Magneto‐optic effect is attracting wide interest as it renders a contactless, potentially power‐free, highly sensitive and spatio‐temporal resolved way in both magnetic material characterization and light manipulation. Intensive explorations exhibit its potential in diverse applications such as optical communication, data storage, phase modulator, optical isolator, and magnetic‐field sensor. Noteworthy, atomically thin two‐dimensional (2D) materials represented by graphene demonstrate the interplay of large shape anisotropy as well as anisotropy in optical and magnetic properties, providing unlimited possibilities for the development of magneto‐optic effect and related applications. Here, we initiate the review with brief summary of the development history of magneto‐optic effect in parallel with the introduction of several typical 2D materials with unique magnetic properties. Thereafter, four important magneto‐optic effects of 2D materials are discussed, including Faraday effect, magneto‐optic Kerr effect, Zeeman effect and Cotton‐Mouton effect. Finally, we refine major challenges in further development of magneto‐optic effects and put forward prospects for several promising candidates of 2D magneto‐optic materials and related applications.

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Transparent hydrogels are key materials for many applications, such as contact lens, imperceptible soft robotics and invisible wearable devices. Introducing large and engineerable optical anisotropy offers great prospect for endowing them with extra birefringence-based functions and exploiting their applications in see-through flexible polarization optics. However, existing transparent hydrogels suffer from limitation of low and/or non-fine engineerable birefringence. Here, we invent a transparent magneto-birefringence hydrogel with large and finely engineerable optical anisotropy. The large optical anisotropy factor of the embedded magnetic two-dimensional material gives rise to the large magneto-birefringence of the hydrogel in the transparent condition of ultra-low concentration, which is several orders of magnitude larger than usual transparent magnetic hydrogels. High transparency, large and tunable optical anisotropy cooperatively permit the magnetic patterning of interference colours in the hydrogel. The hydrogel also shows mechanochromic and thermochromic property. Our finding provides an entry point for applying hydrogel in optical anisotropy and colour centred fields, with several proof-of-concept applications been demonstrated.

chemical properties, 2D materials have long been the forefront of fundamental research and applications in optics and many other fields. [1][2][3][4] For instance, many efforts have been devoted to explore physical paradigms under 2D limit and enable novel optical devices, such as exciton-resonance-enabled lens, [5,6] wideband tunable mode lockers, [7,8] as well as photodetectors and harmonic generators in terahertz regime. [2,9] Noteworthy, together with recently reported 2D ferromagnets, [10][11][12] 2D materials based magneto-optics have attracted great attentions nowadays. Categorized by the interaction between material and incident light under magnetic field, four main magnetooptical effects are investigated commonly, namely the Zeeman effect, Faraday effect, magneto-optical Kerr effect, and Cotton-Mouton effect, in which 2D materials are recognized as promising matters. [13] Among these four magneto-optical effects, the Cotton-Mouton effect is of great benefits for transmitted light modulation in a see-through manner, with the potential applications ranging from sub-micron-scale dynamic light modulators to meter-scale displayable windows in future society. [14,15] The current electro-optical technologies based on liquid crystals face Liquid crystal devices using organic molecules are nowadays widely used to modulate transmitted light, but this technology still suffers from relatively weak response, high cost, toxicity and environmental concerns, and cannot fully meet the demand of future sustainable society. Here, an alternative approach to color-tunable optical devices, which is based on sustainable inorganic liquid crystals derived from 2D mineral materials abundant in nature, is described. The prototypical 2D mineral of vermiculite is massively produced by a green method, possessing size-to-thickness aspect ratios of >10 3 , inplane magnetization of >10 emu g −1 , and an optical bandgap of >3 eV. These characteristics endow 2D vermiculite with sensitive magneto-birefringence response, been several orders of magnitude larger than organic counterparts, as well as capability of broad-spectrum modulation. The finding consequently permits the fabrication of various magnetochromic or mechanochromic devices with low or even zero-energy consumption during operation. This work creates opportunities for the application of sustainable materials in advanced optics.

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Collective behavior widely exists in nature, ranging from the macroscopic cloud of swallows to the microscopic cloud of colloidal particles. The behavior of an individual inside the collective is distinctive from its behavior alone, as it follows its neighbors. The introduction of such collective behavior in two-dimensional (2D) materials may offer new degrees of freedom to achieve desired but unattained properties. Here, we report a highly sensitive magneto-optic effect and transmissive magneto-coloration via introduction of collective behavior into magnetic 2D material dispersions. The increase of ionic strength in the dispersion enhances the collective behavior of colloidal particles, giving rise to a magneto-optic Cotton–Mouton coefficient up to 2700 T–2 m–1 which is the highest value obtained so far, being 3 orders of magnitude larger than other known transparent media. We also reveal linear dependence of magneto-coloration on the concentration and hydration ratios of ions. Such linear dependence and the extremely large Cotton–Mouton coefficient cooperatively allow fabrication of giant magneto-birefringent devices for color-centered visual sensing.

Magnetically influenced light-matter interaction provides a contactless, noninvasive and power-free way for material characterization and light modulation. Shape anisotropy of active materials mainly determines the sensitivity of magneto-optic response, thereby making magnetic two-dimensional (2D) materials suitable in achieving the giant magneto-birefringence effect as discovered recently. Consequently, relationship between magneto-birefringence response and shape anisotropy of 2D materials is critical but has remained elusive, restricting its widespread applications. Here, we report the highly sensitive and largely tunable magneto-coloration via manipulating the shape-anisotropy of magnetic 2D materials. We reveal a quadratic increasing relationship between the magneto-optic Cotton–Mouton coefficient and the lateral size of 2D materials and achieve a more than one order of magnitude tunable response. This feature enables the engineerable transmissive magneto-coloration of 2D materials by tailoring their shape anisotropy. Our work deepens the understanding of the tunability of magneto-optic response by size effect of active materials, offering various opportunities for their applications in vast areas where color is concerned.

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