## 2 Laser-Generated Ultrasound
### Design and Structures
#### 2.2.1 Fiber-Optic Transmitters  
By implementing LGUS transmitters on optical fibers, compact devices for ultrasound generation can be realized. Such miniature transmitters can be used in highly limited space to facilitate specific applications such as IVUS imaging. The fiber-optic LGUS transmitters have suffered from their low photoacoustic conversion efficiency for quite some time. Recently, advances have been made to overcome this obstacle partly by the remarkable progress in developing highly-efficient LGUS transmitters, as detailed in Section 2.1. The photoacoustic conversion efficiency of fiber-optic LGUS transmitters is improved primarily by employing effective absorptive materials. Various applications by virtue of the unique advantages of the fiber-optic LGUS transmitters are demonstrated. Au nanostructures are attempted for fiber-optic LGUS transmission. (i) Fabricated by a focused ion beam technique, highly absorptive Au nanopores can be patterned on the end face of an optical fiber [59]. The miniature LGUS transmitter produces ultrasound with an amplitude of 2.7 kPa and a \(-3\)-dB bandwidth of 7 MHz. (ii) A novel Au nanocomposite, synthesized by mixing the AuNPs into PDMS following a one-pot protocol, is dipped coated on a multi-mode fiber with a core diameter of 400 \(\upmu\)m to realize a broadband miniature fiber-optic LGUS transmitter [46]. The generated ultrasound has an amplitude of 0.64 MPa and a bandwidth of more than 20 MHz, which are significantly improved compared with the work in Reference [59]. Ultrasound imaging of a tissue sample with the resolution of 200 \(\upmu\)m is demonstrated. (iii) Since the absorption layer on the end face of an optical fiber is essential for efficient photoacoustic conversion, numerical simulation based on finite element analysis is investigated to study the photoacoustic generation of AuNPs-PDMS nanocomposites on optical fibers [60]. (iv) The dependency of absorption efficiency of the Au nanospheres and nanorods with different organization and dimensions is simulated [45]. The results provide a useful guide to the design of fiber-optic LGUS transmitters. (v) There are many other explorations of Au nanostructures for fiber-optic LGUS transmitters such as AuNPs-polyethylenimine (PEI) nanocomposites (2.4 kPa reported) [61], AuNPs-PDMS nanocomposites (37 kPa reported) [62] and fiber sidewall ultrasound generation [47]. The carbon-based material is another promising absorber for fiber-optic LGUS transmitters. (i) Through the dissolution of CNTs in PDMS facilitated by functionalization of CNTs with oleylamine, the CNTs-PDMS nanocomposite can be dip coated on optical fibers for efficient LGUS transmission [54]. Figure 3: Schematics of: fiber-optic transmitter (**A**); focused transmitter (**B**); array transmitter with its pitch defined by spatially selective pulsed laser heating (**C**); and array transmitter with its pitch defined by spatially fabricated absorbers (**D**). Blue lines represent short laser pulses. Figure 4 shows SEM pictures of fiber-optic LGUS transmitters using Au nanopores and CNTs-PDMS in References [54; 59], respectively. Strong ultrasound pressure of 3.6 MPa and 4.5 MPa are measured using optical fibers with core diameters of 105 and 200 \(\upmu\)m, respectively. The achieved pressure level is currently the best among all fiber-optic LGUS transmitters. (ii) A novel fiber-optic device based on fiber Bragg gratings is proposed to generate ultrasound at multiple, selected locations along the fiber in a scheme of light tapping where only a particular wavelength is tapped out for ultrasound generation [58]. Graphite powder and epoxy are used as LGUS transmitters in this device. The carbon-based fiber-optic LGUS transmitter typically has higher photoacoustic conversion efficiency than its metallic-based counterpart [63]. Besides broadband and efficient ultrasound generation, the fiber-optic LGUS transmitter possesses many advantages such as compact size, lightweight, ease of use, and immunity to electromagnetic interference.