Lau Wah Tung, Shen Jung-Tsung, Veronis Georgios, Fan Shanhui
Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA. and Ginzton Laboratory, Stanford University, Stanford, California 94305, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2007 Jul;76(1 Pt 2):016601. doi: 10.1103/PhysRevE.76.016601. Epub 2007 Jul 10.
We present the analytic calculation of the cross-spectral density tensor of a thermally radiative planar dielectric slab in extreme near-field, intermediate near-field, and far-field zones. We show that the spatial coherence of the thermal field exhibits distinct features in these zones. At a given wavelength lambda , the coherence length is many orders of magnitude smaller than lambda in the extreme near-field zone, and is roughly lambda/2 in the far-field zone. In the intermediate near-field zone, the coherence length can be much longer than lambda/2 if the loss is small. The physical origin of the short-ranged spatial coherence in the extreme near-field zone is the spatially fluctuating surface charges at the air-dielectric interface. We also demonstrate that in the intermediate near-field zone, the long-ranged spatial coherence is induced by the waveguide modes of the dielectric slab. When the loss is small, the long-ranged coherence falls off approximately as 1/square root x , in contrast to 1/x for a blackbody radiator, where x refers to displacement parallel to the slab surface.
我们给出了热辐射平面介质板在极近场、中间近场和远场区域的交叉谱密度张量的解析计算。我们表明，热场的空间相干性在这些区域呈现出不同的特征。在给定波长λ下，相干长度在极近场区域比λ小许多个数量级，而在远场区域大约为λ/2。在中间近场区域，如果损耗较小，相干长度可能比λ/2长得多。极近场区域短程空间相干性的物理起源是空气 - 介质界面处空间波动的表面电荷。我们还证明，在中间近场区域，长程空间相干性是由介质板的波导模式引起的。当损耗较小时，长程相干性大约以1/√x的形式衰减，而黑体辐射器的衰减形式为1/x，其中x表示平行于平板表面的位移。
