Optical Properties of Organic Thin Films

Particles at scales below or in the order of the wavelength of light show a plethora of scattering patterns due to the interplay of refractive index and size of the particle. At resonance, incident light is trapped inside the structure (resonant Mie-scattering). Typically, organic low refractive-index materials cannot act as optical cavity, their scattering properties differ significantly and have only recently been described as non-resonant Mie-scattering.

In thin films of cyanine dyes with high extinction coefficients both, the real and the imaginary part of the index of refraction vary strongly near the absorption edge of the dye. In nanostructured films that develop during film formation (spinodal dewetting) new scattering effects arise from the interplay of size of the particles, absorption and the strong increase of the refractive index at the absorption edge. In particular, light scattering can be enhanced and the scattering band narrows significantly. Scattering can be further intensified when the dye forms H- or J-aggregates.

Figure: Finite difference time-domain (FDTD) simulation of the near-field scattering field of a cyanine dye droplet.

N. A. Leclaire, et al. Light Scattering Enhancement at the Absorption Edge in Dewetting Droplets of Cyanine Dyes, Adv. Opt. Mater., 2017, 1600903.