Subwavelength Graphene-Based Plasmonic THz Switches and Logic Gates

In this paper, we report on the design procedure for developing subwavelength graphene-based plasmonic waveguide, performing as a THz switch or an AND/OR logic gate. The propagation length of the surface plasmons (SPs), stimulated by a 6 THz TM polarized incident wave along this waveguide with a top graphene layer whose chemical potential is held at <formula formulatype="inline"><tex Notation="TeX">$\mu_{\rm C}=300~{\hbox{meV}}$</tex> </formula> (ON state) is more than 35 times larger than that in the waveguide with <formula formulatype="inline"><tex Notation="TeX">$\mu_{\rm C}= 0~{\hbox{eV}}$</tex></formula> (OFF state). Numerical results, obtained from full wave simulations using a finite element method, also show that the modulation depth density obtained for the straight plasmonic switching waveguide, whose length is just about 20% of the incident wavelength, is larger than those reported to date. Moreover, we also designed a logic AND gate composed of a straight waveguide, a Y-branch switch, and a logic OR gate composed of two face to face Y-branches, whose total lengths are <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{37\%}}$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{45\%}}$</tex> </formula>, and <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{53\%}}$</tex></formula> of the incident wavelength, respectively. Simulations show that the maximum ON/OFF ratios for these subwavelength plasmonic waveguides that occur between their ‘1 1’ and ‘0 0’ logical states are <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{41.37}}$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{39.87}}$</tex></formula>, and <formula formulatype="inline"><tex Notation="TeX">${\sim} {\hbox{40.76}}~{\hbox{dB}}$</tex> </formula>, respectively. These numerical data also show that the modulation depth densities obtained for these devices are also greater than those reported to date. The proposed graphene-based plasmonic switches and gates offer potential building blocks for the future digital plasmonic circuits operating around 6 THz.