## 1 Introduction  
Photoacoustic imaging as a non-invasive detection technology has been widely used in biomedical imaging [1], nondestructive testing [2], and other fields. Due to the thermoelastic effect [3], when a laser beam irradiates a sample, optical absorption in the material causes the light energy to initiate thermal expansion, resulting in deformation of the illuminated area and its surroundings. If the illumination is a fast transient pulse or if the light is modulated periodically, then the material and its surroundings deform correspondingly, thereby generating an acoustic wave. In photoacoustic imaging, structural features or defects in the object can be characterized by detecting reflections or other perturbations in the ultrasonic waves. The spatiotemporal features of the ultrasonic wave are related to those of the incident laser light, but although an intense beam can generate strong acoustic waves, it may damage the material. However, laser intensities far below the damage threshold generate acoustic waves that are relatively weak. The signal-to-noise ratio (SNR) of the reconstructed image can be quantitatively expressed as  
\[\text{SNR}\ \sim\ \text{S}\ \times\ \text{\gamma}\ \times\ \text{A}\ \times\sqrt{\text{P}\times\text{M}}/\text{N} \tag{1}\]  
where S is the photo-generated ultrasound intensity, \(\gamma\) is the sensitivity of the detector, A is the imaging area, P is the number of projections, M is the number of signal averages, and N is the noise floor of the system [4]. Thus, if the excited ultrasound signal intensity, which is proportional to the optical energy, is reduced by a factor of K, to recover the SNR,averaging over K\({}^{2}\) times is needed. Therefore, in order to obtain a higher SNR, researchers tend to increase the magnitude of the generated photoacoustic signal. In current biomedical photoacoustic imaging research, to enhance the imaging quality or increase the imaging depth, the laser pulse energy density used in the experiment is usually higher than the ANSI specified threshold of 20 mJ/cm\({}^{2}\)[5], which can result in ablation of the tissue [6; 7]. It is vital to explore methods for generating strong acoustic waves using energy densities below the laser ablation threshold. To achieve efficient photoacoustic wave generation, [PERSON] et al. used a continuous wave laser with intensity modulation to excite ultrasound waves [8], which greatly improved the SNR and simultaneously kept the sample surface intact. However, this method resulted in the generation of multiple acoustic modes and reflected waves, and interpretation of the signals was complicated. [PERSON] et al. proposed a photoacoustic method that uses a laser array instead of a single light source to excite acoustic waves [9], which increases the intensity of the ultrasonic wave without increasing the energy density illuminating the sample surface. Research of [PERSON]'s group demonstrated that waves can be focused and steered using a spatially distributed laser source, with a spatial light modulator or specially designed computer-generated holograms [10; 11; 12; 13]. They also developed a method using laser-induced phased arrays (LIPAs) to detect defects in metals and provide in-process monitoring during additive manufacturing (AM) [13; 14]. It has also been shown that temporal control of the elements in a linearly arrayed laser source is an efficient way to enhance the intensity of the generated acoustic wave or steer the propagation direction [15; 16; 17; 18; 19; 20; 21]. Although wave steering and signal enhancement by controlling the spatial or temporal distribution of the generated laser source have been demonstrated, the method for manipulating the wavefront of a laser-generated acoustic wave is still uninvestigated. The use of acoustic metamaterials to manipulate the wavefront of propagating acoustic waves from various sources has been proposed and developed.