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

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 WSe on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 × 10 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.

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…Regarding the plasmon resonance, the electric field enhancement factor could be further enhanced by patterning the metal film to form a nanostructure. Wang et al reported a 7000‐fold SHG enhancement of monolayer WSe 2 on an Au film with sub‐20 nm trench (Figure E) . The presence of trenches could provide much stronger near field enhancement compared with planar Au film.…”

“…Copyright 2015, The Royal Society of Chemistry. E, Au trench‐enhanced SHG of monolayer WSe 2 . Copyright 2018, American Chemical Society.…”

“…Surface plasmon is an alternative method to enhance light‐mater interaction thanks to the strong localized field near the metal surface. SHG intensity is proportional to the fourth power of the local field enhancement . Zeng et al demonstrated that SHG of few layers MoS 2 can be enhanced by simply placing MoS 2 on a gold film .…”

“…168 The ability of enhancing light-mater interaction motivates researchers to incorporate those optical cavies to layered TMDs. Recently, TMDs were integrated to various cavities such as the waveguides, 67 photonic crystals, [169][170][171] and plasmonic resonators [172][173][174][175] to enhance their nonlinear performance.…”

“…SHG intensity is proportional to the fourth power of the local field enhancement. 173 Zeng et al demonstrated that SHG of few layers MoS 2 can be enhanced by simply placing MoS 2 on a gold film. 172 They found that F I G U R E 6 A, SHG mapping of monolayer MoS 2 .…”