  The aim of the study is to develop ultra-compact structures enabling an
efficient conversion of single high energy photon (UV) to two lower energy
photons (IR). The proposed structure combines rare-earths doped thin layer
allowing the down-conversion process with a photonic crystal (PhC), in order to
control and enhance the down-conversion using optical resonances. On the top of
the rare-earths doped layer, a silicon nitride (SiN) 2D planar PhC is
synthesized. For that, SiN is first deposited by PECVD. After holographic
lithography and reactive ion etching, a periodic square lattice of holes is
generated on the SiN layer. The PhC topographical parameters as well as the
layers thickness are optimized using Finite-Difference-Time-Domain simulations.
The design and realization of such PhC-assisted down-converter structures is
presented. Optical simulations demonstrate that the PhC leads to the
establishment of resonant modes located in the underneath doped layer, allowing
a drastic enhancement of the absorption of the rare-earth ions without
disturbing the transmission in the visible and near-IR parts of the spectrum,
hence demonstrating the relevance of such an approach.
