Probing crystal axis orientation of birefringent materials via polarized microspectroscopy and anisotropic optical modeling

Due to the intriguing properties of van der Waals materials with in-plane birefringence, the field of low-dimensional polarization dependent devices emerged with manifold applications in optics and (opto-)electronics. For such devices, knowledge about their in-plane crystallographic orientation is essential. We report a fast and non-destructive method for probing the crystal alignment of van der Waals materials with lateral dimensions on the microscale by linearly polarized reflectance microspectroscopy. Based on a standard microscope setup combined with a spectrometer and a polarizer we demonstrate the feasibility for magnifying optics with high numerical aperture. Unlike alternative methods, our approach does not require measuring a sinusoidal signal over a full 360∘ range. Instead, we significantly reduce the experimental effort by evaluating the individual polarized reflectance spectra within a 4 x 4-transfer matrix method framework. The applicability is demonstrated for the biaxial materials MoO3, ReS2, and black phosphorus with varying sample thicknesses. We determine the orientation of their crystal axes within a 5∘ error margin, whereby the reflectance spectra only need to be obtained at 4 different polarization directions.