Photoacoustic Ultrasound Generation on an Optical Fiber Tip Using Gold Nanoparticles as the Target Material

Ultrasonic generators have been used in various applications such as medical imaging and nondestructive testing. The authors have proposed an integration of an ultrasonic generator and a receiver on one piece of optical fiber. The ultrasound wave is steered by the phased array technique, which is inspired by the way bats detecting objects in the dark environment. This application requires an ultrasonic generator featuring a higher frequency, a wider bandwidth and a smaller size. Conventional ultrasonic generators, which are made from piezoelectric materials, have difficulty in achieving the frequency higher than 10 MHz. However, real-time ultrasonic imaging applications need generators featuring frequencies higher than 30 MHz. Recent studies are developing novel ultrasonic generators via the photoacoustic mechanism. An energy absorption layer is applied to absorb light energy and convert it into heat. A mechanical wave is generated by the layer expanding itself due to the thermal expansion phenomenon. The ultrasonic generator based on the photoacoustic principle can exhibit high frequency and wide bandwidth. An ideal energy absorption layer must feature high energy absorption coefficient and high coefficient of thermal expansion (CTE). In this paper, a photoacoustic ultrasound generator on an optical fiber tip was designed, fabricated and tested. Gold nanoparticles (Au NPs) were coated on the end surface of a piece of commercial single mode fiber to be the energy absorption layer. Au NPs were synthesized by the traditional sodium citrate reduction method and the diameter of the Au NP was controlled at 20 nm. Layer by layer (L-b- L) technique was used to coat the Au NPs on the end surface of the fiber. By introducing excitation laser pulses on to the Au NPs, the laser energy was converted into the heat through photothermal effect and finally was turned into a mechanical wave by the thermo-elastic mechanism. The ultrasound generator exhibits very compact size due to the fiber’s miniature size (125 μm in diameter). The results from the experiment showed that an acoustic level of 2.44 kPa at 1.16 mm away from the fiber tip was observed. The laser energy density was 4.244 J/cm2 and the pulse width of the laser was 120 ns.