The aim of this work is to present a new design of high-speed, and ultra-compact optical modulation. The basic idea is to use a Mach-Zehnder interferometer in planar photonic crystal for which one of the two arms was structurally optimized to operate in the regime of slow modes, while the other arm operates in the regime of fast modes. the phase difference between the two optical pulses that travels in both channels strongly depends on the slowdown factor of the optimized photonic crystal slab. The exploitation of the free carriers dispersion effect in silicon provides a π phase difference on a very short length L and with a very low voltage.