The third‐order optical nonlinearity &khgr;(3)of porous silicon has been measured using theZ‐scan technique. Intensity dependent absorption was observed and attributed to a resonant two photon absorption process. The real and imaginary parts of &khgr;(3)have been measured at 665 nm and found to be 7.5×10−9esu and −1.9×10−9esu, respectively. This constitutes a significant enhancement over crystalline silicon. All optical switching based on nonlinear absorption is demonstrated. ©1995 American Institute of Physics.