Simultaneous measurement of complex refractive index and thickness of atomic-layer materials by surface plasmon resonance holographic microscopy

The optical characterization of atomic-layer materials requires the quantitative determination of both their complex refractive indices (RIs) and thickness. However, existing methods rely on preset thicknesses, empirical models, or auxiliary characterization tools. It remains a challenge to jointly retrieve real part n, imaginary part k, and thickness d of ultrathin samples in a wide-field manner and over a broad measurement range. Here, we propose an angle-scanning surface plasmon resonance (SPR) holographic microscopy. This method utilizes the modulation effect of sample parameters (n, k, d) on SPR phase shifts and retrieves them by fitting measured phase shifts with theoretical counterparts at multiple angles. Measurements of monolayer, bilayer, and multilayer graphene samples validate the effectiveness of this method and demonstrate its broad measurement range from monolayer to tens of nanometers. A step-like sample further confirms its wide-field measurement capability. This work provides an optical approach for jointly characterizing the complex RIs and thickness of graphene, with potential applicability to other atomic-layer materials.