摘要
一维光子准晶已在亚波长、亚衍射聚焦及超分辨成像领域不断展现优越性。为丰富及拓展其聚焦特性的应用,本文提出了一种一维光子准晶平V透镜,并研究了材料厚度对其聚焦特性的影响。研究结果表明,该透镜可在第二能带较宽的波长范围内实现亚波长及亚衍射聚焦。本文研究结果将为一维光子准晶平V透镜的设计及应用提供参考。
随着集成光学及微纳光子学的不断发
光子准晶因具有特殊的结构特征(如:二维光子准晶具有长程有序性、旋转对称性、自相似
本文提出了一种V形Fibonacci光子准晶透镜,分析了聚焦特性随电介质厚度变化的规律。
传统光学透镜的分辨率受Rayleigh衍射极限的限制。倏逝波在穿过传统光学透镜后呈指数衰减而不能到达成像面参与成像,其携带的物方信息被丢失。光子晶体作为一种人工微结构材料,其在第二通带可实现负折射,从而增强倏逝波的振幅,修复倏逝波的相位,提高透镜的分辨
在数学上,Fibonacci序列是指:1、1、2、3、5、8、13、21、34、……。在形式上,Fibonacci序列可用递推的形式定义:F(0)=0,F(1)=1,F(2)=1,F(m)=F(m-1)+F(m-2)(m≥2,m∈N

图1 (a) 具有Fibonacci序列的结构示意图 (b) Fibonacci光子准晶平V透镜的二维模型,红色箭头表示平面波的入射方向及穿过透镜后汇聚到焦点的过程,F为透镜的焦距
Fig.1 (a) Structure diagram with the Fibonacci sequence (b) 2D model of the Fibonacci photonic quasi-crystals (PQC) plano-V lens. The red arrows indicate the incident direction of the plane wave and the process of converging to the focal point after passing through the lens. F is the focal distance of the lens
如
为了研究透镜的聚焦特性,首先需分析透镜的光子能带结构,以得到可在透镜中传输的波长范围。因光子准晶不具有周期结构(平移对称性),故其能带结构不可用色散关系表征,本文采用透射谱表征其能带结构。通过传输矩阵法,首先计算光子准晶的透射系
. | (1) |
介质B的厚度在dB∈[10nm, 300nm]范围内变化,所以其传输矩阵MBj(j∈[10, 300],步长Δj=5,不同的j值对应不同厚度下的介质B)可由下式表示:
, | (2) |
其中
, | (3) |
, | (4) |
其中、、、、、和分别是相位差、有效光导纳、电介质厚度、真空波长、电介质折射率、真空介电常数和磁导率。
当入射平面波穿过N层介质后,透射系数可表示为:
, | (5) |
透射率可表示为:
. | (6) |
对于TM模式,表达
. | (7) |
由于本文中入射平面波均为垂直入射,折射角均为0,即TE模式下和TM模式下的透射率T相等,即本文提出的一维光子准晶平V透镜,入射平面光波的偏振方向并不影响透镜的聚焦效果。通过上述方法,可计算得到透镜(dA=dB=100nm)的透射谱,如

图2 具有不同Fibonacci序列的光子准晶的透射谱:(a) F(8) (b) F(9) (c) F(10)
Fig.2 Transmission spectrum of a PQC with different Fibonacci sequences: (a) F(8) (b) F(9) (c) F(10)
由
本文采用有限元法计算分析Fibonacci光子准晶平V透镜的聚焦特性。
进一步仿真计算发现,在F(9)序列光子准晶的第二通带频段(见

图3 dA=100nm,dB变化时Fibonacci光子准晶的透射谱:(a) dB=50nm (b) dB=100 nm (c) dB=200 nm
Fig.3 Transmission spectrum of a PQC when dA=100 nm and dB changes: (a) dB=50 nm (b) dB=100nm (c) dB=200 nm
当dB取不同值时,透镜对入射平面波的聚焦频率范围如下:当dB=50 nm时, f∈[3.139×1

图4 Fibonacci光子准晶平V透镜在λ=752nm且dB变化时的聚焦特性:(a)归一化焦点强度 (b)FWHM 系数 η;(c)焦距
Fig.4 The focusing characteristics of Fibonacci PQC plano-V lens when λ=752nm and dB varies: (a) normalized intensity of focus, (b) FWHM coefficient η, and (c) focal distance
由

图5 dA=dB=100 nm的Fibonacci光子准晶平V透镜对平面波λ=752nm的聚焦场
Fig.5 Focusing field of the Fibonacci PQC plano-V lens with dA=dB=100 nm for a plane wave λ =752nm
由
提出了一种Fibonacci序列光子准晶平V透镜,并分析了透镜随电介质材料厚度变化的亚波长、亚衍射聚焦特性。该透镜可以在一个较宽波长范围内实现亚波长、亚衍射聚焦。研究结果可为一维光子准晶平V透镜的材料选择、结构设计及应用提供参考。
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