Surface plasmon resonance sensor based on photonic crystal fiber filled with silver nanowires

Transcription

1 Optic Applict, Vol. XLI, No. 4, 211 Surfce plsmon resonnce sensor sed on photonic crystl fier filled with silver nnowires XIANGYONG FU, YING LU *, XIAOHUI HUANG, CONGJING HAO, BAOQUN WU, JIANQUAN YAO College of Precision Instrument nd Opto-Electronics Engineering, Tinjin University, Tinjin 372, P.R. Chin, Key Lortory of Opto-Electronics Informtion Technology (Tinjin University), Ministry of Eduction, Tinjin 372, P.R. Chin * Corresponding uthor: luying@tju.edu.cn Silver nnowire filled photonic crystl fiers re proposed in this pper to chieve surfce plsmon resonnce sensors nd overcome the compliccy nd difficulty of coting the holes in the photonic crystl fier. Opticl field distriutions of these fiers t different wvelengths re clculted nd simulted using the finite element method (FEM), nd the sensing properties re discussed in oth res of resonnt wvelength nd intensity detection. Numericl simultion results show tht crefully designed structure of the sensor rings out n excellent effect, with oth spectrl nd intensity sensitivity in the rnge of RIU, etter thn in the cse of similr structures coted with metl film, nd the friction is expected to e simplified. Keywords: surfce plsmon resonnce, photonic crystl fier, silver nnowire, sensor. 1. Introduction The detection nd nlysis of iologicl mterils re widely needed in mny importnt res in production nd dily life, such s medicine, environment, iotechnology, food, etc. Surfce plsmon resonnce-sed iochemicl sensing method is currently the most dvnced rel-time nd lel-free detection technology. Surfce plsmon resonnce (SPR) refers to the excittion of surfce plsmon polritons (SPPs), which re electromgnetic wves coupled with free electron density oscilltions on the surfce etween metl nd dielectric medium (or ir) [1]. SPPs or surfce plsmon wves (SPWs) re TM polrized electromgnetic wves nd propgte long the interfce of metl nd dielectric mterils with electromgnetic field decying exponentilly in oth the metl nd dielectric [2].

2 942 X.Y. FU et l. Surfce plsmons cn e excited y light s the phse mtching condition etween the exciting light nd the surfce plsmons is met. Commonly employed methods for surfce plsmon excittion primrily use prism coupling, plnr wveguide coupling, fier optic coupling, nd grting coupling method. The prism light coupling method is mong the erliest techniques, including the Otto [3] nd Kretschmnn Rether configurtion [4, 5]. Plsmon resonnce depends on the refrctive index in the vicinity of the metl film within 1 nm. When the resonnce occurs, most energy of the exciting light is sored y the surfce plsmons, resulting in shrp decrese in intensity of incident light. The detection of iologicl mterils ound to the metl film using this method hs unprecedented high sensitivity [6]. Progress of iologicl response cn e otined y detecting the ngle [6, 7], spectrl [8] nd phse [9, 1] chrcteristics of the reflected light. For severl decdes of development, there hve een mny prcticl products ville used in detection of iochemicl sustnces. However, lrge size nd high cost gretly limit the ppliction rnge of the prism- -coupling configurtion. With the rpid development of opticl fier mnufcturing process, the need of minituriztion nd integrtion in sensors promotes the reserch nd development of the fier coupled plsmon resonnce sensors, nd mny compct fier-optic SPR sensors hve een studied, such s the metl film coted single-mode fier, multi-mode fier, polriztion mintining fier, tpered fier, Brgg grting, etc. [11 19]. Although the use of opticl fiers provides n ttrctive pltform for the SPR sensing, removing prt cldding of the single-mode fier y physicl or chemicl methods is often needed to mke the fier in close contct with the sustnces under test, which reduces the reliility of the device. However, in multimode wveguides only certin numer of higher-order modes will e phse mtched with plsmon. Thus, sensitivity nd stility of such sensors depend crucilly on lunch conditions [2]. In recent yers, mny specilists nd scholrs put forwrd photonic crystl fier (PCF) sed SPR sensors. The sensing mechnism is through coupling the leky core mode to the plsmon to chieve resonnce sensing. The use of the photonic crystl fier, with its flexile design, mkes it esy to equte the effective index of the core mode to tht of the mteril under test. Thus phse mtching condition etween the core mode nd the plsmon cn e esily chieved t the required wvelength nd then resonnce occurs. Especilly, the friction of sensors does not need removing cldding or tpering fiers s trditionl fiers do. So there is no prolem in the sensor pckge. Mny scientists hve put forwrd lot of surfce plsmon resonnce sensor designs sed on PCFs nd plenty of simultions nd clcultions hve een mde, showing gret dvntges nd good ppliction prospects of the new sensors. In 26, HASSANI nd SKOROBOGATIY proposed PCF-sed SPR sensors nd optimized micro- -fluid design concepts, nd refrctive index resolution of 1 4 RIU cn e chieved in the structure [21]. In 28, HAUTAKORPI et l. proposed nd numericlly nlyzed three-hole photonic crystl fier SPR sensor, with gold film deposited on the inner

3 Surfce plsmon resonnce sensor sed on photonic crystl fier wll of the three holes. Numericl results indicte tht the opticl loss of the Gussin guided mode cn e mde very smll nd tht the refrctive-index resolution for queous nlytes is RIU [22]. Metl nnoprticles nd nnowires (columns), nd other nnostructures cn lso e used to generte surfce plsmon resonnce. SPRs t visile wvelengths hve een reported in chemiclly grown gold nnorods with dimeters of 2 nm [23]. Arrys of prllel nnowires hve lso een suggested s suitle structures for suwvelength imging [24]. Surfce plsmon resonnce sensors sed on photonic crystl fier filled with metl nnowires hve not een reported in the literture. In this rticle, surfce plsmon resonnce sensors sed on photonic crystl fier filled with silver nnowires hve een nlyzed through the finite element method (FEM) y using COMSOL Multiphysics softwre, regulrity of the resonnt wvelength chnging with refrctive index of the smple hs een numericlly simulted, nd resonnt wvelength detection s well s intensity detection sensitivity hve lso een discussed. Numericl results show tht excellent sensing chrcteristics of the structure cn e chieved s the rdius of the silver nnowires is selected in the rnge of 15 nm to 3 nm, with oth spectrl nd intensity sensitivity in the rnge of RIU, etter thn the similr structures coted with metl film. Moreover, the friction of these structures needs no coting process. 2. SPR sensors sed on PCFs filled with silver nnowires of 15 nm rdius It is very difficult to cot the holes in photonic crystl fier with metl film (gold or silver film) of nnometers in thickness of nnoscle, especilly for otining uniform metl film. In this pper, photonic crystl fier filled with silver nnowires is considered s n lterntive to chieve surfce plsmon resonnce nd the sensing properties of this kind of structure re preliminrily studied. LM-8-type photonic crystl fier mde y NKT Compny is used here, with core dimeter of 8 μm, cldding ir hole dimeter of μm, nd lttice pitch of 5.6 μm. A photonic crystl fier selectively filled in the first lyer of the holes with silver nnowires of 15 nm rdius is first clculted nd studied. The fier nd silver nnowires re shown in Fig. 1. Figure 1 represents the fundmentl mode field distriution. Refrctive index of the silic glss fier cn e determined y the Sellmeier eqution, nd the reltive permittivity of silver (or refrctive index) is tken from n opticl hndook [25]. Liquid smple is filled in ll the holes nd the electromgnetic mode of the sensor fier is solved with the finite element method (FEM) (opticl field distriution of the fundmentl mode is shown in Fig. 1, where the rrow indictes the polriztion direction of mgnetic field). The ttenution constnt of the fundmentl mode is

4 944 X.Y. FU et l. Fig. 1. Structure digrm of the LMA-8 PCF which is prtilly filled with silver nnowires (indicted y lue dots) of 15 nm rdius in the first lyer of the holes (). Opticl field distriution of the fundmentl mode (). clculted for different wvelengths of incident light. The wvelength with mximl trnsmission loss cn e identified nd light intensity detection sensitivity t different wvelengths cn lso e investigted. Assume tht the mgnetic field solution otined in opticl fier cn e expressed s Hxyz (,, ) = Hxy (, )e i β z ω t ( ) (1) Here, ω is the ngulr frequency of light, nd t denotes time. The propgtion constnt β cn e expressed s β = n eff k (2) where k =2π/λ is the wve numer with λ eing the free-spce wvelength, nd n eff is the effective index of the mode. The decy in the power of the mode s it propgtes through the sensor fier (towrd the positive z-direction) cn e descried with the formul Pz ( ) = P e α z (3) where P is the power t the reference plne z =. It is esy to demonstrte tht the ttenution constnt α is proportionl to the imginry prt of the effective index, nd α = 2k Im( n eff ) (4) In fct, fier trnsmission loss coefficient is defined s P 1 α loss = log Z Pz ( ) (5)

5 Surfce plsmon resonnce sensor sed on photonic crystl fier Attenution constnt α [1 db/m] Smple index n = 1.33 n = Amplitude sensitivity S [1/RIU] Wvelength λ [ μm] Fig. 2. Reltionship etween wvelength nd ttenution constnt of the fundmentl mode of the LMA-8 PCF with the first lyer of the holes prtilly filled with silver nnowires of 15 nm rdius (). Intensity detection sensitivity curve (). From the ove two equtions, α loss = 1log(e)α = 4.343α cn e otined. As cn e seen, there is only constnt fctor difference. For ese of understnding nd convenience, the power ttenution constnt α is used to quntify the loss of the mode, nd db/m is still used s the unit. For the first lyer of the holes of the LMA-8 PCF eing prtilly filled with silver nnowires of 15 nm rdius, the reltionship etween wvelength nd ttenution constnt of the fundmentl mode is shown in Fig. 2, in which the solid nd dotted curves represent the refrctive indices of the smples, which re 1.33 nd 1.335, respectively. As shown in the figure, there is shrp loss pek in the rnge of 6 7 nm for ech curve. Tht is ecuse the resonnce etween the core mode nd the surfce plsmon mkes gret energy loss of light field in the core. When the refrctive index of the smple chnges from 1.33 to (Δn =.5), the resonnce pek shifts towrd longer wvelength, nd the mount of shift is out 11 nm (Δλ pek ). If the instrumentl pek-wvelength resolution is ssumed to e Δλ min =.1nm, the refrctive index resolution of the corresponding sensor cn e otined s Δλ min R = Δn = RIU Δλ pek (6) Another detection method is known s the power detection t fixed single wvelength, which is lso known s mplitude sed detection method. Assume tht the wvelength of the light is λ, nd the trnsmission length is L, nd then the power detection sensitivity for the refrctive index vrition Δn cn e defined s S( λ ) = PLλ (,, n + Δn ) PLλ (,, n ) PLλ (,, n ) ( Δn ) (7)

6 946 X.Y. FU et l. The sensor length L is typiclly limited y the modl trnsmission loss. A resonle choice of sensor length is L =1/α(λ, n ). Such choice of sensor length results in simple definition of sensitivity for the smll chnges in the nlyte refrctive index [26] S( λ ) PLλ (,, n ) = = PLλ (,, n ) n α ( λ, n ) α( λ, n ) n (8) In Figure 2 we present the mplitude sensitivity curve of the computed PCF-sed SPR sensor filled with silver nnowires of 15 nm rdius. As cn e seen in the digrm, the mximl sensitivity is chieved t 68 nm nd equls 28 RIU 1. It is typiclly sfe ssumption tht 1% chnge in the trnsmitted intensity cn e detected relily, which leds to the sensor resolution of RIU. 3. SPR sensors sed on PCFs filled with silver nnowires of 5 nm nd 3 nm rdii For the rdius of silver nnowires chnging to 5 nm, ttenution spectrum of the fundmentl mode nd the mplitude detection sensitivity curve re shown in Fig. 3 nd 3, respectively. As shown in Fig. 3, there re two close-set resonnce peks nd oth of them re not shrp enough nd not sensitive to the vrition of the smple refrctive index. Amplitude detection sensitivity is not high enough, either. In our opinion, there is degenerte SPW mode round the silver nnowires. If the nnowires re lrge enough, the evnescent wves inside the silver nnowires do not overlp. As the nnowire thickness decreses, the evnescent wves of the otherwise decoupled modes egin to overlp, nd the degenerte SPW mode therefore splits into two different modes. When the rdius of silver nnowires increses to 3 nm, the corresponding fundmentl mode ttenution spectrum nd mplitude detection sensitivity curve cn Attenution constnt α [db/m] Refrctive index n = 1.33 n = Wvelength [ m] Amplitude sensitivity S [1/RIU] λμ λμ Wvelength [ m] Fig. 3. Reltionship etween wvelength nd ttenution constnt of fundmentl mode of the LMA-8 PCF with the first lyer of the holes prtilly filled with silver nnowires of 5 nm rdius (). Intensity detection sensitivity curve ().

7 Surfce plsmon resonnce sensor sed on photonic crystl fier Attenution constnt α [db/m] Refrctive index n = 1.33 n = Amplitude sensitivity S [1/RIU] Fig. 4. Reltionship etween wvelength nd ttenution constnt of fundmentl mode of the LMA-8 PCF with the first lyer of the holes prtilly filled with silver nnowires of 5 nm rdius (). Intensity detection sensitivity curve (). e otined, s shown in Figs. 4 nd 4, respectively. When the refrctive index of the smple chnges from 1.33 to (Δn =.5), the resonnce pek shifts towrd longer wvelength, nd 13 nm of red shift (Δλ pek ) cn e chieved, corresponding to the spectrl resolution of refrctive index of RIU. The mximl mplitude sensitivity is chieved t 7 nm nd equls 22 RIU 1. It is lso ssumed tht 1% chnge in the trnsmitted intensity cn e detected relily, leding to the sensor resolution of RIU. A slight improvement is chieved compred with the rdius of 15 nm. After severl clcultions, the reltionship etween the refrctive index resolution of spectrl detection nd the rdius of silver nnowires is shown in Fig. 5. In the digrm, we cn see the refrctive index resolution is improved s the rdius of silver nnowires increses within the rdius rnge of 1 3 nm. For lrger size of nnowires, the index resolution is no longer improved, or even decresed. Refrctive index resolution [1/RIU] Fig. 5. Reltionship etween the refrctive index resolution of spectrl detection nd the rdius of silver nnowires.

8 948 X.Y. FU et l. Amplitude sensitivity S [1/RIU] r = 5 nm r = 1 nm r = 15 nm r = 2 nm r = 25 nm r = 3 nm r = 35 nm r = 4 nm Fig. 6. Intensity detection sensitivity curve corresponding to the PCF filled with different sizes of silver nnowires. Intensity detection sensitivity curve corresponding to the PCF filled with different sizes of silver nnowires is plotted in Fig. 6. The mximl mplitude sensitivity lso increses s the rdius of silver nnowires increses, nd strts to decrese down to certin rnge, which is similr to the resolution of spectrl detection. 4. SPR sensors sed on PCFs filled with silver nnowires of 15 nm rdius in the second lyer of holes From the contents mentioned ove, we cn see tht fier loss is quite lrge with silver nnowires filled in the first lyer of the holes, which gretly limits the length of the sensing fier (less thn 1 mm). In order to reduce the trnsmission loss, silver nnowires re selectively filled in the second lyer of holes, which is shown in Fig. 7. Figure 7 represents the fundmentl mode field distriution. Fig. 7. Structure digrm of the LMA-8 PCF which is prtilly filled with silver nnowires (indicted y lue dots) of 15 nm rdius in the second lyer of the holes (). Opticl field distriution of fundmentl mode ().

9 Surfce plsmon resonnce sensor sed on photonic crystl fier Attenution constnt α [db/m] Refrctive index n = 1.33 n = Amplitude sensitivity S [1/RIU] Fig. 8. Reltionship etween wvelength nd ttenution constnt of fundmentl mode of the LMA-8 PCF with the second lyer of the holes prtilly filled with silver nnowires of 5 nm rdius (). Intensity detection sensitivity curve (). The reltionship etween wvelength λ nd ttenution constnt of the fundmentl mode α is shown in Fig. 8, in which the solid nd dotted curves represent the refrctive indices of the smples, which re 1.33 nd 1.335, respectively. When the refrctive index of the smple chnges from 1.33 to 1.335(Δn =.5), 11 nm of red shift (Δλ pek ) of the resonnce pek cn e chieved, corresponding to the spectrl detection resolution of RIU. The mximl mplitude sensitivity is chieved t 68 nm nd equls 23 RIU 1. It is lso ssumed tht 1% chnge in the trnsmitted intensity cn e detected relily leding to the sensor resolution of RIU. It is cler tht the fier loss is lrgely reduced, s the silver nnowires re filled in the second lyer of holes, nd the sensor cn rech n order of mgnitude of 2 3 cm in length, while mintining similr sensitivity. 5. Summry Surfce plsmon resonnce sensors sed on photonic crystl fiers filled with silver nnowires of different sizes hve een nlyzed through the finite element method (FEM) y using COMSOL Multiphysics softwre nd the sensing properties re discussed in oth res of resonnt wvelength nd intensity detection. Numericl results show tht excellent sensing chrcteristics of the structure cn e chieved s the rdius of the silver nnowires is selected in the rnge from 15 to 3 nm, with oth spectrl nd intensity sensitivity in the rnge of RIU. Moreover, the friction of these sensor structures needs no coting process. It is lso found tht the sensors with silver nnowires filled in the second lyer of holes cn rech 2 3 cm in length, while hving high sensing sensitivity Acknowledgements The uthors would like to thnk the support of Mjor Stte Bsic Reserch Development Progrm of Chin (973 Progrm) (grnt numer: 21CB32781) nd the Ntionl Nturl Science Foundtion of Chin (grnt numer: ) for the work reported in this pper.

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