Koch-curve fractal plasmonic nanoantennas on VO₂ for near-field plasmonic switching

Abstract We propose fractal plasmonic gold nanoantennas (FPNs) based on the Koch-curve fractal on top of a thin layer of vanadium dioxide (VO 2 ), on a SiO 2 substrate for near-field plasmonic switching. The proposed FPN on the VO 2 thin layer is based on two gold Koch-curve based fractal plasmonic nanopillar segments present next to each other with a small gap between them. The thin VO 2 layer below the FPNs is used as the phase change material for active near-field switching in this fractal plasmonic nanoantenna. The application of heat, electric field or optical radiation leads to the phase transition of the underlying VO 2 layer from its semiconductor to its metallic state, thus leading to an active switching of the field between the two segments of the Koch-curve FPNs from a low (OFF) to a high (ON) value. Exhaustive numerical analysis is carried out by employing Finite Difference Time Domain (FDTD) analysis to determine the E-field enhancement of the proposed switch with VO 2 in its semiconductor (OFF state of the switch) and metallic (ON state of the switch) phase. The intensity switching ratio (ISR) is reported as the ratio of E-field enhancement at the center of the proposed Koch-curve FPNs with VO 2 in its metallic (ON) state to that with VO 2 in its semiconducting (OFF) state. It is shown that, as the number of iterations of the Koch-curve fractal plasmonic antennas is increased, the ISR increases from ∼ 22 for iteration 0 to ∼ 2500 for iteration 2. In addition, spectral tunability is demonstrated by varying the geometrical specifications like the length of the proposed Koch-curve FPN. Moreover, the effect of the thickness of the Koch-curve and the thickness of the VO 2 film for different iterations of the proposed FPN based switch is systematically analysed to obtain high ISRs at the desired wavelength.