that the incident light is substantially absorbed. Landy et al. has shown the fi rst PA, operating in microwave frequency range, where the structure consists of an electric resonator and a cut wire, which independently couple to electric and magnetic fi elds. [ 1 ] Later for the higher frequency ranges, Liu et al. demonstrated an infrared PA system. [ 8 ] The structure is composed of metal-dielectric-metal layers, where the top metal layer is patterned with subwavelength antennas serving as a resonator, and the bottom one is an optical mirror which signifi cantly attenuates the transmittance. The coupling of light to the antennas induces an electric fi eld, while the nearfi eld couplings between the antennas and the metal sheet result in mirror-image charges in the bottom layer. This generates a current loop which induces a magnetic fi eld. [9][10][11][12][13][14] Then, tuning the amplitude and resonance frequency of the electric and magnetic responses can be used to match the impedance of PA to freespace, which minimizes the refl ectance. Hence, minimizing refl ection with impedance matching, while attenuating transmission with a metal sheet leads to perfect absorption. Recently, the dependence of absorbance on a critical coupling condition between resonators and optical mirrors has been investigated to provide a universal way for unity absorbance. [ 15 ] Supporting strong absorbance capabilities, PAs are good candidates for surface enhanced infrared absorption (SEIRA) spectroscopy applications. As the infrared region is accompanied with low radiation damping, PAs engineered at this wavelength window could support plasmonic resonances with high Q-factors, which leads to strong nearfi eld enhancements. This feature is highly advantageous for achieving large spectroscopic signals associated with the molecular vibrational modes of interest. [16][17][18][19][20][21][22] In order to reliably identify the targeted molecules, it is crucial to simultaneously monitor different molecular fi ngerprints. However, PAs' unity absorbance is limited within a narrow spectral window where the plasmonic resonances of their subwavelength antennas lie. This problem could be addressed by utilizing nanoparticle or nanoaperture based confi gurations, supporting multiple resonances. [23][24][25][26][27][28][29][30][31] Recently, different multiband PA structures have been introduced to serve for variety of applications from microwave to mid-infrared frequency ranges. [32][33][34][35][36][37][38][39] A dual-resonant perfect absorber (PA) based on multiple dipolar nanoantenna confi guration is introduced. The PA platform exhibits near-unity (95%-98%) absorbance in dual-resonances. A fi ne-tuning mechanism of dual-resonances is determined via geometrical device parameters of the constituting dipolar elements of the compact PA system. It is also shown that the dual plasmonic resonances are associated with easily accessible and large local electromagnetic fi elds. Possessing large absorbance with strong nearfi elds, the PA system is highly a...

Fractal-shaped nanoantennas have a large potential to enable multiband devices for surface-enhanced spectroscopy due to their scale-invariant geometry that gives rise to strongly enhanced local fields across different spectral ranges with multiscale spatial distributions. In particular, fractal nanoantennas based on plasmonic metals are promising for biodetection applications that extend from the near-infrared across the mid-infrared spectrum. In this context, we introduce novel multiscale resonant structures based on the inverse Cesaro space-filling fractal curve with the remarkable property that the number of resonant bands does not depend on the overall size of the structures. We systematically study their scattering and near-field resonant properties by resorting to full-field finite difference time domain simulations in combination with experimental Fourier transform infrared microspectroscopy. In particular, by investigating a number of gold antennas fabricated by electron-beam lithography on CaF2 substrates, we demonstrate controllable multiband plasmonic resonances from near-infrared to the mid-infrared spectral regions. Moreover, our findings demonstrate that large values of near-field enhancement with hierarchical fractal distributions can be achieved in Cesaro-type nanoantennas across multiple bands that are ideally suited for chemical detection on a small footprint area. In order to demonstrate the full potential of Cesaro fractal nanoantennas for infrared sensing spectroscopy, we show triple band reliable detection of thin poly­(methyl methacrylate) layers with nanoscale thickness. The engineering of Cesaro-type plasmonic nanoantennas provides a novel strategy for the realization of active devices with a large spectral density and reduced footprints that can be conveniently integrated in future plasmonic–photonic active platforms for energy harvesting and optical biosensing.

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