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光以入射角θ0从折射率为n0的光学介质入射于折射率为n1的光学介质,在界面上产生反射和折射,反射角与折射角分别为θ0和θ1,则反射系数r与折射率n、θ0和θ1的关系由菲涅耳反射公式:
$ {r_k} = \frac{{{\eta _{0, k}} - {\eta _{1, k}}}}{{{\eta _{0, k}} + {\eta _{1, k}}}} $
(1) 式中,下标k=p、s,表示p和s的分量光;η为等效折射率,对于p分量光,ηj=nj/cosθj(j=0, 1);对于s分量光,ηj=njcosθj(j=0, 1)。当n0>n1,当θ0>θc(θc为临界角)时,θ1不再是实数角,其余弦值是虚数:
$ \cos {\theta _1} = {\rm{i}}\sqrt {\frac{{n_0^2}}{{n_1^2}}{{\sin }^2}{\theta _0} - 1} = {\rm{i}}\mathit{\Gamma } $
(2) 式中,设$\mathit{\Gamma } = \sqrt {\frac{{{n_0}^2}}{{{n_1}^2}}{{\sin }^2}{\theta _0} - 1} $。
代入(1)式得:
$ \left\{ {\begin{array}{*{20}{l}} {{r_s} = \frac{{{n_0}\cos {\theta _0} - {\rm{i}}{n_1}\mathit{\Gamma }}}{{{n_0}\cos {\theta _0} + {\rm{i}}{n_1}\mathit{\Gamma }}}}\\ {{r_p} = \frac{{{n_0}/\cos {\theta _0} + {\rm{i}}{n_1}/\mathit{\Gamma }}}{{{n_0}/\cos {\theta _0} - {\rm{i}}{n_1}/\mathit{\Gamma }}}} \end{array}} \right. $
(3) 式中,反射系数rk(k=p、s)是复数,其模值丨rk丨=1,因而出现全反射, 而复数rk的幅角就是光经历全反射后的位相改变。p光、s光的位相改变不同,其差值即为位相延迟。这说明对于光场矢量不垂直(平行)于入射面的入射线偏振光,经历全反射后,反射光可变成椭圆偏振光或圆偏振光。
比较(1)式、(3)式可知,在全反射发生的情形下,介质1的等效折射率是纯虚值,即η1, s=in1Γ,η1, p=-in1/Γ。正是这个原因,全反射才出现,同时各分量光产生位相改变,因而有位相延迟,反射光的偏振态一般而言会变为椭圆或圆偏振光。相反地,当θ0 < θc时,θ1依然是实数角,介质1的等效折射率还是实数值,全反射不会发生,位相改变及位相延迟不会出现,反射光的偏振态也不会改变,依旧是线偏振光,只是其偏振方向相对入射光的偏振方向而言有一定的偏转。
如果在界面上镀制周期分布的多层膜,形成1维光子晶体,根据作者的研究表明[19],在带隙范围内,这种1维光子晶体的等效折射率也是纯虚值,而在带隙范围内光被高反射,其程度接近全反射,这样,在θ0 < θc、不发生全反射的情形下,利用敷于界面的1维光子晶体可以实现位相延迟,改变反射光的偏振态,使之由线偏振光(一般而言)变为椭圆或圆偏振光。
如图 1所示,在界面上制成最简单的二元一维光子晶体:ABAB…ABAB,记为(AB)L,L为周期数。A、B是周期内二次元的初始材料,其折射率分别为nA、nB。光以入射角θ0从折射率为n0的光学介质入射,其反射系数r为:
$ {r_k} = \frac{{{\eta _{0, k}} - {Y_k}}}{{{\eta _{0, k}} + {Y_k}}} $
(4) 式中,Yk(k=p, s)是光子晶体与衬底(折射率为nj)的组合体的等效折射率,它与入射角、1维光子晶体中各层膜的折射率和几何厚度、折射角、衬底折射率有关:
$ {Y_k} = \frac{{{\eta _1}\left[ {{m_{22}}{U_{L - 1}}(\chi ) - {U_{L - 2}}(\chi )} \right] + {\rm{i}}{m_{21}}{U_{L - 1}}(\chi )}}{{\left[ {{m_{11}}{U_{L - 1}}(\chi ) - {U_{L - 2}}(\chi )} \right] + {\rm{i}}{\eta _1}{m_{12}}{U_{L - 1}}(\chi )}} $
(5) 式中, m11、m12、m21及m22为周期传输矩阵的矩阵元,意义及其表达式与参考文献[19]中的(7)式相同; χ=(m11+m22)/2, UL-1(χ)、UL-2(χ)是χ的第L-1、L-2阶次第2类切比雪夫多项式(L为周期数,如前所述)[19]。
由(5)式可见,等效折射率Yk是波长λ的函数:Yk=Yk(λ);且Yk是复数。根据参考文献[19]中的结果可知,1维光子晶体的周期数达到很高时,Yk(λ)的实部YRe(λ)在某波长范围内的取值将趋近于或等于零,因而只留下虚部YIm(λ),而对应的波长范围就是光子晶体的带隙。这与全反射的情况类似,说明在光密到光疏入射时,若不发生全反射,则可以在光密介质表面制作多周期多层膜的1维光子晶体,也可以产生位相延迟,使入射的线偏振光(一般而言)变为椭圆或圆偏振光。
利用光子晶体实现位相延迟的讨论
Discussion on phase retardation using photonic crystals
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摘要: 为了设计制造新型的位相延迟器,利用1维光子晶体的特性,在折射率为1.52的玻璃上,镀制了由硫化锌(ZnS)与冰晶石(Na3AlF6)构成的多周期二次元一维光子晶体, 进行了数值模拟计算及理论分析。结果表明,在带隙范围内,1维光子晶体的等效折射率是虚等效折射率; 在斜入射时,带隙内的p光和s光的反射光各自位相增加,出现位相延迟,其偏振态发生改变,由线偏振光变为椭圆(圆)偏振光; 在发生全反射时,光疏媒质的等效折射率是虚等效折射率; 反射光出现位相增加,产生位相延迟,其偏振态发生改变,由线偏振光变为椭圆(圆)偏振光。该延迟器可以改变光的传播方向,改变偏振态的位相,克服了薄膜λ/4波片的缺陷。Abstract: In order to design and manufacture a new type of phase retarder, a multi-period binary 1-D photonic crystal composed of zinc sulfide (ZnS) and cryolite (Na3AlF6) was prepared on the glass with a refractive index of 1.52 by using the characteristic 1-D photonic crystal. Numerical simulation calculation and theoretical analysis were carried out. The analysis results show that, the effective refractive index of one-dimensional photonic crystal is the virtual equivalent refractive index in the band gap range. At oblique incidence, the phase of the reflected light of the p light and s light in the band gap increases, the phase delay occurs, and the polarization state changes from linear polarized light to elliptical (circular) polarized light. When total reflection occurs, the effective refractive index of the optical sparse medium is the virtual equivalent refractive index. The phase of reflected light increases and phase delay is observed at the same time. Its polarization state changes from linear polarized light to elliptical (circular) polarized light. The propagation direction of light and the phase of polarization state can be changed by using the retarder, and the defect of thin film λ/4 wave-plate is solved.
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Key words:
- physical optics /
- phase retardation /
- total reflection /
- imaginary refractive index /
- photonic crystal /
- band gap
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