High-Verdet Constant and Low-Optical Loss Tb³⁺ Doped Magnetite Nanoparticles

Magneto-optical (MO) polymer nanocomposites have emerged as alternatives to conventional MO crystals, particularly in nanophotonics applications, thanks to their better processing flexibility and superior Verdet constants. However, a higher Verdet constant commonly comes with excessive optical loss due to increased absorption and scattering, resulting in a constant or reduced figure-of-merit (FOM) defined as the Verdet constant over optical loss. By doping magnetite (Fe₃O₄) nanoparticles with Tb³⁺ ions, we report a new strategy to enhance the Verdet constant without increasing the optical loss. The Fe₃O₄:Tb³⁺ nanocomposite is one of a kind that simultaneously achieves a state-of-the-art Verdet constant of 5.6 × 10⁵ °/T·m and a state-of-the-art FOM of 31°/T in the near-infrared region.