A solution-processable natural crystal with giant optical anisotropy for efficient manipulation of light polarization

Optical anisotropy (e.g., birefringence and dichroism), a light-matter interaction phenomenon observed three centuries ago in natural Iceland crystal, is paramount for manipulating light polarization in modern optics. To date, various natural birefringent crystals are widely used, but their birefringence is limited to < 0.3. Here, we demonstrate a solution-processable natural crystal C₃H₈N₃I₆·3H₂O with giant birefringence up to 2.8 from visible to infrared spectral region, which hits a new high among natural crystals. 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 next-generation polarized photonic applications in optical communication, 3D imaging, ultra-high-resolution sensing, etc.