二维晶体悬浮液的巨磁双折射效应与可调谐着色
Giant magneto-birefringence effect and tuneable colouration of 2D crystals' suspensions
摘要：
通过光-物质相互作用操纵光的长期追求的目标之一是实现磁场可调谐着色，即所谓的磁致变色效应，由于其非接触和非侵入性，其在光学，生物化学和医学应用方面具有很大的前景。这一目标可以通过磁场控制的双折射来实现，其中颜色是由透射偏振光的相位延迟分量之间的干涉产生的。到目前为止，已经使用电场、材料手性和机械应变证明了双折射可调着色，但是由于透明介质的弱磁光响应或磁响应介质（例如铁磁流体）对可见光的低透射率，磁场控制仍然是难以捉摸的。在这里，我们展示了磁性可调着色的水悬浮液的二维钴掺杂的氧化钛表现出极大的磁双折射效应。悬浮液的颜色可以通过低于0.8 T的中等磁场在可见光范围内在两个以上波长周期内调谐。我们发现，这种巨大的磁色响应是由于偏振光的相位延迟特别大（>3 π），而这又是大的Cotton-Mouton系数（比已知的液晶大三个数量级）、相对高的饱和双折射率（Δ n = 2 x 10^-4）和我们的悬浮液对可见光的高透明度的综合结果。这项工作开辟了一条新的途径，通过工程磁双折射实现可调着色，并可以很容易地扩展到其他磁性2D纳米晶体。所展示的效应可用于各种磁光应用，包括磁场传感器、波长可调谐光学滤波器和透视印刷。
英文摘要：
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.
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