Extraordinary Two-Photon Photoluminescence from Quantum Dots in a Doubly Resonant Plasmonic Nanocavity

Bright two-photon photoluminescence (TPPL) is highly desirable for imaging and optoelectronic applications. The methods to effectively enhance the two-photon emission mainly take advantage of the high local field and large local density of optical states by utilizing excitation resonance and emission resonance in well-designed photonic structures. However, the efficient TPPL process in cavities still remains challenging, since the existing design does not fully take advantage of plasmonic resonances for either excitation or emission enhancement. In this work, we demonstrate a straightforward strategy to enhance the TPPL from CdSe/ZnS quantum dots (QDs) in a convenient double-resonance plasmonic cavity, consisting of a gold nanoparticle on a film with a monolayer QDs spacer. By simultaneously aligning excitation and emission with the plasmonic modes, TPPL enhancement of 7 orders of magnitude is achieved, including 48.0-fold emission enhancement. Moreover, simulations show that the local density of optical states at the emission resonance can be as high as 9 × 103. Our work showcases the potential of nanophotonic systems for nonlinear optical phenomena at the nanoscale.