We propose a photonic crystal biosensor, operating at a single wavelength, based on analysis of resonant guided modes in the spatial Fourier domain. Sensitivities of 65 degrees per RIU and more have been simulated

Bienstman, Peter

Hallynck, Elewout

English

Elewout Hallynck

Peter Bienstman

Crossref

Photonic crystal biosensor in spatial fourier domain

Ghent University Academic Bibliography

Photonic Crystal biosensor in spatial Fourier domainElewout HallynckGhent University - imecPhotonics Research GroupDepartment of Information Technology (INTEC)Sint-Pietersnieuwstraat 419000 Gent, BelgiumEmail: elewout.hallynck@intec.ugent.bePeter BienstmanGhent University - imecPhotonics Research GroupDepartment of Information Technology (INTEC)Sint-Pietersnieuwstraat 419000 Gent, BelgiumEmail: peter.bienstman@intec.ugent.beAbstract—We propose a photonic crystal biosensor, operatingat a single wavelength, based on analysis of resonant guidedmodes in the spatial Fourier domain. Sensitivities of 65 degreesper RIU and more have been simulated.I. INTRODUCTIONLabel-free biosensors today play (and will continue to doso) an important role in a broad range of applications (e.g.health care, military). Many sensing concepts have alreadybeen devised and published [1] but as will become clear inthe following, there is still room for expansion.In this paper, we propose a biosensor based on photoniccrystals (PhCs). The most common photonic crystal basedbiosensors monitor the shift of a resonance in the frequencyspectrum introduced either by a defect (i.e. point or waveg-uide) [2] or guided resonances [3]. The drawback of theseconcepts is that there is a need for a broadband light source.The structure we propose only requires a single wavelengthsource as we monitor a shift in the angular spectrum of alight beam coupled out by the PhC.II. SENSING IN THE SPATIAL FOURIER DOMAINThe working principle of the proposed photonic crystal(PhC) biosensor is based on the coupling of true guided modesto resonant guided modes [4]. It can be seen that in a certainrange of frequencies (depending on the configuration of thePhC) light is coupled out as a Lorentzian beam under a certainangle (see Fig. 1) [5]. By changing the refractive index of theholes and cladding layers of the photonic crystal, the angle ofthis beam for a certain frequency will shift which allows usto use this structure as a sensor.The simulations in this paper have been carried out usingfreely available FDTD based software [6] for a 0.3a thicksilicon PhC slab consisting of holes of 0.8a diameter in atriangular lattice configuration (where a is the lattice constant).A TE-polarised light pulse is coupled in in the ΓM directionof the structure.In order to obtain the power profile of the outcoupled lightwe use the electric and magnetic fields determined by thesoftware to calculate the Poynting vector projected on thevertical direction. By applying a temporal Fourier transformfollowed by a spatial Fourier transform in two dimensions, wecan study the angular spectrum of the Poynting vector for aqfFig. 1. Light is coupled out as a Lorentzian beam with inclination θ andazimuth φ.broad range of frequencies based on only one simulation. Anexample of the angular spectrum of the Poynting vector fora certain frequency can be seen in Fig. 2. We can verify thatthe position of the Lorentzian peak indeed shifts with varyingrefractive index of the PhC.III. SIMULATION RESULTSWhen desiging a photonic crystal, several parameters canbe tuned, each having an influence on the performance of thedevice. The most important parameter in this case is the typeof cladding configuration of the PhC on which we will mainlyfocus. We will also shortly discuss a few other parameters andtheir effects.A. Influence of cladding configurationDepending on the method of analyte delivery and otherfabrication considerations, there are three possible claddingconfigurations: the refractive index only changes in the PhCholes (the no cladding type), both the PhC holes and oneof the cladding layers change while the other one remainsair (asymmetric cladding) or both cladding layers and thePhC holes have their refractive index changed (symmetriccladding). For these three situations, several simulations havebeen carried, each time using a different refractive index sowe can determine a shift per refractive index unit (RIU) of the04590-45-90-90 -45 0 45 90q (degrees)f (degrees)-45 0 45 90-90q (degrees)Data 1.33Data 1.38Lorentzian fit 1.33Lorentzian fit 1.38Fig. 2. Spectral image of outcoupled light (top) with cross-section at φ=0◦(bottom) where the Lorentzian shape can be distinguished. The refractiveindex of the cladding layers above and below the PhC as well as that ofthe holes all vary (symmetric cladding type). The normalised frequency is0.53488.0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.564050607080Normalized frequencyPeak shift per RIU(degrees)SymmetricNo claddingAsymmetric64.9°/RIU51.0°/RIU40.6°/RIUFig. 3. Sensitivity for various cladding configurations. As expected, thesymmetric cladding case is the most sensitive.Lorentzian beam angle. The results of these simulations canbe seen in Fig. 3.As we would expect, the symmetric cladding configurationis the most sensitive with a minimal sensitivity of 65 degreesper RIU. It is recommended to operate the device at ahigh frequency because it can be shown that with increasingfrequency, θ decreases (therefore making it easier to capturelight in a measurement setup) and the width of the Lorentzianshape also decreases which in turn has a positive effect on thelimit of detection.B. Other parametersBesides the different cladding configurations, we have alsoinvestigated the influence of several other parameters.We changed the direction of light excitation from ΓM toΓK and found that the Lorentzian becomes distorted and aportion of the light is also scattered at an angle φ=90◦. As weare unable to properly fit a Lorentzian shape to these angularspectra we can make an estimation of the shift based on thepeak maximum and find out that the sensitivity is roughlyequal to that of the case where we excite light in the ΓMdirection.As expected, the sensitivity increases with increasing holediameter and slightly increases with decreasing slab thickness.The number of periods in the direction of light propagationhas little influence on the beam width.IV. CONCLUSIONBased on the coupling of guided to free space modes,we have proposed a novel photonic crystal based biosensor.The major advantage of this device over resonance basedsensors (e.g. Mach-Zehnder interferometers, ring resonators)is the ability to operate the device at a single wavelength.The structure, with a minimal sensitivity of 65 degrees perrefractive index unit outperforms a surface plasmon resonancesensor in the angular regime [7]. Fabrication, compared to thisSPR sensor, is also less complicated as there is no need formetal deposition or fabrication of a coupling prism.ACKNOWLEDGMENTThe author would like to thank imec for providing a researchgrant. Simulations have been carried out using the StevinSupercomputer Infrastructure at Ghent University.REFERENCES[1] X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, andY. Sun, “Sensitive optical biosensors for unlabeled targets: A review,”Analytica Chimica Acta, vol. 620, no. 1-2, pp. 8 – 26, 2008.[Online]. Available: http://www.sciencedirect.com/science/article/B6TF4-4SJ2WV7-1/2/b631bf29724d89506db27aaac298026a[2] E. Chow, A. Grot, L. W. Mirkarimi, M. Sigalas, and G. Girolami,“Ultracompact biochemical sensor built with two-dimensionalphotoniccrystal microcavity,” Opt. Lett., vol. 29, no. 10, pp. 1093–1095,2004. [Online]. Available: http://ol.osa.org/abstract.cfm?URI=ol-29-10-1093[3] J. H. Schmid, W. Sinclair, J. Garcı´a, S. Janz, J. Lapointe, D. Poitras,Y. Li, T. Mischki, G. Lopinski, P. Cheben, A. Delaˆge, A. Densmore,P. Waldron, and D.-X. Xu, “Silicon-on-insulator guided mode resonantgrating for evanescent field molecular sensing,” Opt. Express,vol. 17, no. 20, pp. 18 371–18 380, 2009. [Online]. Available:http://www.opticsexpress.org/abstract.cfm?URI=oe-17-20-18371[4] S. Fan and J. D. Joannopoulos, “Analysis of guided resonances inphotonic crystal slabs,” Phys. Rev. B, vol. 65, no. 23, p. 235112, Jun2002.[5] E. Hallynck and P. Bienstman, “Photonic crystal biosensorbased on angular spectrum analysis,” Opt. Express, vol. 18,no. 17, pp. 18 164–18 170, Aug 2010. [Online]. Available:http://www.opticsexpress.org/abstract.cfm?URI=oe-18-17-18164[6] A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopou-los, and S. G. Johnson, “MEEP: A flexible free-software package forelectromagnetic simulations by the FDTD method,” Computer PhysicsCommunications, vol. 181, pp. 687–702, January 2010.[7] T. Kan, K. Matsumoto, and I. Shimoyama, “Nano-pillar structure for sen-sitivity enhancement of spr sensor,” in Solid-State Sensors, Actuators andMicrosystems Conference, 2009. TRANSDUCERS 2009. International,21-25 2009, pp. 1481–1484.

https://biblio.ugent.be/publication/2033230/file/3146318.pdf

Photonic crystal biosensor in spatial fourier domain

Abstract

Similar works

Full text

Available Versions

Crossref

Ghent University Academic Bibliography