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. PACS: 42.65; 42.70; 42.87 KTiOAsO4 (KTA) is a recently developed crystal for applications of second-harmonic generation (SHG) and optical parameter amplification or oscillation (OPA or OPO). In comparison to its well-known analog: potassium titanyl phosphateKTiOPO4 (KTP), KTA possesses a larger nonlinear coefficient, higher figure of merit, and lower ionic conductivity [1–4]. These advantages make KTA a superior material to KTP. Recently we have also reported [5] a high damage threshold in KTA induced by self-focusing with a ps laser beam at a wavelength of 532 nm. However, up to now, only few papers [4, 6] are devoted to the study of impurities and defects, which may influence the optical properties of KTA. Cheng et al. reported [4] that the addition of a small amount of Fe2O3 into the melt during the growth promotes the crystallization of single-domain KTA crystals, and an unusually strong optical birefringence increase with Fe2O3 dopant concentration, which leads to blueshift in the SHG cutoff wavelength of as much as 37 nm. The effect of Fe doping on the third-order optical nonlinearities is still unknown. ∗Corresponding author. The nonlinear refraction in SHG and OPA (or OPO) crystals is of interest for the following reasons. The nonlinear refractive index n2, plays an important role in the spatial and temporal pulse evolution in many χ(2) mixing experiments involving ultrashort or high-energy optical pulses. Accurate knowledge of n2 is of direct relevance in assessing spectral and temporal pulse broadening due to self-phasemodulation, chirp reversal and self-compression, the minimum pulse width attainable in ultrashort-pulse SHG and OPA/OPO, and optical damage induced by self-focusing. In this paper, we report an experimental investigation of refractive nonlinearity in undoped and Fe-doped KTA crystals by using a Z-scan technique with linearly polarized 150-fs, 780-nm laser pulses. To the best of our knowledge, this is the first observation of the third-order optical nonlinearity in KTA with fs laser pulses. It has been found that the measured n2 value of Fe-doped KTA is smaller than that of undoped KTA, which indicates that doping with Fe2O3 makes a decrease in the refractive nonlinearity in KTA. The effect of Fe-doping on Kerr nonlinearity in KTA is discussed.