Angular Dependence of the Second-Order Nonlinear Optical Response in Janus Transition Metal Dichalcogenide Monolayers

Abstract

With advances in the growth of Janus transition metal dichalcogenide (TMD) monolayers and potential applications for materials with a permanent dipole moment, we investigate the electronic, linear, and second-order nonlinear optical properties of Janus TMDs using first-principles calculations. We compare our results to available experimental measurements, finding relatively good agreement. We find the appearance of Rashba spin splitting and the mixing of the second-order nonlinear susceptibility components in Janus TMD monolayers contrary to their TMD monolayer counterparts. While the Rashba effect is small, it does modify the electronic band structure near Γ, making Janus TMDs potentially useful for spintronic applications. In analyzing the mixing of the nonlinear optical susceptibility components, which depend on the angular orientation of the experimental setup and the frequency of the measurement, we find enhancement of the effective nonlinear susceptibility. Our elucidation of the susceptibility components of Janus TMD monolayers as dependent on the angular orientation in an experimental setup provides an understanding and interpretation of the second-order nonlinear response.

Nonlinear Dispersion Relation and Out‐of‐Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers

“…where 𝜃 is the incidence angle of excitation, and 𝜙 is the angle between the crystal zigzag direction (x-axis) and the x ′ excitation and detection axis in the laser frame of reference [55]. In contrast, the SHG intensity components stemming from D 3h TMDs are [55,65,71] :…”

“…(2) xxz = 8.4 at ℏ𝜔 = 0.885 eV pumping (see Section S3, Supporting Information), which is comparable to theoretically calculated values. [55] Thus, with our simple approach, we are able to fully map the second-order susceptibility tensor in MoSSe Janus monolayer. MoSSe and WSSe Janus monolayers both belong to the C 3v point group.…”

“…In our experiments, the SH susceptibility of MoSSe and WSSe Janus monolayers at room temperature at exciton resonances is $$\chi ^{(2)}_{yyy}\approx 150 \ \text{pmV}^{-1}$$, and $$200 \ \text{pmV}^{-1}$$, respectively, which is among the largest reported values of second‐order susceptibilities for any TMD monolayer. [ 73 ] In addition, we draw a qualitative and quantitative comparison between the measured dispersion in the MoSSe Janus monolayer and the corresponding calculations available in the literature [ 53–55 ] (see Section , Supporting Information). Reference [55] shows nearly dispersionless results in the exciton‐dominated region, while references [53] and [54] display two distinct resonances that align closely with the measured A and B excitons, providing a good qualitative agreement to the experimental data.…”

“…[46][47][48][49] Second, their atomically thin nature combined with a strong nonlinear response [1,50] allows for efficient frequency conversion without phase matching constraints, [51,52] and thus, provides a viable route towards ultra-broadband classical and quantum sources of light. Janus TMD monolayers further expand such advantages by respectively adding out-of-plane components in the nonlinear optical response, [53][54][55] which introduce an additional functionality to be harnessed in vertical photonics structures, [56,57] and widening the spectral region of resonantly enhanced nonlinear optics to different areas of the visible and near-infrared light. [58] To capitalize on such advantages, understanding the wavelength-dependent second harmonic generation (SHG) efficiency (i.e., the nonlinear dispersion) and mapping the corresponding susceptibility tensor in Janus TMD monolayers is essential.…”

Hybrid Metasurfaces of Plasmonic Lattices and 2D Materials

“…[177] TMDs demon-strate a high nonlinear optical response caused by Pauli blocking and hot electron excitation, and large second and third-order optical nonlinearities that are dependent on the number of layers (Figure 6e). [178,179] Their excellent nonlinear optical absorption can be applied in photonic devices including mode-locking, alloptical switching, and among others. Interestingly, TMDs with noncentrosymmetric crystal structures also possess electron valley coherence and valley-selective circular dichroism, meaning TMDs are deemed to be the prospective materials for valleytronics (Figure 6f).…”