We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at <10 Gb/s, by exploiting
low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM
structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using
the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a
phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral
noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an
order-of-magnitude shorter than most 40 Gb/s MZMs.