Abstract
We introduce broadband waveguide absorbers with near unity absorption. More specifically, we propose a compact non-parity-time-symmetric perfect absorber unit cell, consisting of two metal-dielectric-metal (MDM) stub resonators with unbalanced gain and loss side-coupled to a MDM waveguide, based on unidirectional reflectionlessness at exceptional points. With proper design, light can transport through the perfect absorber unit cell with reflection close to zero in a broad wavelength range. By cascading multiple unit cell structures, the overall absorption spectra are essentially the superposition of the absorption spectra of the individual perfect absorber unit cells, and absorption of ~ 100% is supported in a wide range of frequencies.
Highlights
Near-perfect absorbers show great promise for photovoltaics, thermal emission, invisibility and interferometric applications [1,2,3,4,5,6]
We use a non-PT-symmetric plasmonic-waveguide cavity system, consisting of two MDM stub resonators side coupled to a MDM waveguide [Fig. 2(a)], to realize perfect absorption based on the theory discussed in the previous section
Among different plasmonic waveguiding structures, MDM plasmonic waveguides, which are the optical analogue of microwave two conductor transmission lines [13], are of particular interest [35,36,37,38,39,40], because they support modes with deep subwavelength scale over a very wide range of frequencies extending from DC to visible [41], and are relatively easy to fabricate [42]
Summary
Near-perfect absorbers show great promise for photovoltaics, thermal emission, invisibility and interferometric applications [1,2,3,4,5,6]. In the past few years, it has been proposed using parity-time (PT) symmetric optical structures to attain broadband unidirectional light reflectionlessness at exceptional points by careful modulation of the refractive index profile [1522]. Unidirectional light reflectionlessness can be attained in non-PT-symmetric structures with unbalanced gain and loss (or even without gain) [23,24,25,26,27,28,29] This is due to the fact that exceptional points exist in a larger family of nonHermitian Hamiltonians [23]. We propose a compact non-periodic perfect absorber unit cell, based on the unidirectional reflectionless propagation at exceptional points. In Subsection 3.1, we use this theory to design a perfect absorber unit cell structure based on a non-PT-symmetric plasmonic waveguide system.
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