The effect of anisotropy on cross section spectra of quartz, calcite and corundum spheres of moderate size

Absorption, extinction and scattering cross section spectra of optically uniaxial materials are commonly described by the weighted sum approximation, also known as 1 / 3 −2 / 3 -approximation. Based on our previous work we investigate the deviations of infrared spectra calculated with the weighted sum approx- imation from cross sections generated by finite-difference time-domain (FDTD) simulations of crystalline calcite, quartz and corundum spheres. These materials are astronomically relevant and vary strongly concerning number, strength and resonance frequency of the transition dipole moments. Several effects of anisotropy were observed in the simulated spectra for both single particle spectra and orientation- averaged spectra, like shifted bulk modes and the appearance of additional peaks. These effects were not predicted by the weighted sum approximation.