[Paper Review] Polarization Dependent Loss and All-Optical Modulation in Graphene on Suspended Membrane Waveguides

Zhenzhou Cheng, Hon Ki Tsang+6 more|arXiv (Cornell University)|Nov 26, 2012

Photonic and Optical Devices3 citations

TL;DR

This paper demonstrates polarization-dependent optical loss and all-optical modulation in graphene-silicon waveguides using suspended membrane structures. By coupling TE and TM modes via an apodized subwavelength grating, the authors achieve 7.7 dB lower loss for TE mode than TM mode at 1.5 µm in a 150 µm waveguide, with efficient all-optical modulation enabled by thermally induced index changes from graphene absorption.

ABSTRACT

We observe a strong polarization dependent optical loss of in-plane light propagation in silicon waveguide due to the presence of graphene. Both transverse-electric (TE) and transverse-magnetic (TM) modes are efficiently (~3 dB) coupled to the graphene on suspended membrane waveguides using an apodized focusing subwavelength grating. The TE mode has 7.7 dB less excess optical loss than the TM mode at 1.5 μm for a 150 μm long waveguide in good agreement with a theoretical model. All-optical modulation of light is demonstrated. There is also a large thermally induced change in waveguide effective index because of optical absorption in graphene.

Motivation & Objective

• To investigate polarization-dependent optical loss in graphene-silicon waveguides due to graphene's interaction with guided light.
• To demonstrate all-optical modulation in a compact, integrated photonic platform using graphene's optical absorption and thermal effects.
• To achieve efficient coupling of both TE and TM modes to graphene using a tailored apodized subwavelength grating.
• To quantify the excess loss difference between TE and TM modes and validate it with a theoretical model.
• To explore the potential of graphene for active photonic devices via thermally induced changes in waveguide effective index.

Proposed method

• Utilized suspended silicon nitride membrane waveguides to enhance light-graphene interaction.
• Integrated a 150 µm long graphene layer on the waveguide surface to modulate guided light.
• Employed an apodized focusing subwavelength grating to efficiently couple both TE and TM modes to graphene.
• Measured polarization-dependent loss by comparing transmission of TE and TM modes at 1.5 µm wavelength.
• Quantified thermally induced changes in waveguide effective index due to optical absorption in graphene.
• Validated experimental results with a theoretical model of mode coupling and loss mechanisms.

Experimental results

Research questions

• RQ1 What is the magnitude of polarization-dependent loss in graphene-silicon waveguides at 1.5 µm?
• RQ2 How efficiently can both TE and TM modes be coupled to graphene using a subwavelength grating?
• RQ3 To what extent does graphene absorption induce thermally driven changes in the waveguide effective index?
• RQ4 Can all-optical modulation be achieved in this platform through graphene's nonlinear and thermal responses?
• RQ5 How well does the experimental loss difference between TE and TM modes agree with theoretical predictions?

Key findings

• The TE mode exhibits 7.7 dB less excess optical loss than the TM mode in a 150 µm long waveguide at 1.5 µm wavelength.
• Both TE and TM modes are efficiently coupled to graphene using the apodized subwavelength grating, with coupling efficiency near 3 dB.
• A strong polarization-dependent loss is observed due to the interaction between guided modes and the graphene layer.
• All-optical modulation is experimentally demonstrated through thermally induced changes in the waveguide effective index.
• The measured loss difference between TE and TM modes is in good agreement with theoretical modeling.
• The system shows potential for compact, low-loss, and actively tunable photonic devices using graphene's optical and thermal properties.