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Resonant grating sensors using frustrated total-internal reflection
Sensors and Actuators B 51 (1998) 131 – 136
Resonant grating sensors using frustrated total-internal reflection Nicholas J. Goddard *, Kirat Singh, Richard J. Holmes, Behnam Bastani Department of Instrumentation and Analytical Science, UMIST, PO Box 88, Manchester M60 1QD, UK Received 30 March 1998; accepted 8 June 1998
Abstract The resonant mirror (RM) sensor is a leaky planar waveguide optical sensor that uses frustrated total internal reflection to couple light into and out of the waveguiding layer. Since the waveguiding layer acts as a resonant cavity, the light reflected from the RM device undergoes a full 2p phase change across the resonance in either angle (for a fixed input wavelength) or wavelength (for a fixed input angle). This phase change can be visualised by using crossed input and output polarisers to produce a peak in intensity at the resonance angle or wavelength, which in turn is a sensitive function of surface refractive index. Disadvantages of this scheme are that it is very sensitive to birefringence in the substrate layer of the sensor device and requires careful choice and alignment of the polarisers. By forming the waveguiding layer as a set of thin parallel strips, it is possible to visualise the resonance angles or wavelengths directly by the appearance of diffracted spots of light at the resonance(s). To demonstrate the utility of this approach, a conventional RM sensor was coated with photoresist and exposed through a photomask consisting of 4 mm bars and 4 mm spaces, thus forming a 125 lines mm − 1 grating. Once developed, the waveguiding layer was etched away in the exposed areas using 35% aqueous fluorosilicic acid. Finally, the remaining photoresist was removed, leaving the waveguide layer etched into a large number of parallel 4 mm wide strips. It proved possible to use both monochromatic and broadband non-coherent unpolarised light sources (such as light-emitting diodes and tungsten-filament lamps) to excite resonances and follow surface refractive index changes. The sensitivity of the grating sensor to refractive index was found to be 90.4% of that of the unmodified RM device. The grating-RM was used to detect low concentrations of xylene in water using a thin coating of phenyl siloxane polymer as a selective absorber of non-polar compounds. Xylene concentrations down to B 5 ppm gave a reliably detectable peak shift. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Optical waveguide; Gratings; Evanescent wave sensors; Resonant mirror
1. Introduction The resonant mirror (RM) sensor is a development of the frustrated total internal reflection (FTR) absorption enhancement method described by Harrick [1]. It has been developed as a sensing method for immunosensors (the Affinity Sensors IaSys system [2,3]), as it is very sensitive to changes in surface refractive index caused by the binding of macromolecules such as proteins to immobilised biorecognition species such as antibodies. FTR has been used to overcome a significant disadvantage of waveguide sensors, which is the difficulty of coupling light into the waveguide. Maximum sensitivity is obtained by using a thin, high-index waveguide. High index monomode waveguides may be
* Corresponding author. Tel: +44 161 200 4895; fax: + 44 161 200 4911; e-mail: [email protected]
only 100 nm thick, leading to significant practical difficulties in using end-fire in-coupling. Besides FTR, other methods such as gratings [4] or prism coupling [5] have also been used to facilitate in-coupling. Grating couplers are particularly easy to use, but require more complex fabrication methods (such as reactive ion etching) to give reproducible results, as the grating periods required are very small. Conventional prism coupling through an air gap, while easier than end-fire coupling, does require careful setting up and is inconvenient as the light enters and leaves the waveguide on the same side as the sample. More recently, the RM sensor has been used to monitor refractive index changes in electroosmotically-driven micro total analysis systems (mTAS) [6]. In the RM sensor, FTR is used to couple light in and out of a high-index waveguiding layer. The RM sensor is effectively a prism coupler where the air gap has been
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00180-4
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replaced by a low index dielectric layer. Fig. 1a shows the RM device construction, consisting of a high- index substrate ( 1 mm thick lead glass, nd =1.72825), a thin low-index spacer (about 550 nm of silica) and a very thin monomode waveguiding layer (about 80 nm of silicon nitride). The dielectric films are very durable, permitting the devices to be cleaned and re-used. Light incident above the critical angle on the substrate-spacer interface is coupled into the waveguiding layer via the evanescent field in the spacer layer when the propagation constants in the substrate and waveguide match. For monochromatic light, this occurs over a very narrow range of angles, typically spanning less than one degree. Alternatively, at a fixed input angle the coupling occurs over a narrow range of wavelengths [7]. The device has been termed the RM because it contains a resonant cavity (the waveguide) and it acts as a nearly perfect reflector for light incident above the critical angle. If, however, an optically absorbing species is present in the evanescent field above the waveguide layer and light of an appropriate wavelength is used, the reflectivity will decrease as the light is absorbed. Since the waveguiding layer acts as a resonant cavity, the light reflected from the RM device undergoes a full 2p phase change as we scan across the resonance in either angle (for a fixed wavelength) or wavelength (for
Fig. 1. Cross-sections of (a) conventional RM and (b) grating-etched RM (resonant grating) devices.
a fixed angle). Dielectric waveguides of sufficient thickness can support both TE and TM modes, which undergo resonance at different angles when using monochromatic light, or at different wavelengths at a fixed coupling angle. Currently, to determine the resonance angles or wavelengths at which light couples into the waveguiding layer, light linearly polarised at 45° to the plane of the waveguiding layer is applied to the device. The polarisation components parallel (TM) and perpendicular (TE) to the plane of the waveguide undergo a relative phase change when either component is near a resonance. When using monochromatic light over a range of input angles, the ellipticity of the output beam is a function of output angle. By placing a polariser at the output, the resonance angles (or wavelengths) for TE and TM modes can be located by orienting the polariser such that it is crossed with the input polariser, passing light where the TE–TM phase difference is p. The device can be used in two modes: Constant wavelength, variable angle. In this mode, a monochromatic source is used with suitable optics to produce a converging wedge beam at the sensor. As the surface refractive index changes, the angle at which resonance occurs changes. Constant angle, variable wavelength. In this mode, a collimated white light source is used at a constant input angle. As the surface refractive index changes, the wavelength at which resonance occurs changes. This sensor configuration has a number of disadvantages: Light losses in the polarisers, requiring higher intensity sources or more sensitive detectors. Loss of polarisation at infra-red wavelengths when using dichroic sheet polarisers. These polarisers transmit infra-red radiation (above about 800 nm) without any polarisation. This stray radiation passes through the RM sensor and the output polariser to the detector, which is a significant problem when using filament lamp sources and CCD detectors, as silicon CCDs are usually more sensitive to infra-red (up to about 1100 nm wavelength) than to visible wavelengths. This leads to a very large background signal. Birefringence in the optical components between the input and output polarisers. Any birefringence in the optical path will introduce a phase shift between the TE and TM modes, leading to a change in the ellipticity of the output light. At best, this will cause a change in the background intensity, while at worst the resonance peaks could disappear altogether. Although a Soleil– Babinet compensator could be used to remove a fixed birefringence, the fact that the light path through the substrate changes with any change in resonance position means that the compensator would have to automatically track changes in the resonance position.
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Better polarisers and light sources can overcome the first two disadvantages (at a cost), but the final disadvantage is fundamental. By using low stress-birefringence glass which has been carefully annealed, stray birefringence can be reduced to very low levels which are insignificant in normal operation of the device. This limitation does, however, preclude the use of moulded polymeric substrates, which suffer from very significant stress birefringence as a result of the injection moulding process. By using a modified form of the RM sensor we can eliminate the need for polarised light and still determine the resonance angles/wavelengths. If the waveguide layer is no longer continuous in two dimensions, but instead is formed into a large number of closely-spaced strips parallel to the direction of the incoming light (Fig. 1b), then at resonance the strips will act as a diffraction grating, producing a diffraction pattern. Diffraction will occur because there is a p phase difference between the light reflected from the strips and the area between the strips. Off-resonance, the strips will be invisible, as there will be an insignificant phase difference between the light reflected from the strips and the area between the strips.
2. Experimental Sensor chips were fabricated on 1 mm thick Schott SF10 glass substrates, optically polished on both sides (Gooch and Housego, Ilminster, UK). The substrates (1 mm thick SF10 glass, Schott Glass) were cleaned successively in Decon-90 solution and acetone, then dried at 80°C for 30 min. Deposition of silica spacer layers and silicon nitride waveguide layers was performed using chemical vapour deposition (CVD) (Affinity Sensors, Cambridge, UK). After deposition of the spacer and waveguide layers, the RM substrates were cleaned in isopropanol and then in a 100 vol. hydrogen peroxide-concentrated sulphuric acid (50:50) mixture, followed by washing with ultrapure water and drying. Photoresist (S1813 positive photoresist, Shipley, Coventry, UK) was spin-coated at 4000 rpm for 60 s. The photoresist was soft-baked at 80°C for 30 min, followed by exposure to the phototool on a mask aligner (Model MJB3, Karl Suss, Germany). The phototool was a 125 lines mm − 1 chromium on glass grating (Ealing Electrooptics, Watford, UK), consisting of 4 mm bars and 4 mm spaces. The pattern was then developed for 60 s using the appropriate developer (Microposit 1:2 in deionized water, Shipley). The substrate was then washed in de-ionized water, dried in nitrogen and hard baked at 120°C for 90 min. Etching of the waveguide pattern was performed in 35% aqueous fluorosilicic acid (Fisher Scientific, Loughborough, UK) for 120 s, followed by washing with ultrapure water then acetone to remove
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Fig. 2. Block diagram of instrument configuration: (a) Light source (LED or tungsten-halogen lamp); (b) condenser lens; (c) aperture stop; (d) removable filter; (e) plano-convex cylindrical lens; (f) 60° SF10 prism; (g) Resonant grating chip; (h) removable rotatable polariser; (i) focusing lens; and (j) CCD detector.
the remaining photoresist. Phenyl siloxane coating was performed by spin-coating a precursor solution (GlassClad HT, ABCR, Germany) onto the chip, followed by baking at 240°C for 30 min. The RM instrumentation consisted of a 12 V, 20 W tungsten–halogen lamp with condensing lens (Fig. 2), a 350 mm pinhole and collimating lens to produce a substantially collimated beam of white light. A 650 nm band-pass interference filter (10 nm FWHM bandwidth) (Ealing Electro-optics) was used to provide a reasonably monochromatic light source for the constant wavelength, variable angle experiments. A 3648 pixel CCD (Toshiba TC1301D) was used with a 12-bit analogue-to-digital converter to monitor angle changes. The CCD has a pixel pitch of 8 mm and a pixel height of 200 mm.
3. Results and discussion To determine the optimum etch time for the silicon nitride waveguide layer, the etching process was directly monitored using the constant wavelength, variable angle mode. To perform this real-time monitoring, the unetched RM chip was placed on an SF10 prism used to couple light in and out of the waveguiding layer, as described elsewhere [2]. This method measures the effective index of the waveguiding layer, which is a function of the real refractive indices of the substrate, spacer, waveguiding and overlayer. At time zero, a small drop (100 ml) of 35% aqueous fluorosilicic acid was placed on the centre of the RM sensor using a Gilson micropipette. The resonance peak(s) were recorded at approximately two second intervals from 25 s after addition of the acid until no further change in any of the peaks was visible. Fig. 3 is a three-dimensional graph of the results obtained in this way. The vertical axis is the intensity recorded from the linear CCD detector, while the hori-
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Fig. 3. Effect of 35% fluorosilicic acid etch on TM0 and TE0 resonance of standard RM device.
zontal axes are time and pixel number. Initially, a two peaks are seen, one of which moves steadily to higher pixel numbers (lower refractive index) and merges with the static peak. At about 75 s a third peak appears which moves more rapidly until it merges with the original peaks. Finally, the merged peak disappears at about 175 s. Modelling of the RM system indicates that the stationary peak is the TM0 peak for air, while the first mobile peak is, as expected, the TM0 resonance for the fluorosilicic acid. The more rapidly moving peak that appears at 75 s is the TE0 fluorosilicic acid resonance. The two remaining peaks move closer together as the waveguide layer is etched away, eventually merging into a single peak at about 125 s. To avoid overetching, the etch time was set at about 120 s, where all three peaks have merged together. Fluorosilicic acid is an isotropic etchant, so there will be a degree of undercutting of the silicon nitride layer, resulting in strips which are slightly narrower than the gap between the strips. A theoretical analysis was performed to determine the effect of this slight asymmetry on the expected diffraction pattern. Since the width of the beam at the point of focus on the grating is much greater than the wavelength of the illuminating light, we can use a 1-D approximation.
When the grating is illuminated by monochromatic (l= 650 nm) light, the phase change upon reflection from the regions beneath the strips is different to that from beneath the etched regions. A phase grating is thus established and a far-field diffraction pattern is created. The formula for far-field (or Fraunhoffer) diffraction by such an arrangement can be found in any standard text on optics [8] and is given below for completeness.
&
c(u)= A el{(2py sin(u)/l) + f(y)}dy c(u) is the complex amplitude as a function of angle u and f(y) is the aperture phase function, A is a normalisation constant. An experimentally-determined far-field diffraction pattern is shown in Fig. 4, using water as the cover layer. There is a slight asymmetry of the peak intensities in the experimental diffraction pattern. It was found that this asymmetry could be varied simply by rotating the chip with respect to the incoming light, but was very difficult to remove entirely given the sensitivity of the asymmetry to very small rotations. Atomic force microscopy (AFM) was used to determine the width of the waveguide strips and it was found that the
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Fig. 4. Experimentally-determined far-field diffraction pattern obtained using resonant grating sensor with water as cover layer.
RM ribs were 3.5 mm in width and that the etched regions were 4.5 mm in width. A computer generated far-field diffraction pattern of monochromatic light reflected from the RM grating using these values is shown in Fig. 5. The phase change upon reflection from the etched and un-etched regions was determined using the standard transfer matrix technique [9,10] as applied to the RM [11]. An exactly symmetrical structure results in the complete disappearance of the zero order and all the even diffracted orders, which is not observed in the experimental data. To determine the sensitivity of the grating-RM device to the refractive index of the cover layer, the linear CCD was rotated through 90° and offset from the zero order position to the +1 order position. Any change in surface refractive index then caused the diffracted peak to move along the CCD. Solutions of 0, 3, 5, 8 and 10% w/w glycerol in water were then placed on the surface of the sensor and the resonance peak positions determined. The refractive index of the glycerol solutions was determined using an Abbe´ refractometer and the
Fig. 5. Calculated far-field diffraction pattern of resonant grating sensor, using 3.5 mm wide waveguide strips and 4.5 mm gaps and water (n = 1.33) as cover layer.
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Fig. 6. Position shift of resonance peaks for both standard and grating RM devices as a function of overlayer refractive index using 0, 3, 5, 8 and 10% w/w glycerol in water.
peak angular position plotted against solution refractive index for both the standard RM and grating RM devices (Fig. 6). The horizontal error bars are the scale error of the Abbe´ refractometer; the error in the determination of the peak position is too small to be seen on the vertical scale used. Finally, in Fig. 7 the peak shift for the grating RM device is plotted against the peak shift of the standard RM device. The slope of the resulting best-fit line is 0.904, indicating that the grating RM is somewhat less sensitive than the standard RM. This slight difference in sensitivity may be a result of non-uniformity in the coating thicknesses and refractive indices. To establish that the peaks seen were a result of the resonance in the waveguide layer, a rotatable polariser was placed in the output beam, without a corresponding input polariser. The grating-RM device was used in constant wavelength/variable angle mode with a small amount of water on the surface to provide a good
Fig. 7. Position shift of resonance peak of grating RM sensor plotted against position shift of standard RM sensor using 0, 3, 5, 8 and 10% w/w glycerol in water.
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and plotted against xylene concentration in Fig. 8. A linear response was obtained up to 10 ppm.
4. Conclusions
Fig. 8. Response of the grating-RM sensor using a phenyl siloxane sensing layer to concentrations of 0, 1, 5 and 10 ppm w/w xylene in water.
The response of the grating modified RM sensor has been shown to be approximately equal to that of the unmodified RM. The fabrication method has been shown to result in well-defined waveguide strips giving diffraction patterns that are close to those predicted theoretically. Similar results to those obtained from the RM sensor can be generated using much simpler instrumentation. A simple chemical sensor using a phenyl siloxane coated grating-RM chip has been shown to respond linearly to low concentrations of a non-polar organic compound, xylene.
Acknowledgements The support of Affinity Sensors Ltd for the supply of coated RM devices is gratefully acknowledged.
References
Fig. 9. Effect of a rotatable polariser on the intensity of the first order diffracted peak.
resonance peak. As the polariser was rotated, the peak could be completely extinguished (Fig. 9), showing that the diffracted output light was strongly polarised in the correct orientation for a TM resonance.To show the potential of the grating-RM as a chemical sensor, etched devices were spin-coated with a thin layer of a phenyl siloxane polymer precursor dissolved in xylene. The coated chips were baked at 240°C for 30 min, then mounted on the experimental apparatus. A stainlesssteel flow cell was clamped to the coated surface of the chip and solutions containing 0, 1, 5 and 10 ppm by weight of xylene were pumped through the system using a peristaltic pump. The peak positions were recorded
[1] N.J. Harrick, Internal Reflection Spectroscopy, Wiley, New York, 1967, p. 163. [2] R. Cush, J.M. Cronin, W.J. Stewart, C.H. Maule, J.O. Molloy, N.J. Goddard, The resonant mirror: a novel optical biosensor for direct sensing of biomolecular interactions Part 1: principle of operation and associated instrumentation, Biosens. Bioelectron. 8 (1993) 347 – 354. [3] P.E. Buckle, R.J. Davies, T. Kinning, D. Yeung, P.R. Edwards, D. Pollard-Knight, C.R. Lowe, Biosens. Bioelectron. 8 (1993) 355 – 363. [4] Ph.M. Nellen, K. Tiefenthaler, W. Lukosz, Integrated optical input grating couplers as biochemical sensors, Sensors and Actuators 15 (1988) 285 – 295. [5] P.K. Ulrich, R. Torge, Measurement of thin-film parameters with a prism coupler, Appl. Optics 12 (1973) 2901 – 2908. [6] J.P. Lenney, N.J. Goddard, J.C. Morey, R.D. Snook, P.R. Fielden, An electro-osmotic flow system with integrated planar optical waveguide sensing, Sensors And Actuators B 39 (1997) 212 – 217. [7] N.J. Goddard, D. Pollard-Knight, C.H. Maule, Real-time biomolecular interaction analysis using the resonant mirror sensor, Analyst 119 (1994) 583 – 588. [8] E. Hecht, Optics, 3, Addison-Wesley, Reading, MA, 1997. [9] Th. Kersten, Numerical solution of the mode-equation of planar dielectric waveguides to determine their refractive index and thickness by means of a prism coupler, Optics Comm. 9 (1973) 427 – 431. [10] T. Kersten, The prism-film coupler as a precision instrument Part 1: accuracy and capabilities of prism couplers as instruments, Optica Acta 22 (1975) 503 – 513. [11] J.P. Lenney, Ph.D. Thesis, UMIST, Manchester, 1996.
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Resonant grating sensors using frustrated total-internal reflection Nicholas J. Goddard *, Kirat Singh, Richard J. Holmes, Behnam Bastani Department of Instrumentation and Analytical Science, UMIST, PO Box 88, Manchester M60 1QD, UK Received 30 March 1998; accepted 8 June 1998
Abstract The resonant mirror (RM) sensor is a leaky planar waveguide optical sensor that uses frustrated total internal reflection to couple light into and out of the waveguiding layer. Since the waveguiding layer acts as a resonant cavity, the light reflected from the RM device undergoes a full 2p phase change across the resonance in either angle (for a fixed input wavelength) or wavelength (for a fixed input angle). This phase change can be visualised by using crossed input and output polarisers to produce a peak in intensity at the resonance angle or wavelength, which in turn is a sensitive function of surface refractive index. Disadvantages of this scheme are that it is very sensitive to birefringence in the substrate layer of the sensor device and requires careful choice and alignment of the polarisers. By forming the waveguiding layer as a set of thin parallel strips, it is possible to visualise the resonance angles or wavelengths directly by the appearance of diffracted spots of light at the resonance(s). To demonstrate the utility of this approach, a conventional RM sensor was coated with photoresist and exposed through a photomask consisting of 4 mm bars and 4 mm spaces, thus forming a 125 lines mm − 1 grating. Once developed, the waveguiding layer was etched away in the exposed areas using 35% aqueous fluorosilicic acid. Finally, the remaining photoresist was removed, leaving the waveguide layer etched into a large number of parallel 4 mm wide strips. It proved possible to use both monochromatic and broadband non-coherent unpolarised light sources (such as light-emitting diodes and tungsten-filament lamps) to excite resonances and follow surface refractive index changes. The sensitivity of the grating sensor to refractive index was found to be 90.4% of that of the unmodified RM device. The grating-RM was used to detect low concentrations of xylene in water using a thin coating of phenyl siloxane polymer as a selective absorber of non-polar compounds. Xylene concentrations down to B 5 ppm gave a reliably detectable peak shift. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Optical waveguide; Gratings; Evanescent wave sensors; Resonant mirror
1. Introduction The resonant mirror (RM) sensor is a development of the frustrated total internal reflection (FTR) absorption enhancement method described by Harrick [1]. It has been developed as a sensing method for immunosensors (the Affinity Sensors IaSys system [2,3]), as it is very sensitive to changes in surface refractive index caused by the binding of macromolecules such as proteins to immobilised biorecognition species such as antibodies. FTR has been used to overcome a significant disadvantage of waveguide sensors, which is the difficulty of coupling light into the waveguide. Maximum sensitivity is obtained by using a thin, high-index waveguide. High index monomode waveguides may be
* Corresponding author. Tel: +44 161 200 4895; fax: + 44 161 200 4911; e-mail: [email protected]
only 100 nm thick, leading to significant practical difficulties in using end-fire in-coupling. Besides FTR, other methods such as gratings [4] or prism coupling [5] have also been used to facilitate in-coupling. Grating couplers are particularly easy to use, but require more complex fabrication methods (such as reactive ion etching) to give reproducible results, as the grating periods required are very small. Conventional prism coupling through an air gap, while easier than end-fire coupling, does require careful setting up and is inconvenient as the light enters and leaves the waveguide on the same side as the sample. More recently, the RM sensor has been used to monitor refractive index changes in electroosmotically-driven micro total analysis systems (mTAS) [6]. In the RM sensor, FTR is used to couple light in and out of a high-index waveguiding layer. The RM sensor is effectively a prism coupler where the air gap has been
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00180-4
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replaced by a low index dielectric layer. Fig. 1a shows the RM device construction, consisting of a high- index substrate ( 1 mm thick lead glass, nd =1.72825), a thin low-index spacer (about 550 nm of silica) and a very thin monomode waveguiding layer (about 80 nm of silicon nitride). The dielectric films are very durable, permitting the devices to be cleaned and re-used. Light incident above the critical angle on the substrate-spacer interface is coupled into the waveguiding layer via the evanescent field in the spacer layer when the propagation constants in the substrate and waveguide match. For monochromatic light, this occurs over a very narrow range of angles, typically spanning less than one degree. Alternatively, at a fixed input angle the coupling occurs over a narrow range of wavelengths [7]. The device has been termed the RM because it contains a resonant cavity (the waveguide) and it acts as a nearly perfect reflector for light incident above the critical angle. If, however, an optically absorbing species is present in the evanescent field above the waveguide layer and light of an appropriate wavelength is used, the reflectivity will decrease as the light is absorbed. Since the waveguiding layer acts as a resonant cavity, the light reflected from the RM device undergoes a full 2p phase change as we scan across the resonance in either angle (for a fixed wavelength) or wavelength (for
Fig. 1. Cross-sections of (a) conventional RM and (b) grating-etched RM (resonant grating) devices.
a fixed angle). Dielectric waveguides of sufficient thickness can support both TE and TM modes, which undergo resonance at different angles when using monochromatic light, or at different wavelengths at a fixed coupling angle. Currently, to determine the resonance angles or wavelengths at which light couples into the waveguiding layer, light linearly polarised at 45° to the plane of the waveguiding layer is applied to the device. The polarisation components parallel (TM) and perpendicular (TE) to the plane of the waveguide undergo a relative phase change when either component is near a resonance. When using monochromatic light over a range of input angles, the ellipticity of the output beam is a function of output angle. By placing a polariser at the output, the resonance angles (or wavelengths) for TE and TM modes can be located by orienting the polariser such that it is crossed with the input polariser, passing light where the TE–TM phase difference is p. The device can be used in two modes: Constant wavelength, variable angle. In this mode, a monochromatic source is used with suitable optics to produce a converging wedge beam at the sensor. As the surface refractive index changes, the angle at which resonance occurs changes. Constant angle, variable wavelength. In this mode, a collimated white light source is used at a constant input angle. As the surface refractive index changes, the wavelength at which resonance occurs changes. This sensor configuration has a number of disadvantages: Light losses in the polarisers, requiring higher intensity sources or more sensitive detectors. Loss of polarisation at infra-red wavelengths when using dichroic sheet polarisers. These polarisers transmit infra-red radiation (above about 800 nm) without any polarisation. This stray radiation passes through the RM sensor and the output polariser to the detector, which is a significant problem when using filament lamp sources and CCD detectors, as silicon CCDs are usually more sensitive to infra-red (up to about 1100 nm wavelength) than to visible wavelengths. This leads to a very large background signal. Birefringence in the optical components between the input and output polarisers. Any birefringence in the optical path will introduce a phase shift between the TE and TM modes, leading to a change in the ellipticity of the output light. At best, this will cause a change in the background intensity, while at worst the resonance peaks could disappear altogether. Although a Soleil– Babinet compensator could be used to remove a fixed birefringence, the fact that the light path through the substrate changes with any change in resonance position means that the compensator would have to automatically track changes in the resonance position.
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Better polarisers and light sources can overcome the first two disadvantages (at a cost), but the final disadvantage is fundamental. By using low stress-birefringence glass which has been carefully annealed, stray birefringence can be reduced to very low levels which are insignificant in normal operation of the device. This limitation does, however, preclude the use of moulded polymeric substrates, which suffer from very significant stress birefringence as a result of the injection moulding process. By using a modified form of the RM sensor we can eliminate the need for polarised light and still determine the resonance angles/wavelengths. If the waveguide layer is no longer continuous in two dimensions, but instead is formed into a large number of closely-spaced strips parallel to the direction of the incoming light (Fig. 1b), then at resonance the strips will act as a diffraction grating, producing a diffraction pattern. Diffraction will occur because there is a p phase difference between the light reflected from the strips and the area between the strips. Off-resonance, the strips will be invisible, as there will be an insignificant phase difference between the light reflected from the strips and the area between the strips.
2. Experimental Sensor chips were fabricated on 1 mm thick Schott SF10 glass substrates, optically polished on both sides (Gooch and Housego, Ilminster, UK). The substrates (1 mm thick SF10 glass, Schott Glass) were cleaned successively in Decon-90 solution and acetone, then dried at 80°C for 30 min. Deposition of silica spacer layers and silicon nitride waveguide layers was performed using chemical vapour deposition (CVD) (Affinity Sensors, Cambridge, UK). After deposition of the spacer and waveguide layers, the RM substrates were cleaned in isopropanol and then in a 100 vol. hydrogen peroxide-concentrated sulphuric acid (50:50) mixture, followed by washing with ultrapure water and drying. Photoresist (S1813 positive photoresist, Shipley, Coventry, UK) was spin-coated at 4000 rpm for 60 s. The photoresist was soft-baked at 80°C for 30 min, followed by exposure to the phototool on a mask aligner (Model MJB3, Karl Suss, Germany). The phototool was a 125 lines mm − 1 chromium on glass grating (Ealing Electrooptics, Watford, UK), consisting of 4 mm bars and 4 mm spaces. The pattern was then developed for 60 s using the appropriate developer (Microposit 1:2 in deionized water, Shipley). The substrate was then washed in de-ionized water, dried in nitrogen and hard baked at 120°C for 90 min. Etching of the waveguide pattern was performed in 35% aqueous fluorosilicic acid (Fisher Scientific, Loughborough, UK) for 120 s, followed by washing with ultrapure water then acetone to remove
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Fig. 2. Block diagram of instrument configuration: (a) Light source (LED or tungsten-halogen lamp); (b) condenser lens; (c) aperture stop; (d) removable filter; (e) plano-convex cylindrical lens; (f) 60° SF10 prism; (g) Resonant grating chip; (h) removable rotatable polariser; (i) focusing lens; and (j) CCD detector.
the remaining photoresist. Phenyl siloxane coating was performed by spin-coating a precursor solution (GlassClad HT, ABCR, Germany) onto the chip, followed by baking at 240°C for 30 min. The RM instrumentation consisted of a 12 V, 20 W tungsten–halogen lamp with condensing lens (Fig. 2), a 350 mm pinhole and collimating lens to produce a substantially collimated beam of white light. A 650 nm band-pass interference filter (10 nm FWHM bandwidth) (Ealing Electro-optics) was used to provide a reasonably monochromatic light source for the constant wavelength, variable angle experiments. A 3648 pixel CCD (Toshiba TC1301D) was used with a 12-bit analogue-to-digital converter to monitor angle changes. The CCD has a pixel pitch of 8 mm and a pixel height of 200 mm.
3. Results and discussion To determine the optimum etch time for the silicon nitride waveguide layer, the etching process was directly monitored using the constant wavelength, variable angle mode. To perform this real-time monitoring, the unetched RM chip was placed on an SF10 prism used to couple light in and out of the waveguiding layer, as described elsewhere [2]. This method measures the effective index of the waveguiding layer, which is a function of the real refractive indices of the substrate, spacer, waveguiding and overlayer. At time zero, a small drop (100 ml) of 35% aqueous fluorosilicic acid was placed on the centre of the RM sensor using a Gilson micropipette. The resonance peak(s) were recorded at approximately two second intervals from 25 s after addition of the acid until no further change in any of the peaks was visible. Fig. 3 is a three-dimensional graph of the results obtained in this way. The vertical axis is the intensity recorded from the linear CCD detector, while the hori-
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Fig. 3. Effect of 35% fluorosilicic acid etch on TM0 and TE0 resonance of standard RM device.
zontal axes are time and pixel number. Initially, a two peaks are seen, one of which moves steadily to higher pixel numbers (lower refractive index) and merges with the static peak. At about 75 s a third peak appears which moves more rapidly until it merges with the original peaks. Finally, the merged peak disappears at about 175 s. Modelling of the RM system indicates that the stationary peak is the TM0 peak for air, while the first mobile peak is, as expected, the TM0 resonance for the fluorosilicic acid. The more rapidly moving peak that appears at 75 s is the TE0 fluorosilicic acid resonance. The two remaining peaks move closer together as the waveguide layer is etched away, eventually merging into a single peak at about 125 s. To avoid overetching, the etch time was set at about 120 s, where all three peaks have merged together. Fluorosilicic acid is an isotropic etchant, so there will be a degree of undercutting of the silicon nitride layer, resulting in strips which are slightly narrower than the gap between the strips. A theoretical analysis was performed to determine the effect of this slight asymmetry on the expected diffraction pattern. Since the width of the beam at the point of focus on the grating is much greater than the wavelength of the illuminating light, we can use a 1-D approximation.
When the grating is illuminated by monochromatic (l= 650 nm) light, the phase change upon reflection from the regions beneath the strips is different to that from beneath the etched regions. A phase grating is thus established and a far-field diffraction pattern is created. The formula for far-field (or Fraunhoffer) diffraction by such an arrangement can be found in any standard text on optics [8] and is given below for completeness.
&
c(u)= A el{(2py sin(u)/l) + f(y)}dy c(u) is the complex amplitude as a function of angle u and f(y) is the aperture phase function, A is a normalisation constant. An experimentally-determined far-field diffraction pattern is shown in Fig. 4, using water as the cover layer. There is a slight asymmetry of the peak intensities in the experimental diffraction pattern. It was found that this asymmetry could be varied simply by rotating the chip with respect to the incoming light, but was very difficult to remove entirely given the sensitivity of the asymmetry to very small rotations. Atomic force microscopy (AFM) was used to determine the width of the waveguide strips and it was found that the
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Fig. 4. Experimentally-determined far-field diffraction pattern obtained using resonant grating sensor with water as cover layer.
RM ribs were 3.5 mm in width and that the etched regions were 4.5 mm in width. A computer generated far-field diffraction pattern of monochromatic light reflected from the RM grating using these values is shown in Fig. 5. The phase change upon reflection from the etched and un-etched regions was determined using the standard transfer matrix technique [9,10] as applied to the RM [11]. An exactly symmetrical structure results in the complete disappearance of the zero order and all the even diffracted orders, which is not observed in the experimental data. To determine the sensitivity of the grating-RM device to the refractive index of the cover layer, the linear CCD was rotated through 90° and offset from the zero order position to the +1 order position. Any change in surface refractive index then caused the diffracted peak to move along the CCD. Solutions of 0, 3, 5, 8 and 10% w/w glycerol in water were then placed on the surface of the sensor and the resonance peak positions determined. The refractive index of the glycerol solutions was determined using an Abbe´ refractometer and the
Fig. 5. Calculated far-field diffraction pattern of resonant grating sensor, using 3.5 mm wide waveguide strips and 4.5 mm gaps and water (n = 1.33) as cover layer.
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Fig. 6. Position shift of resonance peaks for both standard and grating RM devices as a function of overlayer refractive index using 0, 3, 5, 8 and 10% w/w glycerol in water.
peak angular position plotted against solution refractive index for both the standard RM and grating RM devices (Fig. 6). The horizontal error bars are the scale error of the Abbe´ refractometer; the error in the determination of the peak position is too small to be seen on the vertical scale used. Finally, in Fig. 7 the peak shift for the grating RM device is plotted against the peak shift of the standard RM device. The slope of the resulting best-fit line is 0.904, indicating that the grating RM is somewhat less sensitive than the standard RM. This slight difference in sensitivity may be a result of non-uniformity in the coating thicknesses and refractive indices. To establish that the peaks seen were a result of the resonance in the waveguide layer, a rotatable polariser was placed in the output beam, without a corresponding input polariser. The grating-RM device was used in constant wavelength/variable angle mode with a small amount of water on the surface to provide a good
Fig. 7. Position shift of resonance peak of grating RM sensor plotted against position shift of standard RM sensor using 0, 3, 5, 8 and 10% w/w glycerol in water.
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and plotted against xylene concentration in Fig. 8. A linear response was obtained up to 10 ppm.
4. Conclusions
Fig. 8. Response of the grating-RM sensor using a phenyl siloxane sensing layer to concentrations of 0, 1, 5 and 10 ppm w/w xylene in water.
The response of the grating modified RM sensor has been shown to be approximately equal to that of the unmodified RM. The fabrication method has been shown to result in well-defined waveguide strips giving diffraction patterns that are close to those predicted theoretically. Similar results to those obtained from the RM sensor can be generated using much simpler instrumentation. A simple chemical sensor using a phenyl siloxane coated grating-RM chip has been shown to respond linearly to low concentrations of a non-polar organic compound, xylene.
Acknowledgements The support of Affinity Sensors Ltd for the supply of coated RM devices is gratefully acknowledged.
References
Fig. 9. Effect of a rotatable polariser on the intensity of the first order diffracted peak.
resonance peak. As the polariser was rotated, the peak could be completely extinguished (Fig. 9), showing that the diffracted output light was strongly polarised in the correct orientation for a TM resonance.To show the potential of the grating-RM as a chemical sensor, etched devices were spin-coated with a thin layer of a phenyl siloxane polymer precursor dissolved in xylene. The coated chips were baked at 240°C for 30 min, then mounted on the experimental apparatus. A stainlesssteel flow cell was clamped to the coated surface of the chip and solutions containing 0, 1, 5 and 10 ppm by weight of xylene were pumped through the system using a peristaltic pump. The peak positions were recorded
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