Abstract
Graphene material, due to its unique conductivity and transparency properties, is utilized extensively in designing tunable terahertz perfect absorbers. This paper proposes a framework to design a tunable terahertz perfect absorber based on fractal triangle-shaped graphene layers embedded into dielectric substrates with the potential for spectral narrowing and widening of the absorption response without the need for geometric manipulation. In this way, the absorption cross-section spectra of the suggested configurations are achieved over the absorption band. First, the defection impact on the single-layer fractal triangle-shaped graphene structure inserted in insulators of the absorber is evaluated. Then, a flexible tunability of the absorbance’s peak is indicated by controlling the Fermi energy. By stacking fractal graphene sheets as a double graphene layer configuration in both the same and cross-states positioning, it is demonstrated that the absorption characteristics can be switched at 6–8 THz with a stronger amplitude, and 16–18 THz with a lower intensity. The impact of changing the Fermi potentials of embedded graphene layers is yielded, resulting in a plasmonic resonance shift and a significant broadening of the absorption bandwidth of up to five folds. Following, the absorption spectra related to the fractal triangle-shaped structures consist of a multi-stage architecture characterized by a spectral response experiencing a multiband absorbance rate and an absorption intensity of over 8 × 106 nm2 in a five-stage perfect absorber. Ultimately, the variations of the absorbance parameter and plasmonic mode under rotating the graphene sheet are explored for single and double fractal triangle-shaped perfect configurations on the absorption band. The presented mechanism demonstrates the tunability of the absorption spectrum in terms of narrowing or broadening and switching the plasmonic resonance by configuring multi-stage structures that can employ a broad range of applications for sensory devices.
Highlights
IntroductionAbsorbers [7,8,9,10] have attracted particular attention due to their optical performance in a wide range of applications, including spectroscopy [11], biosensing [12,13], optical control [14,15], and solar cells [16,17,18]
It is shown that the absorbance level of a fractal absorber developed by a double graphene layer at a similar-state insertion, compared to a monolayer graphene structure, possesses a significant amplification of both the absorption intensity and the shifting of the plasmonic resonance on the absorption band
It denotes that embedding fractal graphene layers into the cross-state positioning configuration provides two separate plasmonic modes with fewer absorption amplitudes than the same-state insertion, due to the admittance mismatching of the graphene sheets
Summary
Absorbers [7,8,9,10] have attracted particular attention due to their optical performance in a wide range of applications, including spectroscopy [11], biosensing [12,13], optical control [14,15], and solar cells [16,17,18] In this context, using conductive structures with sub-wavelength dimensions has the potential to generate localized surface plasmon resonances (LSPR). Due to the role of fractal theory in a wide range of applications, considering this concept in the design of optoelectronic devices leads to high-sensitive absorbers based on the fractal configurations in emerging THz technologies appearing. Interesting results are extracted using the DC voltage bias employed by external sources on the configuration of the proposed multilayer absorbers, which means tuning the amplitude and frequency of the absorption and narrowing or widening the absorption bandwidth of the spectral response and multiband absorption
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