Magnetic field raises χ(3) by 58% in VSe2 and 32% in graphene dispersions, enabling a lab optical switch

A group led by Jimin Zhao at the Institute of Physics, Chinese Academy of Sciences, has used an external magnetic field to control spatial self-phase modulation (SSPM), a third-order nonlinear optical response, in dispersions of 2D flakes. The results were published in National Science Review doi.orgNational Science Reviewdoi.orgOpen source ↗. In ferromagnetic VSe2 the third-order nonlinear optical coefficient χ(3) increased by 58%, and in diamagnetic graphene by 32% doi.orgNational Science Reviewdoi.orgOpen source ↗. The work is an experimental laboratory study on flake dispersions.

Magnetic tuning of χ(3)

According to the authors, all SSPM effects observed so far were intrinsic to the host materials, and no approach to tune or enhance them had been reported. In this work the magnetic field made χ(3) controllable: it could be enhanced or suppressed on demand doi.orgNational Science Reviewdoi.orgOpen source ↗.

For VSe2, χ(3) reached 1.44×10-4 e.s.u. at 1000 mT. For graphene, it reached 2.0×10-7 e.s.u. at 600 mT, which the authors describe as exceeding the record set by Zhao's group with graphene-based SSPM fifteen years earlier doi.orgNational Science Reviewdoi.orgOpen source ↗.

Under the field the optical response became anisotropic: χ(3) depended on the laser polarization, and the field also induced optical birefringence. The authors note that this makes polarization an additional, independent tuning parameter doi.orgNational Science Reviewdoi.orgOpen source ↗.

The authors attribute the effect to collective orientational ordering of the suspended nanoflakes. In the field, VSe2 flakes orient their c-axes parallel to it, while graphene flakes align their basal planes along it. This ordering changes the orientation-averaged anisotropic nonlinearity of the flakes doi.orgNational Science Reviewdoi.orgOpen source ↗.

All-optical switch based on SSPM

The team also demonstrated a magnetic-field-tunable all-optical switch based on SSPM in VSe2. The signal-to-control intensity ratio, which characterizes the "weak light controls strong light" function, was enhanced by up to 340% doi.orgNational Science Reviewdoi.orgOpen source ↗.

Fields below 0.3 T already gave a 100% enhancement. The authors conclude from this that conventional permanent magnets would be sufficient for a device implementation doi.orgNational Science Reviewdoi.orgOpen source ↗.

First author Yixuan Huang said that, instead of only probing the intrinsic nonlinear response of quantum materials, the team achieved its active modulation by an external magnetic field. The authors describe the result as a way to control, rather than merely monitor, laser-induced electron coherence in SSPM doi.orgNational Science Reviewdoi.orgOpen source ↗.