Beyer O, Maxein D, Buse K, Sturman B, Hsieh H T, Psaltis D
Institute of Physics, University of Bonn, Wegelerstr. 8, D-53115 Bonn, Germany.
Phys Rev E Stat Nonlin Soft Matter Phys. 2005 May;71(5 Pt 2):056603. doi: 10.1103/PhysRevE.71.056603. Epub 2005 May 10.
The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNb O3 ) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is beta(p) approximately = 3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Delta t between probe and pump pulses is characterized by a narrow dip (at Delta t approximately = 0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is beta(r) approximately = 0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sigma(r) approximately = 8 x 10(-18) cm(2). The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of approximately = 0.24 ps.
在共线泵浦 - 探测传输实验中，对高功率飞秒光脉冲在铌酸锂晶体（LiNbO₃）中的传播进行了实验和理论研究。发现在很宽的强度范围内，波长为388nm时泵浦传输系数的大幅下降完全符合双光子吸收模型；相应的非线性吸收系数约为β(p) ≈ 3.5 cm/GW。此外，强泵浦脉冲会在776nm处对探测光产生相当大的吸收。探测光传输系数对探测光与泵浦光脉冲之间时间延迟Δt的依赖关系表现为一个窄的凹陷（在Δt ≈ 0处）和一个持续时间长（皮秒时间尺度）的平台。该凹陷是由于涉及泵浦光和探测光光子的直接双光子跃迁引起的；相应的非线性吸收系数约为β(r) ≈ 0.9 cm/GW。平台吸收是由泵浦激发的电荷载流子的存在引起的；776nm处的有效吸收截面约为σ(r) ≈ 8×10⁻¹⁸ cm²。上述非线性吸收参数对偏振不太敏感。对于脉冲持续时间约为0.24ps的情况，未发现热载流子弛豫的具体表现。
