  We present a double-layer dielectric metasurface obtained by stacking a
silicon nanodisc array and a silicon photonic crystal slab with equal
periodicity on top of each other. We focus on the investigation of electric
near-field enhancement effects occurring at resonant excitation of the
metasurface and study its optical properties numerically and experimentally. We
find that the major difference in multi-layer metasurfaces when compared to
conventional single-layer structures appears to be in Rayleigh-Wood anomalies:
they are split into multiple different modes which are themselves spectrally
broadened. As a proof of concept we cover a double-layer metasurface with a
lanthanide-doped up-conversion particle layer and study its interaction with a
1550 nm photoexcitation. We observe a 2.7-fold enhancemed up-conversion
photoluminescence by using the stacked metasurface instead of a planar
substrate, although only around 1% of the up-conversion material is exposed to
enhanced near-fields. Two mechanisms are identified explaining this behavior:
First, enhanced near-fields when exciting the metasurface resonantly, and
second, light trapping by total internal reflection in the particle layer when
the metasurface redirects light into high-angle diffraction orders. These
results pave the way for low-threshold and, in particular, broadband photon
up-conversion in future solar energy and biosensing applications.
