Magnetic Fields Enable Tunable Nonlinear Optical Responses in Two-Dimensional Materials

By applying an external magnetic field to liquid dispersions of 2D flakes, the team achieved active control over the third-order nonlinear optical coefficient χ(3), allowing the optical response to be either enhanced or suppressed on demand.

Spatial self-phase modulation has been extensively investigated in 2D quantum materials to support various photonics devices, particularly all-optical switches with weak-light-control-strong-light functionality. However, all previously observed spatial self-phase modulation effects remained strictly intrinsic to their host materials. As Zhao noted, although his group discovered spatial self-phase modulation in 2D materials and set the record for the largest third-order nonlinear optical coefficient using graphene-based spatial self-phase modulation fifteen years ago, no viable approach had been reported to actively tune or enhance the effect until now.

During the experiments, applying an external magnetic field to dispersions of 2D flakes produced marked enhancements in third-order optical nonlinearity. For ferromagnetic VSe2, χ(3) increased by 58%, reaching 1.44×10-4 e.s.u. at 1000 mT. For diamagnetic graphene, χ(3) was enhanced by 32% to a record value of 2.0×10-7 e.s.u. at 600 mT, surpassing the fifteen-year-old world record.

The study also revealed that the applied magnetic field altered the symmetry of the optical response. Under the external field, χ(3) exhibited an explicit dependence on laser polarization, accompanied by field-induced optical birefringence. This magnetic-field-induced anisotropy allows researchers to use laser polarization as an additional, independent tuning knob for light-matter interactions.

The researchers determined that the tunable spatial self-phase modulation stems from collective field-induced orientational ordering of the suspended nanoflakes. When exposed to the magnetic field, ferromagnetic VSe2 nanoflakes orient their c-axes parallel to the applied field, whereas diamagnetic graphene nanoflakes align their basal planes along the field lines. This collective alignment modulates the orientation-averaged intrinsic anisotropic nonlinearity of the flakes, turning the magnetic field into a macroscopic control handle.

Leveraging this tunability, the research team constructed the first magnetic-field-tunable all-optical switch based on spatial self-phase modulation in VSe2. The signal-to-control intensity ratio, which measures the weak-light-control-strong-light functionality, increased by up to 340%. Even moderate magnetic fields below 0.3 T yielded a 100% enhancement, indicating that conventional permanent magnets can support practical device implementations.

According to first author Yixuan Huang, the investigation establishes a new paradigm for quantum material nonlinear photonics by shifting from passively probing intrinsic responses to actively modulating optical properties with external magnetic fields. The authors concluded that the magnetic-field-controlled switch and record-high optical nonlinearity offer a practical route toward developing high-performance photonic devices.

The experimental setup focused an incident laser beam directly into liquid dispersions of the two-dimensional magnetic flake materials placed within an applied external magnetic field. By systematically tuning both the intensity and polarization of the laser beam, the researchers probed the anisotropic third-order nonlinear optical coefficient across varying field environments.

Subject of Research: Magnetic-field control of third-order nonlinear optical responses in two-dimensional materials

Article Title: Magnetic field controls a third-order nonlinear optical property and laser-induced electron coherence in 2D materials

Article References: Magnetic field controls a third-order nonlinear optical property and laser-induced electron coherence in 2D materials. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: spatial self-phase modulation, nonlinear optics, 2D materials, graphene, VSe2, all-optical switch, quantum materials, birefringence, Magnetic, field, controls, third-order