Figure 15.

(a) General scheme for nonlinear super-resolution photothermal microscopy. (b) Scheme of a focused beam. (c) Scheme showing nonlinear signal amplification when the particle is placed at the center of the heating laser spot. (d) Separation of the nonlinear signals from two nanoparticles separated by a distance smaller than the diffraction limit of a Gaussian beam (the dashed blue line represents the sum of linear signals from two nanoparticles). (e) Linear (top) and nonlinear (bottom) photothermal images of a cluster containing several 90 nm gold nanospheres. Adapted with permission from ref (124). Copyright 2014 John Wiley and Sons.