Superior Nonlinear Optical Response in Non‐Centrosymmetric Stacking Edge‐Rich Spiral MoTe₂ Nanopyramids

Abstract

Transition metal dichalcogenides (TMDs) are of great promise for various nonlinear optical (NLO) applications due to their unique electronic and optoelectronic properties, such as tunable optical bandgap, strong spin-orbit coupling, and exciton effects. However, the desired NLO performances of regular 2H-TMDs are usually restricted by their limited absorption at atomic thickness. With this regard, a structurally novel spiral MoTe 2 (s-MoTe 2 ) nanopyramids is reported with unique and superior NLO response, enabled by their broken inversion symmetry, weak interlayer coupling, exciton resonance, and strong light-matter interaction from the edge-rich 3R-like quasi-multilayer structure. The excellent NLO response over a wide spectral range from the near-infrared to visible region is demonstrated, where second-and third-order NLO responses have been simultaneously observed. Moreover, the secondorder nonlinear susceptibility of s-MoTe 2 is estimated to be around 1-2 order(s) of magnitude larger than those of most reported TMDs. The demonstration of a superior NLO response in such s-MoTe 2 not only paves a new way for designing the best NLO TMD structures, but also greatly prompts their practical applications in micro-nano NLO devices on chips in future.

Emerging Nonplanar van der Waals Nanoarchitectures from 2D Allotropes for Optoelectronics

“…The results agree with the peak position of each exciton shown in reflectance contrast spectra (Figure 6d), where tiny interlayer twist angle leads to weaker interlayer coupling so that exciton-related peak positions hardly shift. [94,121] Besides, SHG and third-harmonic generation (THG) are observed simultaneously (Figure 6e,f), in which the SHG intensity shows a trend of nonlinear enhancement from the bottom to the top of the nanopyramid. [118][119][120][121]132] The enhanced SHG of nanopyramids correlates closely with completely broken inversion symmetry and other internal factors such as constructive superposition of nonlinear dipoles and energy band structure.…”

“…[94,121] Besides, SHG and third-harmonic generation (THG) are observed simultaneously (Figure 6e,f), in which the SHG intensity shows a trend of nonlinear enhancement from the bottom to the top of the nanopyramid. [118][119][120][121]132] The enhanced SHG of nanopyramids correlates closely with completely broken inversion symmetry and other internal factors such as constructive superposition of nonlinear dipoles and energy band structure. [132] More than that, larger lightmatter interaction length and rich edge states should be considered.…”

“…Bottom-up methods (mainly PVD and CVD) are the only way to prepare nanopyramids. [18,59,[110][111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126][127][128] It is impractical to produce type I nanopyramids by top-down methods even though hyperfine nanostructures are produced, for their adjacent vdWLs are covalently bonded with each other. For type II nanopyramids, although there is only vdW interaction between adjacent layers, the regular morphology and the lateral size decreasing monotonously with the increasing layer number would also be the limitations of top-down methods.…”

“…Moreover, as described in the previous section, the symmetry of nanopyramids may be lower than that of the widely reported thermodynamically stable phase due to the interlayer twist angle. [23,118] 3R-like stacking modes are found in a variety of TMD spiral nanopyramids, [59,[119][120][121]132] proving that broken inversion symmetry is prevalent among them and bringing new dawn for nonlinear optics (see Section 4.2.1).…”

“…Nanopyramids driven by a single screw dislocation tend to be asymmetric in most cases (Figure 5f). [59,114,121] Whether the inversion symmetry is recovered or not in the case of multiple screw dislocations is related to the orientation of Burgers vectors and the symmetry of crystal structure (Figure 5g). We are able to distinguish them according to their morphology directly.…”