Reduction of distortion in photothermal microscopy and its application to the high-resolution three-dimensional imaging of nonfluorescent tissues.

A scheme for reducing image distortion in photothermal microscopy is presented. In photothermal microscopy, the signal shape exhibits twin peaks corresponding to the focusing or defocusing of the probe beam when a sample is scanned in the axial direction. This causes a distortion when imaging a structured sample in the axial plane. Here, we demonstrate that image distortion caused by the twin peaks is effectively suppressed by providing a small offset between two the focal planes of the pump and the probe beams. Experimental results demonstrate improvement in resolution, especially in the axial direction, over conventional optical microscopy-even with the focal offset. When a dry objective lens with a numerical aperture of 0.95 is used, the full width at half the maximum of the axial point spread function is 0.6 μm, which is 50% (62%) smaller than the focal spot sizes of the pump (probe) beam. Herein, we present high-resolution three-dimensional imaging of thick biological tissues based on the present scheme.