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Plasmonic resonances in metallic subwavelength structures have been widely exploited for a broad range of applications including nanoantennas, surface-enhanced Raman spectroscopy, chiral metamaterials, metamagnetism and absorbers. The phenomenon of extraordinary optical transmission (EOT) through subwavelength holes or slits based on the surface plasmon resonance is also extensively studied and has been applied for light harvesting. However, most of work about light harvesting devices suffer from many disadvantages such as narrow operating waveband, sensitive to the polarization state of the incident light, narrow accepting angles at a fixed azimuthal angle, which greatly limit their potential applications to spectroscopic detection and phase imaging. In this work, we present a broadband plasmonic resonant absorber in infrared regime. The plasmonic resonant absorber consists of a three-layer structure, i.e. two-dimensional metallic subwavelength hole arrays/dielectric-spacer/ thick metallic film from top to bottom. The designed plasmonic resonator is found to be polarization insensitive and omnidirectional due to the symmetry of the subwavelength hole array structure. The absorption efficiency of such absorber can be optimized by tuning the geometry of the metallic subwavelength structure and the thickness of the dielectric layer in between the two metallic films. The broadband efficient light absorbing property of the plasmonic resonant absorber can be explained by the synergetic effect of plasmonic resonance and Fabry-Perot (FP) resonance. It is shown that the periodic subwavelength metallic holes interact with the incident light to excite the surface plasmons so that the transmitted light intensity is significantly enhanced. The enhancement of the electric field near the metallic surface leads to an improved absorption. Moreover, FP cavity provides a resonant environment for the excited surface plasmons as well as the diffracted waves. As a result, the efficient light absorbing is achieved over a broad waveband. It should be noted that the proposed absorber can be applied to other working wavebands by carefully tuning the geometry of the metallic subwavelength structure and the thickness of the dielectric layer in between two metallic layers. The designed absorber may find important applications in solar cells, photodetectors, thermo-photovoltaic, and thermal emitters.
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