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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.
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