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Mach-Zehnder (MZ) modulators in Silicon-on-Insulator (SOI) are key components for integrated silicon photonic devices. Reducing their energy dissipation is a crucial step for applications of silicon photonics, especially in large data centers. In this work, we combine band-edge slow light structures consisting of silicon waveguide gratings with a periodic (interleaved) p-n junction. The slow-light structures consist of a waveguide grating with wide/narrow sections realized in a 300-nm thick silicon layer, on top of an unetched silicon layer of 50 to 150 nm thickness, fully embedded in SiO2. The grating gives rise to a photonic stop band and to a slow-light region close to the lowest band edge. The profile of the p-n junction varies periodically along the waveguide with interleaved n and p regions. This structure maximizes the spatial overlap between the optical mode and the depletion regions, yielding a further improvement of modulator efficiency beyond the slow-light effect.
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Under an applied reverse bias, the silicon refractive index is modified by the plasma dispersion effect, thus the waveguide grating acts as a phase shifter. The modulator efficiency VpiLpi is strongly improved in comparison with modulators without slow light or with a lateral p-n junction. Thanks to the optimized overlap between electric field and depletion regions, this improvement takes place over a spectral interval that is much larger than the slow-light bandwidth. Insertion losses due to free carriers are also lower than in conventional modulators. The advantage of combining slow-light grating waveguides with an interleaved p-n junction is especially pronounced at low driving voltage (of the order of 1V), where the dissipated energy can be as low as 0.4 pJ/bit over an optical bandwidth of several 10 nm. Thus, the present modulator structure is promising in view of realizing integrated MZ modulators with low power dissipation.
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