  The optical properties of some nanomaterials can be controlled by an external
magnetic field, providing active functionalities for a wide range of
applications, from single-molecule sensing to nanoscale nonreciprocal optical
isolation. Materials with broadband tunable magneto-optical response are
therefore highly desired for various components in next-generation integrated
photonic nanodevices. Concurrently, hyperbolic metamaterials received a lot of
attention in the past decade since they exhibit unusual properties that are
rarely observed in nature and provide an ideal platform to control the optical
response at the nanoscale via careful design of the effective permittivity
tensor, surpassing the possibilities of conventional systems. Here, we
experimentally study magnetic circular dichroism in a metasurface made of
type-II hyperbolic nanoparticles on a transparent substrate. Numerical
simulations confirm the experimental findings, and an analytical model is
established to explain the physical origin of the observed magneto-optical
effects, which can be described in terms of the coupling of fundamental
electric and magnetic dipole modes with an external magnetic field. Our system
paves the way for the development of nanophotonic active devices combining the
benefits of sub-wavelength light manipulation in hyperbolic metamaterials
supporting a large density of optical states with the ability to freely tune
the magneto-optical response via control over the anisotropic permittivity of
the system.
