Selectively Plasmon-Enhanced Second-Harmonic Generation from Monolayer Tungsten Diselenide on Flexible Substrates

TLDR

Monolayer 2D transition‑metal dichalcogenides offer large second‑order nonlinear susceptibility and inversion asymmetry, yet their atomically thin nature limits light interaction length and makes second‑harmonic generation inefficient. The authors fabricated a 150 nm planar surface comprising monolayer WSe₂ and sub‑20 nm gold trenches on flexible substrates, achieving a ~7000‑fold SHG enhancement without peak broadening or background relative to WSe₂ on sapphire. Effective second‑order nonlinear susceptibility of 2.1 × 10⁴ pm V⁻¹ was measured, with a three‑order‑of‑magnitude SHG enhancement preserved over 800–900 nm, tunable by laser polarization, and a flat, ultrathin design that supports scalable, flexible nonlinear converters and integration with other optical components.

Abstract

Monolayer two-dimensional transition-metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this paper, we fabricate a judiciously structured 150 nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20 nm-wide gold trenches on flexible substrates, reporting ∼7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe2 on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 × 104 pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled via selective excitation of the lateral gap plasmon by rotating the laser polarization. Such a fully open, flat, and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form.