最近用FDTD仿真一個結構,FDTD Solutions中金屬的復折射率用折射率n和消光係數k表示,而Opti-FDTD中設置是實部和虛部。因爲對這塊不是很瞭解,以前看的知識忘乾淨了。百度了下,又看了兩篇文章。
金屬折射率爲N=n+ik,其中,實部n爲常規折射率,k爲消光係數;
消光係數k與吸收係數,是真空中波長。
消光係數越大,金屬的吸收越強,那麼,在可見光波段,金屬導體的折射率N的虛部將遠小於遠紅外與微波波段,趨膚深度將會達到波長量級
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.
這裏說非磁性材料的磁導率約爲1,得出的n=n+ik。(百度百科如下,沒找書了,麻煩),磁導率取1應該沒問題
FDTD Solutions+n,k:
當你輸入nk材料時,我們假設你僅對一個波長的結果感興趣。當你仿真寬光譜時,軟件會自動根據所輸入的參數給出一個色散曲線。
‘爲什麼呢?因爲,根據KK理論,有損耗的材料一定有色散!正如超材料,比如能得到等效折射率爲負一, 能不能得到無色散的呢?不能。 因爲必須滿足因果關係。
所以,如果你僅知道一個波長的nk,那麼只有這個波長的結果是正確的,其它波長的結果是否合理已經無關緊要了,因爲你不知道它們對應的材料折射率。如果要知道寬譜結果,材料特性也必須是寬譜的。
需要注意,FDTD軟件的光源頻譜中心是頻率, 中心頻率不對應中心波長。