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Optical sectioning in optical resolution photo acoustic microscopy

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Photoacoustic imaging is one of the recent techniques in the arsenal of bio-medical imaging methods. It cumulates the advantages (and disadvantages) of optics and acoustic. But to compete with the mature pure optical methods, such as the fluorescence microscopy, a three dimensional imaging is mandatory.

 Photoacoustic imaging (PAM) of solid or liquid samples (particularly for biological tissues) is an emerging imaging modality, where light excitation is coupled to acoustic detection via the photoacoustic effect. Samples are illuminated by pulsed light, and acoustic waves generated from the illuminated absorbing regions are detected by acoustic sensors. Several variants are now available (multi detectors, laser scanning ...) particularly the optical resolution photoacoustic (OR-PAM) where the lateral resolution is given by the focused laser beam (µm). Usually the axial resolution is given by the acoustic detector in a confocal position preventing a fast scanning acquisition.

In this context, a joint team of physicists from the LIPhy and the Department of Opto and Cogno-mechatronics from Pusan National University (South Korea) has demonstrated a pure optical sectioning (a good axial resolution) via optical nonlinearity. The classic multiphoton or multiharmonic excitation being not efficient enough at generating detectable ultrasound it is the sound produced by the non-radiative relaxation of chromophore levels populated by excited state absorption (ESA) which has been used. The team brings a proof of concept thanks to popular dyes widely used for biological imaging. A new dye family, presenting a particularly intense excited state absorption, has also been investigated. This work, published at the end of July in Optics Express provides an important step toward a full optical 3D resolution fast scanning photoacoustic microscopy.