Quantitative determination of in-plane optical anisotropy by surface plasmon resonance holographic microscopy

Quantitative determination of in-plane optical anisotropy is essential in finding or designing anisotropic low-dimensional materials and investigating their physical properties. Current determination methods are mostly qualitative or using empirical equations for quantitative calculation. A common weakness of these methods is utilizing light-matter interactions between far-field light and material samples which relies on long interaction distance. However, the thin thickness of low-dimensional material, especially atomic-layer sample, induces an exceeding short light-matter interaction distance and results in low signal-to-noise ratio as well as inaccurate measurement result. In this paper, we propose a novel determination method for in-plane optical anisotropy called azimuthal scanning excitation surface plasmon resonance holographic microscopy. This method utilizes near-field light-matter interactions between material samples and surface plasmon waves oscillating along various in-plane directions. The sample complex refractive indices along all of the in-plane directions can be quantitatively retrieved and thus the magnitude of in-plane optical anisotropy, including birefringence and dichroism, is determined. This method detects the reflection phase shift in surface plasmon resonance regardless of the sample thickness and thus is applicable to ultrathin samples down to atomic-layer. As a demonstration example, monolayer, bilayer and multilayer ReS₂ samples have been used to verify the validity of the proposed method, and we find that the magnitude of in-plane optical anisotropy increases with the decrease of sample thickness. This work provides a precise determination method for in-plane optical anisotropy of thin film samples with various thickness and gives a guidance in finding new anisotropic low-dimensional materials and engineering new polarized nanodevices.