Experimental Investigations Of Optical Nonlinear Absorption And Refraction In Hf: LiNbO₃ Crystal

Lithium niobate (LiNbO₃) crystal is an important multifunctional photoelectric material. There are many studies and applications in the fields of electrooptics, acoustooptics, photorefractive effect and nonlinear optics. There has been considerable interest in investigating the high-order optical nonlinear properties of damage-resistant doped LiNbO₃ series crystals as the nonlinear optical materials. Hafnium-ion (Hf⁴⁺) is the new damage-resistant dopant observed in 2004, so lithium niobate doped with hafnium (Hf:LiNbO₃) crystal will be the new nonlinear optical material. The studies on the optical nonlinear effect in Hf:LiNbO₃ crystal is important significant. In this paper, we performed an experimental investigation of optical nonlinear absorption and nonlinear refraction in Hf:LiNbO₃ crystal mainly using Z-scan with femtosecond laser excitation.The single-beam Z-scan technique is a simple, sensitive method to determine both the nonlinear refractive index and nonlinear absorption coefficient of a given material. In this paper, firstly, the fundamental experimental mechanism and theoretical analytic approach of the single-beam Z-scan technique were described, and the developments of Z-scan were discussed.The Hf:LiNbO₃ crystal used in our experiment is grown by Czochralski method in an air atmosphere under normal pressure along the (001) of the crystal. It is polarized along the c-axis and cut perpendicularly to the c-axis. The thickness is 0.8 mm. The two surfaces were polished. So the sample satisfied the"thin sample"condition. By investigating the transmittance spectrum of the Hf:LiNbO₃ crystal at room temperature, The absorption band edge and the band-gap energy of the crystal was determined.The third-order optical nonlinear effect of Hf:LiNbO₃ crystal is studied by Z-scan with femtosecond laser excitation. The two-photon absorption does not occur in the crystal. This is anticipated because the band-gap energy of the crystal is greater than the energy of two 800 nm photons. The measured closed-aperture Z-scan for the sample exhibits that the Z-scan trace is valley-to-peak, indicating that the laser beam propagating in the crystal undergoes a self- focusing process. Based on the two-band model with a wide band gap, we have calculated the nonlinear refractive index. It has been found that the measuredγvalue is consistent with the theoretical predication. The reason of the third-order optical nonlinear refraction is the aberration of electronic distribution. We have demonstrated that the absorption in the crystal with laser average power up to 3 mW is due to three-photon absorption effect. Our results demonstrate that Hf:LiNbO₃ crystal has potential applications for optical limiter and imaging technology.