Giant and nonreciprocal second harmonic generation from layered antiferromagnetism in bilayer CrI3 (1904.03577v1)

Abstract: Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet, chromium triiodide (CrI3), emerged as the first layered antiferromagnetic insulator in its few-layer form, opening up ample opportunities for novel device functionalities. Here, we discovered an emergent nonreciprocal second order nonlinear optical effect in bilayer CrI3. The observed second harmonic generation (SHG) is giant: several orders of magnitude larger than known magnetization induced SHG and comparable to SHG in the best 2D nonlinear optical materials studied so far (e.g. MoS2). We showed that while the parent lattice of bilayer CrI3 is centrosymmetric and thus does not contribute to the SHG signal, the observed nonreciprocal SHG originates purely from the layered antiferromagnetic order, which breaks both spatial inversion and time reversal symmetries. Furthermore, polarization-resolved measurements revealed the underlying C2h symmetry, and thus monoclinic stacking order in CrI3 bilayers, providing crucial structural information for the microscopic origin of layered antiferromagnetism. Our results highlight SHG as a highly sensitive probe that can reveal subtle magnetic order and open novel nonlinear and nonreciprocal optical device possibilities based on 2D magnets.

• The paper reveals that bilayer CrI₃ exhibits a giant SHG signal driven by its unique layered antiferromagnetic order.
• It shows that the SHG effect is highly sensitive to temperature and magnetic fields, vanishing above the Néel temperature and under ferromagnetic alignment.
• Polarization-resolved measurements indicate a distinctive C₂h symmetry, enabling optical probing of magnetic domain dynamics.

Insights into Nonreciprocal Second Harmonic Generation in Bilayer CrI₃

The paper investigates the nonlinear optical properties of bilayer chromium triiodide (CrI₃), a van der Waals magnet. The paper focuses on the nonreciprocal second harmonic generation (SHG), which is a nonlinear optical process sensitive to symmetries within a material. The bilayer CrI₃ is notable for its layered antiferromagnetic structure, breaking both spatial inversion and time reversal symmetries, and giving rise to a significant nonreciprocal SHG effect.

Key Findings

1. Giant SHG Signal: The SHG observed in bilayer CrI₃ is markedly large, several magnitudes higher than magnetization-induced SHG in bulk crystals and comparable to nonlinear optical materials such as MoS₂. This was attributed to the layered antiferromagnetic order within the CrI₃ bilayers.
2. Temperature and Magnetic Field Dependency: The SHG signal is shown to depend on temperature, vanishing above the Néel temperature (~40 K), confirming the essential role of antiferromagnetic order. Similarly, the signal is suppressed when a magnetic field forces a transition to a ferromagnetically aligned state.
3. Polarization-Resolved SHG: Through polarization-resolved measurements, the paper reveals that the bilayer CrI₃ exhibits C₂h symmetry indicative of a monoclinic stacking order, diverging from the rhombohedral structure of bulk CrI₃ at low temperatures.
4. Magnetic Domain Studies: SHG was employed to visualize magnetic domain dynamics near the metamagnetic transition. Distinct differences were observed in SHG response depending on the direction of magnetic field sweep, supporting the existence of multiple antiferromagnetic ground states.

Implications and Future Directions

The findings present significant implications for the use of 2D magnets in optical and photonic applications, particularly in the field of nonreciprocal and nonlinear optics. The strong c-type SHG effect emphasizes the potential to use such materials for sensitive optical probing of magnetic ordering, especially in systems where traditional methods like neutron diffraction are inadequate. This paper advances the understanding of the interplay between crystal structure and magnetic order, specifically in how stacking influences magnetic interactions.

Given the robustness of 2D materials, future developments may explore integration into optoelectronic devices where control over light-matter interaction at the nanoscale is desirable. Additionally, advancements in this domain could foster new approaches to studying antiferromagnetic materials, possibly leading to novel devices exploiting spintronic and magneto-optical phenomena.

Overall, this research provides a basis for deeper exploration into layered magnetic systems and their emergent optical properties, with a significant prospect for impacting both theoretical models and practical applications in advanced material science and optical engineering.