Detection of photothermal signals from countable and sub-single molecules in liquids

We developed a coaxial beam photothermal microscope, which realize a thermal lens measurement in liquid under an optical microscope. The photothermal signal from countable number of dye molecules could be detected, and they showed good linearity on the number of the molecules. The minimum expected number of the molecules detected was 0.4, half-molecule, and the lower limit of detection calculated from the calibration curve was 0.3. Therefore, we could detect the photothermal signal from sub-single molecule was considered the mechanism of the signal generation may be different from the conventional thermal lens methods.