Measurement of the nonlinear index n(2) of BSO crystals.

If authors will state in their covering communications whether they expect their institutions to pay the publication charge, publication time should be shortened (for those who do). Photorefractive materials are now widely studied for their large nonlinear optical properties. Large gain values in two-beam coupling or degenerate four-wave mixing experiments are easily available using a low-power continuous laser beam, making them attractive materials for such uses as real-time holography, 1 reconfigurable optical connections, 2 and gain media for self-oscillating laser cavities. 3 For these crystals, the photoinduced variation of the re-fraction index can arise from two different origins. The first one is the photorefractive effect (PRE), the second is a nonlinear optical Kerr effect related to the third-order susceptibility tensor. In the case of the PRE the index variation results from the building of a space charge field created by a separation of positive and negative charges inside the crystal. Often considered as slow materials, response times in the nanosecond time scale are now demonstrated for BSO 4 crystals. Even BaTiO crystals and doped photorefractive semiconductors exhibit response time in the picosecond range. On the contrary, in these materials, the n 2 component of the refraction index (n = n 0 + n 2 I), where I is the incident light intensity and n 0 is the linear component of the index) is believed to arise from electronic effects and is expected to have a response time in the femtosecond time scale. Unfortunately , the magnitude of the nonlinear coefficient n 2 is not well determined. We present the results of measurement of n 2 for undoped BSO crystals in the femtosecond domain. To measure the n 2 coefficient we use a classical optical Kerr effect setup described in Fig. 1. In this configuration, an intense laser beam linearly polarized induces a variation of the refraction index in the direction of the optical field. We detect the index variation by measuring the transmission of a weak probe through the crystal placed between polarizer and analyzer. The probe beam is polarized at 45° from the pump beam. The pump and probe beams are produced by a colliding pulse mode-locked dye laser with internal Brewster prisms for group velocity compensation and amplified by a frequency-doubled Nd:YAG laser pumping a chain of dye amplifiers. That produces single amplified pulses of 500-μJ energy at a 620-nm wavelength and a 10-Hz repetition rate. At the …