Formation mechanism of coherent rainbows II
Formation mechanism of coherent rainbows II
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DOI：
10.7498/aps.71.20220455
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发表时间：
2022
影响因子：
1
通讯作者：
Shi Ting-ting;Qian Xuan;Shang Ya-Xuan;Ji Yang
中科院分区：
文献类型：
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作者：
Shi Ting-ting;Qian Xuan;Shang Ya-Xuan;Ji Yang
将白光激光聚焦到液体中，就会出现“相干彩虹”，即形状各异的彩色干涉环。这种连贯彩虹的形成机制如下。激光加热改变了液体的局部温度分布，从而影响了液体的局部折射率，从而影响了光程差，即可能导致激光大角度偏离的热透镜效应。激光波前的曲率以及液体中的对流和气泡也起到了一定的作用，正如这里所报道的那样。由于焦点前的波前曲率为负，焦点后的波前曲率为正，导致焦点附近的相干彩虹不对称。由于局部加热液体中的对流导致温度分布不对称，相干彩虹具有上下不对称的椭圆形。构造了一个考虑波前曲率和热透镜效应的函数，得到了相干彩虹的大小与样品位置的关系。由于激光加热引起的液体不稳定性，液体中的温度分布不再是轴对称的，因此，横向和纵向热凹透镜的焦距不再相同。实验结果表明，相干彩虹在焦点前(后)变化较快(较慢)，且相干彩虹的纵向和横向尺寸不同。在激光中心的水中产生了许多微小的气泡，形成了一个局部的“黑障”，这解释了在相干彩虹的暗区有衍射环和泊松类型的亮点的观测。在许多液体中都观察到了相干彩虹，其中相干彩虹的环状形状和数量取决于液体的性质，如密度、粘度和导热系数。
Focus white light laser into liquids, and "coherent rainbows" show up, namely, colorful interference rings with different shapes. The formation mechanism of such coherent rainbows is the following. 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 also play some roles, as reported here. The wave front curvature results in the asymmetry of the coherent rainbows near the focal point because the wave front curvature in front of the focal point is negative and the wave front curvature at behind 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 lengths of the transverse and longitudinal thermal concave lens are no longer the same. The fitting results agree well with the experimental observations that the coherent rainbows change faster (slower) in front of (behind) the focal point and the longitudinal and transverse dimensions of the coherent rainbows are different. Many tiny bubbles are generated in water at the center of the laser beam, forming a local "black barrier" which explains the observation that there are diffraction rings and Poisson-type bright spot in the dark area of the coherent rainbows. The coherent rainbows have been observed in many liquids where the shape and number of rings of coherent rainbows depend on liquid properties such as density, viscosity and thermal conductivity.