## 4 Summary and Prospects  
In this review, the recent advances that have been made for LGUS transmitters are presented. Table 14 gives an overview of various developed LGUS transmitters. Table 14 provides the development of AOUSTs and their AOUSI applications. The review shows that the performance of LGUS transmitters has been constantly improved in recent years by advances in material science and fabrication technology. From the engineering viewpoint, the performance of LGUS transmitters has been significantly improved in a range of aspects, including transmitting efficiency, imaging resolution, size miniaturization, beam focusing and shaping, and functionality. From the application  
Figure 9: AOUSI cell images (taken at 10 GHz) of: the nuclear region (**C**); and lamellipodium (**D**). The upper row shows white-light images of: the nuclear region (**A**); and lamellipodium (**B**) for comparison. The scale bar of 10 mm applies to all images. The contour of the cell, the structure of the nucleus and the fine details of the fibrillar lamellipodium are clearly observed by the very high acoustic contrast. Reproduced with permission from [104], Nature Publishing Group, 2015.  
viewpoint, LGUS technique has been demonstrated for its potential in a number of applications including biomedical imaging, therapeutic ultrasound, and non-destructive evaluation. The LGUS platform possesses several advantages that can overcome limitations of conventional piezoelectric ultrasound method. (i) Through thermoelastic approach, the LGUS transmitters can generate a broadband ultrasound (from tens of MHz to even hundreds of GHz) provided that a short laser pulse is used to irradiate on an absorptive thin film. This is because the mechanism is based on photoacoustic effect, instead of the resonant characteristics used in piezoelectric transducers. In addition, flat spectral response can be intrinsically achieved. (ii) Imaging with high spatial resolution can be achieved. For ultrasound imaging, high resolution can be provided by either focusing acoustic waves to a diffraction-limited spot size or generating ultrasound in small pitches for phased array implementation. Both approaches can be realized by the LGUS method. Together with its capability to generate ultrasound with high frequency and broad bandwidth, the LGUS platform can provide high resolutions in both the lateral and axial directions. (iii) When combined with a noncontact ODUS sensing technique, LGUS technique provides a completely remote system that can be applied to harsh regions that are difficult to access, such as high pressure and high temperature [106], corrosive, and radioactive environments. Since it is a noncontact approach, it will not interfere with the sample being evaluated. (iv) LGUS technique also offers high tolerance to electromagnetic interference. (v) The fast development in LGUS technology has, in turn, triggered growing interest in constructing AOUSTs and their imaging applications to biomedicine. AOUSI has unique advantages in certain aspects over traditional piezoelectric ultrasound imaging. It seems to be observed that the growing interest and development speed of these techniques in medical imaging are faster than those in industrial applications. Effective and broadband LGUS transmitters should possess high conversion efficiency of radiation energy to heat, large coefficients of thermal expansion, and fast heat diffusion to the surroundings. Deduced by the above review, several principles can be followed when choosing suitable LGUS transmitters. (i) As for the absorbers, metallic or carbon-based materials should have low specific heat capacity for high light-to-heat conversion efficiency. (ii) As for the surrounding medium, materials such as PDMS with large coefficients of thermal expansion are desired for high heat-to-ultrasound conversion efficiency. (iii) The size of absorptive materials has a significant impact on their heat transition to the surrounding medium.