Uncovering hidden structural and mechanical contrast via spectral photoacoustic imaging

Characterizing the mechanical and structural properties of materials is crucial for applications ranging from biomedicine to industrial non-destructive testing. While conventional photoacoustic imaging discards acoustic spectra or restricts spectral analysis to isolated microscopic resonances, we introduce Spectral Photoacoustic Imaging (SPAI) to analyze spatial-frequency information for revealing the mechanical and structural contrasts of macroscopic targets. In our proof-of-concept experiments, we demonstrate that SPAI can reveal contrast from subtle composition differences in commercially graded pencil leads where optical absorption yields negligible difference, extract the structural resonances of sub-surface cavities embedded in opaque 3D-printed blocks, and detect deeply buried internal voids through acoustic mode pattern perturbations. By combining broadband acoustic spectra with frequency-resolved modal patterns, this approach provides a practical framework for noninvasively probing structural and material signatures in complex targets.