## 1 Introduction  
Ultrasound, defined as sound with frequencies from 20 kHz to several GHz, has demonstrated broad applications in medical imaging [1], material science, physics, chemistry, and nanotechnology. In classical medical ultrasound (also known as ultrasonography), an ultrasound pulse is transmitted into the body and the pulse is reflected due to the interfaces between tissues with different acoustic impedances as image contrast. The echo pulse carries back physiological and pathological information inside tissues. Especially, high-frequency ultrasound with frequencies >20 MHz is useful for high-resolution (<100 \(\upmu\)m) biomedical applications such as endoscopic imaging [2], ophthalmology [3], intravascular imaging [4] and dermatology [5]. Magnetic resonance imaging (MRI) systems continue to enhance the resolution up to ~500 \(\upmu\)m [6]. However, routine interventional procedures are restricted due to the high expense of MRI in both the initial investment and the upkeep. Furthermore, MRI has limited temporal resolution, which prohibits its use in fast imaging and rapid diagnosis. X-ray computed tomography is a relatively affordable modality with high resolution of ~400 \(\upmu\)m [7], but it relies on ionizing radiation, making it unsafe for medical monitoring. Optical method is a promising approach for biomedical imaging [8]. Optical coherence tomography (OCT) is an optical technique that can support real-time imaging with high resolution (<10 \(\upmu\)m) [9]; however, OCT suffers from limited penetration depth (<1 mm). Nowadays, the majority of ultrasound imaging systems are safe and portable, and can provide high frame rates beyond 25 frames per second, the threshold for real-time display. It is worth mentioning that photoacoustic imaging derived from ultrasonography is an emerging modality that relies on laser pulses directed on tissues to produce ultrasound [10; 11; 12]. Photoacoustic imaging can, therefore, possess the penetration depth of ultrasound while maps the optical absorption of tissues instead. Presently, piezoelectric transducers are the principle measure to produce biomedical ultrasound for imaging and therapy applications [13; 14]. However, high-resolution ultrasound imaging (<100 \(\upmu\)m) is restricted due to the limited bandwidth of ultrasonic signals generated by the piezoelectric effect. High intensity focused ultrasound (HIFU) is valuable for therapeutic applications [15; 16]. Its applications featuring high-frequency ultrasound for precision therapy also face challenges due to the frailty of piezoelectric materials and impedance mismatch for a large aperture [17]. Capacitive micromachined ultrasonic transducers (CMUTs) are an alternative technique that is promising to produce broadband ultrasound pulses [18; 19; 20; 21], yet there exist technical challenges in realizing two-dimensional (2D) high-frequency CMUT arrays [22]. Laser-generated ultrasound (LGUS) has been investigated over decades [23; 24; 25]. LGUS is with a recent enthusiasm in high-frequency ultrasound and HIFU. LGUS transmitters usually consist of optical absorbers for efficient conversion of light energy into heat, and surrounding media with high coefficients of thermal expansion for generating high-intensity ultrasound. Based on photoacoustic effect, the LGUS transmitters generate ultrasound pulses following the laser profile. If a short laser is employed, short ultrasound pulses in the scale of nanoseconds can be produced. Thus, it is a relatively simple method to create a broadband and high-frequency ultrasound signal. Efforts have been made aiming to optimize laser-ultrasound transduction efficiency in recent years, especially for high-frequency ultrasound generation.