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Stimulation Emission Depleted Photoacoustic

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A chromophore brought in an excited state, when it absorbs light, returns to equilibrium spontaneously by re-emitting light, photons, it is said to fluoresce. But when it is in the condensed phase, its de-excitation can also be done by generating vibrations, phonons, at the origin of a photoacoustic signal. De-excitation can also be stimulated by another light source, it is done to the detriment of the spontaneous emission by depletion of the excited state, it is the phenomenon of Stimulated Emission Depletion (STED ).

When the excitation is localized by a focused laser the STED can improve the localization by "erasing" a part of the excited zone. It is the scanning of this super-resolved zone that leads to super-resolution fluorescence microscopy (F-STED microscopy). F-STED microscopy is now mature, commercial devices are available. It was tempting to try to extend the STED to the photoacoustic domain, which is conceivable since the photoacoustic signal also results from the spontaneous relaxation of the population of the excited state of the chromophores. It is in collaboration with researchers from the University of PUSAN (South Korea) that LIPhy researchers validated the STED effect on the photoacoustic signal, called S-DEST for Sound Stimulated Emission Depletion.
All was not an easy task since, to be an efficient phonon generator, the chromophore should have necessary a low quantum yield, the stimulated de-excitation then hardly competes with the de-excitation by phonons. The researchers came up with the idea of choosing a chromophore with good quantum efficiency but whose phonon emitter level is only reached by absorption from an excited state (ESA). As a result, the stimulated emission only has to fight against the ESA which is carried out with a laser of moderate power. They also showed that this particular S-STED effect made it possible to measure the molecular orientation generated by the pump laser, which can not be done in conventional photoacoustics. The pathway is therefore open to super resolved photoacoustic microscopy. This work, carried out in the laboratory thanks to the funding of the CNRS inter-institute program of the Interdisciplinary Mission (SuperPAM grant 2016) and the "Basic Science Research through the National Research Foundation of Korea (NRF)" program, resulted in a publication in Optics Express of September 2019.

View online : Stimulation Emission Depleted Photoacoustic, Seongho Park, Minju Kim, Jean-Claude Vial, and Kwangseuk Kyhm, Optics Express 2019