Resolution improvement in laser diode-based pump-probe microscopy with an annular pupil filter.

We experimentally demonstrate the use of annular beams to improve lateral resolution in laser-diode-based pump-probe microscopy. We found that the width of the point-spreading function in the case of the annular pump-probe beams is 162 nm, which is 30% smaller than that of the circular beams (232 nm). Furthermore, side lobes were efficiently suppressed at the focal plane since the pump-probe signal is proportional to the product of the two beam intensities. This scheme is demonstrated for the photothermal signal of nonfluorescent gold nanoparticles and the stimulated emission signal of fluorescence beads.