Appl. Phys. B 70, 385–388 (2000) / Digital Object Identifier (DOI) 10.1007/s003409900134 Applied Physics B Lasers and Optics  Springer-Verlag 2000 Nonlinear refraction of undoped and Fe-doped KTiOAsO 4 crystals in the femtosecond regime H.P. Li 1, ∗ , C.H. Kam 1 , Y.L. Lam 1 , F. Zhou 1 , W. Ji 2 1 Photonics Laboratory, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore (Fax: + 65-7912687, E-mail: ehpli@ntu.edu.sg) 2 Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Received: 3 June 1999/Published online: 20 October 1999 Abstract. 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 wave- length. The nonlinear refractive index is determined to be 1.7 × 10 −15 cm 2 /W and 0, 9 × 10 −15 cm 2 /W for undoped and Fe-doped KTA, respectively. No two-photon absorp- tion occurs in these crystals. It is shown that doping with Fe 2 O 3 could weaken refractive nonlinearity of KTA, suggest- ing 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 KTiOAsO 4 (KTA) is a recently developed crystal for ap- plications of second-harmonic generation (SHG) and opti- cal parameter amplification or oscillation (OPA or OPO). In comparison to its well-known analog: potassium titanyl phosphateKTiOPO 4 (KTP), KTA possesses a larger nonlinear coefficient, higher figure of merit, and lower ionic conductiv- ity [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 532nm. 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 Fe 2 O 3 into the melt during the growth promotes the crys- tallization of single-domain KTA crystals, and an unusually strong optical birefringence increase with Fe 2 O 3 dopant con- centration, which leads to blueshift in the SHG cutoff wave- length 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) crys- tals is of interest for the following reasons. The nonlinear refractive index n 2 , plays an important role in the spatial and temporal pulse evolution in many χ (2) mixing experi- ments involving ultrashort or high-energy optical pulses. Ac- curate knowledge of n 2 is of direct relevance in assessing spectral and temporal pulse broadening due to self-phase- modulation, chirp reversal and self-compression, the min- imum 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 re- fractive 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 n 2 value of Fe-doped KTA is smaller than that of undoped KTA, which indicates that doping with Fe 2 O 3 makes a de- crease in the refractive nonlinearity in KTA. The effect of Fe-doping on Kerr nonlinearity in KTA is discussed. 1 Experimental The reliable Z-scan technique, first introduced by Sheik- Bahae et al. [7], is a simple but sensitive single-beam method to determine both the nonlinear refractive index and nonlinear absorption coefficient of a given material. Our Z-scan setup was a standard one [7]. The laser pulses at 780-nm wave- length were delivered by a mode-locked Ti:sapphire laser operating at a repetition rate of 76 MHz. The FWHM pulse duration was 150 fs. The spatial profile of the laser beam was a nearly Gaussian distribution after a spatial filter. The minimum beam waist ω 0 of the focused laser beam was meas- ured to be 13 μ m. The linearly polarized pulses were divided by a beam-splitter into two parts: the reflected one used as a reference to represent the incident light power; and the transmitted one was focused through the sample. Both the beams were recorded by two power probes (Newport 818 SL) simultaneously, and measured by a dual-channel power me- ter (Newport 2832-C) which transferred the digitized signals