SPR 波导耦合

Integrated optical SPR sensor based on mode conversion efficiencyH.S.Pang,T.W.Lee,M.G.Moharam,P.L.Likamwa and H.J.ChoA novel type of integrated optical surface plasmon resonance (SPR)sensor is proposed,for which the operational principle is based on the launching efficiency of eigenmodes in the sensor head.The sensor comprises an inverted-rib-type dielectric waveguide,a portion of which is covered with a thin gold layer.Eigenmodes in the sensor head are coupled modes of a surface plasmon polariton and a dielectric guided wave.The excitation efficiency of the coupled modes varies sig-nificantly depending on the refractive index of the analyte medium on the sensor head.Following this principle,the transmission coefficient of light through the sensor head can be used as a sensitive measure of the variation in the refractive index of the analyte medium.Introduction:Surface plasmon resonance (SPR)-based biomolecular detection techniques are widely used with well developed protocols [1].Current issues in developing SPR-based sensors are throughput enhance-ment and system miniaturisation [2].There have been approaches using SPR imaging systems based on an array of sensor heads on a prism for SPR sensor throughput improvement [3].Integrated optical SPR sensors have a great potential to realise high throughput and highly min-iaturised sensor systems [4,5].For integrated optical SPR sensor heads,absorption types and interferometer types were widely introduced and investigated during the time [5,6].In this Letter,we propose a novel mode-matching-type integrated optical sensor with a small sensor head length and reasonable tolerance in device fabrication.Sensor head scheme and guided modes:The present SPR sensor head was designed based on a singlemode inverted-rib-type dielectric wave-guide with a thin gold layer covering a portion of it,as shown in Fig.1.With this design,the gold layer covered region supports either only one bound mode or one bound mode and one quasi-bound mode depending on the refractive index of an analyte medium on the gold layer.front viewgold layercore cladinverted-rib-type dielectric waveguideFig.1SPR sensor based on inverted-rib-type waveguide,schematic view Inverted rib dimensions:width ¼2m m,height 1.5mm1614121086420–20.4000.30.20.10–0.1–0.2–0.3–0.410–10210abc d–1–2–3–4–5210–1–2–3–4–510–100Fig.2Electric field profiles of eigenmodes in SPR sensor head Two-dimensional plot a SPD-S mode b SPD-A modeThree-dimensional plot c SPD-S mode d SPD-A modeThe bound mode and the quasi-bound mode are coupled modes of surface plasmon polariton (SPP)and dielectric guided wave (DGW)mode.Figs.2a and b demonstrate a representative field profile of the bound mode.As demonstrated in the Figures,the fields of the bound mode on the gold layer and in the dielectric core point in the same direction and the mode is termed as SPD-S (symmetric)mode.The field profile of the quasi-bound mode,shown in Figs.2c and d ,illustrates that the quasi-bound mode has opposite field directions on the gold layer and in a dielectric core.The quasi-bound mode is termed as SPD-A (anti-symmetric)mode.n1460 1.6×101.4×101.2×101.0×108.0×106.0×104.0×102.0×10–2.0×10–4.0×10–6.0×10–8.0×10–1.0×10–1.2×100.012108h ei g ht (y )e l e c t r i cf i e l d (E y )e l e c t r i cf i e l d (E y )1.6×101.4×101.2×101.0×108.0×106.0×104.0×102.0×10–2.0×10–4.0×10–6.0×10–8.0×10–1.0×10–1.2×100.064212108h e i g ht (y )642n1464n1468n1470n1472n1476n1480n1484n1492n1472n1476n1480n1484n1492abFig.3Field profile variation depending on analyte refractive index a SPD-S mode b SPD-A modeRefractive index dependence of mode field profiles:The field profiles of SPD-S and SPD-A modes are sensitively affected by variation of the analyte medium’s refractive index on the gold layer.Figs.3a and b show the electric field profiles of SPD-S and SPD-A modes,respectively,depending on the analyte refractive index.The analyte’s refractive index was scanned from 1.460to 1.492.As illustrated in Fig.3a ,the SPD-S mode shows strong field strength in the dielectric core and relatively weak field strength on the gold layer when the refractive index of the analyte on the gold film is small.Conversely,SPD-A has strong field strength in the dielectric core region in the high refractive index range,as shown in Fig.3b .Mode excitation efficiency and transmittance:The field profile with strong field strength in the dielectric core region,which is similar to the field profiles of the dielectric waveguide (DEWG)mode,makes better mode conversion from DEWG mode to one of the SPD modes.Because the field profiles of the eigenmodes in the sensor head are sig-nificantly affected by the refractive index condition of the analyte on the sensor head,the mode conversion efficiency and finally the transmission characteristics of the sensor head are strongly affected by an analyte medium’s refractive index.In this work,the power conversion efficiency was calculated using the overlap integral equation [7]:a n m ¼2ðþÀð11E i ÂH Ãn m dxdyFig.4represents modal power conversion efficiencies and decay con-stants of SPD-S and SPD-A mode as a function of refractive index.ELECTRONICS LETTERS 31st July 2008Vol.44No.160.500.450.400.35p o w e r c o n v e r s i o n c o e f f i c i e n t0.301.4551.460 1.4651.470analyte refractive indexpower conversion coefficient: DEWG to SPD modesmode decay constantSPD-S mode SPD-A mode1.475 1.480 1.485 1.490 1.4950.250.200.15Fig.4Analyte refractive index dependence of power conversion coefficient and decay constants of SPD-S and SPD-A modesBased on the mode power conversion efficiency and the decay charac-teristics of the eigenmodes in a sensor head,the dependence of the trans-mission coefficient is plotted against the refractive index of the analyte in Fig.5.Each line indicates a different sensor head length.When the sensor head length is short,the transmission characteristic is dominantly affected by mode conversion efficiency.However,for a longer sensor head length,the propagation attenuation of the mode significantly con-tributes to the transmission spectrum.0.240.220.200.180.160.140.120.100.080.060.040.020.00–0.021.4551.4601.4651.470 1.475refractive indextransmittance15 m m 20 m m 100 m m 400 m m1.4801.4851.4901.495t r a n s m i t e d p o w e rFig.5Refractive index dependence of transmissionConclusion:We present a novel type of integrated optical SPR sensor based on field profile matching of a dielectric waveguide mode and the eigenmodes in an SPR sensor head.This type of sensor head has the potential to realise an SPR sensor head with significantly reduced length and requiring reasonable device fabrication tolerance.Further study on the optimisation of sensor dimensions and the refractive indices of each dielectric layer will lead to aqueous analyte operation and sensitivity improvements.#The Institution of Engineering and Technology 200812June 2008Electronics Letters online no:20081684doi:10.1049/el:20081684H.S.Pang,M.G.Moharam and P.L.Likamwa (CREOL:The College of Optics and Photonics,University of Central Florida,4000Central Florida Blvd.,Orlando,FL 32816,USA )E-mail:hbang@T.W.Lee (Center for Computation &Technology,Louisiana State University,216Johnston Hall,Baton Rouge,LA 70803,USA )H.J.Cho (Department of Mechanical,Materials and Aerospace Engineering,University of Central Florida,4000Central Florida Blvd.,Orlando,FL 32816,USA )References1Spadavecchia,J.,Manera,M.G.,Quaranta,F.,Siciliano,P.,and Rella,R.:‘Surface plamon resonance imaging of DNA based biosensors for potential applications in food analysis’,Biosens.Bioelectron.,2005,21,pp.894–9002Homola,J.:‘Present and future of surface plasmon resonance biosensors’,Anal.Bioanal.Chem.,2003,377,pp.528–5393Piliarik,M.,Vaisocherova,H.,and Homola,J.:‘A new surface plasmon resonance sensor for high-throughput screening applications’,Biosens.Bioelectron.,2005,20,pp.2104–21104Harris,R.D.,Luff,B.J.,Wilkinson,J.S.,Piehler,J.,Brecht,A.,Gauglitz,G.,and 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