Multiphoton absorption enhancement by graphene-gold nanostructure.

We present a hybrid graphene-gold nanoantenna designed to enhance multiphoton absorption signals in molecules. The enhancement process involves two key steps: Firstly, the graphene component of the antenna supports molecular absorption in the mid-infrared and terahertz bands. By applying gate voltage, one can adjust the spectral positions of its resonances and select the desired absorption order, determining the number of photons absorbed in a single transition event. Secondly, gold nanorods with carefully tailored geometrical parameters enhance fluorescent single-photon emission. As a proof of concept, we adjust the geometry parameters of the hybrid antenna to the ATTO 700 dye molecule, taking into account its spectrally resolved emission characteristics. We predict a significant local enhancement of the fluorescence signal indicating the highly nonlinear process of N-photon absorption to exceed 5 orders of magnitude for N = 2 and 13 orders of magnitude for higher nonlinearity orders. Our proposed nanoantenna offers a promising platform for the tunable enhancement of highly nonlinear light-matter interactions.