Giant two-photon upconversion from 2D exciton in doubly-resonant plasmonic nanocavity

Photon upconversion through high harmonic generation, multiphoton absorption, Auger recombination and phonon scattering performs a vital role in energy conversion and renormalization. Considering the reduced dielectric screening and enhanced Coulomb interactions, semiconductor monolayers provide a promising platform to explore photon upconversion at room temperature. Additionally, two-photon upconversion was recently demonstrated as an emerging technique to probe the excitonic dark states due to the extraordinary selection rule compared with conventional excitation. However, highly efficient two-photon upconversion still remains challenging due to the limited multiphoton absorption efficiency and long radiative lifetimes. Here, a 2440-fold enhancement of two-photon luminescence (TPL) is achieved in doubly resonant plasmonic nanocavities due to the amplified light collection, enhanced excitation rate, and increased quantum efficiency. To gain more insight into the attractive doubly resonant enhancement in such a plasmon-exciton coupling system, the intriguing thermally tuned excitonic upconversion and optimized amplification factor >3000 are realized at 350 K. Meanwhile, the single resonance enhanced photoluminescence (PL) (~890-fold) and second-harmonic generation (SHG) (~134-fold) are elaborately demonstrated. These results establish a foundation for developing cost-effective, high-performance nonlinear photonic devices and probing fine excitonic states via configuring plasmonic nanocavities.