## 2 Laser-Generated Ultrasound
### Absorptive Materials
#### 2.1.1 Metallic Absorbers  
The LGUS by laser irradiation on metals was first proposed by [PERSON] in 1963 [41], which can be driven by the ablation and the thermoelastic effect. The ablation has to melt the material resulting in plasma formation by applying a large optical power, while the thermoelastic effect does not. Thus, the thermoelastic effect is more desired in practical applications. A thin metal film irradiated by a short pulsed laser beam is capable of inducing a broadband acoustic wave. Furthermore, the adjacent materials of the thin metal film should also be considered for efficient generation of high-frequency ultrasound [42].  
AuNPs are used to optically generate high-frequency ultrasound. There are several examples. (i) A two-dimensional Au nanostructure consisting of AuNPs sandwiched between a substrate and a 4.5-\(\upmu\)m PDMS layer is used to generate ultrasound in the 100 MHz regime [43]. Scanning electron microscope (SEM) picture of the nanostructure is shown in Figure 1. The 2D nanostructure also facilitates the construction of LGUS array transmitters for high-frequency ultrasound imaging. (ii) A one-pot synthesized AuNPs-PDMS nanocomposite for high-amplitude ultrasound generation is demonstrated [39]. The efficiency is optimized through the investigation on various concentrations and thickness of the nanocomposite film. Compared with the pressure generated by an Al thin film, the AuNPs-PDMS transmitter has shown increased efficiency by three orders of magnitude. (iii) The process of heating of AuNPs is studied numerically by finite element analysis [44]. The results can also provide useful guidance for other applications such as photothermal therapy. (iv) AuNPs are tested for fabricating fiber-optic LGUS transmitters [45; 46; 47]. AuNPs can be coated on the end face [45; 46] or on the sidewall [47] of optical fibers. (v) A novel one-dimensional photonic crystal-metallic structure is proposed for complete absorption of optical energy at specific wavelengths for efficient LGUS transmission [40].  
Besides AuNPs, there are several studies on other metals for LGUS transmitters. (i) The laser-generated surface acoustic waves in an Al plate are studied by the finite element model [48]. It is found that the laser parameters are influential factors on the induced ultrasound waves. (ii) Thin Al film coated with reduced graphene oxide (rGO) for its high thermal conductivity is an effective LGUS transmitter, which shows a 64 times enhancement compared to an Al-along transmitter [49]. (iii) The  
Figure 1: (**A**) Scanning electron microscope (SEM) picture of the two-dimensional (2D) Au nanostructure (top view); and (**B**) schematic of the 2D Au nanostructure on a substrate (side view). Reproduced with permission from [43], AIP Publishing, 2006.  
Al states such as the dynamics of temperature and pressure can be studied by heating a submicron Al film with high power nanosecond laser pulses [50]. (iv) Nanostructured germanium (Ge) overlaid with PDMS is fabricated for highly-efficient generation of high-frequency ultrasound [51]. The substrate can be chosen arbitrarily in the fabrication process, allowing flexible applications.