Ultracompact (3 μm) silicon slow-light optical modulator

Wavelength-scale optical modulators are essential building blocks for future on-chip optical interconnects. Any modulator design is a trade-off between bandwidth, size and fabrication complexity, size being particularly important as it determines capacitance and actuation energy. Here, we demonstrate an interesting alternative that is only 3 μm long, only uses silicon on insulator (SOI) material and accommodates several nanometres of optical bandwidth at 1550 nm. The device is based on a photonic crystal waveguide: by combining the refractive index shift with slow-light enhanced absorption induced by free-carrier injection, we achieve an operation bandwidth that significantly exceeds the shift of the bandedge. We compare a 3 μm and an 80 μm long modulator and surprisingly, the shorter device outperforms the longer one. Despite its small size, the device achieves an optical bandwidth as broad as 7 nm for an extinction ratio of 10 dB and modulation times ranging between 500 ps and 100 ps.