  Metamaterials have recently established a new paradigm for enhanced light
absorption in state-of-the-art photodetectors. Here, we demonstrate broadband,
highly efficient, polarization-insensitive, and gate-tunable photodetection at
room temperature in a novel metadevice based on gold/graphene Sierpinski carpet
plasmonic fractals. We observed an unprecedented internal quantum efficiency up
to 100% from the near-infrared to the visible range with an upper bound of
optical detectivity of $10^{11}$ Jones and a gain up to $10^{6}$, which is a
fingerprint of multiple hot carriers photogenerated in graphene. Also, we show
a 100-fold enhanced photodetection due to highly focused (up to a record factor
of $|E/E_{0}|\approx20$ for graphene) electromagnetic fields induced by
electrically tunable multimodal plasmons, spatially localized in self-similar
fashion on the metasurface. Our findings give direct insight into the physical
processes governing graphene plasmonic fractal metamaterials. The proposed
structure represents a promising route for the realization of a broadband,
compact, and active platform for future optoelectronic devices including
multiband bio/chemical and light sensors.
