Nonlinear refraction of undoped and Fe-doped KTiOAsO4 crystals in the femtosecond regime
The third-order optical nonlinearities in undoped and Fe-doped KTA crystals have been measured using the Z-scan technique with femtosecond pulses at 780-nm wavelength. The nonlinear refractive index is determined to be 1.7×10−15 cm2/W and 0,9×10−15 cm2/W for undoped and Fe-doped KTA, respectively. No two-photon absorption occurs in these crystals. It is shown that doping with Fe2O3 could weaken refractive nonlinearity of KTA, suggesting that Fe:KTA will improve the performance of KTA in high-intensity femtosecond laser applications. In addition, the measured nonlinear index of refraction in KTA crystals is about five times lower than that predicted by the two-band theory. One of the reasons for the discrepancy is given as the applicable limit of the simple theory in which a two-parabolic band model has been assumed in the analysis.
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