“Design and detailed study of graphene based plasmonic waveguide”

Electronics industry is dictated by Moore’s law since for more than 50 years. With the size of device reaching nano regime the restriction on the speed of device is coming close to being restricted by the electron speed than physical geometry. Photonics gives a solution to this problem by using photons instead of electrons. However, photonic circuits are very large in size. The need for more speed with smaller footprint is driving the efforts in developing Plasmonics field which combines the advantages of phonics and electronics. However, the need of material which can support longer distance of transmission of plasmons is needed. In the wonder material “graphene” the plasmons are inherent and are easily tunable, it serves as a excellent alternative to conventional plasmonic material. In this thesis work the Surface Plasmon Polaritons(SPP) waveguide using graphene nanoribbon are investigated in terms of its propagation characteristics such as propagation constant, S-parameters, maximum power and figure of merit considering length of propagation and wavelength of surface plasmon. The effect of external electric field on the SPP attribute is explored. Application oriented studies for presence of antidots in waveguide and coupling of plasmons with CNT to be used as plasmonic vias is investigated.

COMSOL Simulation

Lumerical FDTD Simulation

Conclusions:

1. Plasmonic waveguide utilizes evanescent waves confining light to sub-wavelength dimension,
2. Graphene improving the confinement advances the properties of waveguide.
3. The work done suggests the propagation range for plasmon, which depends mainly on chemical potential and the trade-off between real and imaginary part of permittivity.
4. Thickness consideration for simulation suggests a very small affect to the range. The penetration depth study is providing the evanescent leakage of electric field in dielectric. This in particular is important in the designing of plasmonic devices, providing how much miniaturization is possible.
5. Eminent operating frequency using FOM is calculated. In addition to this, best possible range of operating frequencies for wide band applications can be estimated.
6. The trend in field enhancement in power plot and the dispersion relation signifies that the behavior of surface plasmons changes from "photon-like" to "surface plasmon polaritons" to "localized surface plasmon", as the input frequency increases towards the plasma resonance frequency. The compromise between plasmon propagation and frequency of operation is suggested from it. A study of waveguide and edge mode suggested the effects of ohmic losses.
7. The electric field effects on plasmonic waveguide displayed the changes in amplitude for electric field perpendicular to nanoribbon surface which provides a method for amplitude modulation, and change in frequency in electric field parallel to the surface which provides a method for frequency modulation. Further qualitative analysis is needed for deciding the change in frequency due to electric field.
8. The study of large enhancement at the antidot site proposes its use in applications, such as realtime sensing and interconnects for plasmonic circuitry. This study also suggested tradeoff between chemical potential and enhancement needed for different dielectric used.
9. The FEM technique served better in analysis related to edge and waveguide modes and other propagation properties of waveguide.
10. The FDTD technique proved to be better in finding enhancements at the antidots sites, this is because FDTD technique considers scattering phenomena more precisely.
11. The study revealed that, Plasmonic waveguide made from the wonder material graphene shows a great potential for future technology transformations.