Natural Science Foundation of China (No. 61405112), the National High Technology Research and Development Program of China (863 Program) (No.