Bienstman, Peter

Hallynck, Elewout

English

Ghent University Academic Bibliography

Photonic crystal biosensor based onangular spectrum analysisElewout Hallynck and Peter BienstmanPhotonics Research Group, Department of Information Technology, Ghent University - imec,Sint-Pietersnieuwstraat 41, 9000 Gent, Belgiumelewout.hallynck@intec.ugent.beAbstract: The need for cost effective and reliable biosensors in e.g.medical applications is an ever growing and everlasting one. Not only dowe strive to increase sensitivity and detection limit of such sensors; easeof fabrication or implementation are equally important. In this work, wepropose a novel, photonic crystal based biosensor that is able to operateat a single frequency, contrary to resonance based sensors. In a certainfrequency range, guided photonic crystal modes can couple to free spacemodes resulting in a Lorentzian shape in the angular spectrum. ThisLorentzian can shift due to refractive index changes and simulations haveshown sensitivities of 65◦ per refractive index unit and more.© 2010 Optical Society of AmericaOCIS codes: (050.5298) Photonic crystals; (070.4790) Spectrum analysis; (130.6010) Sensors;(280.1415) Biological sensing and sensors.References and links1. M. Huang, A. A. Yanik, T.-Y. Chang, and H. Altug, “Sub-wavelength nanofluidics in photonic crystal sensors,”Opt. Express 17, 24224–24233 (2009).2. A. Densmore, M. Vachon, D.-X. Xu, S. Janz, R. Ma, Y.-H. Li, G. Lopinski, A. Delaˆge, J. Lapointe, C. C. Lueb-bert, Q. Y. Liu, P. Cheben, and J. H. Schmid, “Silicon photonic wire biosensor array for multiplexed real-timeand label-free molecular detection,” Opt. Lett. 34, 3598–3600 (2009).3. P. Debackere, R. Baets, and P. Bienstman, “Bulk sensing experiments using a surface-plasmon interferometer,”Opt. Lett. 34, 2858–2860 (2009).4. K. De Vos, J. Girones, T. Claes, Y. De Koninck, S. Popelka, E. Schacht, R. Baets, and P. Bienstman, “Multiplexedantibody detection with an array of silicon-on-insulator microring resonators,” Photonics Journal, IEEE 1, 225–235 (2009).5. T. Claes, J. Molera, K. De Vos, E. Schacht, R. Baets, and P. Bienstman, “Label-free biosensing with a slot-waveguide-based ring resonator in silicon on insulator,” Photonics Journal, IEEE 1, 197 –204 (2009).6. X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun, “Sensitive optical biosensors for unlabeledtargets: A review,” Analytica Chimica Acta 620, 8 – 26 (2008).7. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light(Princeton University Press, 2008), 2nd ed.8. E. Chow, A. Grot, L. W. Mirkarimi, M. Sigalas, and G. Girolami, “Ultracompact biochemical sensor built withtwo-dimensional photoniccrystal microcavity,” Opt. Lett. 29, 1093–1095 (2004).9. S. Xiao, N. A. Mortensen, “Highly dispersive photonic band-gap-edge optofluidic biosensors,” J. Europ. Opt.Soc. Rap. Public. 06026 Vol 1 (2006).10. 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 resonant grating forevanescent field molecular sensing,” Opt. Express 17, 18371–18380 (2009).11. S. Fan and J. D. Joannopoulos, “Analysis of guided resonances in photonic crystal slabs,” Phys. Rev. B 65,235112 (2002).12. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and S. G. Johnson, “MEEP: A flexible free-software package for electromagnetic simulations by the FDTD method,” Computer Physics Communications181, 687–702 (2010).13. A. Hessel and A. A. Oliner, “A new theory of wood’s anomalies on optical gratings,” Appl. Opt. 4, 1275–1297(1965).14. A. A. Yanik, M. Huang, A. Artar, T.-Y. Chang, and H. Altug, “Integrated nanoplasmonic-nanofluidic biosensorswith targeted delivery of analytes,” Applied Physics Letters 96, 021101 (2010).15. N. L. Thomas, R. Houdre´, M. V. Kotlyar, D. O’Brien, and T. F. Krauss, “Exploring light propagating in photoniccrystals with fourier optics,” J. Opt. Soc. Am. B 24, 2964–2971 (2007).16. T. Kan, K. Matsumoto, and I. Shimoyama, “Nano-pillar structure for sensitivity enhancement of spr sensor,”in “Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International,”(2009), pp. 1481–1484.1. IntroductionOptical biosensing is a research field that, despite already being around for quite a few years,to this day still yields a great deal of scientific output [1, 2, 3, 4, 5]. This comes as no surpriseas there is and always will be a need for cost effective and reliable biosensors and this in manydomains (e.g. medical, food production, military).There are basically two types of photonics based biosensors: fluorescent based and label-free ones. In a fluorescent based sensing scheme, the molecules that we want to measure needto be labeled with a fluorescent or radioactive marker. When illuminating the molecules, thelabels will emit light or a radiowave and depending on the intensity of the captured signal,we can determine the concentration. Although this seems a fairly simple method, the labelingprocedure of the biomolecules is quite complicated. An alternative is making use of label-free sensing. In label-free sensing, biomolecules bond with receptor molecules attached to thesensor. Due to this bonding, the refractive index close to the sensing area will change. Thechange in refractive index is then a measure for the concentration of the biomolecules. Apartfrom this, label-free sensors can also be used for measuring a bulk refractive index (e.g. whenimmersing the sensor in water with a certain NaCl concentration).Quite a few biosensor concepts using photonics have been investigated over the years [6].Here we present a novel sensing method, based on photonic crystals (PhCs) [7]. There arealready several ways to design a PhC biosensor: measuring the shift of a resonance peak inducedby a defect in the PhC [8], monitoring the band gap shift [9] or making use of guided resonances[10]. In the first two cases, light is constantly guided in the plane of the PhC, in the latter case,the direction of the incident and outgoing light is out of the PhC plane. In the sensing conceptdescribed in this paper, the incident light comes in in the plane of the PhC and is then coupledout of the plane, much like a grating coupler (see Fig. 1).zyx4a0.3a4aa21 periods7 periodsEθφFig. 1. Light enters the PhC biosensor by means of a waveguide and is coupled to free spaceas a beam with inclination θ and azimuth φ (left). The simulated structure used for FDTDsimulations is shown in the right figure. All sizes are normalized to the lattice constant a.The grid resolution used is 56 pixels per unit of distance.2. Angular spectrum analysis sensor concept and simulation methodsIn photonic crystals there are two types of guided modes: modes above and below the lightline. The modes below the light line are true guided modes and possess an infinite lifetime. Theother modes (above the light line) are called resonant guided modes [11] and can couple to thecontinuous spectrum of free space modes. It is the latter type that determines the behaviour ofthe proposed structure. Fig. 2 shows the transmission, reflection and radiated (either upwards ordownwards since the structure is symmetric) flux spectra of a 0.3a thick bulk silicon PhC slabcomprising holes of diameter 0.8a in a triangular lattice (where a is the lattice constant), excitedwith x-polarized light in the ΓM direction (see Fig. 1 (right)). The refractive index in the holesas well as that of the cladding layers above and below the PhC slab is 1.33, the refractive indexof water. The resolution is set to 56 pixels per unit of distance. Simulations are carried out usinga freely available FDTD method [12]. Note that the sum of the flux spectra does not equal 1 asthe flux planes defined in our simulations do not constitute a closed volume (the flux plane forout of plane radiation is located at a relatively large distance from the slab and the flux planesfor reflection and transmission only cover the input waveguide).0.2 0.3 0.4 0.5 0.600.51TransmissionReflectionRadiationRegion of interestNormalized frequency (2pic/a)Fig. 2. In a certain frequency region, above the light line, guided modes can couple to freespace modes. The fluxes shown in this figure are calculated for a triangular silicon PhCslab with hole diameter 0.8a, thickness 0.3a, excited in the ΓM direction. The radiationflux represents either the down- or upward direction since the structure is symmetric.In Fig. 2 we see a clearly marked frequency region bound by dips in the out of plane radiatedflux. In order to determine what exactly is happening in this region, we must examine the fieldsthemselves. Fig. 3 shows the calculated angular spectrum of the Poynting vector projected onthe vertical direction for a certain frequency in the region of interest.We can distinguish the Lorentzian shape [13] which we would expect to see due to the res-onant coupling of guided modes to free space modes. As with other diffraction gratings, theangle at which a beam with a certain frequency is radiated, is determined by the momentummatching condition. The angular position of the peak shifts not only in function of frequencybut also due to changes in refractive index which makes it interesting for sensing applications.Now that we have qualitatively determined a shift, we can perform a quantitative analysis.3. Simulation resultsThere are a lot of parameters that can be tuned when designing this structure. In the follow-ing, the results of extensive FDTD simulations are presented for the most important of theseparameters: the type of cladding configuration and the direction of light excitation.04590-45-90-90 -45 0 45 90θ (degrees)φ (degrees)-45 0 45 90-90θ (degrees)Data 1.33Data 1.38Lorentzian fit 1.33Lorentzian fit 1.38Fig. 3. The angular spectrum (left) and cross-section at φ=0◦ (right) of the vertically pro-jected Poynting vector of the proposed structure with a symmetric cladding (i.e. refractiveindex above and below PhC slab equals that of the holes) at normalized frequency 0.53488.A Lorentzian resonance shape is clearly visible.3.1. Different cladding configurationsDepending on fabrication and implementation of microfluidics [1, 14], one can wonder how thesituation with respect to the refractive index in the neighbourhood of the PhC membrane willbecome. Therefore, we have studied the three main situations that can arise: only the refractiveindex in the PhC holes changes and above and below the membrane, there is air (henceforthcalled the no cladding type), one cladding layer consists of air while the other has the samerefractive index as that of the holes (asymmetric cladding) or both cladding layers and holesexperience a change in refractive index (symmetric cladding). For each of these situations,simulations have been carried out for several refractive indices which allows us to determine ashift per refractive index unit (RIU).There are two ways to see a kind of shift: we can look at the angular spectrum for a fixedfrequency by using a spatial Fourier measurement setup [15] or we measure the power for agiven angle while scanning the frequency range. Both methods have been investigated and theresults for the different cladding configurations are presented in Fig. 4. It should however benoted that the major advantage of the first method is that the light source in a fabricated deviceonly needs to be single frequency.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°/RIU0.070.090.110.130.150.17θ (degrees)Normalized frequencyshift per RIUNo claddingAsymmetricSymmetric15 20 25 30 35 40 45 Fig. 4. As expected, the shift for the symmetric cladding configuration is the highest in bothapproaches (left: monitoring angular shift at a fixed frequency; right: measuring frequencyshift at a fixed angle).Like we would intuitively expect, for both measurement methods, the symmetric claddingtype is more sensitive than the asymmetric cladding configuration which is in turn more sensi-tive than the no cladding type. When measuring using the Fourier setup, it is advised to measureat a high frequency because a) the light is coupled out at an angle close to the normal and istherefore easier to capture using a lens and b) as will become apparent in what follows, thewidth of the resonance peak decreases with increasing frequency.Using the other measurement method, we see for the three types a gradual increase of sensi-tivity with increasing angle until it becomes stable at about 25◦ and onwards.3.2. Excitation along the ΓK directionUp until now, we have always excited light in the PhC along the ΓM direction. Exciting it alongthe perpendicular ΓK direction gives a slightly different angular spectrum as shown in Fig. 5.04590-45-90-90 -45 0 45 90θ (degrees)φ (degrees)-45 0 45 90-90θ (degrees)Fig. 5. The angular spectrum at normalized frequency 0.53488 (left) and cross-section atφ=0◦ and normalized frequency 0.569 (right) of the proposed structure with a symmetriccladding. A portion of the light is scattered under an angle φ=90◦ and the Lorentzian shapeis distorted (this is more clearly visible in the right figure).We can see that the Lorentzian shape is distorted (more clearly visible in the right part ofFig. 5) and that now also light is scattered at an angle φ=90◦.As it is difficult to fit a proper Lorentzian to the ΓK spectra we can make an estimation of theshift by simply observing the maxima of the peaks. It turns out that the sensitivity is roughlyequal to that of the structure excited in the ΓM direction.3.3. Other parametersIt is clear that there are still a lot of other properties that can be optimized (e.g. the shape of theunit cell, slab thickness, polarization of incident light, the periodic lattice). We have investigatedthe effect of the number of periods in the direction of light propagation but have found that itis of little influence. The hole diameter has been chosen sufficiently large as to maximize thelight-matter interaction keeping in mind technological limitations for eventual fabrication ofthe device (i.e. minimal distance between adjacent holes). The influence of the slab thicknesshas been investigated briefly and we have found that with decreasing thickness, the frequencyregion in which the Lorentzian shape appears widens and shifts to higher frequencies and thesensitivity also slightly increases.3.4. Noise analysis and detection limitIn simulations, it is virtually impossible to take into account all noise factors that will influencemeasurements (e.g. temperature variations, fabrication errors). In order to already make somepredictions on how the structure will react due to this noise, we can add Gaussian noise tothe simulated signal. By performing a Monte Carlo simulation, i.e. repeatedly determining thepeak position of a noise-ridden signal, it is possible to determine the spread on the calculatedpeak position: by fitting a Gaussian distribution to the determined peak positions we extractthe spread σ . This spread is in turn a measure for the detection limit. Measurements of otherstructures using our Fourier measurement setup have shown, on average, a signal-to-noise ratio(SNR) of 22 dB. The amplitude of the added noise in the Monte Carlo simulation is thereforechosen as to reach the same SNR.15 20 25 30 35 40 450.090.110.130.150.170.19Symmetric cladding layersNo cladding layersAsymmetric cladding layersθ (degrees)σ (degrees)0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.560.511.5-3Normalized frequency Detection limit(10   RIU)2No claddingAsymmetricSymmetricFig. 6. Adding noise in Monte Carlo simulations delivers the spread σ on correct peakdetection (left). The width of the peak, which is in itself dependent on the angle θ , isdirectly related to this σ . Using this information, we can calculate (right) the detectionlimit (considering the worst case/largest σ of the shifted peaks). The detection limit issmallest at a high frequency as this is where the peak width is smallest.Fig. 6 leads us to conclude that the spread σ is practically independent of the cladding con-figuration (variations are due to numerical and statistical errors) but above all, we can say thatthe spread increases with the angle θ . As already mentioned above, it is therefore interesting towork at a high frequency, which is at small angle θ , close to the normal to the PhC membrane.Instead of looking at the sensitivity, a more commonly used figure of merit is the detectionlimit which is defined here as σ /shift. Fig. 6 shows the detection limit in function of the fre-quency for the Fourier setup measurement method. As expected, the detection limit is highestfor high frequencies due to the small peak width at these frequencies. If we would be able toincrease the SNR of the measurement setup, the detection limit could also improve. Simulationshave shown that the detection limit is linearly related to the noise amplitude.4. ConclusionsExploiting the coupling of guided to free space modes, we have devised a new photonic crystalbased biosensor concept. The major advantage of this device over resonance based sensors (e.g.ring resonators) is that it is able to operate at a single frequency. With a simulated sensitivityof 65◦ per RIU, the proposed device performs better than a SPR sensor in the angular regime(i.e. operating at a single wavelength) [16]. Furthermore, when fabricating the device there isno need for a metal deposition step or a coupling prism.If a broadband light source is available, it is possible to operate the device in a reciprocalway by measuring the peak frequency shift at a fixed angle.AcknowledgmentsThe author would like to thank imec for providing a research grant. Also gratitude towards K.Van Acoleyen for information on the Fourier setup.This work was carried out using the Stevin Supercomputer Infrastructure at Ghent University.This research was supported by the Interuniversity Attraction Poles program of the BelgianScience Policy Office, under grant IAP P6-10 photonics@be.

Photonic crystal biosensor based on angular spectrum analysis

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Photonic crystal biosensor based on angular spectrum analysis

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