Giant in-plane anisotropy in novel quasi-one-dimensional van der Waals crystal

Large optical anisotropy is paramount for advancing light manipulation in modern optics. Therefore, there has been an intensive search for materials exhibiting giant optical anisotropy. However, the reported in-plane birefringence of most materials remains relatively low, posing substantial limitations for applications in integrated optics and polarization-sensitive technologies. Here we present a systematic investigation of the in-plane anisotropic properties of the quasi-one-dimensional (quasi-1D) van der Waals (vdW) crystal-Ta₂NiSe₅, employing spectroscopic ellipsometry, angle-resolved polarization Raman spectroscopy, azimuth-dependent reflectance difference microscopy and angle-dependent electronic and optoelectronic techniques. Notably, our study reveals a record-breaking giant in-plane birefringence of up to 2.0 across the visible to infrared spectral region, representing the highest value reported among vdW materials to date. Meanwhile, the physical origin of this extraordinary optical anisotropy is elucidated through first-principles calculations, attributing it to the synergistic effects of significant polarizability contrast and the quasi-1D crystal arrangement. Furthermore, photodetectors based on Ta₂NiSe₅flakes exhibit remarkable performance, including a broad photoresponse spanning 520-2000 nm, ultrafast response time of 75μs, a pronounced dichroic ratio of up to 1.89 and high-resolution polarized light imaging capabilities. Our work not only highlights the immense potential of Ta₂NiSe₅for next-generation polarization-sensitive optoelectronic devices but also inspire innovative approaches for next-generation ultracompact integrated photonics based on quasi-1D vdW materials.