Integrated Optical Sensors

Transcription

1 Integrated Optical Sensors S.V. Pham, M. Dijkstra, A. Hollink, L.J. Kauppinen, H.A.G.M. van Wolferen, R.G. Heideman, M. Pollnau, G.J.M. Krijnen, R.M. de Ridder, P.V. Lambeck and H.J.W.M. Hoekstra Since the start of the group IOMS in the mid-eighties of the last century its research had a strong focus on integrated optical (IO) sensors. A large number of different sensor device concepts, based on for example plasmonics [1] quenching of luminescence [1,2] and a large number of interferometric principles [3, 4] have been proposed and tested; part of the work has led to commercial follow ups. Sensing devices have been designed, fabricated and characterized for the bio-chemical domain and also for measuring deflections of micro-mechanical objects. Below we will discuss the commercialized IO sensor platform, based on a Mach-Zehnder interferometer (MZI), which had an unprecedented sensitivity at the time of development [3]. Next we will show some results of recent work on grating-based, IO label-free enzyme sensing and the IO read-out of stressinduced micro-bridge deflection. As the stress was induced by H 2 gas adsorption by a Pd layer on top of the micro-bridge the device can be used as a H 2 sensor. The IO Mach-Zehnder interferometer Sensing fiber-to-chip interconnect V-groove sawcut interaction window electro-optic phase modulator Au ZnO polarizer Si 3 N 4 SiO 2 SiO 2 fiber Reference Fig. 1. Schematic top view of an MZI sensor EO modulators Silicon Fig. 2. Schematic longitudinal cross section of the MZI sensor (not to scale) A schematic top view of the device is given in Fig. 1, showing the main functional elements: a splitter (Yjunction at the left hand side being the input side), distributing the light equally over a sensing branch incorporating a target-specific bio-receptor layer, and a reference branch; the modulation sections; and the combiner (Y-junction at the output side). The change of the thickness or refractive index of the sensing layer causes a change of the modal phase velocity, which translates into a modal phase change ( ) and so, via interference at the combiner, into a detectable output change. A schematic picture of the longitudinal cross section is given in Fig. 2, showing the main technological features of the device. The main functional features of the device are: The presence of electro-optic (EO) modulation sections makes it possible to overcome the most important problems of conventional interferometers, viz., fringe or directional ambiguity, and sensitivity fading (in extrema of the response curve); moreover, they enable the reduction of noise and drift effects (see Fig. 3). The two branches can be made equal except for the presence of the specific receptor layer in the sensing branch. Owing to this nearly symmetrical implementation the effect of temperature fluctuations on the accuracy is strongly reduced. In order to minimize the effect of thermal gradients, the branch separation is chosen to be small (~40 m). The resolution of the device has been found to be n = 10 8 for the change of refractive index of the (usually water-like) top layer in the sensing section, which corresponds to a resolution t = 0.01 pm of the effective thickness of the receptor layer.

2 Fig. 3. (a) Response of a perfectly balanced MZI to changes in the modal phase difference ; the blue circles indicate points of sensitivity fading and fringe ambiguity, the red circles indicate the points of highest sensitivity. (b) Triangular waveform used to drive the modulator sections. (c) Response of a device with modulation according to (b) applied, for two different values of the modal phase shift in the sensing window. Note that the modulation eliminates fringe ambiguity and sensitivity fading. Label-free enzyme sensing with a Si 3 N 4 grated waveguide optical cavity An important property of a grated waveguide (GWG), which is a waveguide with a grating section, is the occurrence of sharp fringes in the transmission spectrum near the stop-band edges. It is well known that these oscillations are due to Fabry-Perot resonances of Bloch modes propagating in the cavity defined by the grated section [5]. Any small structural changes in the environment of the GWG, which disturb the evanescent field of the GWG propagation mode, will lead to a shift of its transmission spectrum. As an example, we show label-free sensing of PepN enzyme, the Major Suc-LLVY-AMC-hydrolyzing enzyme in Escherichia coli, where the spectral shift of the GWG response is due to the antibody-antigen interaction leading to growth of an ad-layer on it. The GWG setup for the enzyme sensing experiment is shown in Fig. 4a. Figure 4b shows a characteristic transmission spectrum of the GWG, highlighting the (sharpest) peak used for the sensing measurements. Fig.4. (a) The 3D schematic structure of the Si 3 N 4 grated waveguide (GWG) device with a PDMS chamber serving as a closed environment for accurate monitoring of the antibody-enzyme interaction. (b) Characteristic transmission spectrum of the GWG.

3 Fig. 5. (left) Transmission curves monitored following antibody-antigen interaction time. (right) Spectral shift of resonance peak versus reaction time; the binding reaction saturates after ~35 min. The Si 3 N 4 grated waveguides were fabricated using laser interference lithography as described in [6]. To detect a target biomaterial, such as PepN enzyme in this case, its antibody needs to be immobilized on the surface of the GWG device. We followed the standard immobilization process developed by Imenz b.v. [7 ]. Then a polydimethylsiloxane (PDMS) chamber ( cm 3 ) with removable cap was prepared, cleaned by ethanol and placed directly on the device surface. This chamber served as a closed environment (to prevent bulk index changes of the liquid owing to evaporation) for liquid-phase reactions in the next steps of the bio-sensing experiment. Once the antibody was immobilized on the GWG surface and the blocking/washing/drying step right after that was applied, the cell-free extracted (CFE) PepN solution was added and the antibody-antigen interaction was optically monitored through output transmission spectra of the GWG. Figure 5a shows the real-time measurement of the spectral shifts during the antibody-antigen interaction (colored curves). Noise was removed from the spectra using low pass filtering in the Fourier domain (black curves) to enable an accurate determination of the change, p (t) = p (t) p (t o ), of the wavelength of the peak maximum, p. Small changes on the GWG surface, caused by the antibody-enzyme interaction, lead to spectral shifts of the resonant peak, p, as shown in Fig. 5b; the reaction saturates after ~35 min. The total shift was approximately 342 pm, corresponding to the growth of an ad-layer of ~2 nm. Novel mechano-optical sensor based on read-out with a Si 3 N 4 grated waveguide We have recently proposed a compact integrated mechano-optical sensor using a novel and highly sensitive integrated read-out scheme to detect small deflections of a cantilever in close proximity to a grated waveguide (GWG) structure [8]. Here we present the integrated optical read-out of stress induced micro-bridge deflections due to hydrogen gas absorption by a palladium (Pd) receptor layer on top of the micro-bridge. The 3D schematic structure and the cross-section of the GWG-micro-bridge device are shown in Figs. 6a and 6b. The main parts of the device are the Si 3 N 4 grated waveguide and the SiO 2 bridge coated with a 30 nm thick Pd layer. All relevant device dimensions are depicted in the figure. Absorption of H 2 by Pd will cause the cantilever to curl down [9-11], which narrows the GWG-cantilever gap, g, and leads to a stronger interaction between the cantilever and the GWG evanescent modal field, which results in a shift of the transmission spectrum. This effect can be used for the detection of cantilever displacements and thus the concentration of the absorbing gas (see Fig. 6c). Integrated GWG-cantilever devices have been fabricated successfully using MEMS techniques. Details of fabrication process were described in [8]. Instead of producing singly-clamped cantilevers as mentioned in [8], we fabricated devices with a doubly-clamped cantilever and with an aimed gap of g =200 nm. Initial bending of the cantilever was characterized using a white light interferometer. The PDMS chamber placed on top of the device was connected to gas bottles (i.e., N 2 and 1% H 2 -N 2 mixture) through mass

4 flow controllers. The optical performance of the integrated device was monitored using a tunable laser source (Agilent 8164B) with a resolution of 1 pm and an InGaAs photo detector. Fig.6. (a) 3D schematic structure with a PDMS chamber serving as a reaction environment for H2 sensing. (b) Cross section of the GWG-cantilever device. (c) Calculated transmission spectra for various gap sizes, illustrating the sensing principle [2]. Fig.7. Top view white-light interferometry image of the device, showing an initial upbending of ~500 nm at the centre of the micro-bridge. Fig. 8. Transmission curves of the device in responding to (a) absorption and (c) desorption of H 2. (b) Wavelength shift p versus the reaction time. Figure 7 shows a top-view image of the fabricated device, as made with a white light interferometer, indicating an initial bending (upwards, i.e., away from the GWG structure) of the micro-bridge of approximately 500 nm. This initial bending, which leads to a lower sensitivity at low H 2 concentrations (owing to the relatively large gap of g ~ 700 nm), is due to the difference between residual stresses in the SiO 2 base layer and Pd receptor film [8]. Prior to supplying H 2 gas to the measurement chamber, N 2 gas was flushed in during 15 min with a flow rate of 0.5 sccm and optical transmission curves were captured repeatedly every minute. The results showed a stable and reproducible resonance peak at p = nm (see Fig. 8a, curve at t=0), indicating that such a flow rate did not cause any side effects or mechanical vibrations. Noise was removed from the spectrum using low pass filtering in the Fourier domain, enabling accurate and efficient determination of changes in p. Next we supplied the H 2 (1%)-N 2 mixture (flow rate 0.5 sccm) for a longer period of time, during which the transmission spectrum was monitored (see Fig. 8a). The shift p depends almost linearly on time, which can be explained partly by noting that the effect of the initially rapid change of the gap size, g, is compensated by lower values of p / g at larger gap size. After 3.5 hours the flow of the H 2 (1%)-N 2 mixture was switched off and replaced again by a purely N 2 inflow, leading to desorption as indicated by the transmission spectra (see Fig. 8c). Figure 8b shows the peak shifts during a four-hour period. It can be concluded that the desorption takes place at a lower rate (~50%) than the absorption process, and also that no full desorption is achieved during the monitoring period of time. The result provides a proof-of-concept of a novel and compact integrated mechano-optical sensor.

5 Co-operations OptiSense BV, Hengelosestraat 705, 7521 PA Enschede, The Netherlands ( IMEnz Bioengineering BV, LJ Zielstraweg 1, 9713 GX Groningen The Netherlands, ( TST group, University of Twente, Funding This research is supported by MEMSland, a project of the Point One program funded by the Ministry of Economic Affairs, and the STW Technology Foundation through project TOE References 1. H.J.M. Kreuwel, Planar waveguide sensors for the chemical domain, PhD thesis, University of Twente, G.L.J. Hesselink, Luminescence quenching for chemo-optical sensing, PhD thesis, University of Twente, R.G.Heideman and P.V.Lambeck, Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pig-tailed integrated optical phase-modulated Mach-Zehnder interferometer system, Sensors and Actuators B61 (1999), pp P.V. Lambeck, J. van Lith, and H.J.W.M. Hoekstra, Three novel integrated optical sensing structures for the chemical domain, Sensors and Actuators B113 (2006), pp G.J. Veldhuis et al., An integrated optical Bragg-reflector used as a chemo-optical sensors, Pure and Applied Optics 7 (1998), pp. L23-L W.C.L. Hopman et al., Far-field scattering microscopy applied to analysis of slow light, power enhancement, and delay times in uniform Bragg waveguide gratings, Opt. Express 15 (2007), pp IMEnz Bioengineering BV, LJ Zielstraweg GX Groningen The Netherlands, ( ). 8. S.V. Pham, L.J. Kauppinen, M. Dijkstra, H.A.G.M. van Wolferen, R.M. de Ridder, and H.J.W.M. Hoekstra, Read out of cantilever bending with a grated waveguide optical cavity, Phot. Technol. Lett. 23 (2011), pp So V. Pham, Meindert Dijkstra, Henk A. G. M. van Wolferen, Markus Pollnau, Gijs J. M. Krijnen, and Hugo J. W. M. Hoekstra, Integrated mechano-optical hydrogen gas sensor using cantilever bending readout with a Si 3 N 4 grated waveguide, Optics Letters Vol. 36, Iss. 15, pp (2011) 9. S. Okuyama, Y. Mitobe, K. Okuyama, and K. Matsushita, Hydrogen gas gensing using a Pd-coated cantilever, Jpn. J. Appl. Phys., 39 (2000), pp D.R. Baselt, B. Fruhberger, E. Klaassen, S. Cemalovic, C. L. Britton Jr., S.V. Patel, T.E. Mlsna, D. McCorkle, and B. Warmack, Design and performance of a microcantilever-based hydrogen sensor, Sensors and Actuators B: Chemical 88 (2003), pp Z. Hu, T. Thundat, and R.J. Warmack, Investigation of adsorption and absorption-induced stresses using microcantilever sensors, J. Appl. Phys. 90, (2001), pp