Symmetry breaking in 2D materials for optimizing second-harmonic generation

Second-harmonic generation (SHG) is the generation of 2 ω (or half wavelength) light from incident light with frequency ω as a nonlinear optical response of the material. Three-dimensional (3D) SHG materials are widely investigated for developing laser technology to obtain shorter wavelengths in photolithography fabrication of semiconductor devices and the medical sciences, such as for imaging techniques that do not use fluorescent materials. However, to obtain the optimized SHG intensity, the 3D material is required to have no spatial-inversion symmetry (or non-centrosymmetry) and special crystal structure (or so-called phase-matched condition). Recently, engineering symmetry breaking of thin two-dimensional (2D) materials whose 3D structure has the inversion symmetry can offer a breakthrough to enhance the SHG intensity without requiring the phase-matched condition. Over the past decade, many 2D SHG materials have been synthesized to have broken inversion symmetry by stacking heterostructures, twisted moiré structures, dislocated nanoplates, spiral nanosheets, antiferromagnetic order, and strain. In this review, we focus on the recent progress in breaking inversion and rotational symmetries in out-of-plane and/or in-plane directions. The theoretical calculations and experimental setup are briefly introduced for the non-linear optical response of the 2D materials. We also present our perspectives on how these can optimize the SHG of the 2D materials.