We investigate Second Harmonic Generation (SHG) in monolayer WS2 both deposited on a SiO2/Si substrate or suspended using transmission electron microscopy grids. We find unusually large second order nonlinear susceptibility, with an estimated value of deff ~ 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 WS2, 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 deff = 0.77 nm/V for monolayer WS2, which shows good order-of-magnitude agreement with the experimental result.

Two-dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra-thin electronic and optoelectronic materials for novel energy-efficient devices. However, in most thin-film optical applications, there is a long-existing trade-off between the effectiveness of light-matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS 2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS 2 and an aluminum reflector, and can strongly enhance the light-matter interaction within the MoS 2 , increasing the exclusive absorption of monolayer MoS 2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.

Second harmonic generation of single-and few-layer mechanically exfoliated tungsten diselenide (WSe 2 ) samples are studied. The value of the effective second-order nonlinear susceptibility for monolayer WSe 2 is obtained, being three orders of magnitude larger than the values usually reported for other nonlinear bulk crystals. The presence of a monolayer is certified by symmetry analysis of the Raman modes and the occurrence of a direct band gap. Our results on WSe 2 solidify the family of transition metal dichalcogenides as two-dimensional systems with ultra high second-order nonlinear susceptibility.

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We demonstrate the application of two-dimensional materials for ultrashort optical pulse characterization. Monolayer transition metal dichalcogenides, such as tungsten disulfide (WS₂), possess extraordinarily large second-order nonlinear susceptibility, and due to their atomic thickness, have relaxed phase-matching requirements and, hence, an inherently wide bandwidth. Synthesized monolayer WS₂ triangular islands were used to characterize ultrashort optical pulses at the focal point of an objective lens through second-harmonic generation collinear frequency-resolved optical gating.