Sensitivity Enhancement of THz Metamaterial by Decoupling its Resonance from the Substrate's Fabry–Pérot Oscillations

LC-circuit-based resonant metamaterials (MM) operating in the terahertz (THz) are proven to detect the presence of nanoparticles within the capacitive gap of nanoantennas, manifesting a red-shift in the resonance frequency. Sensitivity reduction (reduced red-shift) due to the interaction/coupling of the substrate's Fabry–Pérot (FP) oscillations with the MM resonance is discussed. This new discovery of coupling is more probable and intense in thicker semiconductor substrates used in standard complementary metal-oxide semiconductor processes, due to the high density of the FP oscillations and thus the probability for coupling with the single MM resonance increases. This also results in reducing the quality factor (Q-Factor) of MM resonance, as well as the dielectric response to nanoparticles spread on the MM surface, by reducing the resonance frequency shift (ΔF) and thus the sensitivity of resonant MM. A sensitivity restoration of the MM resonance's red-shift by decoupling it from the FP oscillations after thinning down the substrate by standard backside polishing is shown. The research is based on a combination of rigorous CST system-level simulations along with THz impedance spectroscopy laboratory experiments: up to a fivefold enhancement of sensitivity of the thinned substrate compared with the conventional thick substrates is shown. This work has potential applications in the high-sensitivity detection of nanomaterials and bio-sensing at ultra-low concentrations.