Giant magneto-birefringence effect and tuneable colouration of 2D crystals' suspensions

TL;DR

This paper demonstrates giant, magnetically tunable colouration in suspensions of 2D cobalt-doped titanium oxide, enabling controllable optical effects through large magneto-birefringence for potential applications in sensors and filters.

Contribution

It introduces a new magneto-chromatic effect in 2D nanocrystal suspensions with unprecedented large magneto-birefringence, surpassing previous materials in tunability and transparency.

Findings

Colour can be tuned over two wavelength cycles with magnetic fields below 0.8 T.

The giant effect results from large phase retardation due to high Cotton-Mouton coefficient.

Suspensions exhibit high transparency and saturation birefringence, enabling visible light manipulation.

Abstract

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.