A Liquid-Crystal, Tunable, Ultra-Thin Fabry-Perot Resonator in Ka Band

A tunable, liquid-crystal (LC)-based, subwavelength Fabry-Perot (FP) resonator has been designed and fabricated in the 27-GHz band. The grid mirrors are identical and consist of metallic capacitive meshes characterized by a high filling ratio. This ensures simultaneously a high reflectivity and a strong negative phase of the reflection coefficient on the mirror so as to reduce the resonator thickness as much as possible. The resonator has been filled with a 110-$\mu$ m thick E3 nematic LC. A frequency shift of 6% has been obtained numerically (finite-difference time-domain (FDTD) simulations) using permittivity values measured under 1-T driving magnetic field $(\epsilon _{//}= 2.91, \epsilon_{\perp}= 2.52)$ . An experimental tunability of 2% has been obtained by applying a 10-kHz ac bias voltage of 10 V. This lower value is due to the low driving field $({\sim 0.1}~{\hbox{V}}/ \mu {\hbox{m}})$, the nonperfect polyvinylalcohol (PVA) layer, and the relatively high thickness of the E3 mixture.