LAUSR

We propose the dual resonant optical sum frequency generation (SFG), where the two most singular resonances could be selected, and report from the monolayer (1L-) WSe2 when one (ω_1) of two excitation pulses is resonant to A exciton and their sum frequency (ω_1+ω_2) to D exciton, respectively. The dual-resonant SFG confirms that, under the irradiation of the ω_1- and ω_2-pulses with the same fluence of ~ 1.4×10^10 W/m2, its signal intensity could be enhanced about twenty times higher than the resonant SHG (i.e., 2ω_1 to the D excitonic absorption). Further, the dual resonant SFG intensity of 1L-WSe2 is found to be one order of magnitude higher than the single-resonant SFG intensity of 1L-WS2 under the same condition of two-pulse irradiation. Finally, observations of the dual resonant SFG are thoroughly examined using the real-time time-dependent density functional theory (rt-TDDFT) and the relevant nonlinear optical characteristics are scrutinized using the Greenwood-Kubo formalism.