PhotoAcoustic (PA)spectroscopy is based on the so-called PA effect, which can be defined as the formation and emission of acoustic waves following the absorption of intensity-modulated (typically pulsed) optical radiation by a material.
The underlying physical mechanism of the PA effect is thermoelastic; a portion of the absorbed optical energy is converted into heat following non-radiative molecular relaxation, leading to a rapid transient expansion of the absorber and the generation of an initial pressure which typically propagates in the form of broadband ultrasonic waves in the MHz regime. The emitted PA waves can then be detected by employing the same equipment (e.g., piezoelectric elements) as in conventional ultrasound imaging.
In the linear regime of PA excitation, the peak-to-peak amplitude of the emitted PA waves is directly proportional to the apparent optical absorption coefficient experienced by the excitation wavelength. As a consequence, PA diagnostic techniques can offer excellent optical absorption contrast features deep inside optically turbid media with high sensitivity and spatiotemporal resolution, as the propagating PA waves are attenuated up to three orders of magnitude less compared to NIR optical radiation. Furthermore, the trade-off between imaging depth and spatial resolution can be adjusted according to the acoustic detection bandwidth, as well as, the properties of the optical excitation and acoustic detection elements, permitting thus observations of different specimens at various spatial scales.
The recording of PA signals could successfully uncover hidden underdrawings in paintings and murals, detect invisible restoration operations in artworks, reveal hidden text in multi-layered documents and measure the thickness of opaque paint layers. Therefore, the application of PA technology in CH diagnostics could offer enhanced imaging depth to spatial resolution ratios coupled with high optical absorption sensitivity, overcoming the light scattering limitations of several pure optical techniques.
