Elsevier

Optics Communications

Volume 323, 15 July 2014, Pages 49-53
Optics Communications

Discussion
The study of few-layer graphene based Mach−Zehnder modulator

https://doi.org/10.1016/j.optcom.2014.02.059Get rights and content

Abstract

The investigation of few-layer graphene embedded in silicon waveguide (GESW) exhibits it has a superior performance than monolayer GESW where the effective mode index variation (Δneff) at the given chemical potential change and the number of graphene layers N (N<5) shows in a linear relationship. The results from modal analysis show that the Δneff of quadri-layer GESW can be as large as 0.047, based on which a Mach–Zehnder modulator has been proposed with the advantages of only 16.5 μm arm length, low energy consumption (8 fJ/bit), high extinction ration (31.8 dB) and small applied voltage (<1 V). Furthermore, the working temperature of the proposed modulator can span in a large range from 300 K to 400 K and the modulation performance stays almost unchanged when the input wavelength varies from 1520 nm to 1580 nm.

Introduction

Graphene, the two-dimensional (2D) carbon sheet in honeycomb lattice with only one-atom-layer thickness has attracted lots of interest due to its remarkable mechanical, electric, magnetic thermal and optical properties. In the field of optics, graphene is an excellent platform to transfer surface plasmon in nanometer scale [1] owing to its large active-control of permittivity-function [2] that is not seen in noble metals. In the field of integrated optics, graphene-based fiber laser [3], detector [4] and modulator [5] have already been demonstrated both theoretically and experimentally.

However, most of previous publications [3], [4], [5] emphasized the fact that the one-atom-layer thickness of graphene׳s geometry must be satisfied to ensure graphene׳s unordinary effects. The monolayer graphene is not only difficult to exfoliate from graphite mechanically, but also has very limited light−graphene interaction because its thickness is only 0.335 nm. Some tried to embed several set of monolayer graphene in the same waveguide to enhance the light−graphene interaction, and reported the improved effect with a non-linear relationship [6].

In this paper, we propose to embed graphene in the silicon waveguide (GESW) and the study exhibits that the few-layer GESW has much better performance than the monolayer GESW. A novel Mach−Zehnder modulator with quadri-layer graphene is constructed and the results demonstrate the superior performance with the shorter arm length of 16.5 μm, low energy consumption (8 fJ/bit), high extinction ration (ER) of 31.8 dB and small applied voltage (<1 V). For modulators in long haul communication, the ability to work under extremely rough environments such as low/high temperature and under different wavelengths for (de)multiplexer purpose is of vital importance. Our proposed modulator has also shown the capacity of large temperature tolerance and can work over a large wavelength range.

Section snippets

Optical material property of the few-layer graphene

By defining an effective thickness of graphene, the permittivity of graphene can be derived from the conductivity [8]:ε=1+iδωε0Δd=1Im(δ)ωε0Δd+iRe(δ)ωε0Δdwhere δ is the conductivity of graphene, ω represents the light angular frequency, Δd=0.335nm is the thickness of the monolayer graphene. In the mid-infrared and terahertz range, the optical conductivity of the monolayer graphene is described by the Kubo formula, which can be expressed as follows [7]:δ(ω,μc,Γ,Τ)=ie2π2(ω+i2Γ)[0ε(fd(ε)εfd

TE mode MZI modulator

The enlarged Δneff of the GESW may be convenient for the MZI modulator. The top view and 3D view of MZI modulator based on quadri-layer GESW is shown in Fig. 5(a) and (b), respectively. The graphene sheet can be grown by the chemical vapor deposition (CVD) method and transferred on the silicon-on-insulator substrate, and electrodes can be positioned to connect with quadri-layer graphene and silicon layer.

According to interference theory, transmission of the proposed MZI modulator can be

Conclusion

We have proposed a MZI modulator with enhanced modulation ability by inserting a quadri-layer graphene inside the silicon waveguide. The major advantages of the proposed modulator include the small footprint (80 μm2), high extinction ratio(31.8 dB), low applied voltage (<1 V) and low energy consumption (8 fJ/bit). Moreover, the performance of the proposed modulator remains stable under the variation of temperature and wavelength of the incident light. All of these advantages may promote

Acknowledgements

This work was supported by the National Natural Science Foundation of China (61205054), Zhejiang Provincial Natural Science Foundation of China (Z1110330 and LQ12F05006), the Excellent Young Faculty Awards Program (Zijin Plan) at Zhejiang University.

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