Polarization, a fundamental property of light, has been widely exploited from quantum physics to high-dimensional optics. Materials with intrinsic optical anisotropy, such as dichroism and birefringence, are central to light polarization control, including the development of polarizers, waveplates, mirrors and phase-matching elements. Therefore, materials with strong optical anisotropy have been long-sought. Recently, two-dimensional van der Waals crystals show high optical anisotropy but are mostly restricted to the out-of-plane direction, which is challenging to access in optical engineering. Here we report a two-dimensional van der Waals material, NbOCl₂, that exhibits sharp electronic and structural contrast between its in-plane orthogonal axes. Colossal in-plane optical anisotropy—linear dichroism (up to 99% in ultraviolet) and birefringence (0.26–0.46 within a wide visible–near-infrared transparency window)—is experimentally demonstrated. Our findings provide a powerful and easy-to-access recipe for ultracompact integrated polarization industries.

偏振是光的基本属性，已被从量子物理学到高维光学广泛利用。具有固有光学各向异性的材料（例如二色性和双折射）是光偏振控制的核心，包括偏振器、波片、镜子和相位匹配元件的开发。因此，人们一直在寻找具有强光学各向异性的材料。近年来，二维范德华晶体表现出高光学各向异性，但大多局限于面外方向，这在光学工程中很难实现。在这里，我们报道了一种二维范德华材料 NbOCl ₂ ，它在其面内正交轴之间表现出鲜明的电子和结构对比。实验证明了巨大的面内光学各向异性——线性二色性（在紫外线中高达 99%）和双折射（在宽可见光-近红外透明窗口内为 0.26-0.46）。我们的研究结果为超紧凑集成偏振行业提供了强大且易于使用的方法。