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Optical Contrast of Atomically Thin Films

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journal contribution
posted on 07.03.2019 by Fumin Huang
Here, we provide a comprehensive description of the optical contrast of atomically thin films based on rigorous analytical solutions. The effects of thin film, substrate, and light illumination conditions are fully revealed. The role of the substrate is found to be completely represented by single complex reflectivity, regardless of the structural details. High contrast is realized on low-reflection substrates; however, the phase of the complex reflectivity is critically important. Every thin film has specific reflectivity phase conditions to achieve optimal contrast, which are uniquely defined by the optical properties of the thin film. Extraordinarily, high optical contrast can be achieved on any thin film of any thickness, if the reflectivity of substrates matches the optimal phase conditions. We provide a universal phase map which can be used to determine the optimal phases of any given film. For example, the optimal phase of hexagonal boron nitride films is found to be −90°, which paves the way toward designing high-contrast substrates to visualize this highly transparent two-dimensional material. We also provide detailed discussions on the effects of a range of other factors, including polarizations, incident angles, and the numerical aperture of objectives.