Abstract
Plasmonics offer an exciting way to mediate the interaction between light and matter, allowing strong field enhancement and confinement, large absorption and scattering at resonance. However, simultaneous realization of ultra-narrow band perfect absorption and electromagnetic field enhancement is challenging due to the intrinsic high optical losses and radiative damping in metals. Here, we propose an all-metal plasmonic absorber with an absorption bandwidth less than 8 nm and polarization insensitive absorptivity exceeding 99%. Unlike traditional Metal-Dielectric-Metal configurations, we demonstrate that the narrowband perfect absorption and field enhancement are ascribed to the vertical gap plasmonic mode in the deep subwavelength scale, which has a high quality factor of 120 and mode volume of about 10−4 × (λres/n)3. Based on the coupled mode theory, we verify that the diluted field enhancement is proportional to the absorption, and thus perfect absorption is critical to maximum field enhancement. In addition, the proposed perfect absorber can be operated as a refractive index sensor with a sensitivity of 885 nm/RIU and figure of merit as high as 110. It provides a new design strategy for narrow band perfect absorption and local field enhancement, and has potential applications in biosensors, filters and nonlinear optics.
Highlights
Plasmonics offer an exciting way to mediate the interaction between light and matter, allowing strong field enhancement and confinement, large absorption and scattering at resonance
Substantial absorbers based on different physical mechanisms have been demonstrated theoretically and experimentally in a wide spectral range, which can be generally categorized into broadband absorbers[14,15,16] and narrowband absorbers[17,18,19,20,21] in terms of their absorption bandwidth
While broadband absorbers are generally used in thermo-photovoltaics[22], narrowband perfect absorbers can be used in sensing[9,19,20], absorption filtering[23] and thermal radiation tailoring[24,25]
Summary
Both the narrowband electric and magnetic fields are strongly enhanced in the same spatial region (both E and B enhancements are relatively large at a height t/2 over the silver film, and their hot spots have spatial overlap at around t/2) This can be ascribed to the vertical gap plasmonic mode. This is much like the vertical Split Ring Resonators[41]. Such narrowband resonators will find applications in thermo-photovoltaics, biosensors, nonlinear plasmonics and lasers
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