最近用FDTD仿真一个结构,FDTD Solutions中金属的复折射率用折射率n和消光系数k表示,而Opti-FDTD中设置是实部和虚部。因为对这块不是很了解,以前看的知识忘干净了。百度了下,又看了两篇文章。
金属折射率为N=n+ik,其中,实部n为常规折射率,k为消光系数;
消光系数k与吸收系数,是真空中波长。
消光系数越大,金属的吸收越强,那么,在可见光波段,金属导体的折射率N的虚部将远小于远红外与微波波段,趋肤深度将会达到波长量级
The refractive index of electromagnetic radiation equals
where εr is the material's relative permittivity, and μr is its relative permeability.[31]:229 The refractive index is used for optics in Fresnel equations and Snell's law; while the relative permittivity and permeability are used in Maxwell's equations and electronics. Most naturally occurring materials are non-magnetic at optical frequencies, that is μr is very close to 1, [32] therefore n is approximately √εr. In this particular case, the complex relative permittivity εr, with real and imaginary parts εr and ɛ̃r, and the complex refractive index n, with real and imaginary parts n and κ (the latter called the "extinction coefficient"), follow the relation
and their components are related by:[33]
and:
where is the complex modulus.
这里说非磁性材料的磁导率约为1,得出的n=n+ik。(百度百科如下,没找书了,麻烦),磁导率取1应该没问题
FDTD Solutions+n,k:
当你输入nk材料时,我们假设你仅对一个波长的结果感兴趣。当你仿真宽光谱时,软件会自动根据所输入的参数给出一个色散曲线。
‘为什么呢?因为,根据KK理论,有损耗的材料一定有色散!正如超材料,比如能得到等效折射率为负一, 能不能得到无色散的呢?不能。 因为必须满足因果关系。
所以,如果你仅知道一个波长的nk,那么只有这个波长的结果是正确的,其它波长的结果是否合理已经无关紧要了,因为你不知道它们对应的材料折射率。如果要知道宽谱结果,材料特性也必须是宽谱的。
需要注意,FDTD软件的光源频谱中心是频率, 中心频率不对应中心波长。