Tunable asymmetric Mach-Zehnder interferometer based on spoof surface plasmon polaritons

In this paper, an ultrathin asymmetric Mach-Zehnder interferometer (MZI) based on spoof surface plasmon polaritons (SSPPs) is proposed. The asymmetry in the MZI structure induces a phase difference between the SSPPs waves propagating in the upper and lower arms of the device, leading to interference valleys in the output signal. Additionally, by loading two varactor diodes on the lower arm of the MZI, the position of the interference valleys can be manipulated by varying the bias voltage on the diodes. When the bias voltage applied to the varactor diodes decreases (capacitance increases), there is a remarkably significant redshift in the spectral positions of the three interference valleys within the 5 - 8 GHz frequency band. The transmission coefficient at 5.322 GHz also changes from -19.62 dB to -5.89 dB, and the transmission state switches from the off-state to the on-state. Meanwhile, varactor diodes can be used to modulate the phase of transmitted signals. When the capacitance increases from 0.55 pF to 1.79 pF, the interference valley frequency shifts from 6.325 GHz to 6.196 GHz, and the corresponding phase increases from 137 ° to 180 °, with a modulation range of 43 °. The designed model was processed and measured, and the actual measurement results showed that the position of the interference valley would change with the variation of voltage. Comparing and analyzing the simulation and actual measurement results, it is found that the two correspond well, verifying the rationality and authenticity of the design and proving the reconfigurable characteristics of the proposed SSPPs MZI.