Reconfigurable Mach-Zehnder interferometer for dynamic modulations of spoof surface plasmon polaritons

We propose an ultrathin reconfigurable Mach-Zehnder interferometer (MZI) for realizing dynamic frequency and amplitude modulations of spoof surface plasmon (SSP) signal. Active varactor diodes are integrated in the SSP unit cells on one of the MZI arms to introduce asymmetry to the MZI structure, which can control the interference patterns by varying bias voltages applied on the varactor diodes. We show that the spectral positions of multiple sharp interference dips are very sensitive to the change of diode capacitance, thereby allowing for good frequency modulation. We also demonstrate continuous amplitude modulation by tuning the varactor diodes at multiple selected frequencies. To verify the reconfigurable feature of the proposed SSP MZI, the frequency shift keying (FSK) and amplitude modulations have been experimentally demonstrated on the same structure. The modulation depth of the amplitude modulation can be further improved by designing geometrical parameters of the SSP structure, reaching a significant amplitude change from 0.88 to 0.05 in experiments.