FIG. 1.

Nonlinear optical interactions available for multiphoton image contrast. On the left side of the dashed line are conventional contrast mechanisms: two-photon-excited fluorescence (TPF), second-harmonic generation (SHG), third-harmonic generation (THG), and coherent anti-Stokes Raman scattering (CARS). On the right are additional contrasts accessible with pump-probe microscopy: stimulated Raman scattering (SRS), two-photon absorption (TPA, degenerate or non-degenerate), excited-state absorption (ESA), stimulated emission (SE), ground state depletion (GSD), and cross-phase modulation (XPM).