Theoretical analysis of plasmonic unidirectional propagation at visible frequency based on subwavelength waveguide

A quasi-plasmonic circulator is proposed based on metal-insulator-metal waveguides. Four bus waveguides connected to the corners of a square cavity with dual-narrow-slit waveguides are performed as the input/output ports. According to the enhanced emission and the interference effects, unidirectional propagation for surface plasmon polaritons is achieved. The spectrum and propagation characteristics are numerically investigated by using the finite-difference time-domain method. The transmittance for the matched output port is higher than 71% at the wavelength of 481.2 nm, while the ones for other output ports are all lower than 6%. Therefore, high-transmission and high-isolation are achieved. In addition, the linear relationship between the wavelength and the waveguide length or the refractive index of insulator has also been confirmed.