## 1 Introduction  
Laser-guided focused ultrasound (LGFUS) is a promising alternative to other ultrasound generating modalities (electric, electromagnetic, and piezoelectric) to produce broadband ultrasound signals [1]. LGFUS has proven successful in providing a better alternative to non-invasive procedures for therapeutic purposes and many other biomedical applications [2]. The most prominent mechanism of LGFUS is the thermoelastic effect, also known as the photoacoustic or optoacoustic effect [3], in which laser energy is converted into acoustic energy using the thermal and elastic effect of the materials used in the ultrasound transducer fabrication. Another major characteristic feature of LGFUS is the generation of the shock wave at the focal point with a significant positive phase and long trailing negative phase, as formed in shock wave lithotripsy [4]. These shock waves can be achieved using specially constructed photoacoustic composite transducers for the efficient conversion of laser energy into acoustic energy [2]. Such composite transducers constitute a high optical absorbing material and an elastomeric polymer grown on a substrate [1]. High optical absorbance material is required to absorb maximum light from the source with a minimum reflection, and an elastomeric polymer with a high heat expansion coefficient is needed to convert it into acoustic energy with a high amplitude [1, 5]. CNTs have a high light absorption ratio and generate high temperatures locally under laser exposure [6]. Additionally, CNTs enhanced peak pressure to more than one order of magnitude compared to metals by reducing reflectance and increasing the maximum optical absorbance [7, 8]. Under pulsed laser excitation, CNT compositesgenerate extreme acoustic pressure: eighteen times stronger than a Cr (Chromium) film and five times more potent than a gold nanoparticle composite with the same elastomeric polymer [9]. Furthermore, rapid heat diffusion to the surrounding medium is one of the main characteristics of CNTs [4]. Due to the nanometer spatial configuration of CNTs, thermal energy can be transferred to the adjacent expanding layer. Polydimethylsiloxane (PDMS) has been widely used as a thermally expanding layer because of its high thermal coefficient of volume expansion (\(\alpha\) = 0.92 \(\times\) 10\({}^{-3}\) K\({}^{-1}\)) [6]. Therefore, when CNTs are heated, they immediately transfer most of their absorbed heat energy to the adjacent PDMS layer, which can cause rapid thermal expansion with a high amplitude [4]. LGFUS can produce high frequency (>15 MHz) and acoustic pressure of >50 MPa from a single element lens (6 mm in diameter) via a CNT and PDMS composite transducer on a concave surface [4]. [PERSON] et al. confirmed that CNT-PDMS films can endure a higher optical fluence than a chromium (Cr) film, making them a perfect composite film to apply the maximum available laser energy to optoacoustic excitation. The structural characteristics of the lamination layers define them as excellent materials for high amplitude and high-frequency ultrasound generation [10]. LGFUS has used different techniques to generate shock waves and investigate the effect of different physical parameters on shock wave characteristics. [PERSON] et al. used a small optoacoustic lens of varying diameters to determine the peak positive and negative pressure of the focal waveform (6 mm and 12 mm) and attained a maximum positive pressure of more than 50 MPa [4]. [PERSON] et al. utilized functionalized CNTs and dissolved them into a PDMS solvent before coating them on a substrate. They employed planar optical fibers as the substrate and achieved maximum pressures of 3.6 MPa and 4.5 MPa [11]. Furthermore, experiments using CNTs of different diameters and tube lengths were also carried out to investigate the effect of the thermal conductivity of CNTs on the performance of optoacoustic transducers, resulting in CNTs with a diameter and length of 8 nm and 10-30 \(\mu\)m, respectively [12]. [PERSON] et al.