Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Harnessing plasma absorption in silicon MOS ring modulators


High-bandwidth, low-power and compact silicon electro-optical modulators are essential for future energy-efficient and densely integrated optical data communication circuits. The all-silicon plasma-dispersion-effect ring resonator modulator is an attractive prospect. However, its performance is currently limited by the trade-off between modulation depth and switching speed, dictated by its quality factor. Here we introduce a mechanism to leap beyond this limitation by harnessing the plasma absorption induced in a silicon metal–oxide–semiconductor waveguide to enhance the extinction ratio of a low-quality-factor, high-speed ring modulator. The fabricated devices demonstrate a modulation depth of ~27 dB for a bias of ~3.5 V. Modulation enhancement has been observed for operation frequencies ranging from kilohertz to gigahertz, with data modulation up to 100 Gbit s−1 on–off keying demonstrated, paving a way to the evolution of optical interconnects to 100 Gbaud and beyond per wavelength.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Integrated silicon MOS modulator.
Fig. 2: Static EO response of the MOS ring modulator.
Fig. 3: Frequency response of absorption enhancement.
Fig. 4: EO bandwidths of the inversion and accumulation modes.
Fig. 5: Optical-eye diagrams and EO bandwidth.

Data availability

The data that support the results within this paper are available in the Supplementary Information and at the University of Southampton repository ( Source data are provided with this paper.


  1. Reed, G. T., Mashanovich, G., Gardes, F. Y. & Thomson, D. J. Silicon optical modulators. Nat. Photon. 4, 518–526 (2010).

    Article  ADS  Google Scholar 

  2. Thomson, D. et al. Roadmap on silicon photonics. J. Opt. 18, 073003 (2016).

    Article  ADS  Google Scholar 

  3. Reed, G. T. et al. Recent breakthroughs in carrier depletion based silicon optical modulators. Nanophotonics 3, 229–245 (2014).

    Article  Google Scholar 

  4. Rahim, A. et al. Taking silicon photonics modulators to a higher performance level: state-of-the-art and a review of new technologies. Adv. Photonics 3, 024003 (2021).

    Article  ADS  Google Scholar 

  5. Wang, C. et al. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature 562, 101–104 (2018).

    Article  ADS  Google Scholar 

  6. He, M. et al. High-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s−1 and beyond. Nat. Photon. 13, 359–364 (2019).

    Article  ADS  Google Scholar 

  7. Abel, S. et al. Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon. Nat. Mater. 18, 42–47 (2019).

    Article  ADS  Google Scholar 

  8. Alexander, K. et al. Nanophotonic Pockels modulators on a silicon nitride platform. Nat. Commun. 9, 3444 (2018).

    Article  ADS  Google Scholar 

  9. Kieninger, C. et al. Ultra-high electro-optic activity demonstrated in a silicon-organic hybrid modulator. Optica 5, 739–748 (2018).

    Article  ADS  Google Scholar 

  10. Lu, G. W. et al. High-temperature-resistant silicon-polymer hybrid modulator operating at up to 200 Gbit s−1 for energy-efficient datacentres and harsh-environment applications. Nat. Commun. 11, 4224 (2020).

    Article  ADS  Google Scholar 

  11. Srinivasan, S. A. et al. 60 Gb/s waveguide-coupled O-band GeSi quantum-confined Stark effect electro-absorption modulator. In Optical Fiber Communication Conference (OFC) 2021, Dong, P. et al. (eds) (Optica Publishing Group, 2021);

  12. Mastronardi, L. et al. High-speed Si/GeSi hetero-structure electro absorption modulator. Opt. Express 26, 6663–6673 (2018).

    Article  ADS  Google Scholar 

  13. Srinivasan, S. A. et al. 56 Gb/s germanium waveguide electro-absorption modulator. J. Light. Technol. 34, 419–424 (2016).

    Article  ADS  Google Scholar 

  14. Han, J. H. et al. Efficient low-loss InGaAsP/Si hybrid MOS optical modulator. Nat. Photon. 11, 486–490 (2017).

    Article  Google Scholar 

  15. Jayatilleka, H. et al. Post-fabrication trimming of silicon photonic ring resonators at wafer-scale. J. Light. Technol. 39, 5083–5088 (2021).

    Article  ADS  Google Scholar 

  16. Li, H. et al. A 3-D-integrated silicon photonic microring-based 112-Gb/s PAM-4 transmitter with nonlinear equalization and thermal control. IEEE J. Solid State Circuits 56, 19–29 (2021).

    Article  ADS  Google Scholar 

  17. Nedeljkovic, M, et al. Free-carrier electrorefraction and electroabsorption modulation predictions for silicon over the 1-14-μm infrared wavelength range. IEEE Photonics J. 3, 1171–1180 (2011).

  18. Phare, C. T., Daniel Lee, Y.-H., Cardenas, J. & Lipson, M. Graphene electro-optic modulator with 30-GHz bandwidth. Nat. Photon. 9, 511–514 (2015).

    Article  ADS  Google Scholar 

  19. Nitkowski, A., Chen, L. & Lipson, M. Cavity-enhanced on-chip absorption spectroscopy using microring resonators. Opt. Express 16, 11930 (2008).

    Article  ADS  Google Scholar 

  20. Abel, S. et al. A hybrid barium titanate-silicon photonics platform for ultraefficient electro-optic tuning. J. Light. Technol. 34, 1688–1693 (2016).

    Article  ADS  Google Scholar 

  21. Sharif Azadeh, S., Merget, F., Nezhad, M. P. & Witzens, J. On the measurement of the Pockels effect in strained silicon. Opt. Lett. 40, 1877–1880 (2015).

    Article  ADS  Google Scholar 

  22. Sharma, R., Puckett, M. W., Lin, H. H., Vallini, F. & Fainman, Y. Characterizing the effects of free carriers in fully etched, dielectric-clad silicon waveguides. Appl. Phys. Lett. 106, 241104 (2015).

    Article  ADS  Google Scholar 

  23. Sun, J. et al. A 128 Gb/s PAM4 silicon microring modulator with integrated thermo-optic resonance tuning. J. Light. Technol. 37, 110–115 (2019).

    Article  ADS  Google Scholar 

  24. Hu, C. Modern Semiconductor Devices for Integrated Circuits (Prentice Hall, 2010).

    Google Scholar 

  25. Müller, J. et al. Optical peaking enhancement in high-speed ring modulators. Sci. Rep. 4, 6310 (2014).

    Article  Google Scholar 

  26. Sakib, M. et al. A high-speed micro-ring modulator for next generation energy-efficient optical networks beyond 100 Gbaud. In Conference on Lasers and Electro-Optics, Kang, J. et al (eds) S. (Optica Publishing Group, 2021);

  27. Sakib, M. et al. A 240 Gb/s PAM4 silicon micro-ring optical modulator. In Optical Fiber Communication Conference (OFC) 2022, Matsuo, S. et al (eds) (Optica Publishing Group, 2022);

  28. Zhang, W. et al. Integration of low loss vertical slot waveguides on SOI photonic platforms for high efficiency carrier accumulation modulators. Opt. Express 28, 23143 (2020).

    Article  ADS  Google Scholar 

  29. Zhu, S., Lo, G. Q., Ye, J. D. & Kwong, D. L. Influence of RTA and LTA on the optical propagation loss in polycrystalline silicon wire waveguides. IEEE Photon. Technol. Lett. 22, 480–482 (2010).

    Article  ADS  Google Scholar 

  30. Li, K. et al. Electronic-photonic convergence for silicon photonics transmitters beyond 100 Gbps on-off keying. Optica 7, 1514–1516 (2020).

    Article  ADS  Google Scholar 

  31. Hashemi Talkhooncheh, A. et al. A 100-Gb/s PAM4 optical transmitter in a 3-D-integrated SiPh-CMOS platform using segmented MOSCAP modulators. IEEE J. Solid-State Circuits 58, 30–44 (2023).

  32. Tong, Y. et al. An experimental demonstration of 160 Gbit/s PAM-4 using a silicon micro-ring modulator. IEEE Photon. Technol. Lett. 32, 125–128 (2020).

    Article  ADS  Google Scholar 

  33. Zhang, Y. et al. 240 Gb/s optical transmission based on an ultrafast silicon microring modulator. Photon. Res. 10, 1127–1133 (2022).

    Article  Google Scholar 

Download references


This work was supported by funding from Rockley Photonics and the EPSRC through the Prosperity Partnership (EP/R003076/1), EPSRC Platform Grant (EP/N013247/1), EPSRC Strategic Equipment Grant (EP/T019697/1) and European Commission H2020 PICTURE Project (780930). D.J.T. acknowledges funding from the Royal Society for his University Research Fellowship (UF150325).

Author information

Authors and Affiliations



W.Z. contributed to the idea, simulation, fabrication, device testing and manuscript preparation. K.L. contributed to device testing. M.E., B.C., X.Y., H.D., M.B., D.T.T. and C.G.L. contributed to device fabrication. A.S., G.Y., R.S., A.Z. and G.R. contributed to the discussion, and G.R. manages the Silicon Photonics Group at Southampton. D.J.T. provided high-level project supervision and manuscript revision.

Corresponding author

Correspondence to David J. Thomson.

Ethics declarations

Competing interests

Authors A.S., G.Y., R.S., A.Z., G.R. and D.J.T. are shareholders of Rockley Photonics Holdings Limited (NYSE: RKLY), a global leader in photonics-based health monitoring and communications solutions.

Peer review

Peer review information

Nature Photonics thanks Stanley Cheung and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary discussion, Supplementary Figs. I-1, I-2, I-3, I-4, I-5, I-6, I-7, I-8, II-1, II-2, III-1, III-2, III-3, III-4, III-5, IV-1, IV-2 and IV-3 and Supplementary Tables III-1 and IV-1.

Supplementary Data

Original data for Fig. I-1.

Supplementary Data

Original data for Fig. I-2, Fig. I-3, Fig. I-4.

Supplementary Data

Original data for Fig. I-5.

Supplementary Data

Original data for Fig. I-6.

Supplementary Data

Original data for Fig. I-7.

Supplementary Data

Original data for Fig. I-8.

Supplementary Data

Original data for Fig. III-1.

Supplementary Data

Original data for Fig. III-3/4/5.

Supplementary Data

Original data for Fig. I-1.

Supplementary Data

Original data for Fig. I-2.

Supplementary Data

Original data for Fig. I-3.

Source data

Source Data Fig. 1

Source data of image of the device in Fig. 1.

Source Data Fig. 2

Source data of optical transmission data in Fig. 2.

Source Data Fig. 3

Source data of optical transmission and image containing data in Fig. 3.

Source Data Fig. 4

Source data of bandwidth and eye diagram image of Fig. 4.

Source Data Table.1

Source data of bandwidth and eye diagram image of Fig. 5.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Zhang, W., Ebert, M., Li, K. et al. Harnessing plasma absorption in silicon MOS ring modulators. Nat. Photon. 17, 273–279 (2023).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing