Deterministic fractal antennas are employed to realize multimodal plasmonic devices. Such structures show strongly enhanced localized electromagnetic fields typically in the infrared range with a hierarchical spatial distribution. Realization of engineered fractal antennas operating in the optical regime would enable nanoplasmonic platforms for applications, such as energy harvesting, light sensing, and bio/chemical detection. Here, we introduce a novel plasmonic multiband metamaterial based on the Sierpinski carpet (SC) space-filling fractal, having a tunable and polarizationindependent optical response, which exhibits multiple resonances from the visible to midinfrared range. We investigate gold SCs fabricated by electron-beam lithography on CaF 2 and Si/SiO 2 substrates.Furthermore, we demonstrate that such resonances originate from diffraction-mediated localized surface plasmons, which can be tailored in deterministic fashion by tuning the shape, size, and position of the fractal elements. Moreover, our findings illustrate that SCs with high order of complexity present a strong and hierarchically distributed electromagnetic near-field of the plasmonic modes. Therefore, engineered plasmonic SCs provide an efficient strategy for the realization of compact active devices with a strong and broadband spectral response in the visible/mid-infrared range. We take advantage of such a technology by carrying out surface enhanced Raman spectroscopy (SERS) on Brilliant Cresyl Blue molecules deposited onto plasmonic SCs. We achieve a broadband SERS enhancement factor up to 10 4 , thereby providing a proof-of-concept application for chemical diagnostics.

Metamaterials have recently established a new paradigm for enhanced light absorption in stateof-the-art photodetectors.Here, we demonstrate broadband, highly efficient, polarizationinsensitive, 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 nearinfrared 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 | ≈ 20 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.Nowadays, the conversion of light into electrical signals is at the heart of several technologies. Notwithstanding the high level of maturity, the large scale, and diversity of application areas, the need for a compact photodetection platform with higher performance in terms of speed, efficiency, spectral range, as well as flexibility, semitransparency, and compatibility with complementary metal-oxide-semiconductor (CMOS), is becoming more eminent 1 .Such features can be greatly fulfilled by combining functional systems with metamaterials 2 (i.e. structured materials on the subwavelength scale with engineered electromagnetic properties). This leads to the concept of metadevices 2 , a logical extension of the metamaterial paradigm, where interactions are nonlinear and the response is dynamic, using systems with spatially variable elements. Generally, such metadevices can operate at low voltages, which is a competitive advantage over conventional technology exploiting bulk and expensive electrooptical crystals, often non-integrated in silicon photonics. However, the bandwidth of such metamaterials is gen-erally narrow due to their resonant nature and most of them are working in the infrared band. Mastering control in the optical regime is technologically a more challenging task, due to the expensive and demanding fabrication of nanostructures, resulting from the need of highresolution electron-or ion-beam techniques. Also, devices working in a wide spectral range are important for multiband photodetection 3 , on-chip sensing of multiple analytes 4 , as well as multiplexed fluorescence and Raman detection 5 . Hybridization of low-dimensional carbon allotropes with plasmonic metamaterials 6-16 is known to provide broadband and strong light-matter interactions, improvement of the nonlinear response due to the electromagnetic field enhancement given by the metamat...

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