光在非透明介质中的扩散
Diffusion of light in semitransparent media
摘要：
光漫射通常与厚的不透明介质有关。事实上，多重散射是必要的扩散制度的发病和这样的条件一般不符合几乎透明的介质。尽管如此，在足够长的时间，在一个无限的薄板运输仍然是由一个多散射过程，其完整的表征是缺乏。在本文中，我们表明，经过短暂的瞬态，透射强度的均方宽度仍然表现出一个简单的线性增加随时间的扩散理论预测，即使在光学厚度低至1。有趣的是，这种线性增长预测不依赖于板的厚度，也不依赖于其折射率对比度，但这种简单的近似在弹道扩散制度的准确性还没有调查到目前为止。通过蒙特卡罗模拟，我们发现明确的证据表明，边界条件发挥了积极的作用，在重新定义的扩散系数的渐近值直接修改的统计分布在这种介质中的光传输。在这方面，我们证明了需要区分一组内在的和有效的输运参数，其关系和相互作用与边界条件仍有待充分理解。
英文摘要：
Light diffusion is usually associated with thick, opaque media. Indeed, multiple scattering is necessary for the onset of the diffusive regime and such condition is generally not met in almost transparent media. Nonetheless, at long enough times, transport in an infinite thin slab is still determined by a multiple scattering process whose complete characterization is lacking. In this paper we show that, after a short transient, the mean square width of the transmitted intensity still exhibits a simple linear increase with time as predicted by diffusion theory, even at optical thickness as low as one. Interestingly, such linear growth is predicted not to depend neither on the slab thickness nor on its refractive index contrast, yet the accuracy of this simple approximation in the ballistic-to-diffusive regime hasn't been investigated so far. By means of Monte Carlo simulations, we find clear evidence that boundary conditions play an active role in redefining the very asymptotic value of the diffusion coefficient by directly modifying the statistical distributions underlying light transport in such media. In this respect, we demonstrate the need to distinguish between a set of intrinsic and effective transport parameters, whose relation and interplay with boundary conditions remains to be fully understood.
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