Angle- and Polarization-Independent Structural Color Based on Controlled Phase and Gain Margins in Ultrathin Transparent Dielectrics

Fascination with nature's color palette has propelledmankind'sefforts to imitate and surpass it. Apart from the traditional interestsin brighter and more fade-resistant colors, the present time addsthe need for doing so without impacting the environment based on nontoxicmaterials. Although most commercial colorants are based on chemicalpigments that absorb wavelengths of light matching their electronicand molecular transitions, the development of nanofabrication techniquesin the last decades has spurred researchers to study structural colors,where color can be made out of colorless materials by carefully designedmicrometer and nanoscale inclusions. Although, a myriad of such configurationshave been reported for the production of color, in most cases, theysuffer from angle or polarization dependence or require expensivematerials or impractical fabrication techniques that are incompatiblewith large-scale production. Recently, some approaches have been proposedfor the use of random media to mitigate these effects. Here, we demonstratean approach based on colorless mirrors and transparent lossless dielectricsand show how this can be harnessed to produce a full color paletteby using a subwavelength bilayer structure composed of thin filmsof titanium and its oxide for the production of inexpensive structuralcolor. Furthermore, we introduce an additive manufacturing processthat creates 58 different hues from 4 primaries in only three sequentialevaporations. Based on this color palette, we reproduce Van Gogh'sself-portrait on a 1 sq in. area providing proof of the applicabilityof the platform for large-scale, low-cost, and environmentally friendlyproduction of structural colors.