Millimeter-wave spoof surface plasmon polariton waveguide with uniform-depth transition for liquid material detection

In this paper, we propose a spoof surface plasmon polariton (SPP) waveguide with uniform-depth transition at 500–730 GHz. It is excited by a standard waveguide with simple offsets in the vertical and longitudinal directions, eliminating the need for the conventional tapering-depth transition. Since the SPP waveguide with strong field confinement is sensitive to the surround environment, this SPP waveguide can be applied for the detection of liquid materials. The liquid material injected into the SPP grooves behaves as a resonator, leading to a stopband. The frequency resolution can reach 15 GHz for a variation of 0.1 in the dielectric constant of the liquid materials. To further verify the concept of the proposed SPP waveguide with uniform-depth transition, a scaled structure at 50–66 GHz frequency band is designed and fabricated. The sample was fabricated by 3D printing technique with surface metallization. The measured reflection coefficient is below -10 dB and the measured insertion loss is about -4 dB within 50–66 GHz.