The seminal work of R.B. Wood (1902), who discovered anomalies in the reflection spectra of sub-wavelength metallic gratings, triggered the field of plasmonics, where ultra-thin metallic sheets laterally structured on a sub-wavelength scale, so called metallic meta-surface, are under operation. The goal of the field has extended considerably in the last decades and has aimed at arbitrary control over the amplitude, phase and polarization… of light waves at the sub-wavelength scale. All-dielectric meta-surfaces consisting in nano-structured thin films of high index dielectric material, are attracting much attention, owing to their capability to achieve the same goal as their metallic counterpart, yet with an enhanced efficiency (especially for the manipulation of strong optical resonances), being freed from significant energy dissipation as encountered in metallic nano-structures. All dielectric meta-surfaces have been around for quite a while, but were named differently (photonic crystal dielectric membranes or high index contrast gratings). Unless rare exceptions, the literature reports on structures with non-broken vertical symmetry. In the present contribution we emphasize that breaking the vertical symmetry of all-dielectric meta-surfaces provides a widely enhanced degree of freedom for the control of spatial routes and spectral characteristics of light, which depends, to an essential extent, on the local density of photonic states in the thin nano-structured dielectric film. As an enlightening illustration, we concentrate on a dielectric meta-surface formed by two super-imposed identical evanescently coupled gratings, with adjustable gap distance and lateral alignment. We show that this remarkably simple meta-surface can provide any local density of photonic states from zero (Dirac cone) to infinity (ultra-flat zero curvature dispersion characteristics), as well as any constant density over an adjustable spectral range. Exemplifying applications will illustrate the great potential of this new approach.
In this communication, we present the potentialities offered by 2D photonic crystals to trap and absorb photons in thin silicon layers. We will specifically focus on the impact of the photonic crystal unit cells symmetry, and the possibility to increase light absorption and generated photocurrent using multi-periodic and pseudo-disordered photonic nanostructures.
In silicon-based solar cells, a substantial part of the energy losses is related to the charge carriers thermalization in the UV-blue range and the week carriers collection at these wavelenghts. To avoid this issue, we introduce a new concept which combines a rare-earths doped thin layer with a photonic crystal (PC) layer, allowing an efficient conversion from UV-blue photons to near-IR photons. We report on the feasibility of such a nanostructured down-converter module using an active rare-earth doped CaYAlO4 thin layer and a silicon nitride PC on top. By means of optical numerical simulations, the promising potentialities of the concept are demonstrated.