Abstract
Plasmonic effect using a cross-hair can convey strongly localized surface plasmon modes among the separated composite nanostructures. Compared to its counterpart without the cross-hair, this characteristic has the remarkable merit of enhancing absorptance at resonance and can make the structure carry out a dual-band plasmonic perfect absorber (PPA). In this paper, we propose and design a novel dual-band PPA with a gathering of four metal-shell nanorods using a cross-hair operating at visible and near-infrared regions. Two absorptance peaks at 1050 nm and 750 nm with maximal absorptance of 99.59% and 99.89% for modes 1 and 2, respectively, are detected. High sensitivity of 1200 nm refractive unit (1/RIU), figure of merit of 26.67 and Q factor of 23.33 are acquired, which are very remarkable compared with the other PPAs. In addition, the absorptance in mode 1 is about nine times compared to its counterpart without the cross-hair. The proposed structure gives a novel inspiration for the design of a tunable dual-band PPA, which can be exploited for plasmonic sensor and other nanophotonic devices.
Highlights
With the ability of efficaciously transforming electromagnetic (EM) waves from free space into the sub-wavelength scale, plasmonic nanomaterials disclose a broad range of applications in nanophotonics [1,2,3,4,5,6]
This comes from the fact that the optical characteristics of proposed plasmonic perfect absorber (PPA) support excellent impedance matching from air to the metal–air composite; such an elevated absorptance is achieved in visible and near-infrared regions
We can substantiate that mode 2 is mainly corresponded to the interaction between the incident EM wave, gaps among nanometals and cavity parts in the silver-shell nanorods (Fabry–Perot resonance), which is related to the localized Surface plasmon resonance (SPR), gap plasmon resonance (i.e., GPR) and cavity plasmon resonance (i.e., CPR)
Summary
With the ability of efficaciously transforming electromagnetic (EM) waves from free space into the sub-wavelength scale, plasmonic nanomaterials disclose a broad range of applications in nanophotonics [1,2,3,4,5,6]. Several PPAs based on all-metal or composite MNP-dielectric nanorod structures for refractive index sensing application have been designed [52,53] These nanosensors operated only in a single-band, restricting the working spectrum, especially, ranging in both visible and near-infrared regions. We demonstrate strongly enhanced EM wave distribution on the gaps, metal-shell cavities, Nanomaterials 2020, 10, 493 metal and cross-hair surfaces, which can simultaneously support the SPR, GPR, CPR and LPR modes and attain dual-band unity absorptance with high tunability and sensitivity. The physical origin of dual-band perfect absorption peaks is related to the SPR, GPR, CPR and LPR modes that simultaneously occur in the proposed PPA based on the plasmonic effect arising from the cross-hair/nanorod combination. We believe that the proposed structure gives a novel inspiration for the design of a tunable dual-band absorber, which can be applied to plasmonic sensors and other nanophotonic devices
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