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A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor


Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only 20 MHz (refs 10, 11), although it has been predicted theoretically that a 1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

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Figure 1: Schematic diagram showing the cross-sectional view of a MOS capacitor waveguide phase shifter using ‘silicon-on-insulator’ technology.
Figure 2: Phase shift Δφ versus drive voltage VD of the MOS capacitor phase shifter in Fig. 1 at a wavelength of λ = 1.55 µm for different phase shifter lengths.
Figure 3: Integrated silicon optical modulator and measured drive-voltage-dependent output optical intensity.
Figure 4: Frequency dependence of the optical response of a silicon MZI modulator.
Figure 5


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We thank M. Salib for help in process design and optical loss testing; M. Morse, A. Barkai, S. Tubul and D. Tran for technical assistance in device fabrication; A. Alduino for backend processing; and S. Koehl for data collection software. Special thanks go to D. Elqaq, M. Gill, S. Pang and B. Venkateshwaran for contributions during the early stages of this research. Finally, we thank G. Reed for discussions.

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Correspondence to Ansheng Liu.

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Liu, A., Jones, R., Liao, L. et al. A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor. Nature 427, 615–618 (2004).

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