Bandwidth-modulated SSPP-SIW hybrid filter integrated with graphene using spin-coated ion gel

Abstract Spoof surface plasma polaritons (SSPPs) and substrate integrated waveguides (SIWs) are compact waveguiding structures with high and low cut-off responses, respectively, which together enable a new kind of non-resonant bandpass filters. Recently, tunable SSPP and SIW devices on printed circuit boards (PCBs) with surface-mounted graphene have gained considerable interest for attenuation modulation with graphene intrinsic absorption. In this work, a SIW-SSPP hybrid filter is integrated with graphene in a new configuration on PCBs, achieving tunable bandwidth at Ka-band. The proposed bandpass filter consists of a microstrip-fed SIW and periodic SSPP slots on its top electrode. Graphene is designed on a polyimide carrier to sweep the SSPP effective permittivity over the slots, which red-shifts the SSPP cut-off frequency and enahnces the slow-wave effect significantly. Therefore, the graphene absorption of slow waves are significantly enlarged. A novel integration method is developed to combine the wet-transferred graphene with the hybrid filter by spin-coating a uniform ion gel layer stably. Also, ion gel is used to gate graphene and change its chemical potentials. Under a low bias voltage sweeping from −0.5 to 1 V, the 3 dB bandwidth of the proposed filter is continuously modulated from 45% to 14%, and the corresponding transmittance modulation is as large as 20 dB at 30 GHz. Such approach provides a novel integration method for graphene and other 2D materials on both PCBs and semiconductor substrates and paves a new way to modulate the frequencies of SSPP devices.