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 der Waals materials inherently support considerable out-of-plane birefringence. However, 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 biaxial van der Waals materials Ta₂NiS₅, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 μm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta₂NiS₅ show strong in-plane anisotropy with a long propagation length (>20 μm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.