PROCEEDINGS ARTICLE | April 15, 2011
Proc. SPIE. 7982, Smart Sensor Phenomena, Technology, Networks, and Systems 2011
KEYWORDS: Gold, Optical fibers, Nanoparticles, Ultrasonography, Finite element methods, Aluminum, Photoacoustic spectroscopy, Acoustics, Absorption, Nanorods
The low energy-conversion efficiency in photoacoustic generation is the most critical hurdle preventing its wide
applications. In recent studies, it was found that the selection of the energy-absorbing layer material and design of the
acoustic generator structure both determine the photoacoustic conversion efficiency. The selection of the absorbing
material is based on its optical, thermal, and mechanical properties. In this research, we calculated and compared the
conversion efficiencies of six different absorbing film materials: bulk aluminum, bulk gold, graphite foil, graphite
powder-resin mixture, gold nanospheres, and gold nanorods. The calculations were carried out by a finite element
modeling (FEM) software, COMSOL Multiphysics. A 2D-axisymmetric model in COMSOL was built up to simulate a
3-layer structure: optical fiber tip, light absorbing film, and surrounding water. Three equations governed the thermo-elastic
generation of ultrasonic waves: the heat conduction, thermal expansion and acoustic wave equations. In "thick-film"
generation regime, majority of the laser energy is absorbed by the film and converted to high-frequency film
vibration, and the vibration excites the ultrasound wave in the adjacent water, while the water would not be heated
directly by the laser. From the results of this FEM simulation, the acoustic signal generated by gold nanosphere (or
nanorod) film is over two times stronger than that generated by graphite powder-resin film of the same thickness. This
simulation provides a strong support to the absorbing material selection for our proposed fiber ultrasound generator.
