Visible to mid-infrared giant in-plane optical anisotropy in ternary van der Waals crystals

Birefringence is at the heart of photonic applications. Layered van derWaals materials inherently support considerable out-of-plane birefringence. How-ever, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxialvan der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 mu m. The in-plane birefringence Delta n approximate to 2 and0.5 in the visible and mid-infrared ranges is one of the highest among van derWaals materials known to date. Meanwhile, the real-space propagating wave-guide modes in Ta(2)NiS(5)show strong in-plane anisotropy with a long propalgation length (>20 mu m) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on vander Waals materials.