Metal-free ultrathin terahertz absorber with independently tunable dual narrow bands

A technique is introduced to precisely control the resonance behaviour of a metal-free graphene-based terahertz absorber by independently tuning the dual resonant peaks. The proposed ultrathin absorber features a multilayer configuration, with two isolated resonator layers of patterned graphene on dielectric (SiO2) substrates and a thin graphite sheet at the bottom serving as a reflector. The stacked arrangement enables independent tunability of the high-absorptivity resonant peaks at 7.33 and 9.34 THz. The structure, with a thickness of just lambda/15 of the free-space wavelength, offers a compact design suitable for space-constrained applications. Its symmetrical geometry ensures polarization insensitivity and stable performance for incident angles up to 60 degrees. Simulated results, analyzed via CST Studio and validated with an equivalent circuit model, demonstrate excellent thermal stability. Furthermore, the narrowband response of the proposed absorber improves its sensitivity to refractive index (RI) variations induced by biomolecular interactions, validating its suitability in biosensing applications. The absorber demonstrates peak absorption across both the resonant frequencies with an analyte optimized at 1.5 mu m thickness. Sensitivity levels of 1.1 and 1.05 THz/RIU along with figure-of-merit values of 2.11 and 2.23 are recorded for the lower and upper bands, respectively. The absorber offers enhanced selectivity due to low values of full width at half maximum. High Q-factors of 12.85 and 19.3 confirm its strong potential for RI sensing.