Photoacoustic resonance by spatial filtering of focused ultrasound transducers

The effect of a spherically focused ultrasound (US) transducer (TD) on photoacoustic (PA) measurements is analytically investigated using the concept of a virtual point detector. The derived analytical results indicate that the limited numerical aperture (NA) of the PA detector takes on the role of spatial filtering of the induced PA waves, which leads to the occurrence of a peak frequency in the PA spectrum. The mathematical description of this phenomenon is similar to the result of resonance peaks of light propagation in dielectrics. This PA resonance peak depends on the NA of the US TD and the absorption coefficient of the PA source. Experimentally measured PA spectra from an ink solution using a frequency-domain PA system verify the PA resonance effect. Finally, we discuss the possibility that previously reported PA resonances interpreted as PA source viscosity might be actually caused by the US TD's spatial filtering.