Leaf-Petal Shaped Tunable Metasurface Perfect Absorber with Refractive Index Sensing Capability

This work presents a graphene-based tunable metasurface perfect absorber (MPA) with an absorptivity (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$A(f)$</tex>) of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{9 9. 8 6 \%}$</tex> at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3. 5 5 ~ T H z}$</tex>. The narrowband absorber provides a bandwidth of only 2.93 %. The proposed MPA is ultrathin at just <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.5 \mu ~\mathrm{m}$</tex>, equivalent to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{g} / 17.12$</tex>, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{g}$</tex> is the guided wavelength at 3.55 THz. The periodicity of the MPA is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8 \mu ~\mathrm{m}\left(\lambda_{g} / 5.35\right)$</tex>. The MPA is polarization-insensitive due to its symmetrical design and maintains performance up to a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{6 0}^{\circ}$</tex> incidence angle for both TM and TE polarizations. With an increase in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu_{c}$</tex> from 0.3 to 1 eV, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$A(f)$</tex> response shows a blue shift with a frequency tunability from 2.74 to 4.88 THz. The MPA also shows excellent sensing capability when loaded with analytes of different refractive indices <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(n)$</tex>. The proposed refractive index sensor has a sensitivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.35 \text{THz} / \text{RIU}$</tex> and a quality factor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 0. 3 6}$</tex>. Hence, the proposed work could be used as an absorber or refractive index (RI) sensor in the terahertz (THz) regime.