Design And Application Of Passive/Active Mach-Zehnder Interferometers Based On Spoof Surface Plasmon Polaritons

Spoof surface plasmon polariton(SSPP)in microwave and terahertz frequencies successfully imitates the strong confinement of the surface plasmon polariton(SPP)in optical frequencies,thereby realizing strong confinement to the electromagnetic field.When SSPP propagates along the structured metal-medium interface,the energy is tightly confined in subwavelength regime perpendicular to the interface and have an enhancement,which makes SSPP supremely sensitive to changes in dielectric environment around the interface.The MachZehnder interferometer(MZI)has a symmetrical physical structure.When its two arms are in the same dielectric environment,the entire interferometer is equivalent to an ordinary transmission line.As one arm’s dielectric environment changes,the electromagnetic waves transmitted on the two arms will have a phase difference,leading to interference of the two coupled waves.Tiny change in dielectric environment can cause significant change in output electromagnetic field amplitude;hence MZI also has high sensitivity.In this paper,the advantages of SSPP and MZI are superimposed,and passive and active MZI structures based on microwave SSPP has been proposed.The main innovations include:1.Passive MZI sensors based on SSPP have been proposed and designed.The SSPP MZI can modulate the amplitude and phase of the output electromagnetic wave by placing different dielectric samples on its sensing arm.Different relative permittivity of different samples will bring different interference patterns of the transmission amplitude curve,so the permittivity of the detected sample can be distinguished by the position of the output spectrum valley.This paper focuses on the sensitivity of the designed MZI sensor.It has been proved that after combining SSPP and MZI,the SSPP MZI is significantly more sensitive than the single-arm SSPP waveguide,and its sensitivity is greatly improved than that of the traditional MZI based on microstrip.This paper also discusses the influence of structural parameters of the detected sample and MZI arm length on the sensitivity,proving that the proposed MZI sensor based on SSPP can achieve a high sensitivity with a small size.2.Active reconfigurable MZI structures based on SSPP have been proposed and designed.By introducing a varactor diode on some SSPP units of the sensing arm and changing the bias voltage applied on the diode in real time,the amplitude and phase of the output electromagnetic wave can be manipulated dynamically.The output spectrum of MZI has multiple interference valleys and the position of the interference valleys is very sensitive to the diode capacitance change.Therefore,this active SSPP MZI can realize binary frequency shift keying(2FSK)with flexible selectable carrier frequency and continuous transmission amplitude modulation at multiple frequencies.Experiments have proved that the reconfigurable MZI devices proposed in this paper can successfully achieve 2FSK and continuous amplitude modulation,in which the transmission coefficient at a specific frequency can be adjusted from 0.05 to 0.88.