The Excitation of Ultrasound by Laser Radiation in Water Using an Optical Fiber Laser Converter with a 2D Colloidal Crystalline Coating

Abstract

The technology of applying a colloidal single-layer coating of transparent polystyrene (PS) Ø 1 μm spheres at the tip face of a quartz fiber has been proposed and tested. Such a coating plays, in a light absorbing liquid, the role of a converter of pulsed laser radiation into acoustic radiation. The generation of ultrasound in water using a converter based on a quartz fiber 1 mm in diameter with a 2D colloidal crystalline coating consisted of polystyrene spheres with a diameter of ~1 μm at the fiber end was investigated. When excited by laser radiation (λ = 1.064 µm), coating of polystyrene spheres created in the liquid a laser thermal microstructure with a characteristic size of fractions of ~λ and a maximum temperature up to 10−2 degree at an energy in a short laser pulse of ~0.005 J. This short-lived thermal microstructure generated sound pulses in the liquid in the approximately 0.2–4 MHz range. The results of the experimental study of this effect are reported. The proposed laser radiation converter with colloidal coating of the optical fiber distal tip by a single layer of transparent spheres can be used for the development of new laser microtools for studying, processing of various objects in microsurgery, microstructuring of the surface, spot cleaning and restoration of objects of art and history.

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

“…A methodology for calculating the main characteristics of the generated ultrasound (the frequency spectrum and the angular diagram) is presented (Part 2). Part 3 (Results) contains the results of calculating US parameters in two opposite cases: large spheres with a diameter of 200 µm in a highly absorbing medium (ink solution in water at a wavelength of 0.532 µm, light absorption coefficient α ≈ 10 −2 µm −1 ) and small spheres with a diameter of ≈ 1 µm in a weakly absorbing medium (pure water at a wavelength of 1.064 µm, α ≈ 10 −5 µm −1 ), as well as comparison of the results of calculations with the corresponding experimental results from the papers [8,9]. Part 4 (Discussion) is devoted to the analysis of the comparison of theoretical and experimental results both from the point of view of the applicability of the theoretical consideration and from the viewpoint of the possibilities and prospects for the development of ultrasound generation technology using fiber systems coated by transparent spheres.…”

“…No detailed enough research aimed at testing and exploiting such ideas has been reported in the literature yet. In our earlier works [8][9][10][11], we carried out experimental studies in two relatively limiting cases: large spheres with a diameter of 200 µm in a highly absorbing medium (ink solution in water at a wavelength of 0.532 µm, light absorption coefficient α ≈ 10 −2 µm −1 ), and small spheres with a diameter of 0.96 µm in a weakly absorbing medium (pure water at a wavelength of 1.064 µm, α ≈ 10 −5 µm −1 ). The considered media were chosen because they are the simplest phantoms of biological media.…”

“…Naturally, for effective suppression of microorganisms, the generated ultrasound, in addition to high frequency (several MHz), must be sufficiently intense. For this, it is logical to focus the radiation emerging from the fiber, for example, using transparent microlenses or transparent microspheres deposited on the fiber end [8,9]. No detailed enough research aimed at testing and exploiting such ideas has been reported in the literature yet.…”

“…In the next sections, based on the qualitative analysis made above, we present results of the quantitative consideration of US generation using coatings from transparent spheres and compare them with the experimental data [7][8][9] on the generation of high-frequency US with excitation through a layer of focusing microspheres.…”

“…Beam structure from a system of spheres. For a system of small spheres (0.96 µm), such an analysis was carried out in [9]. It was shown that, in a weakly absorbing medium, a laser beam breaks up into a three-dimensional interference periodic system of prolate spheroids, the minimal size of which (and, consequently, the maximal light intensity) is determined by beam focusing by a single sphere.…”