On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index...

$: On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index sensor array$
On-chip temperature compensation inan integrated slot-waveguide ring

resonator refractive index sensor array

Kristinn B. Gylfason,1,∗ Carl Fredrik Carlborg,1

Andrzej Kazmierczak,2 Fabian Dortu2, Hans Sohlstrom,1

Laurent Vivien,3 Carlos A. Barrios,4 Wouter van der Wijngaart,1

and Goran Stemme 1

1Microsystem Technology Laboratory, School of Electrical Engineering,KTH - Royal Institute of Technology, Osquldas vag 10, SE-100 44 Stockholm, Sweden

2Applied Photonics Department, Multitel a.s.b.l.,Rue Pierre et Marie Curie 2, B-7000 Mons, Belgium

3Institut d’Electronique Fondamentale, CNRS UMR 8622, Bt. 220,Universite Paris-Sud 11, F-91405 ORSAY cedex, France

4ISOM, Universidad Politecnica de Madrid, ETSI Telecomunicacion,Ciudad Universitaria s/n, ES-28040 Madrid, Spain

∗[email protected]

http://www.ee.kth.se/mst

Abstract: We present an experimental study of an integrated slot-waveguide refractive index sensor array fabricated in silicon nitride onsilica. We study the temperature dependence of the slot-waveguide ringresonator sensors and find that they show a low temperature dependence of−16.6 pm/K, while at the same time a large refractive index sensitivity of240 nm per refractive index unit. Furthermore, by using on-chip tempera-ture referencing, a differential temperature sensitivity of only 0.3 pm/K isobtained, without individual sensor calibration. This low value indicatesgood sensor-to-sensor repeatability, thus enabling use in highly parallelchemical assays. We demonstrate refractive index measurements duringtemperature drift and show a detection limit of 8.8× 10−6 refractive indexunits in a 7 K temperature operating window, without external temperaturecontrol. Finally, we suggest the possibility of athermal slot-waveguidesensor design.

© 2010 Optical Society of America

OCIS codes: (130.6010) Sensors; (230.5750) Resonators; (120.6810) Thermal effects.

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1. Introduction

Refractive index sensing is widely used for real-time monitoring of chemical processes and,when used in combination with separation techniques such as liquid chromatography or cap-illary electrophoresis, universal solute detection systems can by created [1]. Refractive indexsensing is also used for label-free monitoring of bio-molecular interactions on surfaces, forexample in the commercially successful surface plasmon resonance (SPR) based sensors [2].Most commercial separation tools and SPR sensors are built from bulky discrete componentsand placed in centralized laboratories, but integrated planar lightwave circuit technology holdsgreat promise for the development of mobile low cost sensor arrays for highly parallel real timeanalysis. For use in such integrated systems, planar waveguide ring resonator sensors are inter-esting for their small footprint, high quality factors, and ease of integration with other on-chipoptical and fluidic functions.

To be of analytical relevance [3] and present a viable alternative to current technology [4],novel sensors need to achieve a detection limit of the order of 10−6 refractive index units (RIU)or less. When considering that commonly used waveguide materials, and the liquid samplesof interest, have thermo-optic coefficients of magnitudes 10−5–10−4 RIU/K, it is clear thatminimizing temperature interference is a fundamental aspect of refractive index sensor design.Any uncompensated sensor would require external temperature stabilization to the order of10–100 mK to reach the required detection limit.

Three approaches have been used for thermal noise reduction: active temperature control,athermal waveguide design, and temperature drift compensation by on chip referencing [4].

The first approach, active control of system temperature, is commonly implemented withexternal Peltier heat pumps. Given a suitable thermal and electronic design, temperature sta-bility in the 10 mK range is feasible. However, the required components add to the cost, size,and complexity of the the sensor system, and limit the cost benefits gained by employing sili-con micro-fabrication. We note, however, that for measuring chemical reaction rates, absolutetemperature control is required, since rate constants are temperature dependent.

In the second approach, athermal waveguide sensors are designed by taking advantage of thedifferent polarity of the thermo-optic coefficients of liquid samples and solid waveguide mate-rials. For example, water has a negative thermo-optic coefficient of κH2O = −10−4 RIU/K [5],while silicon nitride and oxide have a positive value of κSi3N4 = κSiO2 = 10−5 RIU/K [6, 7].By balancing the fraction of light propagating in each material, the temperature dependenceof the waveguide effective index can be eliminated. Since athermal waveguides are intrinsi-cally temperature compensated, the compensation is not frequency limited by the thermal timeconstants of the sensor chip. The main drawbacks are that the design is often very sensitive tochip-to-chip fabrication variation and is solvent dependent, since thermo-optic coefficients ofliquid solvents vary significantly.

Athermal planar waveguide wavelength filters with solid polymer top cladding have beenstudied for almost two decades [8], but we are not aware of any report of integrated refractiveindex sensors using athermal planar waveguides. Recently, however, Suter et al. [9] reported onnon-integrated athermal ring resonator refractive index sensors using liquid filled silica capillar-ies with air cladding. In this case, the capillary wall is the waveguide core and light is coupledin via a manually positioned optical fiber taper.


Si

SiO2

H2O

Si3N4 Si3N4 Si3N4Si3N4 H2OH2O

ws wgwr

tc

tbwr wr w’rws

> 0 < 0

> 0

> 0

Fig. 1. A schematic cross section of the coupling region of a slot-waveguide ring resonatorrefractive index sensor. To the left is the straight bus waveguide and to the right the bent ringwaveguide. The opposite polarity of the thermo-optic coefficients κ of the solid waveguidematerials and the liquid sample, utilized in athermal sensor design, is indicated on the ringwaveguide end face.

A possible explanation for the absence of reports on athermal planar sensors is that the lowrefractive index of liquid samples, compared to that of the solid waveguide materials, makesconventional strip sensor waveguides more asymmetric than polymer clad filter waveguides.Thus, the technique of thinning the waveguide core to push power up into the polymer topcladding, as employed in filter design, when employed to sensors, mostly pushes power downinto the solid substrate. Furthermore, since the thin-core waveguides operate close to cut-off,the athermal operation point is sensitive to fabrication variances.

This limitation can be overcome by use of the recently developed slot-waveguides [10], inwhich an adjustable fraction of the optical mode can be confined in the liquid top cladding byadjusting the slot width. This new design parameter enables the design of athermal single modewaveguides without thinning the core and has already been applied to wavelength filter designin [11]. Figure 1 shows a schematic cross section of the coupling region of a slot-waveguidering resonator refractive index sensor. To the left is the straight bus waveguide and to the rightthe bent ring sensor waveguide.

In the third approach, temperature drift compensation by on-chip references, multiple iden-tical sensors in good thermal contact with each other are integrated on the same substrate. Ifthe fluidic system allows injecting the sample of interest to one sensor, and a reference sam-ple to another, differential measurements can be made. Such designs are solvent independentand tolerant to chip-to-chip fabrication variation. However, for highly parallel operation, thefabrication method must yield repeatable sensor-to-sensor thermal sensitivity within each chip,so that time consuming thermal calibration of each sensor can be avoided. Furthermore, thetemperature compensation is limited in frequency by the thermal time constants of the sensorchip.

A sensor system might employ all or any mix of the three temperature interference reduc-tion methods discussed. For example, active temperature control can compensate for a residual


temperature sensitivity of an athermal waveguide design. However, to deliver on the promiseof compact, low cost, integrated optics for refractive index sensing, active temperature controlis best avoided, and on on-chip temperature compensation techniques pursued instead.

Here, we present an integrated slot-waveguide refractive index sensor array, designed for highrefractive index sensitivity and temperature compensation by on-chip referencing. Our experi-ments include, to our knowledge, the first reported thermal sensitivity study of slot-waveguiderefractive index sensors and the first reported implementation of a slot-waveguide sensor arraythermally compensated by on-chip referencing. We demonstrate the ability of the sensor sys-tem to operate during temperature drift and verify that our fabrication process yields sufficientlyrepeatable temperature sensitivity for on-chip compensation without individual sensor calibra-tion. Furthermore, we discuss the possibility of trading refractive index sensitivity in exchangefor fully athermal slot-waveguide sensors.

2. Sensor chip design

Figure 2 is a top view of the layout of the optical circuit, occupying a chip area of 3×7 mm2.Light enters at a 10◦ angle to the surface normal, via the surface grating coupler (c). We use afully etched grating designed for a high coupling efficiency, a large coupling angle tolerance,and simple fabrication [12]. The propagating light is then split, by a multi-mode interferencesplitter (b), into eight channels: REF1, which has no sensor and is used for alignment and laseramplitude compensation, REF2, which is coupled to a reference slot-waveguide ring resonatorcovered with silicon dioxide top cladding, and channels M1 to M6, containing the sensing sites,where openings are etched in the silicon dioxide top cladding to allow liquid sample accessdown to the slot-waveguide ring resonators (a).

The optical devices are etched into a silicon nitride film of thickness tc = 300 nm (refer toFig. 1). The distribution network consists of 900 nm wide channel waveguides, and channel-slot mode converters [13] are used for conversion between the two waveguide types before andafter the ring resonator coupling regions, where the bus slot-waveguides have rail widths ofwr = 400 nm and a slot width of ws = 200 nm. The coupling gap is wg = 350 nm. In the sensingring we employ asymmetric slot-waveguides [14] with the inner rail widened to w′

r = 550 nm[15], for high optical confinement in the slot and low bending loss.

The pitch between the channels is 750 μm and each waveguide output is focused onto asingle pixel of an InGaAs photo-diode array at the output edge [16]. The orientation of theinput grating and splitter, perpendicular to the output edge, minimizes stray light illuminationof the detector array.

For sample delivery, a microfluidic channel network in poly(dimethylsiloxane) (PDMS), witha separate fluid channel to each sensor, is bonded on top of the silicon chip [17]. The totalsilicon chip area is 15×40 mm2, most of which is used as support for the fluidic channels andconnections in the PDMS layer. The overall size of the optical circuit, i.e. the pitch between thesensors and the distance of the sensors from the output edge, is also determined by the needfor sufficient spacing between fluidic channels, to avoid cross channel leakage, and a minimumchannel width, to allow manual alignment of the microfluidics to the optics under a microscope.

3. Sensor chip fabrication

The integrated optical components of the sensor chip are fabricated by standard silicon micro-fabrication methods. First, a bottom cladding layer of thickness tb = 3.26 μm was grown bywet thermal oxidation of a silicon substrate at 1100◦C. A 300 nm thick silicon nitride film wasthen deposited by low pressure chemical vapor deposition (LPCVD) at 800◦C from NH3 andSiH2Cl2 precursors. Then, the optical device layer was patterned in the silicon nitride film, byelectron beam lithography and dry etching. We employed a negative electron beam resist (ma-


Integrated optical components

500 µm

REF1 M1 M2 M3 M4 M5 REF2M6

Grating couplerSplitterSlot ring-resonatorSlot wave guide

1 µm

70 µm20 µm

1 µm

7 mm

3 m

m

(a)

(b) (c)

(a) (b) (c)

Fig. 2. A top view of the layout of the optical chip: Light is injected at the surface gratingcoupler (c) and split, by the multi-mode interference splitter (b), to the six sensing channelsM1–M6 and the two reference channels REF1 and REF2. Inset are an optical micro-graphof the splitter (b); and electron micro-graphs of the grating coupler (c), and a slot-waveguidering resonator (a), with an enlargement of the coupling region.

N 2403, Micro Resist Technology GmbH, Germany), exposed with a Raith-150 electron beamwriter using a 10 μm aperture at an acceleration voltage of 25 kV, as a mask for etching in aCHF3/CF4/O2 plasma.

A negative electron beam resist is particularly well suited for electron beam patterning ofnarrow optical waveguides, since only a small fraction of the surface needs to be exposed.Previously, we employed a positive resist (PMMA) and an additional lift-off step to the sameeffect [15], but we found the simplified process more repeatable, even though the patterningresolution achievable with the negative resist is only just sufficient for our purpose. Since thesmallest feature of our design is the 200 nm wide waveguide slot, standard deep UV lithographycould be used for mass-production [18].

A dense top cladding layer of a suitable refractive index is needed to protect the opticaldistribution network and cover the input grating coupler. Our choice of LPCVD tetraethylorthosilicate (TEOS) deposited at 720◦C is dictated by the need for void free filling of thethrough-etched input grating groves (with an aspect ratio of 3/5). A top cladding thickness of530 nm, in combination with the 3.26 μm bottom cladding, was previously shown to give op-timum input coupling efficiency [12]. Openings in the top cladding down to the sensors werepatterned by optical lithography and wet etching in buffered hydrofluoric acid. The thermallyoxidized bottom cladding provides a suitable etch stop, since the etch rate selectivity to nondensified TEOS is about 1:8.

The process is finalized by dicing the substrate. To avoid particle contamination of the sen-sors, the chip surface was covered with a protective layer of photoresist during dicing. From amass production perspective, it is important that no polishing of the output edge is needed afterdicing. This is achieved by limiting the divergence of the output beams by inverted waveguide


tapers at the output edge [19] and by a proper choice of lens and detector array in the detectoroptics [16]. Since handling of individual sensor chips is avoided, they can be mass produced atlow cost using standard silicon micro-fabrication.

4. Experiments

Light from a mechanically tunable, external cavity, diode laser (TSL-210V, Santec, Japan) wascoupled into the chip from free space optics above the surface. The minimum wavelength stepof the laser is 1 pm and the tuning range is 1260 – 1360 nm. TE polarized light was selected byan in-line fiber polarization controller between the laser and the free space optics. After passingthrough the chip, light is collected from the diced chip edge by a lens focusing the eight outputsonto a linear photo-diode array (XLIN-1.9-016-TE0, Xenics, Belgium) — each output onto asingle pixel. The silicon sensor chip is clamped to an aluminum platform with a Peltier heatpump attached to its back side. The temperature of the system is read out by a Pt1000 platinumresistance thermometer attached to the aluminum platform, and temperature stability to within0.1 K is achieved by a feed-back amplifier driving the Peltier heat pump. A detailed descriptionof the optical setup has been published in [16].

Liquid flow was controlled by off-chip syringe pumps, one for each flow channel. Duringoperation, the pumps supply a continuous flow of deionized (DI) water to the chip, and samplesare injected into the flow using in-line injection valves. A detailed description of the fluidicsetup has been published in [17].

To determine the refractive index limit of detection of the sensor array, three experimentswere performed:

In the first experiment, we determined the temperature sensitivity of the slot waveguide sen-sors operating in still standing DI water. Using the Peltier heat pump, we raised the temperatureof the chip from 23.0◦C to 33.0◦C, in steps of 2.0 K, and then straight back to 23.0◦C, whilemonitoring the sensor resonance wavelength. We tracked the response of both sensor M1 andM2, to study the temperature compensation ability of the system, when referencing one sensoragainst the other. The laser continuously swept a wavelength range of 2.1 nm with a 20 pmwavelength step.

In the second experiment, we determined the refractive index sensitivity, by injecting a dilu-tion series of ethanol in DI water into channel M1. The shift of each dilution from the refractiveindex of pure water is obtained from [20] and listed in Table 1. During injections, thermal driftwas monitored by a DI water filled reference channel (M4). The laser swept a wavelength rangeof 4 nm with a 10 pm step.

Table 1. Mass percentage (mass of ethanol/total mass of solution) of the injected ethanolcalibration solutions and the corresponding shift from pure water refractive index.

Mass percentage Refractive index shift5.97 0.00373.98 0.00241.99 0.00120.994 0.00060.500 0.0003

In the third experiment, we raised the temperature of the chip from 22.5◦C to 31.5◦C in stepsof 3.0 K, while repeatedly injecting 2% ethanol plugs, to demonstrate the ability of the systemto compensate for temperature variations during measurement.


The simple mode structure of the single mode ring resonators allows us to use the Levenberg-Marquardt algorithm1 to fit a Lorentzian model to the data points around resonance, and therebyreduce the effective wavelength error of the measurement well below the wavelength step ofthe laser [21]. Furthermore, we can reduce the interfering influence of reflections from waveg-uide discontinuities on the estimated resonance wavelength, by cascading a Fabry-Perot cavitymodel [22] to the Lorentzian resonator model. The presence of such parasitic reflections is welldocumented in previous experimental work, and particularly visible in the spectra published in[23], where end-fire light coupling at the chip edge is employed. The problem is less apparentin [24], where surface grating couplers are employed at both input and output.

To isolate the contribution of the parasitic reflections, we first collected transmission spectraof dry ring resonators, since no light is coupled to the rings when operating in air. The spectraindicated that two parasitic Fabry-Perot cavities contribute. The extracted model parameters forthe shorter cavity fit well to the length of the output waveguide from the edge of the opening inthe top cladding, at the sensing ring, to the output edge of the chip. We suspect that the longercavity corresponds to the section from the input grating coupler to the edge of the sensor open-ing, however, due to the complicated lightwave circuit in this section this hypothesis is difficultto confirm. Figure 3 shows example spectra, the fitted combined models, and the extracted ringresonance wavelengths of sensor M1 operating in DI water at two different chip temperatures.

5. Results

Figure 4(a) and (b) show ring resonance wavelengths as functions of time during temperaturestepping, for channels M1 and M2, respectively. In Fig. 4(c) and (d) we plot the same datasets as functions of temperature, to quantify the temperature sensitivity of the two sensors. Lin-ear regression yields temperature sensitivities of −16.7±0.2 pm/K and −16.4±0.1 pm/K forchannels M1 and M2, respectively. The larger standard error on the estimate of the M1 tempera-ture sensitivity is due to the somewhat poorer linearity of the M1 response. This poorer linearityis in turn due to a poorer fit of the combined ring resonator and Fabry-Perot cavity model tothe M1 spectra, yielding a larger error in the ring resonance wavelength estimate. The randomscatter in both channels is, however, dominated by the limited temperature stability (0.1 K) ofthe temperature controller, and averages out in the regression. From this measurement, we findthat the differential temperature sensitivity for slow temperature drift, without individual sensorcalibration, is only of the order of 0.3 pm/K, that is a 55 times improvement.

Figure 5(a) shows the resonance wavelength shifts of sensor M1, and the reference M4, asfunctions of time during injections of the dilution series of ethanol into a running buffer ofDI water in M1. The mass percentage of each injection is indicated above the correspondingpeak. The three lines represent channels: M1; M4; and M1−M4, that is M1 compensated bysubtracting from it the drift observed in M4.

The inset of Fig. 5(a) shows a magnification of the measured baseline noise of the com-pensated signal M1−M4. The standard deviation of the measured total system noise is onlyσ = 0.7 pm, even though the laser wavelength step is 10 pm in this measurement. This signif-icant noise reduction is due to the model fitting employed, which averages out wavelength andamplitude errors. We noticed that a further decrease of the wavelength step did not significantlyreduce the noise level, indicating that other noise sources also contribute to the system noise.

Figure 5(b) shows the corresponding resonance wavelength shifts observed in M1−M4 asa function of the refractive index shift of the injected solutions. As expected, the ring reso-nance shift is proportional to the refractive index shift and the slope of the line fitted yields therefractive index sensitivity of the sensor: S = 240±10 nm/RIU.

Figure 5 (c) demonstrates the ability of the system to compensate for temperature variation

1As implemented by the MATLAB® nlinfit function.


1309 1309.2 1309.4 1309.6 1309.8 1310 1310.2 1310.4 1310.6 1310.8 13110

0.1

0.2

0.3

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0.6

0.7

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1

Wavelength [nm]

Tran

smis

sio

n [a

rb.]

1310.18 1310.22 1310.26

0.08

0.09

0.1

0.11

0.12

23 °C33 °C

MeasurementModel

Fig. 3. Example transmission spectra of sensor M1 operating in DI water at two differenttemperatures. The wavelength step in this particular measurement was 20 pm. The solidline is a combined Lorentzian and double cavity Fabry-Perot model. The obtained qualityfactor of this device was 3000 and the arrows indicate the extracted resonance wavelengths.The inset shows an enlargement of the region around resonance at 33◦C

during measurement. The figure shows the resonance wavelength shift of sensors M1 and M2as a function of time, with the DI water filled M2 serving as a temperature reference. Repeatedshots of 2% ethanol are injected in M1, while the temperature of the chip is varied from 22.5◦Cto 31.5◦C.

6. Discussion

The measured negative temperature sensitivity of the slot-waveguide ring resonators confirmsthe ability of slot-waveguides to guide and confine a significant mode fraction in the low indexslot. A comparison of the thermo-optic coefficients of water, silicon nitride, and silicon oxideshows that, to obtain a negative temperature coefficient, more than 10% of the mode power mustpropagate in the liquid sample. Indeed, numerical simulations of the design have indicatedthat 27% of the optical power of the slot-waveguide quasi-TE mode should propagate in thesample [25]. The high refractive index sensitivity of 240 nm/RIU obtained in our measurementsalso indicates that a large mode fraction propagates in the liquid sample.

Even though our waveguide design was optimized for high refractive index sensitivity ratherthan athermal operation, the obtained absolute temperature sensitivity of −16.6 pm/K, on aver-age, is still rather low. In fact, the negative sign indicates that, from an athermal design perspec-tive, the waveguides are slightly overcompensated, and that by reducing the amount of power


50 100 150 200

1310.2

1310.25

1310.3

1310.35

1310.4

Time [min]

Reso

nan

ce w

avel

eng

th [n

m]

50 100 150 200

1309.7

1309.75

1309.8

1309.85

−16.7 ± 0.2 pm/K

22 24 26 28 30 32 34

−16.4 ± 0.1 pm/K

Temperature [°C]

23°C

25°C

27°C

29°C

31°C

33°C

23°C

(a) M1 (c) M1

(b) M2 (d) M2

Fig. 4. The left panels show the resonance wavelengths of (a) channel M1, and (b) channelM2, as functions of time during temperature stepping from 23.0◦C to 33.0◦C and a jumpback to 23.0◦C. The right panels show the corresponding resonance wavelengths of (c)channel M1, and (d) channel M2, as functions of temperature. The slopes of the fitted linesyield the temperature sensitivities of the sensors.

propagating in the liquid sample, by adjusting the slot width, a fully athermal design could beaccomplished. For comparison, the athermal capillary based ring resonator sensors reported bySuter et al. [9] show temperature sensitivities from 17.2 pm/K down to 5.4 pm/K, while at thesame time a refractive index sensitivity of only 3.6 nm/RIU, two orders of magnitude lower thanpresented here. In contrast, the planar polymer ring resonator sensors reported in [26], show amuch larger magnitude of temperature sensitivity of −80 pm/K, since the polymer core and theliquid cladding have thermo-optic coefficients of the same polarity and thus do not compensateeach other.

In Fig. 5(a), we note that the baseline of M1 red shifts slightly with each injection, whilethe reference M4 blue shifts 14 pm during the same period. We attribute the red shift of theM1 baseline to the known solvent absorption of PDMS [27]. During each injection, a smallfraction of the ethanol is absorbed, which then slowly diffuses out again during the subsequentDI water flush and lifts the baseline. To avoid any effect of the PDMS solvent absorption on therefractive index sensitivity calibration, care was taken to reach a plateau level in each injection.Furthermore, the slope of the refractive index sensitivity is drawn through the plateau levelsof the injections, and not referred to the drifting base line. The good linearity of the observedresponse and the close agreement with the refractive index sensitivity of 210 nm/RIU reportedin [15], for a resonator of the same design but with no PDMS layer, further supports the validityof the calibration. The slightly higher sensitivity observed in this work is most likely due to


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M1M4

M1−M4

0 0.002 0.004−0.2

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Refractive index shift

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th s

hift

[nm

]240 ± 10 nm/RIU

2% 2% 2% 2%

22.5°C 25.5°C 28.5°C 31.5°C

Time [min]150 200 250 300

M1M2

M1−M2’

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0

5

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set

fro

m b

asel

ine

[pm

]

M1−M4

3σ = 2.1 pm

(b) (a) (c)

M1−M4

M2’

Fig. 5. (a) The resonance wavelength shifts of sensors M1, M4, and their difference, asfunctions of time during injections of a dilution series of ethanol into a running buffer ofDI water in M1. The inset shows a magnification of the measured baseline noise of thedifferential signal. (b) The corresponding shifts observed in M1−M4 as a function of therefractive index shift of the injected solution. (c) Resonance wavelength shifts of channelsM1, M2, and their difference, for repeated injections of 2% ethanol in M1 during a 9 Ktemperature transient. The dashed line M2′ indicates an interpolated M2 signal with crosstalk from the injections in M1 removed.

deviations in waveguide dimensions as a result of changes in the lithography process (fromlift-off in [15] to negative resist in this work). The blue shift of M4 corresponds to a sensortemperature increase of 0.8 K, and is most likely due to an increase in ambient temperatureduring measurement.

Following the convention of using three standard deviations of the total system noise as ameasure of the sensor resolution [28], we obtain a refractive index limit of detection of

D =3σS

=2.1 pm

240 nm/RIU= 8.8×10−6 RIU, (1)

without calibration or active temperature control. Since the differential temperature sensitivityis only 0.3 pm/K, we have a temperature operating window of 7 K, in which the system noiseis dominated by other factors than temperature. If on-chip referencing would not be employed,the operating window would be only 0.1 K.

Due to the higher index contrast between silicon and water than between silicon nitrideand water, higher refractive index sensitivities can be obtained in silicon slot-waveguide sen-sors [29]. For example, numerical calculations have predicted silicon slot-waveguide sensorsensitivities of 348 nm/RIU for a 104 nm slot width [24], and 490 nm/RIU for a 40 nm slot


width [23]. However, from a sensing point of view, the obtainable detection limit of the sens-ing system is a more relevant figure of merit [28], and as evident from (1) above it dependsboth on the refractive index sensitivity and on the total system noise, which in turn dependsstrongly on the quality factor of the sensing resonator [28]. Thus, even though the silicon slot-waveguide reported in [24] experimentally showed a refractive index sensitivity of 298 nm/RIU,the low observed quality factor (330) of the sensing ring resonator yielded a detection limit of4.2×10−5 RIU, almost five times that of the silicon nitride device in this work. We note, though,that high quality silicon slot-waveguide ring resonators with solid polymer top cladding havebeen demonstrated in [30], and given appropriate design adjustments for a liquid water topcladding such devices should be able to improve on the refractive index limit of detection pre-sented here.

As seen in Fig. 5(c), an increase in temperature yields a common mode blue shift of bothchannels that is effectively canceled in the differential signal, while the injection of a highrefractive index sample in M1 yields a clear differential red shift, as expected. We notice, how-ever, that the injection in M1 creates slight cross-talk in M2. We believe this effect is due to thesolvent absorption of PDMS, as discussed above, and the close proximity of reference chan-nel M2 to the measurement channel M1 in this experiment (in contrast to reference channelM4 in Fig. 5(a)). To avoid these cross-talk effects on the differential signal, we interpolate thereference signal over the injection time, before subtracting it from M1 for compensation. Theinterpolation is indicated by the dashed line M2′ in the figure.

7. Conclusions

We have presented an integrated slot-waveguide refractive index sensor array and, to our knowl-edge, the first thermal sensitivity study of slot-waveguide refractive index sensors. The sensorsshow a temperature dependence of only −16.6 pm/K, on average, and at the same time a highrefractive index sensitivity of 240 nm/RIU. Furthermore, on-chip temperature compensation,by referencing the sensors to each other, yields a differential temperature sensitivity of only0.3 pm/K.

We demonstrated the ability of the sensor system to measure during temperature drift andshowed that our fabrication process yields sufficiently repeatable sensor-to-sensor temperaturesensitivity for on-chip compensation without individual sensor calibration, thus avoiding timeconsuming calibration and enabling use in highly parallel chemical assays.

Our main result is that a slot-waveguide refractive index sensor array, utilizing on-chip tem-perature referencing, can deliver a refractive index limit of detection of D = 8.8×10−6 RIU ina 7 K temperature operating window, without external temperature control or individual sensorcalibration.

Finally, we suggested the possibility of athermal slot-waveguide refractive index sensors, bytuning the slot width to balance the fraction of light propagating in the liquid sample. Athermalslot-waveguide sensors would be able to accurately measure liquid sample refractive indexunder rapid temperature variation.

Acknowledgments

K. B. Gylfason acknowledges support of the Steinmaur Foundation, Liechtenstein. This workis done within the FP6-IST-SABIO project (026554) funded by the European Commission.


On-chip temperature compensation in an integrated slot-waveguide ring resonator refractive index...

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