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...

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We present a metamaterial-based perfect absorber (PA) that strongly supports four resonances covering a wide spectral range from 1.8 µm to 10 µm of the electromagnetic spectrum. The designed perfect absorber has metal–dielectric–metal layers where a MgF2 spacer is sandwiched between an optically thick gold film and patterned gold nanoantennas. The spectral tuning of PA is achieved by calibrating the geometrical parameters numerically and experimentally. The manufactured quad-band plasmonic PA absorbs light close to the unity. Moreover, the biosensing capacity of the PA is tested using a 14 kDa S100A9 antibody, which is a clinically relevant biomarker for brain metastatic cancer cells. We utilize a UV-based photochemical immobilization technique for patterning of the antibody monolayer on a gold surface. Our results reveal that the presented PA is eligible for ultrasensitive detection of such small biomarkers in a point-of-care device to potentially personalize radiotherapy for patients with brain metastases.