D-shped photonic crystl fiber plsmonic refrctive index sensor bsed on gold grting JUNJIE LU, 1 YAN LI, 1,* YANHUA HAN, 1 YI LIU, 1 AND JIANMIN GAO 2 1 Deprtment of Optoelectronics Science, Hrbin Institute of Technology, Weihi, 264209, Chin 2 School of Energy Science nd Engineering,, Hrbin Institute of Technology, Hrbin 150001, Chin *Corresponding uthor: liy@hit.edu.cn Received XX Month XXXX; revised XX Month, XXXX; ccepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX In this work, we proposed high resolution D-shped photonic crystl fiber (PCF) surfce plsmon resonnce (SPR) sensor bsed on gold grting. Gold grting is introduced to modulte the resonnce wvelength nd enhnce the refrctive index (RI) sensitivity. Structure prmeters of PCF nd gold grting re nlyzed by finite element method (FEM) for optimizing the SPR sensor. The simultion results indicte tht ir hole pitch, ir hole dimeter nd gold thickness nd grting constnt hve little influence on the sensitivity of refrctive index, which reduces the requirement of precise processing. For improving the resolution of RI sensing, two-feture (2F) interrogtion method which combines wvelength interrogtion nd mplitude interrogtion is used nd the mximum theoreticl resolution of SPR sensor reches to 5.98 10-6 RIU in rnge of 1.36-1.38 nd the wvelength sensitivity reches to 3340nm/RIU. The proposed SPR sensor shows potentil pplictions on developing high-sensitivity, rel time nd fst-response SPR-RI sensor. OCIS codes: (060.2370) Fiber optics sensors; (240.6680) Surfce plsmons; (060.5295) Photonic crystl fibers; (280.4788) Opticl sensing nd sensors. http://dx.doi.org/10.1364/ao.99.099999 1. INTRODUCTION Surfce plsmon resonnce (SPR) is considered s promising method to detect the tiny refrctive index (RI) chnge, since SPR is extremely sensitive to permittivity of surrounding environment. During the pst decde, high sensitivity nd rel-time detection mke the SPR sensor widely used in chemicl nd biologicl sensing[1, 2]. Recent yers, SPR sensor bsed on photonic crystl fiber (PCF) hs ttrcted lot of ttention since the first SPR sensor ws proposed by Jorgenson in 1993, where the fiber core ws coted with the gold film by removing section of the fiber cldding to exhibit the plsmonic response[3]. PCF bring new vitlity to the fbriction of SPR sensors since its unique bility of controlling the evnescent wve penetrtion. When phse mtching condition is met, surfce plsmon polritons (SPP) mode cn be excited nd results in strong resonnce bsorption. By detecting the resonnce wvelength, unknown nlyte RI cn be clculted. For investigting the performnce of SPR sensor, mny PCF structures, such s dul-core PCF[4], D-shped PCF[5, 6] nd birefringent PCF[7], hve been well studied in the lst decde. For improving the performnce of the PCF-SPR sensors, some theoreticl designs of metl coted lrge ir hole PCF-SPR sensors hve been reported[8, 9]. For instnce, Hssni nd Skorobogtiy designed PCF with lrge ir holes, nd they coted the inner surfces of the ir holes with metl films to fbricte the PCF-SPR sensor in which the lrge ir holes were used s micro chnnels. The simultion results showed tht the resolution of their sensor could rech 10 4 RIU[10]. For relizing extremely high sensitivity, lrger dimeter ir holes nd multichnnel nlyte-filled PCF structure is proposed. Fn et l. presented high sensitivity RI sensor bsed on two lrge ir holes chnnels filled with nlyte which wvelength sensitivity reches to 7040nm/RIU[11]. Most of the reported PCF SPR sensors re coted with the multiple metl lyer nd liquid is filled inside the ir-holes for incresing the sensitivity, which is difficult in terms of fbriction. Moreover, since the metl film is coted inside the ir holes, it is difficult to precisely control the thicknesses of the metl film nd it is lso time-consuming to fill nd re-fill the nlyte from the ir holes. In this pper, we proposed SPR sensor bsed on D-shped PCF, which is coted with gold grting for improving the RI sensitivity. Gold grting results in more loss of core mode nd by chnging the grting constnt, the resonnce wvelength could be modulted. The resonnce wvelength is sensitive to the surrounding refrctive index of gold grting, which shows promising potentil in RI sensing. Simultion results show tht the RI sensitivity is 3 times s much s norml D-shped PCF-SPR sensor with gold film. By investigting the structure prmeters of PCF, simultion results show tht PCF structure hs little influence on the sensitivity of SPR sensor. Furthermore, the two-feture (2F) interrogtion method is used to provide higher resolution for RI sensing. In RI rnge of 1.36-1.38, the wvelength sensitivity reches to 3340nm/RIU, nd the mximum theoreticl resolution reches to 5.98 10-6 RIU, which is much higher thn those of wvelength nd intensity interrogtion method.
2. DESIGN AND ANALYSIS The proposed PCF-SPR sensor is shown in Fig.1 (). There re two type ir holes in the PCF which is side polished with 3μm depth for creting plne sensing re. Before fbricting the gold grting, gold film should be coted on the plne surfce of D-shped PCF by mgnetron sputtering or pulsed lser deposition. Then gold grting cn be fbricted by using electron-bem lithogrphy or photolithogrphy. The cross section of the SPR sensor is shown in Fig. 1(b). The dimeter of smll ir holes is ds=0.8µm nd the dimeter of lrge ir holes is dl=1.6µm, respectively. The lttice pitch between ll ir holes is Λ=2.3µm. The thickness of the gold grtings is hg=40nm. The gold grting constnt is d0=1µm nd duty rtio is η=0.5. The used fiber mteril is fused silic nd the RI is determined by Sellmeier eqution [12] s: 14.61RIU -1. The gold grting cn enhnce the loss of the core mode which results in high mplitude sensitivity. The electric field of gold grting SPR sensor is shown in Fig. 2(c), which resonnce wvelength is 1562.56nm. Besides the y-polrized core mode we lso cn see the SPP mode ppers on the interfce of gold grting nd dielectric s shown in Fig. 2 (d). There is strong coupling between core mode nd SPP mode, when the resonnce wvelength is 1562.56nm. Therefore, strong coupling results in high confinement loss nd high mplitude sensitivity. 2 A1 A2 A3 2 B1 B2 B3 n( ) 1, (1) where A1=0.696166300, A2=0.407942600, A3=0.897479400, B1=4.67914826 10-3 µm 2, B2=1.35120631 10-2 µm 2 nd B3=97.9340025µm 2. The complex dielectric constnt of gold is given by Johnson nd Christy[13]. The confinement loss is clculted by using the imginry prt of the effective RI: (2) 4 =8.686 k0 Im( n eff ) 10 db/cm, where k0 = 2π/λ is the wvenumber nd Im(neff) is the imginry prt of effective index. The Gussin bem propgtes long the z-direction nd mode nlysis is performed in the XY plne. Fig. 2. () Resonnce wvelength of gold film SPR sensor nd gold grting SPR sensor with RI from 1.36 to 1.37. (b) The electric field of full coted gold film SPR sensor. (c) The electric field of gold grting SPR sensor. (d) Intensity distribution of SPP mode. Fig. 1. Schemtic of proposed PCF-SPR sensor () 3D view (b) crosssection of sensor We investigted the sensitivity difference between SPR sensor with gold film nd gold grting. As shown in Fig. 2 (), when the thickness of gold is 40nm nd the RI of nlyte chnges from 1.36 to 1.37, the resonnce wvelength of SPR sensor with full coted gold film shifts from 1389.40nm to 1397.14nm, which mens the wvelength sensitivity is 774nm/RIU which is not idel for RI sensing. The electric field of full coted gold film SPR sensor is shown in Fig. 2(b). Since the wvelength of common commercil brodbnd lser source is less thn 2μm, resonnce pek more thn 2μm is bndoned. However, when the gold film is replced with gold grting, which could be fbricted by lser direct writing or photolithogrphy, the resonnce wvelength shifts significntly s shown in Fig. 2(). The wvelength difference is 33.54nm s the resonnce wvelength shifting from 1562.56nm to 1596.10nm, which mens the wvelength sensitivity is 3354nm/RIU. The simultion result shows tht by introducing the gold grting, the wvelength sensitivity of the SPR sensor is 4 times of norml gold film SPR sensor. Moreover, the mplitude sensitivity of gold film SPR sensor is -2.66RIU -1 nd the gold grting SPR sensor is - 3. RESULTS AND DISCUSSIONS Considering the prcticl fbriction of the PCF, the dimeter of ir holes is investigted. The confinement loss spectr is shown in Fig. 3(), when the dimeter of smll holes (ds) chnges from 0.7µm to 0.9µm nd the mximum loss chnges from 149.83dB/cm to 173.48 db/cm. The wvelength sensitivity could be clculted s[14]: pek Sw, (3) n where Δλpek is the difference between two resonnce wvelength nd Δn is the vrition of nlyte RI. As shown in Tble 1, different dimeter of smll ir hole hs little influence on the wvelength sensitivity. Moreover, we lso investigted the sensitivity vrition, when the dimeter of lrge ir holes (dl) chnges. Fig.3 (b) illustrtes the loss spectr of gold grting SPR sensor with different dimeters of lrge ir holes. With incresing of the dimeter, the loss slightly deduces nd the resonnce wvelength shifts to shorter wvelength. But the dimeter of lrge ir hole hs limited influence on the wvelength sensitivity s shown in Tble 2. The lttice pitch (Λ) lso ws investigted nd the loss spectr is shown in Fig.3 (c). With incresing of the pitch, the resonnce wvelength shows slightly blue shift nd confinement loss decreses. The reson is tht lrge pitch results in chnging of the phse mtching condition. As shown in Tble 3, different lttice pitch hs slightly influence on wvelength sensitivity. The prmeters of gold grting lso ply vitl role in performnce of PCF-SPR sensors. As shown in Fig.3 (d), with incresing of the gold grting thickness, more energy of core mode is used for overcoming the dmping loss nd the resonnce wvelength shifts towrds to shorter wvelength. When the thickness of gold grting increses, the sensitivity slightly increses s shown in Tble 4. To optimize the performnce of gold grting SPR sensor, the resonnce wvelength is
selected round 1550nm nd the structure prmeters of PCF this should be ds=0.8µm, dl=1.6µm, Λ=2.3µm nd the thickness of gold should be hg=40nm. Fig. 3. Vrition of confinement loss s function of wvelength with vrying () dimeter of smll ir holes ds, (b) dimeter of lrge ir hole dl (c) lttice pitch of ir holes Λ nd (d) gold grting thickness hg, when the RI chnges from 1.36 to 1.37 Tble 1. Reltionship between ds nd wvelength sensitivity (Sw) ds (µm) 0.7 0.8 0.9 Sw(nm/RIU) 3320 3354 3390 Tble 2. Reltionship between dl nd wvelength sensitivity (Sw) dl (µm) 1.5 1.6 1.7 Sw (nm/riu) 3382 3354 3328 Tble 3. Reltionship between Λ nd wvelength sensitivity (Sw) Λ (µm) 2.25 2.3 2.35 Sw (nm/riu) 3368 3354 3344 Tble 4. Reltionship between hg nd wvelength sensitivity (Sw) hg (nm) 30 40 50 Sw (nm/riu) 3244 3354 3412 The gold grting prmeters were lso investigted for improving the performnce of the SPR sensor. As shown in Fig. 4 (), different grting constnt (d0) results in different resonnce wvelength, nd the wvelength sensitivity lso chnges s shown in Tble 5. We cn lso observe tht with incresing of grting constnt, the loss increses which results in higher mplitude sensitivity. Therefore, the grting constnt cn be used to modulte the resonnce wvelength. It is possible for SPR sensors working in expected wvelength such s common commercil brodbnd lser. Figure 4 (b) depicts the loss spectr of gold grting with different duty rtio (η). We cn see tht with incresing of duty rtio, the confinement loss increses significntly. When the duty rtio increses, the full width t hlf mximum (FWHM) reduces, which cn get better signl-to-noise (SNR) rtio. However, the sensitivity slightly decreses s shown in Tble 6. Thus, the grting constnt is set s d0=1µm nd the duty rtio could be set s η=0.5. Fig. 4. Vrition of confinement loss with vrying () grting constnt d0, (b) duty rtio η, when the RI chnges from 1.36 to 1.37. Tble 5. Reltionship between d0 nd Wvelength sensitivity (Sw) d0 (µm) 0.9 1.0 1.1 Sw (nm/riu) 3108 3354 3564 Tble 6. Reltionship between η nd Wvelength sensitivity (Sw) η 0.4 0.5 0.6 Sw (nm/riu) 3196 3354 3176 Since the SPR sensor is extremely sensitive to the chnge of surrounding environment RI, we investigted the trnsmission spectr by chnging the nlyte RI. From Fig. 5() we cn see tht with incresing of the RI the resonnce wvelengths re red-shifted. Therefore, the nlyte RI could be detected by mesuring the shift of resonnce wvelength. As shown in Fig. 5(b), with incresing of nlyte RI, the resonnce wvelength shows good liner response nd the wvelength sensitivity reches to 3340nm/RIU. In ddition, with incresing of nlyte RI, the effective index of SPP mode is close to tht of core mode, which results in more energy loss of core mode. Therefore, the mplitude lso cn be used to sensing the RI s convenient nd cost effective method. The mplitude sensitivity cn be defined from[15]: 1 1,n sa RIU, (4),n n where α(λ,n) is the loss t RI of n nd α(λ,n) is the loss difference between two djcent nlyte RIs.
Figure 5 (c) shows the mplitude sensitivity of the proposed SPR sensor clculted by Eq. (4). From Fig. 5(c), mximum mplitude sensitivity cn be obtined t the wvelength of 1660nm, which is - 69.3RIU -1. Bsed on the theory mentioned in Ref.[6], the two-feture (2F) sensitivity combines wvelength sensitivity nd mplitude sensitivity for enhncing the RI sensitivity. As embedded imge shown in Fig.5 (), if we detect the RI difference Δn of nlyte nd resonnce wvelength shifts from A to B, the 2F sensitivity cn be defined s: AB x y S2 F =. By normlizing Δx nd Δy, we cn mke wvelength sensitivity nd mplitude sensitivity into one dimension. For exmple, when nlyte RI chnges from 1.37 to 1.375, wvelength shifts from 1596.10nm to 1612.44nm nd loss shifts from 144.42dB/cm to 153.24dB/cm, which mens Δx=16.34nm, Δy=8.82dB/cm nd Δn=0.005, respectively. We ssume the length of sensing re is 1cm. If the wvelength resolution is 0.02nm (AQ3617B) nd mplitude resolution is 0.05dB, the RI chnge will bring 817 dt intervls for the x-xis (DIx=16.34/0.02=817) nd 176 dt intervls for the y-xis (DIy=8.82/0.05=176). Therefore, the 2F dt intervls DI2F=(817 2 +176 2 ) 1/2 =836. Finlly, the mximum theoreticl resolution of wvelength (Rw), mplitude (R) nd R2F cn be clculted by R DI DI DI. x y Results show tht 2F resolution (R2F=5.98 10-6 RIU) is higher thn theoreticl wvelength resolution (Rw =6.12 10-6 RIU) nd theoreticl mplitude resolution(r =2.84 10-5 RIU). Fig. 5. () Vrition of confinement loss s function of wvelength with different nlyte RI.(b) Wvelength sensitivity with the nlyte RI rnge from 1.36 to 1.38, (c) mplitude sensitivity with the vrition of nlyte RI. (5) (6) 4. CONCLUSION A prcticl D-shped photonic crystl fiber RI sensor bsed on surfce plsmon resonnce is demonstrted by putting gold grting on the flt plne. FEM hs been used to nlyte the performnce of this proposed SPR sensor. The simultion results show tht PCF structure prmeters hve little influence on the performnce of sensor. The gold grting is used to enhnce the sensitivity nd tune the resonnce wvelength. Using the 2F interrogtion method, the mximum theoreticl resolution of RI is improved significntly nd reches to 5.98 10-6 RIU, which is much higher thn wvelength nd mplitude resolution. Driving on the dvntge of ltest nnofbriction technique, the proposed structure cn be utilized for environmentl, biologicl nd biochemicl sensing pplictions. Funding Informtion. Ntionl Key Reserch nd Development Progrm of Chin (Grnt No. 2017YFF0209801); Nturl Science Foundtion of Shndong Province (Grnt No. ZR2018MF031); Ntionl Nturl Science Foundtion of Chin (Grnt No. 11504070). References [1] P. Singh, "SPR Biosensors: Historicl Perspectives nd Current Chllenges," Sensors & Actutors B Chemicl 229, 110-130 (2016). [2] A. A. Rift, G. A. Mhdirji, Y. M. Su, Y. G. Shee, R. Ahmed, D. M. Chow, nd F. R. M. 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