Importantly, the optical contrast on semitransparent gold is enhanced by a factor between 5 and 16 with respect to the case of an opaque gold substrate for wavelengths λ > 550 nm (see the inset of Figure  1b where the ratio between the contrasts

is given). These results indicate that enhanced visualization and thickness estimation of mica flakes can be achieved on semitransparent gold substrates. selleck The dependence of the optical contrast on the thickness of the mica flakes is shown in Figure  1c for three representative wavelengths (λ = 475, 550, and 650 nm) and for the two thickness values of the gold layer, i.e., 20 nm (continuous lines, semitransparent gold) and 300 nm (dashed lines, opaque gold). The optical contrast shows an oscillatory behavior characteristic of multilayered structures [5], with an enhanced signal for semitransparent gold (compare continuous and dashed lines of the same color). The oscillatory behavior of the optical contrast is due to an oscillatory behavior of the mica reflectance spectrum, which can be translated Ivacaftor into an oscillatory change in the color of the mica flakes perceived by the human eye. Indeed, for a standard observer the chromaticity of the color of a material under white illumination can be defined by the parameters x and y given by [7]: (6) where the tristimulus X, Y, and Z are defined from the reflectance spectrum

as: (7) Here, , , and are the so-called color matching functions of a standard observer [7]. In Figure  1d, we show the calculated evolution of the chromaticity of RAS p21 protein activator 1 the mica flakes’ color in the xy chromatographic space as a function of the mica thickness in the 0- to 300-nm range. The black and red lines correspond to the semitransparent and opaque gold layers, respectively. According to these results, we expect a gradual change of color as the mica thickness increases in the thin range below approximately 50 nm. This gradual change is almost reversed back for thicker layers, between 50 and 100 nm, and then evolves to larger and fastest

chromaticity changes with the thickness from 100 to 300 nm. In the case of an opaque gold substrate (red line in Figure  1d), the evolution of the chromaticity of the mica flakes is qualitatively similar but restricted to a narrower space of colors, thus making increasingly difficult to achieve a precise optical characterization on this type of substrates. It is worth mentioning that the theoretical contrast that can be achieved on semitransparent gold substrates is between half and three halves of the contrast that can be achieved on SiO2 substrates [2, 3], in which single mica layers can be detected. This makes reasonable the detection of a few mica layer sheets on semitransparent gold substrates. Methods We verified the theoretical predictions discussed above by fabricating thin mica flakes on semitransparent gold films and characterizing them by optical and atomic force microscopy.