Optimizing Nonlinear Optical Visibility of Two-Dimensional Materials

Abstract

Two-dimensional (2D) materials have attracted broad research interests across various nonlinear optical (NLO) studies, including nonlinear photoluminescence (NPL), second harmonic generation (SHG), transient absorption (TA), and so forth. These studies have unveiled important features and information of 2D materials, such as in grain boundaries, defects, and crystal orientations. However, as most research studies focused on the intrinsic NLO processes, little attention has been paid to the substrates underneath. Here, we discovered that the NLO signal depends significantly on the thickness of SiO in SiO/Si substrates. A 40-fold enhancement of the NPL signal of graphene was observed when the SiO thickness was varied from 270 to 125 nm under 800 nm excitation. We systematically studied the NPL intensity of graphene on three different SiO thicknesses within a pump wavelength range of 800-1100 nm. The results agreed with a numerical model based on back reflection and interference. Furthermore, we have extended our measurements to include TA and SHG of graphene and MoS, confirming that SiO thickness has similar effects on all of the three major types of NLO signals. Our results will serve as an important guidance for choosing the optimum substrates to conduct NLO research studies on 2D materials.

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…The third factor is the substrate effect. Xianchong et al reported that SHG intensity can be increased about 100‐fold by varying the SiO 2 thickness of the SiO 2 /Si substrate, which was attributed to the constructive interference effect . Another important reason is the excitonic effects.…”

“…Xianchong et al reported that SHG intensity can be increased about 100-fold by varying the SiO 2 thickness of the SiO 2 /Si substrate, which was attributed to the constructive interference effect. 72 Another important reason is the excitonic effects. There were numerous papers reported that SHG could be enhanced dramatically when the excitation laser was in resonance with the excitonic peak.…”

An Insightful Picture of Nonlinear Photonics in 2D Materials and their Applications: Recent Advances and Future Prospects

“…[ 39,40 ] The higher conversion efficiency in NLO material can be enhanced by the use of the MoS 2 3R phase. To increase the 2DLMs SHG interaction power by optical techniques like interference, [ 41,42 ] wave‐guides, [ 43 ] and cavities, [ 44,45 ] can be adopted. The 2D based TMDCs non‐centrosymmetric materials make them auspicious candidates for ultra‐compact light highly efficient conversion photonic based nanodevices.…”

Dispersion of the nonlinear susceptibility of MoS2 and WS2 from second-harmonic scattering spectroscopy

“…On the other hand, a direct calibration using an estimated incident intensity at the beam waist was also proposed to derive the TMD susceptibility [10][11][12], but this approach appears to be delicate due to very precise identification of all beam parameters that must be integrated. The effect of the substrate can also strongly modify the emitted SH signal due to additional interference contributions [12,13], and strain artifacts are known to result in SHG variations [14]. In terms of TMD materials, a few studies have highlighted the effect of crystalline quality on discrepancies between the published χ (2) values between mechanically exfoliated samples and those obtained from chemical vapor deposition (CVD) [10,15].…”