  Thin Bi2Te3 flakes, with as few as 3 quintuple layers, are optically
visualized on the SiO2-capped Si substrates. Their optical contrasts vary with
the illumination wavelength, flake thickness and capping layers. The maximum
contrast appears at the optimized light with the 570nm wavelength. The contrast
turns reversed when the flake is reduced to less than 20 quintuple layers. A
calculation based on the Fresnel law describes the above observation with the
constructions of the layer number-wave length-contrast three-dimensional (3D)
diagram and the cap thickness-wavelength-contrast 3D diagram, applicative in
the current studies of topological insulating flakes.
