Generation of High-Frequency Ultrasound in a Liquid upon Excitation by Laser Radiation through a Light Guide with a Converter of Transparent Spheres

One of the important tasks in optoacoustics today is the development of methods and tools for generating high-frequency ultrasound (above 1 MHz) in liquids and other media. To expand the frequency range of ultrasound, it was proposed to use coatings consisting of focusing spheres on a fiber tip. The methodology of calculating the ultrasound spectra depending on the sphere size, index of refraction, and parameters of laser radiation was developed. Two cases of small and large spheres in strongly and weakly absorbing media were simulated. The experimental results were analyzed in the approximations allowing a fairly accurate estimation of the spectrum and indicatrix of the generated ultrasound upon laser excitation through a converter based on a coating of transparent spheres. A good agreement between the model and experimental result was obtained.