Abstract
Produced by light interacting with structures at micrometer or nanometer levels, structural color can be utilized to investigate details of material structures. In this article, we studied the pattern of interference color generated from repetitive aragonite–conchiolin double layers on colorless nacreous pearls. Based on qualitative wave analysis and quantitative electromagnetic computation, we theoretically concluded such patterns are mainly determined by aragonite layer thickness. We also demonstrated how to predict the aragonite layer thickness and estimate the conchiolin refractive index variation on a Tahitian pearl with near-colorless body color and strong iridescence. We believe this approach offers a new perspective to study nanostructures in biology and mineralogy.
Highlights
Produced by light interacting with structures at micrometer or nanometer levels, structural color can be utilized to investigate details of material structures
The left expression in the equation is optical path difference (OPD) arising from the aragonite–conchiolin double layer, where n is the refractive index (RI), d is the submicroscopic layer thickness, with subscripts 1 and 2 referring to media aragonite and conchiolin, respectively. θt is the angle of transmission and could be calculated by the angle of incidence θi using Fresnel Equations
Given d 1 is the dominant factor in OPD expression, we expected that aragonite layer thickness could be estimated by interference pattern observed on round pearl
Summary
Produced by light interacting with structures at micrometer or nanometer levels, structural color can be utilized to investigate details of material structures. We demonstrated how to predict the aragonite layer thickness and estimate the conchiolin refractive index variation on a Tahitian pearl with near-colorless body color and strong iridescence. Because the interference equations cannot take amplitude variations into consideration, we switched to electromagnetic method to compute the theoretical reflectance[14], and calculated the color perceived by the CIE standard observer[2] Such a color perception is described by CIE lightness (L), chroma (C), and hue (H). To visualize the numerical description, we further synthesized CIELCH colors via 8-bit computer graphics, and generated series of interference pattern simulations on round colorless pearl, with aragonite layer thickness ranging from 275 to 600 nm at 25 nm increment (Fig. 1d). In order to validate our theoretical predications, we need to conduct destructive analysis on nacre to see whether the differences in iridescent patterns correlate with differences in aragonite layer thickness
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