Tunable plasmon-enhanced second-order optical nonlinearity in transition metal dichalcogenide nanotriangles

The development of nanomaterials with a large nonlinear susceptibility is essential for nonlinear nanophotonics. We show that transition metal dichalcogenide (TMD) nanotriangles have a large effective second-order susceptibility $[{\ensuremath{\chi}}⁽²⁾]$ at midinfrared to near-infrared frequencies owing to their broken centrosymmetry. ${\ensuremath{\chi}}⁽²⁾$ is calculated within the density-matrix formalism that accounts for dissipation and screening. ${\ensuremath{\chi}}⁽²⁾$ peaks in the vicinity of both two-photon resonances (specified by the geometry) and plasmon resonances (tunable via the carrier density). Aligning the resonances yields the values of ${\ensuremath{\chi}}⁽²⁾$ as high as ${10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{V}$. These findings underscore the potential of TMD nanotriangles for nonlinear nanophotonics, particularly second-harmonic generation.