Design of a Differential Spoof Surface Plasmon Sensor for Dielectric Sensing and Defect Detection

A new design of a highly confined Spoof Surface Plasmon (SSP) line splitter-based differential phase sensor is presented for the THz frequency region. The position, orientation and shape of the stem cells are properly optimized to realize highly confined, compact SSP design. The designed differential splitter facilitates the relative phase difference between the transmission coefficient phase corresponding to the reference arm and the test arm. The dispersion characteristic i.e, asymptotic frequency response of stem shaped unit cell founds to be quite improved over its conventional counterpart. The incorporated splitter shaped topology provides a symmetric differential sensing environment in both THz and MW region. The proposed sensor facilitates quite high sensitivity due to its improved electric field confinement than that of the microstrip counterpart. The obtained phase difference changes with a relative change in the effective dielectric constant of the test sample at a specified frequency. The design is then scaled accordingly, and the prototype is fabricated to validate its defect detection performance in the microwave (MW) frequency region. In the present scenario, FR-4 samples (with thickness 0.8 mm, 1.6 mm) having two different defect profiles are considered for experimental validation. It is found that the developed sensor can produce substantial phase difference even for the small presence of the defect. In the present case, the measured average phase differences corresponding to the FR-4 sample having 1 mm void distributed uniformly with an interspacing of 1.25 mm, are 74.02° and 92.35° and 2.5 mm spacing between the same void gives phase difference of 31.18° and 40.26° for 0.8 mm and 1.6 mm sample thickness, respectively over the frequency range of 8–12 GHz.