基于超材料的中波紅外寬帶偏振轉(zhuǎn)換研究

金柯 劉永強(qiáng) 韓俊 楊崇民 王穎輝 王慧娜

(西安應(yīng)用光學(xué)研究所,西安 710065)

基于超材料的中波紅外寬帶偏振轉(zhuǎn)換研究

金柯?劉永強(qiáng) 韓俊 楊崇民 王穎輝 王慧娜

(西安應(yīng)用光學(xué)研究所,西安 710065)

(2017年1月18日收到;2017年5月1日收到修改稿)

基于硅納米塊陣列和亞波長金屬光柵,硅納米塊長軸與金屬光柵夾角為45°,本文設(shè)計(jì)了一種高效、寬帶偏振轉(zhuǎn)換結(jié)構(gòu).模擬計(jì)算表明該結(jié)構(gòu)實(shí)現(xiàn)了線偏振光90°旋轉(zhuǎn),在3.4-4.5μm波段偏振轉(zhuǎn)換率大于60%,在3-5μm光譜范圍內(nèi)的轉(zhuǎn)換對比率大于104.由于該結(jié)構(gòu)光學(xué)性能優(yōu)異,制備難度低,可以應(yīng)用于光傳輸控制.

偏振轉(zhuǎn)換,亞波長金屬光柵,超材料,納米塊

1 引 言

光學(xué)偏振技術(shù)在光網(wǎng)絡(luò)和光信息系統(tǒng)中廣泛應(yīng)用[1,2].中波紅外是光偏振技術(shù)應(yīng)用的重要波段,如紅外通信、紅外預(yù)警、紅外成像、紅外光譜、紅外遙感等,所以對中波紅外波段的偏振控制就顯得十分重要[3,4].光偏振態(tài)轉(zhuǎn)換已經(jīng)是偏振控制的重要內(nèi)容,目前利用雙折射材料制作的波片可以把線偏振光轉(zhuǎn)換成橢圓或圓偏振光[5].此外,利用法拉第磁光效應(yīng)可以旋轉(zhuǎn)線偏振光的偏振方向,旋轉(zhuǎn)角與磁感應(yīng)強(qiáng)度和光穿越介質(zhì)的長度成正比[6].但是這些器件往往尺寸較大,不利于光子集成.

超材料通常指人工周期結(jié)構(gòu),它具有天然材料所不具備的奇異的電磁特性[7-10].超材料在電磁波偏振態(tài)控制和光子集成方面成為非常有潛力的新技術(shù)[11,12].利用超材料技術(shù)可以使線偏振光的偏振方向旋轉(zhuǎn)已經(jīng)被證實(shí),而且控制偏振光的性能顯著[13].超材料結(jié)構(gòu)包括亞波長微結(jié)構(gòu)陣列,具有亞波長狹縫及微孔陣列的金屬膜[14]等.目前人們對超材料偏振轉(zhuǎn)換器進(jìn)行了廣泛研究.Cong等[15]利用三層不同方向的金屬光柵實(shí)現(xiàn)了90°偏振轉(zhuǎn)換.Huang等[16]利用間隔一定距離的SiN膜和金屬膜及其上的矩形孔陣列(矩形孔長軸正交)實(shí)現(xiàn)了線偏振光90°偏振轉(zhuǎn)換.Cheng等[17]利用石墨烯超材料設(shè)計(jì)實(shí)現(xiàn)了波段可調(diào)的反射偏振轉(zhuǎn)換.Dong等[18]利用H型結(jié)構(gòu)超表面陣列單元實(shí)現(xiàn)了GHz波段的線性極化轉(zhuǎn)換圓極化波.Wu等[19]提出了一種陣列方形諧振腔,實(shí)現(xiàn)了GHz波段的反射偏振轉(zhuǎn)換.但是這些方法制作工藝復(fù)雜、轉(zhuǎn)換效率低、工作帶寬窄,限制了實(shí)際應(yīng)用.

本文提出一種基于超材料的陣列結(jié)構(gòu),在硅基底一面設(shè)計(jì)長方體硅納米塊陣列,另一面設(shè)計(jì)亞波長金屬光柵,納米塊長軸與金屬光柵夾角45°.經(jīng)過精確的模擬計(jì)算,結(jié)果表明設(shè)計(jì)模型實(shí)現(xiàn)了90°偏振轉(zhuǎn)換,在中波紅外波段具有偏振轉(zhuǎn)換效率高、波段寬的優(yōu)點(diǎn).

2 理論模型

圖1為研究模型結(jié)構(gòu),其中圖1(a)為模型截面示意圖,圖1(b)為納米塊陣列示意圖.模型分三層：上層為長方體硅納米塊陣列,納米塊高3μm,長1μm,寬0.4μm,納米塊陣列周期為1μm;中間層為硅基底;下層為金屬光柵,光柵材料為金,周期0.5μm,厚度0.6μm,占空比0.4,金屬光柵柵條與納米塊長軸夾角為45°.入射光為X軸方向偏振的線偏振光,入射方向沿-Z軸方向.本文采用時(shí)域有限差分(FDTD)法模擬,由于陣列的周期結(jié)構(gòu),計(jì)算時(shí)Z軸方向采用完全匹配層截?cái)嚯姶艌?X軸,Y軸方向采用周期邊界條件.硅的相對介電常數(shù)為11.56,金的色散采用Drude模型,,其中等離子體頻率ωp為1.38×1016rad/s,自由電子平均碰撞時(shí)間τ為33 fs[20].

圖1 偏振轉(zhuǎn)換模型Fig.1.Structure of polarization transform ation.

由(2)式可以看出僅當(dāng)δ=π及θ=π/4時(shí)等式成立,此時(shí)光偏振方向從平行于X軸的偏振方向完全轉(zhuǎn)換成平行于Y軸的偏振方向,說明入射線偏振光實(shí)現(xiàn)了90°偏振轉(zhuǎn)換.所以這就是本文設(shè)計(jì)模型中X軸與納米塊長軸夾角為45°的原因.

圖2 入射光的納米塊長軸和短軸分量示意圖Fig.2. Schem atic of com ponents along long-and short-axis.

由于同時(shí)滿足相位差δ=π的波段非常窄,這樣其他頻率的光只有部分實(shí)現(xiàn)了90°偏振轉(zhuǎn)換.此外,硅折射率在中波紅外約3.5,導(dǎo)致硅-空氣界面的反射率高達(dá)30%.為了提高偏振轉(zhuǎn)換后的透射光對比率及其透過率,我們在基底背面設(shè)計(jì)了亞波長金屬光柵,柵條方向平行于入射偏振光偏振方向.這樣經(jīng)過90°偏振轉(zhuǎn)換的偏振光可以透過金屬光柵,偏振方向沒有轉(zhuǎn)換的光被反射.我們采用有效介質(zhì)理論設(shè)計(jì)金屬光柵來提高透過率[21].對于偏振方向垂直光柵的透射光,金屬光柵等效薄膜的折射率為,其中, nw為金屬折射率,nf為光柵槽介質(zhì)折射率,f為金屬光柵占空比.這樣我們按照光學(xué)薄膜理論設(shè)計(jì)合適占空比的金屬光柵作為硅的減反射膜層,金屬光柵等效薄膜的折射率滿足為空氣折射率,ns為基底折射率.為了使反射率最小,金屬光柵厚度應(yīng)滿足h=λ0/(4n),λ0為中心波長.

3 結(jié)果與分析

圖3為透射光中不同偏振分量的透過率.Tx為透射光中偏振方向平行于X軸的線偏振光透過率,即偏振方向平行入射光偏振方向時(shí)光的透過率;Ty為透射光中偏振方向平行于Y軸的線偏振光透過率,即偏振方向垂直于入射光偏振方向時(shí)光的透過率.從圖3可明顯看出峰值波長3.698μm處透過率達(dá)到87%,在3.6-3.9μm波段透過率Ty大于80%, 3.4-4.5μm波段透過率Ty大于60%,3.2-4.8μm波段透過率Ty大于40%.可見采用本文設(shè)計(jì)的模型結(jié)構(gòu)可以實(shí)現(xiàn)寬帶、高效的90°偏振轉(zhuǎn)換.從圖4可以看出,透射光不同偏振分量透過率Ty和Tx在3-5μm光譜范圍內(nèi)的對比率大于104,說明本文提出的模型結(jié)構(gòu)經(jīng)90°偏振轉(zhuǎn)換后偏振光的偏振度非常高.

圖3 (網(wǎng)刊彩色)透射光中不同偏振分量的透過率Fig.3.(color on line)Transm ission of d iff erent polarization com ponents.

圖5為透射光在納米塊長軸和短軸的場分量之間產(chǎn)生的相位差.從圖中可以看出相位差δ=π在峰值波長3.698μm處,而且隨著波長增加相位差線性下降.這是因?yàn)榧{米塊長軸和短軸方向的等效折射率為定值(忽略硅折射率色散影響),所以在這兩個(gè)方向的光程差也為定值,在光程差一定時(shí),隨著波長增加相位差將減小.

圖6(a)為峰值波長3.698μm的入射偏振光電場矢量在納米塊陣列上表面的一個(gè)陣列周期內(nèi)的分布圖,圖6(b)為峰值波長3.698μm的入射偏振光透過納米塊陣列后的電場矢量在一個(gè)陣列周期內(nèi)的分布圖.其中圖6(a)的電場矢量分布基本平行于X軸,圖6(b)的電場矢量分布基本平行于Y軸.說明入射光的偏振方向旋轉(zhuǎn)了90°,從平行于X軸方向偏振態(tài)轉(zhuǎn)換為平行Y軸方向偏振態(tài).

圖4 透過率Ty和Tx的對比率Fig.4.Calcu lated transm ission contrast ratio.

圖5 納米塊長軸和短軸兩個(gè)場分量的相位差δFig.5.Transm ittance phase differenceδbetween the two field com ponents.

圖6 (網(wǎng)刊彩色)(a)峰值波長時(shí)在納米塊上的表面電場矢量分布;(b)峰值波長時(shí)透過納米塊后的電場矢量分布Fig.6.(color on line)(a)D istribu tion of electric field vector on the su rface of nanorod at the peak wavelength; (b)d istribu tion of electric field vector through the nanorod at the peak wavelength.

圖7為不同硅納米塊高度下的透過率,可見隨著納米塊高度增加,峰值波長紅移.這是因?yàn)榧{米塊高度增加,滿足相位差δ=π的波長隨著增大.圖8為不同硅納米塊寬度下的透過率,可見隨著納米塊寬度增加,峰值波長紅移.這表明通過調(diào)節(jié)納米塊高度及寬度就可以調(diào)節(jié)偏振轉(zhuǎn)換波段.從圖7和圖8可以看出,在每條曲線最高透射峰右側(cè)有一個(gè)明顯的透射谷.這是因?yàn)閷τ诠杌?納米塊陣列可以等效為一層低折射率膜層,當(dāng)?shù)刃庸鈱W(xué)厚度滿足λ/2整數(shù)倍時(shí),透過率為極小值.在其他波長時(shí)等效層對基底有增透作用,在滿足λ/4奇數(shù)倍時(shí)為透射極大值.所以透射谷為此處波長滿足納米塊等效層光學(xué)厚度等于λ/2整數(shù)倍所致.

圖7 (網(wǎng)刊彩色)不同硅納米塊高度下的透過率Fig.7.(color on line)Transm ission at d iff erent heights of silicon nanorod.

圖8 (網(wǎng)刊彩色)不同硅納米塊寬度下的透過率Fig.8.(color on line)Transm ission at d iff erent w id ths of silicon nanorod.

4 結(jié) 論

在硅基底兩個(gè)表面分別設(shè)計(jì)硅納米塊陣列和亞波長金屬光柵,硅納米塊長軸與金屬光柵夾角為45°.通過精確的FDTD模擬計(jì)算,結(jié)果表明該結(jié)構(gòu)實(shí)現(xiàn)了偏振光90°偏振轉(zhuǎn)換,并且這種結(jié)構(gòu)在中波紅外區(qū)域具有偏振轉(zhuǎn)換效率高、波段寬、對比率高的優(yōu)點(diǎn).本文所提結(jié)構(gòu)光學(xué)性能優(yōu)異,制備難度低,對于光傳輸控制有重要的應(yīng)用價(jià)值.

[1]Zhao H J,Yang S L,Zhang D 2009 Acta Phys.Sin.58 6236(in Chinese)[趙華君,楊守良,張東 2009物理學(xué)報(bào)58 6236]

[2]Chen J,Yan L S,Pan W,Luo B,Guo Z 2011 Acta Opt. Sin.31 1224001(in Chinese)[陳娟,閆連山,潘煒,羅斌,郭振2011光學(xué)學(xué)報(bào)31 1224001]

[3]Sundaram C M,Prabakaran K,Anbarasan P M,Ra jesh K B,M usthafa A M 2016 Chin.Phys.Lett.33 64203

[4]Dong C,Li B,Li H X,Liu H,Chen M Q,Li D D,Yan C C,Zhang D H 2016 Chin.Phys.Lett.33 74201

[5]W ang P,Shang Y P,Li X,Xu X J 2015 Chin.J.Lasers 42 116002(in Chinese)[王鵬,尚亞萍,李霄,許曉軍2015中國激光42 116002]

[6]LiC Z,W u B J 2010 Acta Opt.Sin.30 3153(in Chinese) [李崇真,武保劍2010光學(xué)學(xué)報(bào)30 3153]

[7]Han J F,Cao X Y,Gao J,Li S J,Zhang C 2016 Acta Phys.Sin.65 044201(in Chinese)[韓江楓,曹祥玉,高軍,李思佳,張晨2016物理學(xué)報(bào)65 044201]

[8]W ang G D,Liu M H,Hu X W,Kong L H,Cheng L L, Chen Z Q 2014 Chin.Phys.B 23 017802

[9]Fan Y N,Cheng Y Z,Nie Y,W ang X,Gong R Z 2013 Chin.Phys.B 22 067801

[10]Schu rig D,M ock J J,Justice B J,Cumm er S A,Pend ry J B,Starr A F,Sm ith D R 2006 Science 314 977

[11]Yang H H,Cao X Y,Gao J,Liu T,LiW Q 2013 Acta Phys.Sin.62 064103(in Chinese)[楊歡歡,曹祥玉,高軍,劉濤,李文強(qiáng)2013物理學(xué)報(bào)62 064103]

[12]Li S J,Gao J,Cao X Y,Zhang Z,Zheng Y J,Zhang C 2015 Opt.Express 23 3523

[13]Huang Ch P 2015 Opt.Express 23 251150

[14]Genet C,Ebbesen T W 2007 Nature 445 39

[15]Cong L,CaoW,Zhang X,T ian Z,Han J,Zhang W 2013 Appl.Phys.Lett.103 171107

[16]Huang C P,W ang Q J,Y in X G,Zhang Y,Li J Q,Zhu Y Y 2014 Adv.Opt.M ater.2 723

[17]Cheng H,Chen S Q,Yu P,Li J X,X ie B Y,Li Z C, T ian JG 2013 Appl.Phys.Lett.103 223102

[18]Dong G X,Shi H Y,X ia S,Li W,Zhang A X,Xu Z, W ei X Y 2016 Chin.Phys.B 25 084202

[19]W u J L,Lin B Q,Da X Y 2016 Chin.Phys.B 25 088101

[20]Zhu Z H,Liu K,Xu W,Luo Z,Guo C C,Yang B,M a T,Yuan X D,YeW M 2012 Opt.Lett.37 4008

[21]Liao Y L,Zhao Y 2014 Opt.Quan t.E lectron.46 641

(Received 18 January 2017;revised m anuscript received 1 May 2017)

Middle-wave infrared and broadband polarization conversion based on metamaterial

Jin Ke?Liu Yong-Qiang Han Jun Yang Chong-M in Wang Ying-Hui Wang Hui-Na

(X i’an Institute of App lied Op tics,X i’an 710065,China)

The polarization state is one of the m ost im portant basic properties of the electrom agnetic wave.Researchers have m ade great eff orts to manipulate it.Control of the polarization state of an electrom agnetic wave is a p rom ising promotion for figuring outmany practical engineering problem s in in frared remote sensing,op tical communication and infrared target recognition.In this paper,we propose a w ide-band and high-efficient linear-polarization converter on the basis of them etam aterial,which is com prised of silicon nanorod array and subwavelength m etal grating that can realize a 90°polarization converter of linearly polarized light and is com posed of silicon nanorod array cascade subwavelength m etal grating:on one side of design located is the cuboid silicon nanorod array,on the other side of the design the subwavelength m etallic grating on the silicon substrate,and the angle between silicon nanorod array and subwavelength metal grating is 45°.Because of the deference in geometrical dimension between the long axis and the short axis of the nanorod,results of the equivalent refractive index of the long axis direction and the short axis direction are different, and the anisotropic birefrigent effect is formed.Based on the Jones matrix,the feasibility of polarization converter is described.The polarization converter efficiency and polarization state of the structure are simu lated and analyzed by the finite-difference time-dom ain method.And the variation characteristics of polarization converter transm ittance are simulated under several nanorod with different heights and w idths.In order to im prove the contrast ratio and the transm ission,the effectivem edium theory is used to design them etal grating for im p roving the transm ission.According to the theory of op tical thin fi lm,we design the subwavelength m etal grating with suitab le duty cycle as the antireflection coating.The simulation results show that the structure can realize 90°rotation of linearly polarized light,the polarization converter efficiency is greater than 60%in a spectral range of 3.4-4.5μm and the contrast ratio is greater than 104in a spectral range of 3-5μm.This structure can effectively realize the 90°polarization conversion in the spectral range of medium wave in frared and has the advantages of high conversion efficiency and high contrast ratio. In addition,the range of spectral of polarization conversion can be changed by ad justing the height and w idth of the nanorod.It can be app lied to op tical transm ission control of optical network and optical information system,because of its excellent optical perform ance with the advantages of high polarization conversion efficiency and w ide band in the m id-in frared waveband and low preparation diffi cu lty.

polarization converter,subwavelength metal grating,metamaterial,nanorod

PACS:42.25.Bs,78.20.Bh,42.25.Ja,78.20.Ci DO I:10.7498/aps.66.134201

?通信作者.E-m ail:jinkegoodm an@163.com

PACS:42.25.Bs,78.20.Bh,42.25.Ja,78.20.Ci DO I:10.7498/aps.66.134201

?Corresponding author.E-m ail:jinkegoodm an@163.com