GB2185308A

GB2185308A - Optical waveguide material sensor

Info

Publication number: GB2185308A
Application number: GB08600522A
Authority: GB
Inventors: Jolyon Peter Willson
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-01-10
Filing date: 1986-01-10
Publication date: 1987-07-15
Also published as: GB8600522D0; GB2185308B

Abstract

A biochemical sensor device comprises a planar monomode waveguide 11 one surface of which is coated with a metal film 13 which in turn is coated with a dielectric film 14. The dielectric is as elective absorber of a material to be sensed. Light propagated through the waveguide interacts with surface plasmons in the metal film, the degree of interaction being a function of the electric field conditions adjacent the metal film. The attenuation is measured by a photodetector 16. A waveguide supporting two guided modes, only one of which exhibits resonance, may be employed, the non-resonant mode providing a reference signal. <IMAGE>

Description

SPECIFICATION Optical sensor device This invention relates to optical sensors, e.g. for chemical, biochemical or biological analysis.

Surface plasmon resonance is an optical surface phenomenon that has recently been employed in the construction of sensors. A surface plasmon is a surface charge density wave at a metal surface. A physical description of the phenomenon is given by H. Raether in Phys. Thin Films, 1977, 74 pp 237-244. The resonance can be observed when light interacts with a thin metal film applied to a smooth surface of a transparent, typically glass, body. Light reflected internally from the surface exhibits a minimum intensity for a particular (resonant) angle of incidence, this angle determined by the dielectric conditions adjacent the metal film and the properties of the metal film itself.

In a prior art sensor using this phenomenon, a metal film is applied to one surface of a glass prism. Such a device is described in Electronics Letters, 8th Nov. 1984, 20, No. 23, pp 968 to 970. In this device the resonant angle is determined by varying the angle of incidence and measuring the intensity of the reflected light. Such an arrangement is of course relatively bulky and requires a high degree of precision in the manufacture of its optical parts.

The object of the present invention is to minimise orto overcome these disadvantages.

According to one aspect of the invention there is provided a sensor device responsive to the presence of one or more materials, the device including an optical waveguide at least part of the surface of which is adapted to allow light propagated in discrete words along the waveguide to interact with surface plasmons thereby causing attenuation of the light, the degree of attenuation being characteristic of the presence or absence of the one or more materials, and means for detecting said attenuation.

According to another aspect of the invention there is provided a sensor device responsive to the presence of one or more materials, the device including an optical waveguide for the propagation of light in one or more discrete guided modes, a metal film disposed on at least a portion of the waveguide surface, and a dielectric film supported on the metal film, wherein the relative dielectric constraints of the waveguide, the metal and the dielectric and the thicknesses of the metal and dielectric films are such that at least one said guided mode interacts with surface plasmons in the metal film thereby causing attenuation of that mode, the degree of attenuation being characteristic of the presence or absence of the one or more materials adjacent the dielectric film.

Whilst plasmon resonance has been conventionally observed under unguided propagation conditions which can be described using ray optics, we have found that resonance can- also be observed for discrete guided mode propagation. Under the latter conditions ray optics are inappropriate for describing the physical process involved.

In an optical waveguide light propagates in one or more discrete modes. Aguided mode has a characteristic wave vector which is a function of the waveguide construction.

Plasmon resonance is observed when the component of the guided mode wave vector parallel to the metal/dielectric interface (Kx) is equal to the surface plasmon wave vector (Ksp) as given by the following equation:

where W is the optical frequency, C the free space velocity of light and Em is the real part of the dielectric constant of the metal. E1 is the dielectric constant of the waveguide and e2 is the dielectric constant of a dielectric applied to the metal e is the characteristic angle of the guided mode at the metal/dielectric interface. Thus the value of the wave vector at resonance is a function of both dielectric constants, the optical wavelength and of the metal.

We have found that, if the cladding of a multimode or monomode waveguide is sufficiently reduced in thickness that the evanescent field of the guided mode or modes extends from the core, surface plasmons can be excited in a thin metal film deposited on to the waveguide surface.

Embodiments of the invention will now be described with reference to the accompanying drawings in which Figures 1 and 2 are respectively sectional and plan views of a waveguide sensor arrangement; and Figures 3 and 4 show respectively alternative sensor constructions.

Referring to Figures 1 and 2, the sensor arrangement includes a planar monomode optical waveguide 11 supported on a substrate 12. The waveguide 11 is coated with a very thin metal film 13, typically 500 to 1000 A (0.05 to 0.1 microns), which film is in turn coated with a thin dielectric film 14. We prefer to employ vacuum evaporated (or sputtered) gold or silver as the coating metal 13.

Light is launched into the waveguide from a monochromatic light source 15, e.g. a light emitting diode or a laser, and is propagated along the waveguide 11. Light emitted from the waveguide 11 is received by a photodetector 16.

Light propagating along the waveguide 11 interacts with surface plasmons, the degree of interaction having a function of inter alia, the electric field condition adjacent the metal film 13.

When the waveguide is contacted with a material that is selectively absorbed by the dielectric 14 the electric field conditions adjacent the metal film 13 are altered causing a corresponding change in the plasmon resonance condition. This change in the resonance condition is detected and measured as a change in the output light intensity via the photodetector 16.

In a modification of the arrangement of Figure 1, only half of the surface of the device is coated with the dielectric so as to define two paths one of which has no dielectric film. This path forms a reference or control path to provide a differential sensor. In a further construction the sensor may comprise a plurality of light paths each sensitive to a different material.

Figure 3 shows a further sensor arrangement in which differential detection may be employed. A thin film optical waveguide 21 is formed on a substrate 22, the waveguide including a Y coupler whereby light launched into the guide via a light source 23 is split into first and second paths 24, 25 respectively. The waveguide defining one path (25) is coated with a thin film 26 comprising a silver or gold film and a dielectric film. Thus, light travelling along the path 25 displays surface plasmon resonance whilst light travelling along the other path 24 is unaffected. Light propagated via the paths 24 and 25 is received by a respective photodetector 27, 28. The outputs of the detectors 27 and 28 are coupled to a differential amplifier 29.Variation of the plasmon resonance condition in response to the presence of a selectively absorbed material in contact with the film 26 produces a corresponding change in the amplifier output.

Light may be launched into the waveguide by a number of techniques. Thus converging lenses may be used, or light may be led to and from the waveguide via optical fibre couplings. In a further construction the waveguide itself can provide the necessary coupling.

Figure 4 shows a waveguide structure that facilitates launching of the input light. The structure comprises a waveguide section 31 having input and output diffraction gratings 32 and 33 at the ends thereof. The waveguide 31 is provided with a surface metal film 34 which in turn is coated with a dielectric film 35. Light from a light source (not shown) is focussed by a lens 36 on to the input grating 32 whereby the light is propagated along the waveguide whereby plasmon interaction with the metal film 34 takes place. The propagated light is emitted from the waveguide via the output grating 33 and the light intensity pattern is monitored by a photodetector array 37.

In a further application a waveguide which supports two guided modes, only one of which exhibits resonance, may be employed. In such a device the non-resonant mode is angularly resolved to provide a reference signal from the attenuated resonant mode.

The sensor arrangements described herein may be employed in chemical, biochemical and biological applications. They are particularly adapted to forensic use as their small size renders them responsive to trace quantities of materials. in particular the dielectric film may contain antibodies for detecting binding of antigens to the antibodies. The availability of one or more reference light paths will allow the effect of the solution in which the antigens are contained to be compensated for.

Claims

1. A sensor device responsive to the presence of one or more materials, the device including an optical waveguide at least part of the surface of which is adapted to allow light propagated in discrete modes along the waveguide to interact with surface plasmons thereby causing attenuation of the light, the degree of attenuation being characteristic of the presence or absence of the one or more materials, and means for detecting said attenuation.

2. A sensor device responsive to the presence of one or more materials, the device including an optical waveguide for the propagation of light in one or more discrete guided modes, a metal film disposed on at least a portion of the waveguide surface, and a dielectric film supported on the metal film, wherein the relative dielectric constraints of the waveguide, the metal and the dielectric and the thicknesses of the metal and dielectric films are such that at least one said guided mode interacts with surface plasmons in the metal film thereby causing attenuation of that mode, the degree of attenuation being characteristic of the presence or absence of the one or more materials adjacent the dielectric film.

3. A sensor device as claimed in claim 2, wherein said metal film is 0.05 to 0.1 microns thick.

4. A sensor device as claimed in claim 2 or 3, wherein said metal film comprises silver or gold.

5. A sensor device as claimed in any one of claims 1 to 4, wherein said waveguide is a planar waveguide.

6. Asensor device as claimed in any one of claims 1 to 5, wherein said waveguide is a monomode waveguide.

7. A sensor device as claimed in any one of claims 1 to 6 wherein said waveguide incorporates diffraction gratings whereby light is conveyed to and from the waveguide.

8. A sensor device substantially as described herein with reference to and as shown in Figures 1 and 2, or Figure 3, or Figure 4 of the accompanying drawings.