We present experimental demonstration of fast all-optical switching in a one-dimensional photonic crystal nanocavity embedded in a Silicon waveguide. The transmission of the device is tuned by injecting free carriers into the nanocavity region using an optical pump beam. By strongly confining light in the photonic crystal nanocavity the sensitivity of light to small refractive index changes is enhanced. The small cavity volume (~0.1 μm3) and unpassivated sidewalls enable ultra-fast switching speeds with low pulse energies. Using a pulse energy of only 60pJ, a refractive index change of approximately 10-2 is obtained. This small index change, due to the high confinement nature of the cavity structure, leads to a strong change in transmission spectrum. Consequently, the resonance is shifted up to its full-width-at half-maximum (~7.5nm), and the transmission of the device is modulated by 71% with a time response of less than 1.5 ns. Such a device could open the door to the large-scale integration of ultra-fast modulators and switches.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.