[Paper Review] Graphene nano-ribbon waveguides

Songbai He, X. Zhang+1 more|arXiv (Cornell University)|May 28, 2013

Plasmonic and Surface Plasmon Research13 references3 citations

TL;DR

This paper proposes a low-loss graphene nano-ribbon waveguide structure integrating a graphene layer, silica buffer, and silicon substrate to enable efficient terahertz plasmonic waveguiding at 30 THz. It demonstrates single-mode operation, reduced propagation loss, and a high figure of merit, with coupled ribbon configurations enabling ultra-small nano-ring cavity designs for integrated photonic circuits.

ABSTRACT

Graphene as a one-atom-thick platform for infrared metamaterial plays an important role in optical science and engineering. Here we study the unique properties of some plasmonic waveguides based on graphene nano-ribbon. It is found that a graphene ribbon of finite width leads to the occurrence of coupled edge mode. The single-mode region of a single freestanding graphene ribbon is identified at a fixed frequency of 30 THz. A low-loss waveguide structure, consisting of a graphene layer, a silica buffer layer and silicon substrate is proposed to reduce the propagation loss and obtain a high figure of merit for future integration of waveguide devices. Furthermore, two coupled ribbon configurations, namely, side-side coupling and top-bottom coupling, are investigated. As a device example, a nano-ring cavity of ultra-small size is designed.

Motivation & Objective

• To design a low-loss, high-performance plasmonic waveguide using graphene nano-ribbons for terahertz applications.
• To reduce propagation loss in graphene-based waveguides through a multilayer structure with silica buffer and silicon substrate.
• To identify the single-mode operating region of a freestanding graphene ribbon at 30 THz.
• To investigate coupled ribbon configurations (side-side and top-bottom) for enhanced mode control and device integration.
• To demonstrate a practical device application via an ultra-small nano-ring cavity design.

Proposed method

• A multilayer waveguide structure is proposed, consisting of a graphene layer, a silica buffer layer, and a silicon substrate to suppress radiation and propagation losses.
• Theoretical modeling is performed using electromagnetic wave equations to analyze surface plasmon polaritons in finite-width graphene ribbons.
• The single-mode operation region is identified by solving the dispersion relation for a freestanding graphene ribbon at a fixed frequency of 30 THz.
• Two coupled ribbon configurations—side-by-side and top-bottom—are analyzed to study mode hybridization and field confinement.
• A nano-ring cavity is designed by closing a coupled ribbon waveguide structure to achieve subwavelength mode confinement.
• The figure of merit (FoM) is calculated as a key performance metric to evaluate waveguide efficiency and integration potential.

Experimental results

Research questions

• RQ1 What is the single-mode operating bandwidth of a freestanding graphene nano-ribbon at 30 THz?
• RQ2 How does the inclusion of a silica buffer and silicon substrate affect propagation loss in graphene waveguides?
• RQ3 What are the field confinement and mode characteristics in side-by-side and top-bottom coupled graphene ribbon configurations?
• RQ4 Can a low-loss, high-figure-of-merit waveguide be achieved in a compact, integrable geometry?
• RQ5 What is the feasibility of realizing an ultra-small nano-ring cavity using coupled graphene ribbons?

Key findings

• A single-mode region is identified for a freestanding graphene ribbon at 30 THz, enabling stable, low-loss propagation of surface plasmon polaritons.
• The proposed multilayer structure (graphene/silica/silicon) significantly reduces propagation loss compared to freestanding configurations.
• The figure of merit (FoM) is enhanced due to improved mode confinement and reduced radiative losses, enabling better integration potential.
• Coupled ribbon configurations (side-side and top-bottom) support hybridized modes with strong field localization, suitable for compact devices.
• An ultra-small nano-ring cavity is successfully designed with subwavelength dimensions, demonstrating potential for on-chip photonic integration.
• Theoretical analysis confirms that the waveguide supports high-quality factor modes with low propagation loss, validating its suitability for nanophotonic applications.