Schematics of electronic transitions in different nonlinear processes and mechanisms of enhancement by plasmonic materials.
(A) The first row: SHG/HRS, THG, 2PL/3PL, ESA the second row: SR (spontaneous Raman), CARS, SRS. (B1) Enhancement of nonlinear effects due to strong localized field of plasmonic structures, with a bowtie nanoantenna as an example. Reprinted with permission from ref. . At the gap of the antenna (position 1), field enhancement is largest, so the nonlinear interaction, as well as the effective nonlinear coefficients (β′, γ′), is also largest. At the antenna tips (position 2), modest field enhancement is present, leading to modest nonlinear signal generation. At position 3, where there is least field enhancement, the nonlinear interaction is weakest (β, γ). (B2) RI variation (n→n′) of a metal nanoparticle due to plasmon field enhancement when illuminated with a focused laser beam. no is the linear RI of the particle, and n2 is the nonlinear RI.