ABSTRACT Enhancing inherently‐weak light‐matter interactions in two‐dimensional (2D) photoactive materials is critical for fundamental research and developing next‐generation nanophotonic and quantum optical devices. Conventional approaches utilizing plasmonic nanostructures of at least tens of nanometers in thickness, however, suffer from significant mode and physical size mismatch with the atomically thin 2D materials, limiting both the enhancement factor and device miniaturization. Here, we overcome these constraints by downscaling the thickness of plasmonic nanostructures to the nanometer limit. Using sub‐5‐nm‐thick single‐crystal gold nanoribbon arrays (GNRAs), we achieve extreme optical confinement and enhancement that can strongly boost atomic‐scale light‐matter interactions, as evidenced by a ≈ 860‐fold enhancement of Raman scattering in a monolayer WS 2 . The ultrathin GNRAs open opportunities for creating high‐performance and transparent 2D‐material‐based photonic devices without compromising the overall thickness. Specifically, we realize an ultrathin optical frequency converter (3.5‐nm‐thick GNRA/monolayer WS 2 ) exhibiting a 143‐fold enhancement in second‐harmonic generation, and an ultrathin photodetector (3.6‐nm‐thick GNRA/four‐layer MoTe 2 ) with a high responsivity of ≈ 0.69 A/W and a detection limit down to ≈ 580 pW. The nanometer‐thick gold with ultra‐tight optical confinement opens up a new chapter for extreme light‐matter interactions and developing ultrathin photonic and optoelectronic devices.
Zhu et al. (Thu,) studied this question.