Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers

• Published in: Scientific reports Vol. 4; no. 1; p. 5530
• Main Authors: Janisch, Corey, Wang, Yuanxi, Ma, Ding, Mehta, Nikhil, Elías, Ana Laura, Perea-López, Néstor, Terrones, Mauricio, Crespi, Vincent, Liu, Zhiwen
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 02-07-2014; Nature Publishing Group
• Subjects: 639/301/119/544; 639/624/400/385; 639/925/357/1018; Aperture; Crystals; Electron microscopy; Humanities and Social Sciences; multidisciplinary; Science; Symmetry; Transmission electron microscopy; Tungsten
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep05530

We investigate Second Harmonic Generation (SHG) in monolayer WS
2
both deposited on a SiO
2
/Si substrate or suspended using transmission electron microscopy grids. We find unusually large second order nonlinear susceptibility, with an estimated value of d
eff
~ 4.5 nm/V nearly three orders of magnitude larger than other common nonlinear crystals. In order to quantitatively characterize the nonlinear susceptibility of two-dimensional (2D) materials, we have developed a formalism to model SHG based on the Green's function with a 2D nonlinear sheet source. In addition, polarized SHG is demonstrated as a useful method to probe the structural symmetry and crystal orientation of 2D materials. To understand the large second order nonlinear susceptibility of monolayer WS
2
, density functional theory based calculation is performed. Our analysis suggests the origin of the large nonlinear susceptibility in resonance enhancement and a large joint density of states and yields an estimate of the nonlinear susceptibility value d
eff
= 0.77 nm/V for monolayer WS
2
, which shows good order-of-magnitude agreement with the experimental result.