Second Harmonic Generation Control in 2D Layered Materials: Status and Outlook

Abstract

Second harmonic generation (SHG) as an essential nonlinear optical effect, has gradually shifted its research trend toward the integration and miniaturization of photonic and optoelectronic on‐chip devices in recent years. 2D layered materials (2DLMs) open up a new research paradigm of nonlinear optics due to their large second‐order susceptibility, atomically thin structure, and perfect phase‐matching. However, 2DLMs are facing a bottleneck of weak SHG conversion efficiency limit caused by short light–matter interaction lengths at a nanoscale. Moreover, advances in integrated on‐chip SHG devices based on 2DLMs rely on the continuing development of novel strategies with tunable and efficient SHG responses. Here, this review provides a comprehensive overview of recent progress in exploring highly efficient and tunable SHG responses in 2DLMs. Various modulation and enhancement strategies for the SHG response of 2DLMs are extensively studied and systematically discussed, which can be classified into two categories: symmetry breaking and light‐matter interaction enhancement. Moreover, remaining challenges and outlooks toward further extending and realizing the practical applications of 2DLMs in nonlinear on‐chip integrated devices with SHG modulation and enhancement characteristics are discussed.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7

“…Moreover, the magnitude of potential offset increases from monolayer to trilayer (ABA stacking), indicating that the OP spontaneous polarization is superimposable, despite the antiparallel arrangement (AB stacking) of each layer. Since the SHG response is proportional to spontaneous polarization, Nb 3 SeI 7 nanoflakes are expected to deliver second-order nonlinearity in both 3D bulk and 2D nanoflakes down to the monolayer limit.…”

Optical Second Harmonic Generation of Low-Dimensional Semiconductor Materials

“…Several reviews have been conducted on the nonlinear optical properties of LDMs [ 3 , 4 , 26 , 27 ], with a few recent papers offering in-depth discussions specifically on the SHG in 2D materials [ 28 , 29 , 30 ]. In this paper, we provide a thorough review of recent advancements in the investigation of highly efficient and adjustable SHG processes in LDMs.…”

Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide

“…The SHG intensities exhibit relatively weak increases with fluctuations as the excitation wavelengths increase, reaching a maximum under excitation at 950 nm. The relative variations of SHG intensities may be influenced by three factors: the resonance effect of SHG with the electronic structure of the material, the reabsorption effect of the material itself on SH photons, and the reflection of the SH signal by the surfaces of the substrate. − These effects explain that when the excitation frequency aligns with the lowest optical transition frequency of the material, there is an enhanced SHG intensity due to constructive interference between different electronic states. As the excitation wavelength moves away from this resonant condition toward longer wavelengths where there is less overlap with energy levels of interest in NbSe 2 flakes, we observe a decrease in SHG intensities.…”