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 $\left[{\chi }^{\left(2\right)}\right]$ at midinfrared to near-infrared frequencies owing to their broken centrosymmetry. ${\chi }^{\left(2\right)}$ is calculated within the density-matrix formalism that accounts for dissipation and screening. ${\chi }^{\left(2\right)}$ 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 ${\chi }^{\left(2\right)}$ as high as ${10}^{-6}\mathrm{m}/\mathrm{V}$. These findings underscore the potential of TMD nanotriangles for nonlinear nanophotonics, particularly second-harmonic generation.

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• Dissipative dynamics
• Electrical conductivity
• Electron relaxation
• Optical & microwave phenomena
• Optical conductivity
• Photonics
• Plasmonics
• Plasmons
• Quantum transport
• Transport phenomena
• 0-dimensional systems
• Functional materials
• Nanostructures
• Quantum master equation
• Tight-binding model