Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a ...layered antiferromagnetic insulator in its few-layer form, opening up opportunities for various functionalities in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.
Abstract
Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ...ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe
2
, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.
Graphene with massless Dirac fermions can have exceptionally strong third-order optical nonlinearities. Yet reported values of nonlinear optical susceptibilities for third-harmonic generation (THG), ...four-wave mixing (FWM) and self-phase modulation vary over six orders of magnitude. Such variation likely arises from frequency-dependent resonance effects of different processes in graphene under different doping. Here, we report an experimental study of THG and FWM in graphene using gate tuning to adjust the doping level and vary the resonant condition. We find that THG and sum-frequency FWM are strongly enhanced in heavily doped graphene, while the difference-frequency FWM appears just the opposite. Difference-frequency FWM exhibited a novel divergence towards the degenerate case in undoped graphene, leading to a giant enhancement of the nonlinearity. The results are well supported by theory. Our full understanding of the diverse nonlinearity of graphene paves the way towards future design of graphene-based nonlinear optoelectronic devices.
Controllable synthesis of high‐quality hexagonal boron nitride (h‐BN) is desired toward the industrial application of 2D devices based on van der Waals heterostructures. Substantial efforts are ...devoted to synthesize h‐BN on copper through chemical vapor deposition, which has been successfully applied to grow graphene. However, the progress in synthesizing h‐BN has been significantly retarded, and it is still challenging to realize millimeter‐scale domains and control their morphologies reliably. Here, the nucleation density of h‐BN on Cu is successfully reduced by over two orders of magnitude by simply introducing a small amount of silicon, giving rise to large triangular domains with maximum 0.25 mm lateral size. Moreover, the domain morphologies can be modified from needles, tree patterns, and leaf darts to triangles through controlling the growth temperature. The presence of silicon alters the growth mechanism from attachment‐limited mode to diffusion‐limited mode, leading to dendrite domains that are rarely observed on pure Cu. A phase‐field model is utilized to reveal the growing dynamics regarding B‐N diffusion, desorption, flux, and reactivity variables, and explain the morphology evolution. The work sheds lights on the h‐BN growth toward large single crystals and morphology probabilities.
Large domains of hexagonal boron nitride (h‐BN) are deposited on silicon‐doped Cu substrate. A rich variety of h‐BN domain morphologies can be delicately controlled through the growth temperature, from fractal tree patterns to dendritic triangles.
In order to study the influence of profile modification on the internal flow field，temperature field and stress field of the gearbox system，the gear with or without modification are accurately ...modeled by Pro/E. The fluid field of gearbox system is calculated by overset grid method，and the steady calculation result is transmitted to the boundary of the gear to solve the temperature field and the stress field，i.e. thermal-fluid-solid coupling numerical simulation is used to study the gearbox oil bath lubrication. The lubrication volume fraction of the end part of gear pair，internal pressure variance of the gearbox，temperature and stress distribution of the gear solid body are compared and analyzed under different modification conditions. The results show that when b=1（i.e. straight-line modification），the temperature field and stress field of the gear are not improved much，and the modification effect is not evident. Then, when b=2, and when b=1.22，the modification show the best effect. No matter what kind of mod
The authors of this Article have become aware of the following recent paper that reports the electrically tunable degenerate four-wave mixing in graphene-covered SiN waveguides: “Alexander, K., ...Savostianova, N. A., Mikhailov, S. A., Kuyken, B. & Van Thourhout, D. Electrically tunable optical nonlinearities in graphene-covered SiN waveguides characterized by four-wave mixing. ACS Photon. 4, 3039–3044 (2017).” This reference has been added as ref. 33 in the new sentence “Very recently, degenerate four-wave mixing in graphene-covered SiN waveguides with ion-gel gating was experimentally studied33.” Accordingly, the previous sentence has been amended to read as “Few such experiments have been reported, although they have been suggested in theoretical work28–32” and all subsequent references have been renumbered. These changes have been made in the online versions.
Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI.sub.3) has been shown to be ...a layered antiferromagnetic insulator in its few-layer form.sup.1, opening up opportunities for various functionalities.sup.2-7 in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI.sub.3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG.sup.8-11 and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far.sup.12,13 (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI.sub.3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C.sub.2h crystallographic symmetry--and thus monoclinic stacking order--in bilayer CrI.sub.3, providing key structural information for the microscopic origin of layered antiferromagnetism.sup.14-18. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.
Topological properties in quantum materials are often governed by symmetry and tuned by crystal structure and external fields, and hence, symmetry-sensitive nonlinear optical measurements in a ...magnetic field are a valuable probe. Here, we report nonlinear magneto-optical second harmonic generation (SHG) studies of nonmagnetic topological materials including bilayer WTe2, monolayer WSe2, and bulk TaAs. The polarization-resolved patterns of optical SHG under a magnetic field show nonlinear Kerr rotation in these time-reversal symmetric materials. For materials with 3-fold rotational symmetric lattice structure, the SHG polarization pattern rotates just slightly in a magnetic field, whereas in those with mirror or 2-fold rotational symmetry, the SHG polarization pattern rotates greatly and distorts. These different magneto-SHG characters can be understood by considering the superposition of the magnetic field-induced time-noninvariant nonlinear optical tensor and the crystal-structure-based time-invariant counterpart. The situation is further clarified by scrutinizing the Faraday rotation, whose subtle interplay with crystal symmetry accounts for the diverse behavior of the extrinsic nonlinear Kerr rotation in different materials. Our work illustrates the application of magneto-SHG techniques to directly probe nontrivial topological properties, and underlines the importance of minimizing extrinsic nonlinear Kerr rotation in polarization-resolved magneto-optical studies.
Monolayers of Fe.sub.3GeTe.sub.2 exhibit itinerant ferromagnetism with an out-of-plane magnetocrystalline anisotropy; ionic gating raises the ferromagnetic transition temperature of few-layer ...Fe.sub.3GeTe.sub.2 to room temperature.
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