Mechanisms of the anomalous Pockels effect in bulk water
Mechanisms of the anomalous Pockels effect in bulk water
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DOI：
10.1007/s10043-018-0407-6
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发表时间：
2018-02
期刊：
影响因子：
1.2
通讯作者：
S. Yukita;Yuto Suzuki;Naoyuki Shiokawa;Takayoshi Kobayashi;E. Tokunaga
中科院分区：
文献类型：
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作者：
S. Yukita;Yuto Suzuki;Naoyuki Shiokawa;Takayoshi Kobayashi;E. Tokunaga
“反常”普克尔斯效应是在电解质水溶液中通过两个电极之间的光束被施加在电极之间的交流电压偏转的现象：偏转角与电压成比例，使得入射光束交替地改变其方向。这种现象，在散装水中的普克尔斯效应，显然与非线性光学中所相信的相矛盾，即，宏观反转对称性应当被破坏以发生诸如一阶电光效应的二阶非线性光学效应，即，Pockels效应为了澄清潜在的机制，对电极材料的影响的依赖性进行了研究，发现与铂电极的Pockels系数是两个数量级小于铟锡氧化物（ITO）电极。实验证实，这些电极上的双电层（EDL）中的界面水的泡克尔效应显示出类似于本体水中的效应的电极依赖性，而该效应取决于AC电压的频率，使得界面信号随频率减小，但本体信号随频率增加到221 Hz。这些实验结果导致的结论是，光束偏转是由于在体水区域的折射率梯度，这是暂时形成的界面水的泡克尔斯效应时，施加交流电场的EDL。折射率梯度是由扩散层扩展到体区域，由于其电荷偏置的离子分布作为体水的反转对称性的破坏因子而引起的。这种机制与非线性光学原理并不矛盾。
The “anomalous” Pockels effect is a phenomenon that a light beam passing between two electrodes in an aqueous electrolyte solution is deflected by an AC voltage applied between the electrodes: the deflection angle is proportional to the voltage such that the incident beam alternately changes its direction. This phenomenon, the Pockels effect in bulk water, apparently contradicts what is believed in nonlinear optics, i.e., macroscopic inversion symmetry should be broken for the second-order nonlinear optical effect to occur such as the first-order electro-optic effect, i.e., the Pockels effect. To clarify the underlying mechanism, the dependence of the effect on the electrode material is investigated to find that the Pockels coefficient with Pt electrodes is two orders of magnitude smaller than with indium tin oxide (ITO) electrodes. It is experimentally confirmed that the Pockels effect of interfacial water in the electric double layer (EDL) on these electrodes shows an electrode dependence similar to the effect in bulk water while the effects depend on the frequency of the AC voltage such that the interfacial signal decreases with frequency but the bulk signal increases with frequency up to 221 Hz. These experimental results lead to a conclusion that the beam deflection is caused by the refractive index gradient in the bulk water region, which is formed transiently by the Pockels effect of interfacial water in the EDL when an AC electric field is applied. The refractive index gradient is caused by the diffuse layer spreading into the bulk region to work as a breaking factor of inversion symmetry of bulk water due to its charge-biased ionic distribution. This mechanism does not contradict the principle of nonlinear optics.