Nonlinear frequency-mixing photoacoustic imaging of a crack

We present a technique for nonlinear photoacoustic imaging of cracks by laser excitation with intensity modulation at two fundamental frequencies combined with detection at mixed frequencies. By exploiting the strong dependence of the photoacoustic emission efficiency on the state—open or closed—of the contacts between the crack faces, remarkably enhanced image contrast is observed, ∼20 times higher than in linear photoacoustic images at the highest of the fundamental frequencies.