A Differential Active SLSP-Based Microwave Sensor for Liquid Characterization

A high-sensitivity differential microwave sensor based on spiral spoof localized surface plasmon (SLSP) is designed for liquid characterization. Two polydimethylsiloxane (PDMS) microfluidic channels are attached to the etched SLSP resonators. The spiral SLSP resonator structure has a diameter of less than 1/38 of wavelength, thus possessing deep sub wavelength property and strong binding ability to the electric field. The electric field is concentrated on the SLSP slot to allow strong field–liquid interaction. The differential configuration is employed to suppress the influence of environmental factors, and the liquid permittivity can be retrieved by the difference between two resonance frequencies. The differential SLSP sensor is fabricated and verified by experiment, and the results demonstrate that the proposed sensor possesses both a high sensitivity of 0.52% and a compact size (
$0.13\lambda _{{0}}\,\,\times \,\,0.09{\lambda }_{{0}}$
). Further, an active amplification circuit is introduced to increase the magnitude of resonance (
$\sim $
10 dB), thus improving the sensing accuracy.