Optical anisotropy, a spatially asymmetric light–matter interaction that manifests itself as birefringence and dichroism, is paramount for manipulating light polarization in modern optics. So far, various natural birefringent crystals are widely used, but their birefringence is limited to <0.3. Here we demonstrate a solution-processable natural crystal C₃H₈N₆I₆·3H₂O with giant birefringence up to 2.8 within the visible to infrared spectral region. Combining critical point analysis and the first-principles calculations, we reveal that this giant optical anisotropy mainly comes from the linear (I₃)⁻ structural units in a parallel arrangement, which maximizes the difference of polarizability along the different crystallographic axes. This work highlights the potential of natural polyiodide crystals as an outstanding platform to satisfy the increasing demand for photonic applications that exploit polarization in optical communication, three-dimensional imaging, ultrahigh-resolution sensing and other tasks.

光学各向异性是一种空间不对称的光与物质相互作用，表现为双折射和二向色性，对于现代光学中操纵光偏振至关重要。迄今为止，各种天然双折射晶体被广泛应用，但其双折射仅限于<0.3。在这里，我们展示了一种可溶液加工的天然晶体C ₃ H ₈ N ₆ I ₆ ·3H ₂ O，在可见光至红外光谱区域内具有高达2.8的巨大双折射。结合临界点分析和第一性原理计算，我们揭示了这种巨大的光学各向异性主要来自平行排列的线性(I ₃ ) ^-结构单元，这使得沿不同晶轴的偏振率差异最大化。这项工作凸显了天然聚碘晶体作为出色平台的潜力，可以满足对光通信、三维成像、超高分辨率传感和其他任务中利用偏振的光子应用日益增长的需求。