THz Metamaterial\nSensitivity Enhancement by Reduction\nof Substrate’s Fabry–Perot Oscillations Using\nBack Plates as an Optical De-Coupler

This work presents a new method for the enhancement of\nsensitivity\nin Terahertz (THz) spectroscopy on metamaterial (MM) in terms of its\nresonance frequency shift (Δ<i>F</i>), by attaching\nthe dielectric back plate to the MM’s silicon (Si) wafer. The\ndielectric back plates are designed to minimize the Fresnel reflections\nat the backside of the substrate, identical to a broadband antireflective\n(AR) plate tailored for THz. Utilizing broadband AR technology, we\ndemonstrate the concept of decoupling MM resonance from the substrate’s\nFabry–Pérot (FP) oscillations. This is done by effectively\ncoupling the THz light out of the high-permittivity substrate, resulting\nin the improved quality factor of the MM resonance and overall plasmonic\nenhancement on the metasurface. The back plate acts as a surface plasmonic\nenhancer to the THz MM by increasing the field intensity on the front\nmetasurface, leading to enhancement of dielectric response (MM’s\nΔ<i>F</i>). This makes the MM resonance ultrasensitive\nto the minor changes of particle size/concentration under test spread\non the metasurface, contributing to enhanced resonance Δ<i>F</i>. The plate also makes the Si substrate optically lossless,\nenabling the full effect of MM resonance shift and increasing the\nresonance Δ<i>F</i> by 8-fold compared with MM’s\nfabricated on conventional Si substrates. This research is backed-up\nwith system-level CST simulations and experimental THz impedance spectroscopy\nof the MM. This method and chip structure is CMOS compatible having\npotential applications for any resonant MM fabricated on a substrate\naimed to maximize dielectric sensitivity for biosensing and nanoparticle\nTHz spectroscopy.