We present a double-layer dielectric metasurface obtained by stacking a silicon nanodisk 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 enhanced 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.

We explore the usage of pulse sequence optimization to boost the quantum properties of topological defects in molecular graphenoids at high temperatures. We reach spin-lattice relaxation times on the same order as those of the best quantum devices in the literature, ∼1 ms at room temperature. The coherence time is shown to be heavily affected by the hyperfine interaction and by the high concentration of hydrogen atoms in particular. We test and compare the applicability and performance of different decoupling sequences in enhancing the coherence, identifying the best-performing sequences for the purposes of robust state initialization and coherence optimization. Coherence times up to 30 μs are reached, and we provide insight into the system-environment interaction mechanisms, with a semiclassical model that considers the nuclear bath as a source of a classical random noise and the dynamical decoupling as a filter function. Full deconvolution of the noise spectrum of the bath is obtained, and we show the noise density has a Lorentzian shape whose parameters describe the nuclear-bath dynamics.

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