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Finite element characterisation of graphene-silicon hybrid waveguides

Mathivanan, Sathya Narayanan (2019). Finite element characterisation of graphene-silicon hybrid waveguides. (Unpublished Doctoral thesis, City, University of London)

Abstract

Recent years have witnessed remarkable progress in nanoscience and nonlinear optics research, paving entry of 2D materials such as graphene, phosphorene, silicene, and so on. Graphene has extraordinary optical, electrical, mechanical and thermal properties. The uniqueness of graphene lies in its modulation depth (> 90%) and signal attenuation (< 5%), achieved by controlling its Fermi level through gate voltage. It is envisioned that graphene-based modulators would be the ultimate high-speed modulator of the future, performing at speeds up to 10 times faster than existing ones.

The accuracy of such solutions lies in determining the permittivity (εr) of materials used. Graphene possesses complex conductivity (Ϭ(ω) = Ϭ1 + jϬ2) and permittivity (ε(ω) = ε1 + jε2). Using Kubo formalism, we derived an analytical method for finding the conductivity of graphene (Ϭg,interband +Ϭg,intraband) and then the complex permittivity (εg). The values of ε(ω) are plotted as a function of chemical potential (μ, eV), wavelength (λ, nm) and thickness of graphene layers (t(g)).

Benchmarking was carried out using two solvers viz., complex and perturbation to ascertain the suitability of the method. Effective mode index (n(eff)) and absorption (α) are calculated for quasi-TE and quasi-TM guided modes of the waveguide. We found that the waveguide performance parameters are highly influenced by the position of graphene layers in the waveguide and the thickness and type of dielectric material that encapsulate the graphene layers. Two positions viz., graphene-as-top layer and graphene-as-slot layer were analysed. Three dielectric materials, hBN, Al2O3 and HfO2, classified as low-, high-and very-high index, respectively, are chosen. For operation wavelength range (1.3{1.7μm) and for varying dielectric layer thickness from 5 to 70 nm, the plots for neff and α (dB/μm) are obtained.

Performance parameters such as extinction ratio (ER) and insertion loss (IL) were calculated for varying dielectric thickness (5-70 nm). ER and IL are achieved within the ranges 20-70 and 3-4 dB/μm, respectively. We inferred that to enhance the modal properties (n(eff) and α), graphene-as top layer waveguide can have a combination of very high-index (εr = 25) dielectric material (thickness > 15 - 20 nm) encapsulating graphene whereas a very high-index dielectric with reduced thickness (< 5 - 10 nm) encapsulating graphene is suitable for graphene-as-slot layer waveguide.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Electrical & Electronic Engineering
URI: http://openaccess.city.ac.uk/id/eprint/22237
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