Figure 3.

(I) Scheme of a PHI microscope offering both backward and forward configurations. The forward detection mode requires an additional microscope objective. (II) Photothermal image of single 10 nm gold NPs in (a) backward and (b) forward detection modes. Scale bar: 1 μm. In both images, the narrow distribution of photothermal signals confirms the narrow size distribution expected for single nanoparticles. (III) Dependency on modulation frequency of the photothermal signal in (a) forward and (b) backward detection. At higher modulation frequencies, the photothermal signals in both detection modes are similar, whereas at lower frequencies, the photothermal signal is higher in the forward detection mode. Panels (I–III) are reprinted with permission from the ref (24). Copyright 2006 American Physical Society. (IV) Photothermal microscopy of a mixture of single 10 and 30 nm gold NPs and 100 nm polystyrene beads: (a) in-phase and (b) out-of-phase signal using a single laser beam and (c) PHI signal using separate pump and probe beams. In the in-phase image, a constant background and strong scattering by polystyrene beads are visible. In the out-of-phase image, there is a minor leakage from the strongly scattering objects whereas in the photothermal image using two separate laser beams, the signal-to-noise is about 1 order of magnitude better than with a single laser beam. Reprinted with permission from ref (26). Copyright 2014 AIP Publishing.