王玲玲+熱西代古麗·吾吉艾合買提+林琪+翟翔+劉桂東
收稿日期:20131121
基金項(xiàng)目:國(guó)家自然科學(xué)基金資助項(xiàng)目(11074069, 61176116)
作者簡(jiǎn)介:王玲玲(1955-),女,河北撫寧人,湖南大學(xué)教授
通訊聯(lián)系人,Email:llwang@hnu.edu.cn
摘要:應(yīng)用時(shí)域有限差分方法(FDTD)研究了基于金屬介質(zhì)金屬(MIM)波導(dǎo)缺陷諧振環(huán)結(jié)構(gòu)的傳輸特性. 該結(jié)構(gòu)由一通道波導(dǎo)和位于通道上方的缺陷諧振環(huán)組成,與無(wú)缺陷諧振腔結(jié)構(gòu)相比,缺陷諧振環(huán)結(jié)構(gòu)破壞了環(huán)形腔原有的共振模式,從而呈現(xiàn)出新穎的濾波特性. 當(dāng)缺陷尺寸發(fā)生改變時(shí),諧振環(huán)有效長(zhǎng)度發(fā)生變化,通過(guò)調(diào)整缺陷的尺寸,可以有效調(diào)節(jié)濾波波長(zhǎng),其數(shù)值計(jì)算值與傳輸線理論值基本吻合. 此外,通道波導(dǎo)與諧振環(huán)間的耦合強(qiáng)度在一定程度上依賴于缺陷的位置,因此通過(guò)調(diào)節(jié)缺陷的位置可以有效控制濾波強(qiáng)度. 與其他濾波器相比,此結(jié)構(gòu)在不改變結(jié)構(gòu)總尺寸的情況下,可調(diào)節(jié)濾波波長(zhǎng),實(shí)現(xiàn)了更寬頻段的濾波,并有效調(diào)節(jié)其透射率.當(dāng)缺陷尺寸設(shè)定為某一特定值時(shí),能實(shí)現(xiàn)模式間的簡(jiǎn)并,提高濾波性能.
關(guān)鍵詞:光學(xué)器件;MIM波導(dǎo);時(shí)域有限差分方法;傳輸線理論;濾波器
中圖分類號(hào):O469 文獻(xiàn)標(biāo)識(shí)碼:A
Study of the Transmission Characteristics
of the Structure of Defective Rectangular Ring
Resonator Based on MIM Waveguide
WANG Lingling, REXIDAIGULI.Wujiaihemaiti, LIN Qi, ZHAI Xiang, LIU Guidong
Abstract: The transmission characteristics of the structure of defective rectangular ring resonator based on metalinsulatormetal waveguide were investigated in the finite difference time domain method. This structure consists of a waveguide channel and a defective rectangular ring resonator,which is parallel to the waveguide.Compared with the perfect rectangular ring resonator, the structure with defect destroys the symmetry of the resonant modes in the resonator, which results in a novel filter function of the complex resonator. The effective length of the structure depends on the size of the defect, so filtering wavelength can be tuned by adjusting the dimension of the defect. The numerical simulation results are essentially in agreement with the transmission line theory calculation results. In addition, the coupling strength between the waveguide channel and defective rectangular ring resonator is dependent on the defect location, which is useful in the control of the transmittance at filtering wavelength. Compared with the other filtering structures, our structure can realize more broadband segment filtering, effectively adjust the filtering wavelength and control the transmittance without changing the overall size. When the size of detect is chosen on a certain value, a new double degenerated mode appears, which improves the filtering properties of the structure. It has potential application in integrated optics due to its miniaturization and simple fabrication process.
Key words: optical devices; MetalInsulatorMetal(MIM) waveguide; finite difference time domain method; transmission line theory; filter
表面等離激元(Surface Plasmon Polariton,SPPs)是入射光子與金屬表面自由電子相互作用形成的非輻射電磁模式,是沿著金屬介質(zhì)表面?zhèn)鞑サ南烹姶挪?,在納米量級(jí)上具有顯著的局域增強(qiáng)效應(yīng)[1-3]. 利用SPPs的這一特殊性質(zhì),可以有效實(shí)現(xiàn)亞波長(zhǎng)量級(jí)上的電磁傳輸與調(diào)控. 基于SPPs的納米光子器件是實(shí)現(xiàn)納米全光網(wǎng)絡(luò)的基礎(chǔ),那么,怎樣實(shí)現(xiàn)在納米尺度上對(duì)SPPs有效調(diào)控成為該領(lǐng)域研究者關(guān)注的熱點(diǎn). 例如,基于金屬納米顆粒陣列[4]或金屬納米線[5]的SPPs波導(dǎo)已經(jīng)在理論上提出并在實(shí)驗(yàn)上獲得驗(yàn)證,但此類結(jié)構(gòu)能量損失大,有效傳播距離小,難以獲得應(yīng)用. 而基于金屬絕緣體金屬(MetalInsulatorMetal,MIM)的SPPs波導(dǎo)結(jié)構(gòu)可以避免出現(xiàn)輻射模和泄漏模,有效地將電磁波局域在亞波長(zhǎng)結(jié)構(gòu)內(nèi),從而實(shí)現(xiàn)光在納米尺度內(nèi)的有效傳輸[6]. 近年來(lái),關(guān)于MIM結(jié)構(gòu)的功能器件,如光分束器[7],定向耦合器[8],布拉格反射器[9-10],濾波器[11-18]等已有報(bào)道. 研究者利用諧振腔的共振特性設(shè)計(jì)了多種濾波結(jié)構(gòu),基于MIM結(jié)構(gòu)的環(huán)形濾波器[16-18]具有選頻特性好,結(jié)構(gòu)緊湊等優(yōu)點(diǎn),通過(guò)調(diào)節(jié)其結(jié)構(gòu)參數(shù),例如諧振腔尺寸,諧振腔與MIM波導(dǎo)的耦合距離,有效折射率分布等,實(shí)現(xiàn)其濾波特性的調(diào)節(jié).
近年來(lái),基于MIM矩形諧振腔結(jié)構(gòu)的表面等離子體波導(dǎo)濾波器的研究指出,通過(guò)調(diào)整諧振腔的長(zhǎng)度,可以有效地濾掉特定波長(zhǎng)[19],且能量損耗小. 然而,這種結(jié)構(gòu)由于尺寸的限制,無(wú)法實(shí)現(xiàn)更寬頻段的濾波. 目前,理論上提出一種基于MIM波導(dǎo)填充諧振環(huán)結(jié)構(gòu), 該結(jié)構(gòu)由一通道波導(dǎo)和與通道耦合的諧振腔組成,SPPs在該諧振腔內(nèi)傳播時(shí)發(fā)生共振耦合形成駐波,通過(guò)在環(huán)中引入金屬結(jié)構(gòu)改變諧振腔的耦合長(zhǎng)度,實(shí)現(xiàn)濾波帶寬的調(diào)控,基于該結(jié)構(gòu)的波分復(fù)用器也隨之提出[20]. 以上研究中,均通過(guò)改變結(jié)構(gòu)尺寸達(dá)到調(diào)控濾波波長(zhǎng). 為了使結(jié)構(gòu)更加緊湊、工藝更加簡(jiǎn)單,本文提出并在數(shù)值上證實(shí)了基于缺陷諧振環(huán)MIM表面等離激元波導(dǎo)結(jié)構(gòu)濾波器,該結(jié)構(gòu)由一通道波導(dǎo)和位于通道上方的缺陷諧振環(huán)組成, 采用時(shí)域有限差分(FDTD)方法,通過(guò)改變?nèi)毕莸膸缀纬叽?,模擬計(jì)算該結(jié)構(gòu)的透射譜及共振模式下的磁場(chǎng)分布,并與傳輸線模型的計(jì)算結(jié)果進(jìn)行比較,以明確該濾波器的傳輸特性. 結(jié)果表明,缺陷的設(shè)置破壞了環(huán)腔結(jié)構(gòu)諧振腔原有的對(duì)稱性,影響原有的幾種共振模式,從而出現(xiàn)了新穎的濾波特性. 其濾波特性依賴于缺陷的尺寸,通過(guò)改變?nèi)毕莩叽缈梢杂行д{(diào)節(jié)濾波波長(zhǎng),并且當(dāng)缺陷尺寸設(shè)定為某一特定值時(shí),有些模式間發(fā)生簡(jiǎn)并,可以提高該結(jié)構(gòu)的濾波性能. 此外,缺陷諧振環(huán)結(jié)構(gòu)的部分諧振模式依賴于缺陷的位置,當(dāng)缺陷位置不同時(shí),通道波導(dǎo)與缺陷諧振環(huán)之間的耦合強(qiáng)度不同,對(duì)不同共振波長(zhǎng)下的透射率有一定的影響.
1數(shù)值模擬與結(jié)構(gòu)分析
基于缺陷諧振環(huán)MIM表面等離激元波導(dǎo)結(jié)構(gòu)濾波器的結(jié)構(gòu)如圖1(a)所示.在數(shù)值模擬過(guò)程中,設(shè)通道波導(dǎo)和諧振環(huán)寬度均為d=50 nm, 通道波導(dǎo)與缺陷諧振環(huán)之間的耦合寬度為t=20 nm,環(huán)長(zhǎng)度L=300 nm,諧振環(huán)的缺陷寬度為a,深度為b. 通道波導(dǎo)和缺陷諧振環(huán)中填充介質(zhì)均為空氣(εd=1). 灰色部分為金屬Ag,其相對(duì)介電常數(shù)在可見(jiàn)光到近紅外波段可以采用Drude模型[21-22]進(jìn)行計(jì)算:
式中:εω為入射頻率無(wú)限大時(shí)對(duì)應(yīng)的介電常數(shù),其值約為3.7; ωp為金屬表面電荷發(fā)生集體振蕩的本征頻率,其值約為1.38×1016rad/s; γ為金屬中電荷發(fā)生集體振蕩的阻尼系數(shù),其值約為2.73×1013rad/s; ω為入射波頻率. 使用FDTD Solution 6.0軟件進(jìn)行模擬計(jì)算,計(jì)算步長(zhǎng)設(shè)定為dx=dy=2 nm,邊界條件均采用完全匹配層(PML). S處放置橫磁波模式波源,Q處放置能量監(jiān)控器.
采用傳輸線理論分析該濾波結(jié)構(gòu)的共振條件時(shí)[23],等效電路如圖1(b)所示.缺陷諧振環(huán)的等效阻抗定義為:
Zequ=R+iω(Lm+Le)+1/iωC.(2)
式中:等效電阻R=leff/σ b,leff=4(L-2d)+2b,σ=iω(εm-ε0),leff為缺陷環(huán)有效長(zhǎng)度,σ為等效電導(dǎo)率;Lm=μ0(L-2d)2為諧振環(huán)的磁場(chǎng)電感;Le=Leff/ω2εmd為諧振環(huán)的電子自感;C=ε0d/a 為缺陷結(jié)構(gòu)的等效電容. 諧振環(huán)濾波器的等效電路如圖1(b)所示,等效阻抗Zequ作為負(fù)載,加載在特征阻抗為Z0=β dε0ω的傳輸線上,則廣義阻抗可表示為:
Z=ZL=ZR=Z0(Zequ/2)-iZ0tan (β leff/2)Z0-i(Zequ/2)tan (β leff/2).(3)
式中:傳播常數(shù)β=neff /ε0ω可以由MIM波導(dǎo)的色散關(guān)系εdkm+εmkdtanh(-ikdd/2)=0得出,k0=2π/λ為真空中的波矢,km=(β2-εmk20)1/2,kd=(β2-εd k20)1/2.當(dāng)復(fù)阻抗匹配,即Z*L=ZR時(shí),可以得到共振條件和相應(yīng)的共振波長(zhǎng)λm.
2結(jié)果討論與分析
當(dāng)結(jié)構(gòu)未引進(jìn)缺陷(即a=b=0)時(shí),由圖2 (a) 的透射譜可以看出,3個(gè)波谷對(duì)應(yīng)的共振波長(zhǎng)分別為λ=681,747,1 372 nm. 圖2(b) 為λ=681 nm時(shí)的磁場(chǎng)分布,磁場(chǎng)強(qiáng)度主要集中在環(huán)形腔四邊中心位置,對(duì)應(yīng)TM2f模式. 圖2(c) 為共振波長(zhǎng)λ=747 nm時(shí)對(duì)應(yīng)的TM2c模式,磁場(chǎng)強(qiáng)度主要集中在環(huán)形腔4個(gè)頂點(diǎn)位置,該模式是由于環(huán)形腔的4個(gè)轉(zhuǎn)角使SPPs發(fā)生反射共振,導(dǎo)致相應(yīng)的電磁能量有效地局域在環(huán)形腔內(nèi)形成的. 圖2(d) 為共振波長(zhǎng)λ=1 372 nm時(shí)對(duì)應(yīng)TM1模式. 根據(jù)傳輸線理論可以預(yù)測(cè),改變環(huán)形腔的有效長(zhǎng)度可以調(diào)控濾波器的頻率特性. 結(jié)構(gòu)中引入缺陷,在不改變結(jié)構(gòu)總尺寸大小的情況下,能改變共振環(huán)的有效長(zhǎng)度,從而達(dá)到調(diào)節(jié)濾波波長(zhǎng)的目的.
為了驗(yàn)證上述預(yù)測(cè)理論,設(shè)缺陷寬度與深度相等,即a=b,研究缺陷邊長(zhǎng)對(duì)傳輸特性的影響. 結(jié)果顯示,當(dāng)0 圖4分別給出TM1g,TM2g模式濾波波長(zhǎng)與缺陷寬度a及缺陷深度b的關(guān)系. 固定缺陷深度b=50 nm,改變?nèi)毕輰挾萢,發(fā)現(xiàn)隨著a的增大,TM2g模式對(duì)應(yīng)的濾波波長(zhǎng)沒(méi)有明顯變化,而TM1g所對(duì)應(yīng)的共振波長(zhǎng)隨寬度a單調(diào)遞增,如圖4(a)所示.固定缺陷寬度a=50 nm,改變?nèi)毕萆疃萣時(shí),透射譜上出現(xiàn)3個(gè)波谷,分別對(duì)應(yīng)TM2g,TM2c和TM1g模式,其中TM2g,TM1g模式對(duì)應(yīng)的共振波長(zhǎng)隨深度b單調(diào)遞增,如圖4(b)所示.因此,通過(guò)改變?nèi)毕莸慕Y(jié)構(gòu)參數(shù)可以調(diào)節(jié)濾波波長(zhǎng).
最后研究缺陷設(shè)置在不同位置時(shí)該結(jié)構(gòu)的傳輸特性. 在其他參數(shù)不變的情況下,取缺陷尺寸a=b=150 nm. 圖5(a)和(b)分別為正立的凹字形結(jié)構(gòu)與朝右的凹字形結(jié)構(gòu)的透射譜.由圖5可知,波長(zhǎng)分別為716,864,1 314,1 830 nm處出現(xiàn)波谷,當(dāng)共振波長(zhǎng)分別為716,864 nm時(shí)發(fā)生二級(jí)諧振,對(duì)應(yīng)模式分別為TM2c和TM2g;當(dāng)波長(zhǎng)分別為1 314,1 830 nm時(shí)發(fā)生一級(jí)諧振,分別對(duì)應(yīng)TM1n和TM1g模式. 圖6(a)~圖6(h)為在2種情況下,4種模式對(duì)應(yīng)的磁場(chǎng)分布,由圖6(a)和(e)可知,波長(zhǎng)716 nm對(duì)應(yīng)TM2c模式,磁場(chǎng)均局域在諧振腔的4個(gè)角并無(wú)差異,然而,TM2g,TM1n,TM1g模式的磁場(chǎng)分布不同,其中TM2g,TM1g模式磁場(chǎng)局域在缺陷里,如圖6(d)和圖6(h)所示,這是由缺陷諧振環(huán)表面的環(huán)形電流引起的. 此外,對(duì)于2種不同缺陷位置,缺陷環(huán)與通波導(dǎo)間的耦合強(qiáng)度不同.
與正立的凹字形結(jié)構(gòu)相比,朝右的凹字形缺陷在TM1n模式下諧振環(huán)與通道之間的耦合較弱,而TM1g模式下缺陷諧振環(huán)與通道波導(dǎo)間的耦合強(qiáng)度較強(qiáng),因此調(diào)節(jié)缺陷位置可以有效控制諧振強(qiáng)度.
3結(jié)論
本文應(yīng)用時(shí)域有限差分方法研究了基于MIM波導(dǎo)缺陷諧振環(huán)結(jié)構(gòu)的傳輸特性. 結(jié)果表明,當(dāng)設(shè)置缺陷時(shí),破壞了環(huán)腔結(jié)構(gòu)諧振腔原有的對(duì)稱性,影響原有的幾種共振模式,從而出現(xiàn)了新穎的濾波特性. 當(dāng)改變?nèi)毕輰挾群蜕疃葧r(shí),可以有效調(diào)節(jié)不同模式所對(duì)應(yīng)的濾波波長(zhǎng). 將缺陷尺寸調(diào)節(jié)到特定值時(shí),產(chǎn)生了新的共振模式,提高了該結(jié)構(gòu)的濾波性能. 最后研究了缺陷位置對(duì)共振模式的影響,缺陷位置不同時(shí),通道波導(dǎo)與缺陷諧振環(huán)之間的耦合強(qiáng)度不同,因此通過(guò)調(diào)節(jié)缺陷的位置可以有效控制濾波強(qiáng)度. 以上結(jié)果將有助于設(shè)計(jì)復(fù)合結(jié)構(gòu)濾波器,在集成光學(xué)器件設(shè)計(jì)方面具有潛在的應(yīng)用價(jià)值.
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[13]XIAO S, LIU L, QIU M. Resonator channel drop filters in a plasmonpolaritons metal[J]. Opt Express, 2006, 14(7): 2932-2937.
[14]LU H, LIU X, MAO D,et al.Tunable bandpass plasmonic waveguide filters with nanodisk resonators[J]. Opt Express, 2010, 18(17): 17922-17927.
[15]WANG G, LU H, LIU X,et al.Tunable multichannel wavelength demultiplexer based on MIM plasmonic nanodisk resonators at telecommunication regime[J]. Opt Express, 2011,19(4): 3513-3518.
[16]HOSSEINI A, MASSOUD Y. Nanoscale surface Plasmon based resonator using rectangular geometry[J]. Appl Phys Lett, 2007, 90(18): 181102.
[17]WANG T, WEN X, YIN C,et al.The transmission characteristics of surface plasmon polaritons in ring resonator[J]. Opt Express, 2009, 17(26): 24096-24101.
[18]PENG X, LI H, WU G,et al.Research on transmission characteristics of aperturecoupled square ring resonator based filter[J]. Opt Commun, 2013, 294: 368-371.
[19]王玲玲, 張振, 王柳,等.基于矩形諧振腔MIM波導(dǎo)結(jié)構(gòu)的表面等離子體帶阻濾波器[J]. 湖南大學(xué)學(xué)報(bào):自然科學(xué)版,2012, 39(5): 65-68.
WANG Lingling, ZHANG Zhen, WANG Liu,et al.Study of the surface plasmon bandstop filter based on the structure of rectangular resonator MIM waveguide[J]. Journal of Hunan University: Natural Sciences,2012, 39(5): 65-68.(In Chinese)
[20]ZAND I, MAHIGIR A, PAKIZEH T,et al.Selectivemode optical nanofilters based on plasmonic complementary splitring resonators[J]. Opt Express, 2012, 20(7): 7516-7525.
[21]LIN X, HUANG X. Toothshaped plasmonic waveguide filters with nanometeric sizes[J]. Opt Lett, 2008, 33(23): 2874-2876.
[22]HAN Z, FORSBERG E, HE S. Surface plasmon Bragg gratings formed in metalinsulatormetal waveguides[J]. IEEE Photon Technol Lett, 2007, 19(2): 91-93.
[23]ZAND I, ABRISHAMIAN M S, BERINI P. Highly tunable nanoscale metalinsulatormetal split ring core ring resonators (SRCRRs)[J]. Opt Express, 2013, 21(1): 79-86.
[24]ECONOMOU E N. Surface plasmons in thin films[J]. Phys Rev,1969, 182: 539-554.
[13]XIAO S, LIU L, QIU M. Resonator channel drop filters in a plasmonpolaritons metal[J]. Opt Express, 2006, 14(7): 2932-2937.
[14]LU H, LIU X, MAO D,et al.Tunable bandpass plasmonic waveguide filters with nanodisk resonators[J]. Opt Express, 2010, 18(17): 17922-17927.
[15]WANG G, LU H, LIU X,et al.Tunable multichannel wavelength demultiplexer based on MIM plasmonic nanodisk resonators at telecommunication regime[J]. Opt Express, 2011,19(4): 3513-3518.
[16]HOSSEINI A, MASSOUD Y. Nanoscale surface Plasmon based resonator using rectangular geometry[J]. Appl Phys Lett, 2007, 90(18): 181102.
[17]WANG T, WEN X, YIN C,et al.The transmission characteristics of surface plasmon polaritons in ring resonator[J]. Opt Express, 2009, 17(26): 24096-24101.
[18]PENG X, LI H, WU G,et al.Research on transmission characteristics of aperturecoupled square ring resonator based filter[J]. Opt Commun, 2013, 294: 368-371.
[19]王玲玲, 張振, 王柳,等.基于矩形諧振腔MIM波導(dǎo)結(jié)構(gòu)的表面等離子體帶阻濾波器[J]. 湖南大學(xué)學(xué)報(bào):自然科學(xué)版,2012, 39(5): 65-68.
WANG Lingling, ZHANG Zhen, WANG Liu,et al.Study of the surface plasmon bandstop filter based on the structure of rectangular resonator MIM waveguide[J]. Journal of Hunan University: Natural Sciences,2012, 39(5): 65-68.(In Chinese)
[20]ZAND I, MAHIGIR A, PAKIZEH T,et al.Selectivemode optical nanofilters based on plasmonic complementary splitring resonators[J]. Opt Express, 2012, 20(7): 7516-7525.
[21]LIN X, HUANG X. Toothshaped plasmonic waveguide filters with nanometeric sizes[J]. Opt Lett, 2008, 33(23): 2874-2876.
[22]HAN Z, FORSBERG E, HE S. Surface plasmon Bragg gratings formed in metalinsulatormetal waveguides[J]. IEEE Photon Technol Lett, 2007, 19(2): 91-93.
[23]ZAND I, ABRISHAMIAN M S, BERINI P. Highly tunable nanoscale metalinsulatormetal split ring core ring resonators (SRCRRs)[J]. Opt Express, 2013, 21(1): 79-86.
[24]ECONOMOU E N. Surface plasmons in thin films[J]. Phys Rev,1969, 182: 539-554.