06- Creating color and appearance of surfaces in real and Fourier space by tailored disorder

The aim of our experiment-theory project is to study the influence of spatial correlations on disorder in plasmonic and dielectric nanostructures by investigating long- and short-range disorder as well as fractal and quasicrystalline arrangements. We want to determine the relationship between two-point correlation functions, k-dependent optical properties, and the wavelength-dependent bidirectional reflection distribution function (BRDF), in order to design the color and appearance of surfaces by tailored disorder.

The idea is to not only allow for creating different spectral behavior that would represent the different colors from the CIE 1931 color space, but also allow for the different appearances of the surfaces, namely the angle- and polarization-dependent spectral reflectances.

In particular, we are going to use metallic and dielectric nanoparticles in regular and disordered 1D and 2D arrangements. Specifically, we will utilize magnesium, gold, aluminum, nickel, silver, as well as dielectrics such as Al₂O₃, SiO₂, and MgH₂ for the scattering nanoantennas. Rayleigh-Wood anomalies crossing with plasmon dispersions can enhance or suppress spectral and angular scattering for certain polarizations in given directions. Tailored spatial disorder functions with given two-point correlation functions such as long- and short-range disorder with Gaussian or rectangular disorder distributions in size, position, and orientation of the nanoantennas will give the possibility to tailor the optical responses.

Generating the structures is carried out by using electron-beam lithography, colloidal hole-mask or etching lithography, and further nanostructuring techniques. Measuring their optical responses in a newly designed simultaneous Fourier- as well as real-space spectral and polarization-resolved scatterometer gives the necessary angle-resolved BRDF data. A special treat of using magnesium as plasmonic material is its ability to perform reversible phase transitions from metallic (Mg) to dielectric (MgH₂) upon hydrogenation and subsequent dehydrogenation. This allows us to create surfaces that can change their colors and even their appearances, similar to chameleons.

In close collaboration with Mercator-Fellow Prof. Dr. Sergei Tikhodeev in Moscow, we will develop and utilize our theory for predicting the spectral, angular, and polarization-dependent BRDF data. In detail, we will be using a coupled-dipole model as well as the resonant state expansion technique, which we will extend towards predicting the angular-dependent far-field spectra of disordered systems.

Additionally, for the first time, we aim at predicting ab initio the color appearance of the tailored disordered surfaces by solving Maxwell’s equations in combination with complex dielectric material functions. This would allow for bridging the gap towards high-level virtual reality rendering software, where until now mostly empiric and heuristic models are used to describe the appearance of surfaces.