  One of the long sought-after goals in manipulation of light through
light-matter interactions is the realization of magnetic-field-tuneable
colouration, so-called magneto-chromatic effect, which holds great promise for
optical, biochemical and medical applications due to its contactless and
non-invasive nature. This goal can be achieved by magnetic-field controlled
birefringence, where colours are produced by the interference between
phase-retarded components of transmitted polarised light. Thus far
birefringence-tuneable coloration has been demonstrated using electric field,
material chirality and mechanical strain but magnetic field control remained
elusive due to either weak magneto-optical response of transparent media or low
transmittance to visible light of magnetically responsive media, such as
ferrofluids. Here we demonstrate magnetically tuneable colouration of aqueous
suspensions of two-dimensional cobalt-doped titanium oxide which exhibit an
anomalously large magneto-birefringence effect. The colour of the suspensions
can be tuned over more than two wavelength cycles in the visible range by
moderate magnetic fields below 0.8 T. We show that such giant magneto-chromatic
response is due to particularly large phase retardation (>3 pi) of the
polarised light, which in its turn is a combined result of a large
Cotton-Mouton coefficient (three orders of magnitude larger than for known
liquid crystals), relatively high saturation birefringence (delta n = 2 x
10^-4) and high transparency of our suspensions to visible light. The work
opens a new avenue to achieve tuneable colouration through engineered magnetic
birefringence and can readily be extended to other magnetic 2D nanocrystals.
The demonstrated effect can be used in a variety of magneto-optical
applications, including magnetic field sensors, wavelength-tuneable optical
filters and see-through printing.
