相干彩虹的形成机制(续)  

Formation mechanism of coherent rainbows(Ⅱ)

作　　者：施婷婷 钱轩[1,2] 尚雅轩 姬扬[1,2] Shi Ting-Ting;Qian Xuan;Shang Ya-Xuan;Ji Yang(State Key Laboratory of Superlattices and Microstructures,Institute of Semiconductors,Chinese Academy of Sciences,Beijing 100083,China;Center of Materials Science and Optoelectronics Engineering,University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院半导体研究所,半导体超晶格国家重点实验室,北京100083 [2]中国科学院大学材料科学与光电技术学院,北京100049

出　　处：《物理学报》2022年第15期121-129,共9页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:12174378,12004376)资助的课题。

摘　　要：将白光激光聚焦照射不同液体,可以形成不同形状的彩色干涉环,即“相干彩虹”.形成相干彩虹的原因是激光加热使液体局部温度分布发生变化,影响了折射性质,形成了光程差,从而使不同波长的光各自发生干涉(热透镜效应).本文确定了激光束的波前曲率对相干彩虹的形状和大小的影响,揭示了样品(纵向厚度很薄的液体)在凸透镜焦点前后相同距离处产生的相干彩虹有显著差别的原因:焦点前波前曲率为负,焦点后波前曲率为正.通过构建一个包含波前曲率和热透镜效应的函数,很好地拟合了实验结果.数值模拟还定量说明了激光加热导致的液体不稳定性(发生对流)使得相干彩虹的形状具有上下不对称性,而中心过热区(产生气泡)使得相干彩虹的内部出现暗区,以及在这个暗区中观察到的内疏外密的衍射环和泊松亮斑.When white light laser is focused on liquids,colorful interference rings with different shapes,i.e.“coherent rainbows” can be seen.The formation mechanism of such coherent rainbows is described as follows.The laser heating changes the local temperature distribution of the liquid,which affects the refractive index locally and thus the optical path difference,i.e.thermal lens effect which may cause the laser to go off at large angles.The curvature of the laser wavefront and the convection and bubbles in the liquid can also play some roles as reported here.The wave front curvature results in the asymmetry of the coherent rainbows in the near of the focal point because the wave front curvature in front of the focal point is negative and the wave front curvature in the rear of the focal point is positive.The coherent rainbow has an oval shape with up-down asymmetry because convection in the locally heated liquid leads to an asymmetric temperature distribution.We construct a function including wave front curvature and thermal lens effect,and obtain the relationship between the size of the coherent rainbows and the position of the sample.As a result of the liquid instability induced by the laser heating,the temperature distribution in the liquid is no longer axisymmetric,thus,the focal length of the transverse and longitudinal thermal concave lens are no longer the same.The fitting results accord well with the experimental observations that the coherent rainbows change faster(slower) in front of(rear) the focal point and the longitudinal and transverse dimension of the coherent rainbow are different.Many tiny bubbles are generated in water in the center of the laser beam,forming a local “ black barrier” which explains the observation that there are diffraction rings and Poisson-type bright spots in the dark area of the coherent rainbows.The coherent rainbows can be observed in many liquids where the shape and number of rings of coherent rainbows depend on liquid properties such as density,viscosity and thermal c

关 键 词：相干干涉 衍射 气泡 泊松亮斑 

分 类 号：O436.1[机械工程—光学工程]