Resonantly Enhanced Optical Birefringence in Ultrathin High-Index WS₂ Metasurfaces

Optical birefringence plays an important role in the manipulation of polarization states of light. However, the weak birefringence in common birefringent materials restricts the device thickness to tens of microns for desired phase retardation. Although recent advances in dielectric metasurfaces have enabled remarkable birefringence using structurally anisotropic nanostructures, the refractive index contrast between the dielectric and the air cladding fundamentally limits the thickness of metasurfaces to a fraction of a micron to obtain the required phase retardation. Here, birefringent resonances in high-index tungsten disulfide (WS2) metasurfaces are utilized to push the thickness limit down to the deep subwavelength scale. The inherent high-index property of WS2 enables birefringent resonances in WS2 metasurfaces which consist of anisotropic nanostructures with a thickness of only 50 nm. Such birefringent resonances enhance the light-matter interaction and produce an unprecedented birefringence (Delta n(eff)) about 4. As a result, it is experimentally realized that circular-to-linear polarization conversion with a normalized efficiency of approximate to 90% within an incident angle range up to +/- 10 degrees. The results break the fundamental limit of birefringent devices and provide strategies for creating ultimate thin polarization optical devices.