Three-photon luminescence assisted by excitonic energy transfer in gold nanoparticle-WS₂ monolayers

2D transition metal dichalcogenides coupled to semiconductor quantum dots or metallic nanoparticles give rise to novel energy transfer pathways and intricate nonlinear optical properties. In this article, by using multiphoton microscopy, we observe photoluminescence signals in the green or blue light spectrum (higher than 2 eV) emitting from the gold nanoparticles that are coupled to tungsten disulfide (WS₂) monolayers, even when the excitation energy is as low as 1 eV. The pump power dependence indicates that these photoluminescence signals involve the absorption of three photons not by the gold nanoparticles or WS₂ alone but through a one-photon intraband excitation in the gold nanoparticle and a two-photon excitation in WS₂ that, via the excitonic resonance energy transfer, delivers the energy to the gold nanoparticle by inducing an interband excitation. We suggest ways to improve the light emission efficiency of this hybrid structure for a potential room temperature upconversion emitter.