Large pure refractive nonlinearity of nanostructure silica aerogel at near infrared wavelength

The nonlinear refraction coefficient of the third-order susceptibility of silica aerogel was estimated to be /spl sim/-1.5/spl times/10/sup -15/ m/sup 2//W (/spl sim/-3.67/spl times/10/sup -9/ esu) by a signal-beam z-scan technique with /spl sim/1 ps Ti:sapphire lasers. The third-order nonlinear refraction coefficient of nanostructure silica aerogel is almost five orders larger than the nonlinear (/spl chi//sup 3/) refraction of bulk material. The large nonlinear refraction originates mainly from the direct excitation to the surface trapped states, and the one-photon resonance process.