Fig. 2.
Validation of the third virtual wavelength. (a) Schematic of one-color two-photon absorption and two-color two-photon absorption. (b) SFG in BBO crystal after successful pulse synchronization. Three colors (from left to right): 415, 463.5, and 525 nm, corresponding to λ1/2, λ3/2, and λ2/2, respectively. (c) Normalized two-photon action cross section of SR101. (d) Fluorescence intensity at the virtual excitation Iλ1+λ2 versus the time delay between the two constituent laser pulses. (e) ex vivo imaging of a brain tissue slice (staining with SR101). Left, λ1 = 830 nm only; middle, λ2 = 1050 nm only; right, λ1 and λ2 simultaneously. (The time delay is optimized.)