  Magnetic 2D materials hold promise to change the miniaturization paradigm of
unidirectional photonic components. However, the integration of these materials
in devices hinges on the accurate determination of the optical properties down
to the monolayer limit, which is still missing. By using hyperspectral
wide-field imaging we reveal a non-monotonic thickness dependence of the
complex optical dielectric function in the archetypal magnetic 2D material CrI3
extending across different length scales: onsetting at the mesoscale, peaking
at the nanoscale and decreasing again down to the single layer. These results
portray a modification of the electronic properties of the material and align
with the layer-dependent magnetism in CrI3, shedding light into the
long-standing structural conundrum in this material. The unique modulation of
the complex dielectric function from the monolayer up to more than 100 layers
will be instrumental for understanding and manipulating the magneto-optical
effects of magnetic 2D materials.
