Peculiarities of light absorption by spherical microcapsules

Abstract

We address the peculiarities of optical radiation absorption in a poly-layer spherical microparticle which simulates an inorganic/organic poly-shell absorbing microcapsule with a water-based cargo. By means of the finite-difference time-domain technique, the spatial distribution of absorbed light power in microcapsules of various sizes and internal structure is numerically simulated. We have engineered the optimal structure of a capsule that provides for light absorption enhancement and consists of a strong refracting transparent coating and an absorbing layer which covers a liquid core. The proposed multi-layer microcapsule prototype provides for a manifold increase in the absorbed light power density in comparison with usual single-layer absorbing capsule. We show that for wavelength-scale microcapsules it is optimal to use a material with the refractive index larger than 2 as an outer shell, for example, titanium dioxide. The highest values of the absorbed power density can be obtained in microcapsules with absorbing shell thickness of approximately λ/10. When laser radiation is scattered by a dimer constituted by two identical absorbing microcapsules the absorbed power density can be maximized by choosing proper dimer spatial configuration. In the case of strongly absorbing particles, the absorption density is maximal if the distance between capsules is about one capsule diameter. For weakly absorbing particles, light absorption increases when particles in the capsule are in geometrical shades of each other.