A sensor based on the planar-polarization interferometer

A

ELSEVIER

PHYSICAL

Sensors and Actuators A 68 ( 1998) 384-387

A sensor based on the planar-polarization interferometer Yu.M. Shirshov a,*, S.V. Svechnikov a, A.P. Kiyanovskii a, Yu.V. Ushenin a, E.F. Venger a, A.V. Samoylov a, R. Merker b aInstitute of Semiconductor Physics, National Academy of Sciences of Ukraine, prospect Nauki 45, 252650 Kiev, Ukraine b Con-QUIP GmbH, Bertolt-Brecht-Allee 22, D-01309 Dresden, Germany

Abstract

We have successfully used a single-beamplanar interferometerbasedon the silicon-silicon dioxide-silicon nitride-phosphosilicate glass multilayer structure. s- and p-polarizations of the samelight beam are used as its individual arms.The high sensitivity of the device when serving as a refractometerand au immunosensoris demonstrated.The experimental results are shown to be in a good agreementwith those 0 1998Elsevier ScienceS.A. All rights reserved. calculatedwithin the framework of a transversesynchronism. Keywords: Sensors; Interferometers; Planar polarization

1. Introduction Use of optical methods for detection of slight changes in the refraction index of a liquid and/or of molecular layers adsorbed on a smooth surface seems to be a promising line of developing fermentation and immunological biosensors [ I]. Of all the optical approaches used in this connection, the interference principle provides the highest sensitivity. For instance, refractive-index changes of about 10m4 were detected in Ref. [2] for a liquid. The authors of Ref. [3] have demonstrated that a direct registration of small (about 1000 D) pesticide molecules dissolved in water is possible through their immediate binding to the antibodies immobilized at the optical waveguide walls. The advantage of such devices lies in the possibility of increasing the sensitivity (practically without limit) by increasing the length of the active region where the evanescent wave is interacting with a polarizable medium. However, the design of a planar interferometer with spatially separated beams (used in Ref. [ 31) has a grave drawback consisting in the necessity of using optical splitters and connectors of high precision. One can overcome this drawback by using light beams of different polarization in the operation and comparison channels. This means that for an s-polarized beam the magnetic vector in the evanescent-wave region weakly interacts with the medium polarization, so it can serve as a reference beam. Contrary to this, a p-polarized beam strongly interacts with an ambient, so it can serve as an operating beam. The two corresponding * Corresponding author. E-mail: [email protected]

electric fields add up in a natural way at the waveguide output end, and the outgoing beam ellipticity is then to be measured. 2. Experimental

procedure

We have developed and tested a planar-polarization interferometer that uses a single linearly polarized light beam. The polarization azimuth was 45” relative to the surface of a multilayer flat waveguide. This waveguide was a layered structure on a silicon substrate (Fig. 1) . It included a thermally formed silicon dioxide buffer layer (its thickness, d, was 1.2 pm and refractive index, IE,was 1.46)) a silicon nitride waveguide layer (d=0.19 km, n = 2.0) and a phosphosilicate glass (PSG) covering layer (d = 0.12 Frn) . The chip surface area was 10 mm X 12 mm. The chip edges were processed in a specific way to improve light leading both in and out without mwl \

FL--7

Fig. 1. General view of the multilayer structure as an optical wal

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-7 Jy

s

14

Fig. 3. Variations of the output signal intensity due to the changes in the solution refractive index. photo diode Fig. 2. Optical diagram of a sensor based on the planar-polarization interferometer.

special matching elements (prisms or gratings). To provide a contact with the substancestudied, a window was etched (forming a microcuvette) in the central areaof the chip covering layer. Its edges,2 to 8 mm long, were normal to the light beam.The liquid to be testedwas flow-injected into the hollow enclosedwith a rubber O-ring. The sensoroptical diagram is shown in Fig. 2. A polarized He-Ne laser beamis focusedwith a cylindrical lens onto the input face of the optical waveguide, so the amplitudesof the s- andp-componentsare equal. After the beamhasinteracted with the medium studied,both s- andp-polarized beamcomponents are led out through the interferometer output end. They form a wave of elliptic polarization, the parametersof which are determinedby both the sensordesign and the optical constantsof the medium studied. Variations in its refractive index and/or the thickness of the adsorbedmolecular layer lead to a continuous change of the phase difference between the s- and p-polarized components at the device output. This will result in periodic changesof the output wave ellipticity, which areregisteredwith a linearpolarizer (placed immediately at the light beam output) and a detector of the output beamintensity. A dichroic film (d = 0.2 mm, quenching degree98%) servedas a polarizer. The input window of a glassoptical fibre (0.5 mm in diameter) waspressedimmediately againstthe polarizer. The distancebetweenthe interferometer output end and the fibre input window did not exceed 1 mm. Light detection after passing the optical fibre wascarried out with aphotomultiplier of moderateefficiency.

(2)

Here A(pP,,A(pP2,Aqs,, Apse,,, ‘pP,and cpsare to be found in a self-consistent way from the synchronism condition. They are functions of n,, ni, n, and d. From Eqs. ( 1) and (2) one not only can calculate the number m of interferencefringes that can be observedwhen the refractive index of the solution studied changesfrom IZ, ton, (m= [QsP(n,) -@Jn,)]/2~),butcanalsopredictan explicit form of the I,, ( t) curve for a given ?Zi( t) . Fig. 3 shows the experimental lout(t) curve. It was taken when the liquid in the device was gradually changedin the following sequence:water-aqueoussolution of sodium chloride-water-aqueous solution of sugar-water. The refractive index of water ( measuredindependentlywith anAbbe refractometer) was 1.3330.Both solutions were preparedin such a way that their refractive indices were 1.3348.One can see from Fig. 3 that the number of interferencefringes is N 3.0, whatever the substancedissolved. This fact indicates that no ordered surfaceaggregateexists in this caseat the solutionsilicon nitride interface. Similar experimentshave beencarried out (in a wide range of refractive-index changes) with solutions of glycerol. The parameterm was also calculated from Eqs. ( 1) and (2). Both the calculated and experimentally found m values (along with the maximum ni value) are given in Table 1. One can see from Table 1 that the calculated m values agreerather well with the experimental ones.This is an evidencethat the synchronismtheory may be applied to describe devicesof the type used. An attempt at parameter optimization for the proposed planar-polarization interferometerseemsto be of interest.To

3. Results and discussion

The thicknessesof both buffer and covering layers used by us much exceededthe light wavelength, hln2, in the solution studied. So a well-known approachwas used, basedon the concept of transversesynchronism [ 41. A straightforward calculation [5,6] gives for the output signal parametersthe following expressions:

~(t),ut=kZi,Sin@~',,(fZi) where

(1)

Table 1 Experimental (m,,,) and calculated (m,,,) values of the number of interference fringes obtained for a sequence of water and aqueous solutions

Solution

ni

mexp

mcdc

Glycerol (1) Glycerol (2) Sodium chloride sugar

1.3573 1.3660 1.3338 1.3379

11.8 32.3 3.3 2.4

10.4 33.4 2.8 2.1

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this end one can use a number of interferencefringes, mcalc (calculated for the case of ni change from nil to ni2) as a sensitivity measureof a device usedas a refractometer.We have calculated the m value, for replacing water with an aqueoussolution of glycerol, in a standarddevicefor different waveguide layer thicknesses.The greater the m values, the higher the device sensitivity. Fig. 4 showsthe m dependence on the waveguide layer thickness. It is unexpectedly nonmonotonic, with a pronounced peak. This meansthat there exists,in principle, a thicknessvalue for a single-modewaveguide at which the resulting gap betweenthe s- andp-polarized beamsreachesits maximum. In our case such a value would be 1.3-l .4 pm, so the thicknessd = 2 pm usedin our experimentswas not optimum. The abovefact seemsto beof interest.Someconsiderations can be applied in this connection. Decreasingthe waveguide thicknessresults in an increaseof the numberof wave reflections from its walls. Consequently, the total time during which anelectromagneticwave is interacting with apolarized medium increases.But after the optimum thicknessvalue has beenpassed,the conditions for a guided wave mode to exist seemto becomeworse- the waveguidebecomestoo thin. Fig. 5 illustrates how the output signal changes under bovine serumalbumin (BSA) adsorptiononto the waveguide surface. (The phosphatebuffer solution in the cuvette was replacedby a 50X 10m5g ml-’ BSA solution in this case.) From Fig. 5 one can seethat there arefive oscillations of the

I

01

dopt 0.15

I

0.2

output signal intensity. This correspondsto a phase-differencechangeexceeding 10~. We note that an attemptto wash the adsorbedBSA layer out failed: no output signal oscillations were detectedwhen a pure buffer was substituted for the solution of BSA. Assumingthat a linear dependenceexists between the BSA adsorptionand the concentration of molecules in the solution, and taking into account that the device usedcandetecta phasechangeabout0.01n, one can estimate the device maximum sensitivity as being about 10 ng ml-‘. This may be sufficient for direct detection of pesticides in water solutions. (The sameconclusion was also drawn in Ref. [3].)

4. Immunoreaction Fig. 6 showstheexperimentalcurve obtainedby theplanarpolarization interferometer.Case1 correspondsto a flow cell of water. The output signal doesnot change. Case 2 is the response for replacing the contents of the microcell with buffer solution. Case3 correspondsto the phasechangeunder adsorption of mouse anti-immunoglobulin (mouse ol-IgG) onto the waveguidesurface.(The ol-IgG solution had a concentration of 50 X 10e5 g ml-‘.) We note that an attemptto wash out the adsorbedmouse cr-IgGlayer by the buffer solution failed (case4) : no changes in output signal were observed.Case5 illustrates theresponse of theplanar-polarizationinterferometercausedby the immunoreaction between mousecr-IgG and mouse immunoglobulin (mouse IgG). Our attemptto wash this complex layer out failed (case 6), which proves that an immunoreaction takesplace betweena-IgG and IgG.

,

d

Fig. 4. Dependence of the number of interference fringes on the waveguide layer thickness.

1

ox)0 Fig. 5. Adsorption of BSA: 1, transition from water to the buffer solution; 2, injection of BSA solution in the same buffer.

I 40M

I

t. min

84a40

Fig. 6. Phase changes caused by the adsorption of mouse cl-IgG onto the waveguide surface and by the immunoreaction between mouse m-IgG and mouse IgG as a function of time (see text).

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5. Conclusions

The possibility of developing a single-beam planarpolarization interferometerhasbeendemonstrated.High sensitivity of the device when serving as a refractometer,immunosensor and enzyme sensor is demonstrated. The experimental results are shown to be in a good agreement with those calculated within the framework of a transverse synchronism. The advantageof such devices lies in the possibility of increasing the sensitivity by increasing the length of the active region where the evanescentwave is interacting with a polarizable medium.

Acknowledgements

This work waspartially supportedby the Grant PL 965131 INCO-Copernicus.

References [ l] W. Huber, R. Barnert, Ch. Fattinger, J. Hubscher, H. Koller, F. Muller, D. Schlatter, W. Lukosz, Direct optical immunosensing, Sensors and Actuators B 4 (1992) 122-126. [2] Th. Schubert, N. Haase, H. Kuck, R. Gottfried-Gottfried, Refractive index measurements using an integrated Mach-Zehnder interferometer, Sensors and Actuators A 60 (1997) 108-l 12. [3] T.T. Shipper, R.P.H. Kooyman, R.J. Heideman, J. Greve, Feasibility of highly sensitive optical waveguide immunosensors for pesticide detection: physical aspects, Tech. Digest, 5th Int. Meet. Chem. Sensors, Rome, Italy, July 1994, vol. 2, pp. 1021-1024. [4] T. Tamir (Ed.), Guided-Wave Optoelectronics, Springer, Berlin, 1988.

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semiconductorsmaterials. His areasof interests are physics of semiconductors,functional andintegratedoptoelectronics. Aleksandr P. Kiyanovskiy was born on 8 March, 1959.Since

1982he hasbeenwith the Institute of SemiconductorPhysics of the National Academy of Sciencesof Ukraine as an engineer. His areasof expertise cover optical devices including planar semiconducting interferometers.He is now working on planar semiconducting interferometersincluding design and development. Yuri V. Ushenin was born on 6 August, 1948.Since 1973he has been with the Institute of SemiconductorPhysics of the National Academy of Sciencesof Ukraine asan engineerand a senior researcher (since 1992). His areas of expertise include systemsof data collection and processing,software design (Pascal, Delphi) and optical devices including SPR and planar semiconducting interferometers. He is now working on surfaceplasmon resonanceapparatusandplanar semiconducting interferometers including design and construction. Evgenii F. Venger was born on 19 April, 1947.He received

Biographies

the MS. degreefrom Vinnitsa Educational College in 1970. Since 1978 he has been with the Institute of Semiconductor Physics, National Academy of Sciencesof Ukraine (Kiev) asjunior researcher,senior researcher(since 1988)) leading researcher (since 1990) and head of the Department of SemiconductorHeterostructures(since 1992). He is aDoctor of Phys.-Math. Sci. ( 1990) andaprofessor( 1992). His areas of research activity include spectroscopyof surface vibrational and plasma polaritons in semiconductor structures; computersimulation of relaxation processesin ion-implanted semiconductors; and optical properties of ultra-dispersed systems.

Yurii M. Slrirshov was born in 1940. He received his MSc.

Anton V. Samoylov was born on 13 Sept., 1974. Since 1997

in physics and mathematicsfrom Chemovtsy University in 1963. Since then he has been with the Institute of Semiconductor Physics, where he received his Ph.D. in physics in 1970 and Doctor of Physics in 1991. His areasof scientific activity include the physics of surfaces,molecular electronics, gas sensorsand biosensors.

he has been a postgraduatestudent at the Institute of Semiconductor Physics-of the National Academy of Sciencesof Ukraine (Kiev). His areasof interest include softwaredesign and development of optical devices including planar semiconducting interferometers. He is now working on planar semiconducting interferometers including design and development.

Sergey V. Svechnikov was born on 21 July, 1926. He was

head of departmentat Kiev Polytechnical Institute, Ukraine ( 1957-1961) ; head of the Microelectronics Department in the Institute of SemiconductorPhysicsof the National Academy of Sciencesof Ukraine (1961-1980); deputy director, head of the Optoelectronics Division (1980-1991); and director, head of the Optoelectronics Department (1991present). His researchfields of interestarephysical processes in optoelectronic structures,physics of low-dimensional systems, recombination processes,defects and impurities in

RolfMerker was born in 1949. He graduatedfrom the Kiev

Polytechnical Institute, Electron-Technics faculty, in 1973. He was then a postgraduatein the Institute of Semiconductor Physics, National Academy of Sciencesof Ukraine (Kiev). He received the degreeof Candidateof Phys-Math. Sci. in 1976 for researchon the electrical properties of recombination thermocentresin silicon. His areasof scientific activity are microelectronics,functional and integratedoptoelectronics and design of analytical equipment.