Three-photon and Four-photon Absorption in Lithium Niobate and Lithium Tantalate by Z-scan Technique

In the past two-decade optical rectification of femtosecond near-infrared pulses in lithium niobate (LN) has become the most widespread way to generate high energy terahertz (THz) pulses in the low part of the THz spectrum (0.1 – 2 THz). LN has excellent nonlinear optical properties [1] for a variety of applications. Beside LN, lithium tantalate (LT), another ferroelectric crystal from the same symmetry class, is also a material of interest in terms of THz generation [2]. They share similar nonlinear optical properties, large nonlinear coefficients and high damage thresholds. At 800 nm pumping (typical for the widespread used Ti:sapphire lasers), LT is even more promising, since three-photon absorption is mostly suppressed in LT due to its larger band gap. One of the main limitation factors on the maximum usable pump intensity is the free-carrier absorption in the THz range induced by multiphoton absorption of the pump.