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一维光子准晶平V透镜  PDF

  • 郭嘉威
  • 谭威
  • 谢建斓
  • 刘建军
湖南大学 物理与微电子科学学院,微纳光电器件及应用教育部重点实验室,低维结构物理与器件湖南省重点实验室,湖南 长沙 410082

中图分类号: O734

最近更新:2021-10-18

DOI:10.11972/j.issn.1001-9014.2021.05.004

  • 全文
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  • 参考文献
  • 作者
  • 出版信息
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目录contents

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。

引言

随着集成光学及微纳光子学的不断发

1-11,微透镜与微透镜阵12-15因其体积小、重量轻、便于集成化及阵列化等优点而成为研究热点。光子晶体透镜作为微透镜的杰出代表之一,在亚波长、亚衍射聚焦及超分辨率成像方面具有良好的性16-20,且准周期光子晶体(又称为光子准晶)透镜的聚焦及成像特21-27优于周期光子晶体透镜。

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

28-30;一维光子准晶具有Fibonacci序31-33、Thue-Morse序列结34-36)而使其散射子具有多种位置特征。因此,与周期光子晶37-38相比,光子准晶在设计过程中可调节的参数更多,且可实现广泛应用,如:透21-27、光39-47、棱48、滤波3949-50、传感434651-52、空芯光束的产53及拓扑光子器54等。目前,光子准晶透镜亚波长、亚衍射聚焦及超分辨成像的研究主要集中于二维光子准21-2427,而一维光子准晶透镜的研究则相对薄弱。相对于二维光子准晶结构,一维光子准晶结构更简单,在制作上也相对容易,便于推广应用。另外,与当前研究较多的超表面平面透55-56相比,一维光子准晶透镜也具有一定优势。超表面平面透镜一般是二维结构,制备较复杂,且其周期结构使得制备误差对光学性能影响较大,而一维光子准晶透镜则可避免这些问题。虽我们已开展了一维光子准晶的聚焦特性研究,但仅限于平凹透25-26及圆形光57。为丰富及拓展一维光子准晶聚焦特性的应用,其它结构的一维光子准晶透镜尚待研究。

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。

1 模型与算法

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

16-20,从而实现亚波长聚焦。类比周期光子晶体,光子准晶在第二通带也会发生负折25-26。这种具有传播和增强倏逝波性能的透镜可提高成像分辨率及聚焦效果。

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

58。若把两种不同介质材料(设为A、B)按Fibonacci序列排布即可构成Fibonacci光子准晶。如1a),若m取不同的数值,则可得到不同的Fibonacci光子准晶。设F(0)=B,F(1)=A,则F(2)= F(1)+ F(0)=AB,F(3)= F(2)+ F(1)=ABA,F(4)= F(3)+ F(2)=ABAAB……。根据现实功能需要,取不同材料、厚度、层数及外形结构与尺寸,可设计不同的Fibonacci光子准晶器件(如本文设计的一维光子准晶平V透镜,如1b))。

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

1a),设电介质材料A、B分别为SiO2、Ge,其折射率分别为nA=1.56,nB=4.00,其厚度分别为dAdB。透镜的横截面,如1b)所示,其宽度、高度、V形平台宽度及底座高度分别设为LhL1h1。V形坡面采用光栅结构。平面光波从透镜底部入射,并汇聚于透镜上方。设焦点半宽度(full width at half maximum)FWHM=ηλ,系数η值越小,其焦点质量越好。

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

59。对于TE模式,由于介质A的厚度为dA=100nm不变,其传输矩阵MA可由下式表示:

MA=cosδA                iηAsinδAiηAsinδA          cosδA . (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:

MBj=cosδBj                iηBjsinδBjiηBjsinδBj          cosδBj (2)

其中

δk=-2πλnkdkcosθt(k=A,Bj) (3)
ηk=ε0μ0nkcosθt(k=A, Bj) (4)

其中δkηkdkλnkε0μ0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:

t=2η0Aη0+Bη0ηN+1+C+DηN+1 (5)

透射率可表示为:

T=t2 . (6)

对于TM模式,表达式(5)和(6)同样适用,表达式(4)需要改为:

ηk=ε0μ0nk/cosθt(k=A, Bj) . (7)

由于本文中入射平面波均为垂直入射,折射角θt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2所示。

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)

2可知,Fibonacci序列的光子准晶可产生四大带隙(见区域:Ⅰ、Ⅱ、Ⅲ、Ⅳ),其所在的频率范围分别为f∈[1.717×1014 Hz, 2.832×1014 Hz],f∈[4.293×1014 Hz, 4.786×1014 Hz],f∈[5.578×1014 Hz, 6.542×1014 Hz],f∈[7.961×1014 Hz, 8.753×1014 Hz]。 F(8)中各带隙边缘存在些许弯曲,即带隙尚未完全成形。F(9)中各带隙边缘无弯曲,即带隙已完全成形。随着光子准晶层数的进一步增加,F(10)中各带隙边缘无弯曲,虽带隙保持稳定,但相对于F(9),其层数增加接近1倍,导致制备成本及难度大幅度增加。因此,选用F(9)作为透镜的序列数,既能达到较好的聚焦效果,又能节省材料且方便制备。

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。

2 结果与讨论

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2b)灰色区域,ffocus∈[2.832×1014 Hz , 4.293×1014 Hz ](此时dB=100 nm))可发生聚焦。不过,实际制备误差将导致聚焦频率发生偏离。为此,改变dB∈[50 nm, 200 nm],可计算得到透镜对入射平面波的公共聚焦波段范围。以dB=50nm、100nm、200nm为例,透射光谱如3所示。

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

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∈[3.139×1014 Hz, 4.858×1014 Hz];当dB=100 nm时,f∈[2.832×1014 Hz, 4.293×1014 Hz];当dB=200 nm时,f∈[2.742×1014 Hz, 4.135×1014 Hz]。因此,公共聚焦频率范围为fcfocus∈[3.139×1014 Hz, 4.135×1014 Hz](见3灰色区域)。从该频段中取平面波f=3.989×1014 Hz(对应波长λ=752 nm),研究材料厚度变化对透镜聚焦特性的影响。固定材料SiO2的厚度dA=100 nm,且以ΔdB=5 nm为间隔改变材料Ge的厚度dB∈[10 nm, 300nm],焦点的归一化焦点强度、FWHM系数η及焦距随dB变化的规律,如4所示。

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

4可知,随着dB的增加,焦点的归一化焦点强度、FWHM系数η及焦距均呈振荡趋势。由图 (a)可知,在dB=10 nm、100 nm、185 nm、265 nm、275 nm时,焦点强度取得局部最大值。由图 (b)可知,当dB∈[20 nm,65 nm]时,0.5λ<FWHM<λ,透镜实现了亚波长聚焦,而其它情况下,FWHM<0.5λ,突破了衍射极限,也即透镜实现了亚衍射聚焦。由图 (c)可知,当dB=80nm、185nm时,焦距取得局部最大值。综合4可知,当dA=dB=100nm时,焦点强度最大,且FWHM=0.367λ趋于最小,实现了最佳的亚衍射聚焦特性,如5所示。

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

图5可知,一维光子准晶平V透镜的聚焦点低于V形高度,使该透镜在超导纳米线单光子探测领域具有重要的潜在应用。超导纳米线单光子探测器的探测效率与纳米线对光子的吸收率密切相关,提高纳米线对光子的吸收率即可提高探测器的探测效

60。若将超导纳米线单光子探测器的纳米线直接制作在透镜的V形凹槽内部,使纳米线置于透镜焦点处,则大部分的光子汇集于纳米线上,从而可大幅提升探测器的探测效率。另外,相较于一维光子准晶平凹透25-26,本文提出的一维光子准晶平V透镜最佳焦点的半高宽FWHM更小,焦点旁瓣更小,且该透镜可调参数更多,这意味着可以更加细微地去调节透镜结构,从而实现更好的亚波长、亚衍射聚焦效果。

3 结论

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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