In a study published in Nature Communications, a research group led by Prof. LI Dabing and Prof. LI Shaojuan from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed and experimentally demonstrated the presence of broadband, low-loss, giant in-plane birefringence in a biaxial van der Waals materials Ta₂NiS₅, spanning an ultrawide-band from visible to mid-infrared wavelengths.

Birefringence is the difference in the speed of light between two crystallographic axes, which has important applications in polarization control, ultra-confined light coupling, and non-linear optics. Despite large birefringence is very essential, the currently birefringence crystals such as YVO₄, MgF₂ and liquid crystals have relatively small birefringence, and the relatively large volume of these materials is not applicable to compact integrated photonic applications.

Compared to the bulk crystals, layered van der Waals materials highlight with atomic flat smooth surfaces, strong light-matter interactions and compatibility with the current silicon photonic technology, which makes them the promising candidates for next generation on-chip compact nanophotonic 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.

The research team predicted crystals that might support giant in-plane birefringence in terms of structural anisotropy and electronic polarizability, and through first-principles calculations, spectroscopic ellipsometry, Fourier transform infrared spectroscopy and near-field tests, they revealed that Ta₂NiS₅ exhibits in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges, which is one of the highest among van der Waals materials known to date.

Furthermore, the research team revealed the anisotropic waveguide transmission from visible to mid-infrared band with low-loss in Ta₂NiS₅ by real-space nano-imaging. In the mid-infrared band, the propagation length of the waveguide modes can exceed 20 μm, and exhibit elliptical isofrequency contour, which shows a significant potential to the directional control of light for nanophotonic applications.

This study paves the way for utilizing layered biaxial chalcogenides as broadband giant birefringent material to develop subwavelength integrated optics in the future.