Figure 1.

(I) Principle of thermal lens microscopy. The photothermal signal is either positive or negative, depending on the positive or negative offset between the coaxial pump and probe beams, as the thermal lens acts as a diverging or converging lens. The signal vanishes for zero offset. Reprinted with permission from ref (81). Copyright 2000 The Japan Society of Applied Physics. (II) Principle of photothermal microscopy: divergence of the probe beam (purple) induced by the thermal lens (white) around the nanoparticle (yellow) due to the illumination of the heating beam (green). Reprinted from ref (5). Copyright 2019 American Chemical Society. (III) Photothermal microscopy is insensitive to nonabsorbing scatterers such as latex beads. (Left) Differential interference contrast (DIC) image and (middle and right) photothermal image of a sample consisting a mixture of single 300 nm latex beads, single 80 nm gold nanopsheres, and single 10 nm gold nanospheres. In the DIC image, the strong scattering objects are the latex beads, weakly scattering objects are single 80 nm gold nanoparticles, and single 10 nm gold nanoparticles are not visible. In the photothermal image at low excitation power (middle), only single 80 nm gold nanoparticles are visible, and in the photothermal image at high excitation power (right), both types of gold nanoparticles are visible, but the strongly scattering latex beads are not visible. Reprinted with permission from ref (20). Copyright 2002 The American Association for the Advancement of Science.