Barbano E C, Harrington K, Zilio S C, Misoguti L
Appl Opt. 2016 Jan 20;55(3):595-602. doi: 10.1364/AO.55.000595.
We report on the third-harmonic generation (THG) of tightly focused femtosecond laser pulses at the interfaces of a cuvette filled with organic solvents. Such a system presents four interfaces separating two materials of different refractive indices and third-order nonlinear susceptibilities where the THG takes place because the symmetry around the focus is broken. We selected two cuvettes (silica and B270 crown glass) filled with different organic solvents (acetone, chloroform, and dimethyl sulfoxide) in order to have a variety of interfaces with different linear and nonlinear optical properties. For some of the peaks, the self-focusing modifies the expected cubic power law dependence for THG and as a consequence the four peak profiles may be quite uneven. Although the THG is due to the electronic part of the nonlinear susceptibility, it can suffer from the influence of the self-focusing effect, a Kerr nonlinearity that can have both instantaneous electronic and slow nuclear contributions. This mixture of two distinct third-order nonlinear processes was never considered for such interfaces. All the THG signals could be understood by taking into account the self-focusing effect. Furthermore, the nonlinear refractive indices, n(2), and third-order nonlinear susceptibilities of the solvents, χ((3)), could be determined simultaneously by the THG signals using the cuvette walls as a reference.
我们报道了在装有有机溶剂的比色皿界面处，紧聚焦飞秒激光脉冲的三次谐波产生（THG）现象。这样一个系统存在四个界面，分隔着两种具有不同折射率和三阶非线性极化率的材料，由于焦点周围的对称性被打破，三次谐波产生就发生在这些界面处。我们选择了两个装有不同有机溶剂（丙酮、氯仿和二甲基亚砜）的比色皿（石英和B270冕牌玻璃），以便获得具有不同线性和非线性光学性质的各种界面。对于某些峰值，自聚焦改变了三次谐波产生预期的立方幂律依赖关系，结果四个峰值轮廓可能相当不均匀。虽然三次谐波产生是由于非线性极化率的电子部分，但它可能会受到自聚焦效应的影响，自聚焦效应是一种克尔非线性效应，它既有瞬时电子贡献，也有缓慢的核贡献。对于这样的界面，从未考虑过这两种不同的三阶非线性过程的混合情况。考虑到自聚焦效应，所有的三次谐波产生信号都可以得到解释。此外，使用比色皿壁作为参考，通过三次谐波产生信号可以同时确定溶剂的非线性折射率n(2)和三阶非线性极化率χ((3))。
