Adaptive X-Band Permittivity Sensing Using a Waveguide-Integrated 3D-Printed All-Dielectric Slotted 1-D Photonic-Crystal Resonator

This paper presents an all-dielectric, 3D-printed permittivity sensor based on a slotted one-dimensional photonic crystal (1D PhC) resonator embedded inside a standard WR90 waveguide. The basic resonator is made of alternating cyclic olefin copolymer (COC, εr= 2.3) and air layers forming Bragg reflectors around a central defect. Among the supported resonant modes, the even mode is utilized for sensing, as it exhibits a high electric-field confinement at the defect region with additional lower-power side lobes distributed in the COC layers along the resonator axis. The operating principle of the proposed sensor relies on introducing five thin longitudinal slots: a central slot located at the field maximum and two pairs of side slots positioned at locations where the electric field side-lobes are approximately 5 dB and 10 dB lower. By selectively filling individual slots or slot combinations with the material under test, the sensitivity can be adaptively tailored to materials with different permittivity and loss. Higher perturbation is achieved by filling slots at high-field regions in order to enhance the sensitivity for low-loss and low-permittivity materials such as polymers. In contrast, lower-power perturbation reduces the interaction with high-loss materials, which preserves resonance visibility and prevents excessive attenuation in the measured response. Moreover, each selected slot configuration produces a distinct resonance shift, enabling multi-response permittivity sensing from a single resonant mode excitation. Full-wave simulations and experimental measurements using several solid and liquid materials validate the proposed concept, demonstrating reliable material discrimination and adaptable sensitivity. A high sensitivity of 320 MHz per unit permittivity is achieved when all slots are utilized.