Emission enhancement in dielectric nanocomposites.

We show that radiative decay in dielectric nanocomposite is greatly affected by the shape and arrangement of its constituents. For the same fractions of constituting materials the radiative decay rate can differ by orders of magnitude. E.g. for the case of diluted luminescent nanoparticles with refractive index of 1.43 inside a host with refractive index of 3.5 we show that one can achieve broadband Purcell factors of up to 40, which is of the same order as what can be achieved for a single frequency with deliberately patterned high Q dielectric resonators. Further, we discuss the fact that the Purcell factor is a spatially inhomogenous quantity and we present a simple way to calculate the average Purcell factor in arbitrary nanocomposites based on the reciprocity theorem and a quasistatic approximation. It is shown that in dielectric nanocomposites a low index luminescent medium should occupy gaps between high index particles to achieve maximal Purcell enhancement.