  Nanoscale dielectric resonators and quantum-confined semiconductors have
enabled unprecedented control over light absorption and excited charges,
respectively. In this work, we embed luminescent silicon nanocrystals (Si-NCs)
into a 2D array of SiO2 nanocylinders, and experimentally prove a powerful
concept: the resulting metamaterial preserves the radiative properties of the
Si-NCs and inherits the spectrally-selective absorption properties of the
nanocylinders. This hierarchical approach provides increased photoluminescence
(PL) intensity obtained without utilizing any lossy plasmonic components. We
perform rigorous calculations and predict that a freestanding metamaterial
enables tunable absorption peaks up to 50% in the visible spectrum, in
correspondence of the nanocylinder Mie resonances and of the grating condition
in the array. We experimentally detect extinction spectral peaks in the
metamaterial, which drive enhanced absorption in the Si-NCs. Consequently, the
metamaterial features increased PL intensity, obtained without affecting the PL
lifetime, angular pattern and extraction efficiency. Remarkably, our
best-performing metamaterial shows +30% PL intensity achieved with a lower
amount of Si-NCs, compared to an equivalent planar film without nanocylinders,
resulting in a 3-fold average PL enhancement per Si-NC. The principle
demonstrated here is general and the Si-NCs can be replaced with other
semiconductor quantum dots, rare-earth ions or organic molecules. Similarly,
the dielectric medium can be adjusted on purpose. This spectral selectivity of
absorption paves the way for an effective light down-conversion scheme to
increase the efficiency of solar cells. We envision the use of this
hierarchical design for other efficient photovoltaic, photo-catalytic and
artificial photosynthetic devices with spectrally-selective absorption and
enhanced efficiency.
