Study on Fabrication and Applications of Ultrasound Transducers Using Carbon Nano Materials with Optoacoustic Effect

Abstract

The optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light). The optoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphones or piezoelectric sensors. The time variation of the electric output (current or voltage) from these detectors is the optoacoustic signal.

In the generation of ultrasound, pulsed laser beams are irradiated on a material and absorbed by it. The absorbed optical power is converted to heat, leading to rapid localized temperature increase. This results in rapid thermal expansion of a local region, leading to generation of ultrasound into the medium. However, to obtain the ultrasound with high acoustic pressure, the materials to fabricate ultrasound transducers should have high efficiency of optical energy absorption and high thermal conductivity.

In this dissertation, the composite of carbon nanotubes (CNTs) and poly-dimethylsiloxane (PDMS) were coated on a polymethyl methacrylate (PMMA) plate to make optoacoustic transducers. Because of the high efficiency of optical energy absorption and high thermal conductivity, they can transfer the absorbed heat energy to a surrounding medium very rapidly and effectively.

The generated ultrasound waves by the optoacoustic transducer when irradiated a pulsed laser beam were measured using a needle hydrophone. It was shown that the generated ultrasound waves had the typical waveform of the blast wave. Some propagation characteristics of the blast wave were measured and compared with the calculated results. It was found that the propagation speed and attenuation of the wave are different from those of usual sound. And, from the comparison of the measured and the calculated acoustic fields, it is assumed that every point on the transducer surface produces almost same waveform.

In order to get much higher acoustic pressure by focusing, an optoacoustic film transducer was fabricated by coating carbon nanotubes (CNTs) and poly(dimethylsiloxane) (PDMS) on a surface of a flexible optical polyethylene terephthalate (PET) sheet with thin thickness. A line-focused optoacoustic source was made using the film transducer, and its characteristics were investigated. Very high pressure about 35 MPa in maximum was obtained by the source. It was demonstrated that the source can engrave a line trace on a chalk surface.

The results suggested that the composite of light-absorbing material (CNTs) and elastomeric material (PDMS) can be used to fabricate an optoacoustic transducer and generate blast wave-like shock waves efficiently when the transducer is irradiated by a pulsed laser beam. The line-focused optoacoustic source fabricated by using the film transducer gives very high peak pressure at its focal position. From the line trace engraved on a chalk by the effect, it is suggested that the acoustic pressure for micro-scale ultrasonic fragmentation and biomedical therapy could be obtained by the line-focused optoacoustic source.