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We have observed a large optically induced absorption in porous silicon. The figure of merit for third order optical nonlinearity is of the order of 10 .3 (esu 9 cm), a value as large as that of a photorefractive material such as BaTiO3. By exploiting this phenomenon, we have made simple demonstrations of optical hysteresis, parallel image processing, and all-optical logic gates. These results point to the possibility of expanding the applications of Si semiconductors from electronics to all-optical technologies.A great deal of interest has been generated by the demonstration of the feasibility of electro-optical or all-optical logic devices, such as quantum well self-electro-optic effect devices (SEEDs), I-3 guided-wave devices, 4'5 or optical bistable etalons 6-8 for future optical-processing and optical communication systems. The materials extensively used for these optical devices are III-V or II-VI semiconductors, dielectrics, or organic materials. 9 However, the stability of these materials is not as good as Si semiconductors and the large-scale integrated circuit (LSI) fabrication processes for these devices are not as highly developed as those for Si devices. Therefore, optoelectronic or all-optical processing systems using these materials have not been widely implemented.Recently, porous silicon fabricated by electrochemical anodization has been reported to exhibit visible luminescence at room temperature) ~ The photoluminescence intensity is very strong with a quantum yield of several percent. 11 Therefore there are many studies being conducted with this material, in which strong electroluminescence has been demonstrated, leading to various applications such as flat panel displays) 2-~8 Moreover, electrochemical anodization can be included in an LSI fabrication process, so porous Si also seems to be a promising candidate for the fabrication of optoelectronic integrated circuits (OEIC), such as optical interconnection systems, on Si substrates.In this paper we report the first observation of a large optical nonlinear absorption change in porous Si at room temperature under weak continuous wave (CW) laser excitation. This large optical nonlinearity allows one to control light with light. This finding should lead to new applications for porous St, such as optical phase conjugation, optical memories, and optical switching. We demonstrate here all-optical logic gates (inverters and NOR functions), 17 optical hysteresis, and Optical parallel processing for the first time in porous St, thus suggesting the expansion of applications of Si semiconductors from electronic technologies to all-optical technologies.

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