## 1 Introduction
Laser-ultrasound transduction efficiency can be described by \(p_{0}\) = \(\Gamma A_{\text{e}}\)[26], where \(p_{0}\) is the local pressure immediately after the laser radiation; \(\Gamma\) = (\(\beta v_{\text{s}}^{2}\))/\(C_{\text{p}}\) is defined as the Grueneisen coefficient, \(\beta\) is the thermal coefficient of volume expansion, \(v_{\text{s}}\) is the speed of sound, and \(C_{\text{p}}\) is the specific heat; \(A_{\text{e}}\) = \(\upmu_{\text{a}}F\) is the volumetric optical absorption, where \(\upmu_{\text{a}}\) is the optical absorption coefficient and \(F\) is the optical fluence. High optical absorption ensures high optical energy density deposited on the absorber, low specific heat provides high temperature rise under the same optical absorption, and large thermal coefficient of volume expansion produces strong pressure under the same temperature rise. (i) Materials with high optical absorption coefficient and low specific heat to reach local high temperature in the laser irradiation region are investigated. (ii) Absorbing materials with a unique spatial configuration such as nanoscale composition have been chosen to allow fast heat transfer into the expanding surroundings and thus the generation of high-frequency ultrasound. (iii) Polydimethylsiloxane (PDMS) has been considerably adopted as an attractive expanding matter in virtue of its high coefficients of thermal expansion [27]. LGUS transmitters are favorable for high-frequency arrays for high-resolution ultrasound imaging compared with traditional piezoelectric ultrasound transducers [28; 29]. In addition, efficient and broadband laser-generated focused ultrasound enables broad HIFU applications such as high-precision therapy [30] and micro-droplet ejection [31]. Furthermore, the advantage of contactless excitation furnished by LGUS transmitters permits unique applications that may not be attained by conventional ultrasound transmitters. Based on optical generation and detection of ultrasound, all-optical ultrasound transducers (AOUSTs) and their application to all-optical ultrasound imaging (AOUSI) are realized. As mentioned above, LGUS transmitters can generate a broadband and high-frequency ultrasound signal. For optical detection of ultrasound (ODUS), optical resonance can be used to realize sensitive and broadband ultrasound detection. There are several types of ODUS detectors such as microring resonators [32], Fabry-Perot (FP) etalons [33], and Mach-Zehnder interferometers [34]. The optical detection platform intrinsically facilitates broadband ultrasound detection up to hundreds of MHz [32; 35]. Therefore, AOUSTs hold the promise for high-resolution ultrasound imaging compared with piezoelectric transducers. In addition, unlike the piezoelectric transducer, the detection sensitivity of ODUS detectors is inherently independent of their active area, which is optically defined on an order of microns. Although the reduced optical active area may impair the optical property such as quality factors and thus the ultrasound detection sensitivity [36; 37], there exist approaches to recover the degraded optical property [38]. High sensitivity is thus preserved with small active area. On the other hand, the LGUS transmitters can be realized with a similar size, for example, using a point-like laser source or light-absorbing material for ultrasound excitation. Such features can be utilized to construct a miniature AOUST probe without sacrificing its sensitivity, which facilitates a range of applications such as endoscopic ultrasound imaging and minimally invasive clinical procedures. The possibility of noncontact operation of AOUSTs exhibits another advantage over conventional transducers. In this review article, various progress of LGUS transmitters and AOUSTs in recent years is presented. In Section 2, the development of LGUS transmitters is described. A variety of absorbers such as carbon nanotubes (CNTs) and gold (Au) nanoparticles (NPs) (AuNPs) are employed for efficient photoacoustic transmission.