  Photoacoustic imaging and sensing have been studied extensively to probe the
optical absorption of biological tissue in multiple scales ranging from large
organs to small molecules. However, its elastic oscillation characterization is
rarely studied and has been an untapped area to be explored. In literature,
photoacoustic signal induced by pulsed laser is commonly modelled as a bipolar
"N-shape" pulse from an optical absorber. In this paper, the photoacoustic
damped oscillation is predicted and modelled by an equivalent mass-spring
system by treating the optical absorber as an elastic oscillator. The
photoacoustic simulation incorporating the proposed oscillation model shows
better agreement with the measured signal from an elastic phantom, than
conventional photoacoustic simulation model. More interestingly, the
photoacoustic damping oscillation effect could potentially be a useful
characterization approach to evaluate biological tissue's mechanical properties
in terms of relaxation time, peak number and ratio beyond optical absorption
only, which is experimentally demonstrated in this paper.
