Abstract
Extraordinarily high optical contrast is instrumental to research and applications of two-dimensional materials, such as, for rapid identification of thickness, characterisation of optical properties, and quality assessment. With optimal designs of substrate structures and light illumination conditions, unprecedented optical contrast of MoS2 on Au surfaces exceeding 430% for monolayer and over 2600% for bilayer is achieved. This is realised on custom-designed substrates of near-zero reflectance near the normal incidence. In particular, by using an aperture stop to restrict the angle of incidence, high-magnification objectives can be made to achieve extraordinarily high optical contrast in a similar way as the low-magnification objectives, but still retaining the high spatial resolution capability. The technique will allow small flakes of micrometre size to be located easily and identified with great accuracy, which will have significant implications in many applications.
Highlights
Optical contrast spectroscopy is a valuable tool for easy location and identification of the layers of two-dimensional
It has been demonstrated that optical contrast of thin films is completely determined by the underlying substrate and the illumination conditions of light[15]
In addition to optimising the design of the structures, we systematically investigate the effects of light illuminating conditions, including the numerical apertures (NA) of objectives and the aperture stops (AS)
Summary
Optical contrast spectroscopy is a valuable tool for easy location and identification of the layers of two-dimensional (2D) materials[1,2,3,4,5]. Machine vision has been applied to allow rapid, automated and reliable identification of a flake’s presence[13]. Large optical contrast is beneficial in many applications. It allows a rapid and accurate identification of the layer number of 2D materials and enables the development of sensors for ultrasensitive label-free molecular sensing[14]. The role of substrate is fully represented by a complex reflectivity r = r0 e iφ. Both the amplitude r0 and the phase φ are important.
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