Two-dimensional (2D) isotropic transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2), have exhibited numerous fascinating nonlinear optical behaviors which make TMDCs promising candidates for emerging optoelectronic device applications. However, direct utilization of 2D TMDCs for nonlinear optics is still a great challenge due to its limited light-matter interaction length at the atomic layer thickness scale and the poor field confinement with low nonlinear conversion efficiency. Although some strategies have been reported to enhance the optical second harmonic generation (SHG) process in 2D TMDCs, the complicated fabrication processes are not suitable for manufacturing cost-effective nonlinear optical nano-devices.
In this work, the joint international research team including Wuhan National Lab for Optoelectronics and University of Nebraksa-Lincoln recently report for the first time a strong anisotropic enhancement of SHG in monolayer MoS2 by integrating with one-dimensional (1D) titanium dioxide nanowires (NWs). The SHG emission from the hybrid nano-structure exhibits more than two orders of magnitude enhancement compared with that of the bare MoS2 layer on SiO2/Si substrates. The SHG enhancement was attributed to the localized electric field enhancement in the hybrid structure and the absence of substrate-induced doping and dielectric screening effects of the suspended MoS2 layer, resulting in a strong light-matter coupling efficiency. In addition, the SHG signal can be effectively modulated by varying the polarization of the incident laser beam, leading to a strong anisotropic SHG enhancement. Research team found that one-dimensional NW-induced crystal lattice deformation is a key factor which leads to strong anisotropic SHG enhancement in atomic layer of MoS2. This method can be also exploited to manipulate other nonlinear optical processes such as third-harmonic generation and high-order harmonic generation, in 2D isotropic materials. This work are expected to achieve precise and selective control of the optical properties of low-dimensional materials, making them widely used in the next generation of low-cost and high-efficiency polarization-dependent nanoscale optical devices such as nano-stress sensors and tunable photonic devices. More information could be found in the recently published paper in Nano Letters (“Anisotropic Enhancement of Second Harmonic Generation in Monolayer and Bilayer MoS2 by Integrating with TiO2 Nanowires” 10.1021/acs.nanolett.9b01933).
https://pubs.acs.org/doi/10.1021/acs.nanolett.9b01933
