Fig. 1: Nonlinear changes in the optoacoustic spectrum and signal as a function of fluence.

A homogeneous agar cube with uniform reduced scattering coefficient \({{\rm{\mu }}}_{{\rm{s}}}^{\prime} =4{\pm 1{\rm{cm}}}^{-1}\) and absorption coefficient \({{\rm{\mu }}}_{{\rm{a}}}=0.1\pm 0.02{{\rm{cm}}}^{-1}\) was illuminated with a laser operating at 800 nm at different light fluences. a The raw optoacoustic signal as a function of time (depth). The signals corresponding to the edge of the cube closer to transducer is weaker than the signals from the edge closer to illumination source due to light attenuation. b Diagram of the location of the phantom in relation to the illumination source and transducer in terms of both time and distance. c Enlarged plot of the signal from the edge closest to the illumination source. d The same signals from panel c normalized to the corresponding fluences. e The spectra of the signals in panel a. f The signals from panel e normalized to the corresponding fluences. g The optoacoustic spectrum amplitude at different frequencies as a function of fluence