SINGLE MODE OPTICAL FIBRE SURFACE PLASMA WAVE CHEMICAL SENSOR

A.J.C. "'ubb, F.P. Payne, R. Millington and C.R. Lowe

Introduction During the past decade interest in the use of surface plasma waves for chemical analysis

has grown from being a laboratory demonstration to the availability of commercial instruments which

are capable of measuring a wide range of chemical analytes at extremely low concentrations [l]. The

plasma wave sensor exploits the extreme sensitivity of surface plasma waves to small changes in the

refractive index of the material surrounding them. The surface plasma wave consists of a TM wave guided at the interface of a metal and surrounding dielectric (Fig. 1).

.

Fig. 1 A plasma wave is a TM surface wave guided by the interface between a metal and a dielectric.

The plasma wave decays evanescently on either side of the interface, with an exponential decay length

into the dielectric of only a few hundred nm. Consequently, the plasma wave is very sensitive to

changes in the refractive index near the metal surface, which manifest themselves in a change in the

attenuation of the plasma wave. The effective index of the plasma wave is described by

where E, and ed are the dielectric constants of the metal and dielectric respectively. Commercial

instruments exploit the Kretschmann configuration as a convenient way of exciting the surface plasma

wave (Fig. 2 next page) [2]. In this approach one surface of a glass prism is coated with a thin layer of

silver or gold, typically about 50 nm thick. When light of a suitable wavelength and angle of incidence

impinges on the metal surface from within the prism a sharp dip in the reflection is seen corresponding

to the excitation of a surface plasma wave at the boundary of the metal and the surrounding environ-

ment. The required angle of incidence is given by

r

Andrew Tubb and Frank Payne are with Cambriage University Engineering Department. Roger Millington and Chris Lowe are with Cambridge University Institute of Biotechnology.

Fig. 2 Surface plasma wave excited using the Kretschmann configuration. The refractive index of the glass prism is n.

nP nkosin& =

where k, is the free space wave number of the incident light. A small change in the refractive index

below the metal surface results in a change in the plasma wave effective index np and from equation 2

this results in a change in the angle of the incident light needed to excite the plasma wave. In commer-

cial instruments the change in 8 is measured by means of a photodiode array, and associated electron-

ics. The incident angle must be very accurately controlled, typically to within about 0.1", and the

wavelength of the incident light to within about 1 nm. The high price, and lack of portability of com-

mercial plasma wave sensors has led to a search for simpler and cheaper alternatives wi

level of sensitivity. The most promising of such devices have been based on optical waveguides, either

in the form of multimode fibres [3], or integrated optical waveguides [4]. Very promising results have

been demonstrated with these devices, which exhibit a similar degree of sensitivity to commercial

instruments. Never the less, all these devices suffer from a number of disadvantages, which stem from

the very precise conditions needed to excite a plasma wave, as implied by equations 2 and 3. In the case

of multimode fibres this is achieved by carefully controlling the launch conditions of the light into the

fibre, whilst in integrated optical devices, dielectric buffer layers are placed between the waveguide and

the metal layer to provide phase matching to the plasma wave [4]. The wavelength of the light must

also be controlled to withii a nm.

In this paper, we present a new optic& fibre plasma wave sensor which overcomes all these diffi-

culties. This is based on a tapered single mode fibre whose surface is coated with a layer of silver. The

changing cliameter along the taper results in a distributed phase matching between the fibre and plasma

modes, which guarantees the excitation of a surface plasma wave over an extremely broad range of

wavelengths from 600 to about 1000 nm.

The device The sensor is illustrated in Fig. 3, below. It consists of a tapered single mode fibre with a

layer of silver evaporated on to the tapered surface. The adhesion of the silver is improved by first

evaporating a 3 nm layer of chromium. The thickness of the silver layer is about 50 nm.

silver layer

fig. 3. Silvercoated tapered optical fibre.

The fibre was tapered using similar methods for producing fused fibre couplers [5] . In the device

described here the fibre parameters were as follows: initial fibre diameter 125 pm; initial core diameter

3pm; core refractive index 1.469; cladding refractive index 1.458. The diameter at the taper waist was

about 10 pm, over a length of several mm. The tapered sensor was packaged so that the aqueous solu-

tion under analysis could be circulated around it. The effective index of the plasma wave is a strong

function of wavelength and external refractive index. A surface plasma wave is excited when the

effective index of the propagating fibre mode matches that of the plasma wave. The variation of both

the taper diameter and the silver thickness produce a distributed phase matching condition that varies

along the taper. A more detailed understanding of the operation of this device may be obtained from

Fig. 4, where we have plotted the real part of the effective indices of the fundamental fibre mode

along the tapered fibre, and of the surface plasma wave for different external refractive indices.

effective index 1.48

1.46 : .._____________ ~ _ _ _ _ _ _ _ _ _

I

0 2 4 6 8 1.36l .

fibre radius pn Q

Fig. 4 Effective indices of fibre and plasma modes along tapered fibre at a wave- length of 700 nm for different extemal refractive indices: (a) 1.39; (b) 1.37; (c) 1.33.

4 / 3

From Fig. 4 we see that for higher extemal refractive indices the phase matching between the fibre and

plasma waves occurs nearer the start of the taper, where the effective index of the fibre mode changes

slowly, resulting in a longer interaction length for the plasma wave, and a higher attenuation. For lower

refractive indices there is always a phase matching point towards the taper waist.

Experimental results The main application for this device is as an environmental sensor for measur-

ing low level pollutants, particularly in an aqueous environment, by immobilisation of the analyte

under investigation on the metal surface, which modifies the refractive index seen by the surface

plasma wave. Accordingly, the sensitivity of the device to changes in the extemal refractive index is of

paramount interest. To this end we have measured the spectral ttansmission through the sensor as a

function of changes in the extemal refractive index. The results are shown in Fig. 5.

transmission

\ methanol I

400 500 600 700 800 900 1,000 1,100

wavelength nm Fig. 5. Transmission through tapered plasma wave sensor for different extemaI refractive indices: methanol (1.326), acetone (1.357) and propanol (1.375).

From Fig. 5 we can see the very broad nature of the plasma resonance, ranging from 650 to 1000 nm.

Over this range the transmission through the device is very sensitive to changes in refractive index for

values near that of water (1.33). In particular, the best response occurs at wavelengths around 350 - 800

nm, where cheap semiconductor lasers are readily available.

In Fig. 6 we show the variation with refractive index of transmission through the sensor at a wave-

length of 730nm, derived from a halogen lamp and monochromator. The response is almost linear,

with changes in refractive index of less t h y 5 x being detectable. The refractive indices cover a

range of values close to those relevant for aqueous samples, and the wavelength used is close to that

available from cheap laser diodes as used in CD players.

4 / 4

transmission 25 I 1

20

15

10

5 -

-

-

-

0 ’ ’ ‘ . ‘ . ’ ’ ’ . ‘ . ’ refractive index

1.32 1.33 1.34 1.35 1.36 1.37 1.38

fig. 6. Transmission as a function of external refractive index.

Conclusions We have demonstrated a new type of single mode optical fibre plasma wave sensor with a

very broad spectral response ranging over 400 nm. This is achieved by a dismbuted phase matching

between the fibre and plasma modes, resulting from the tapering of the fibre. The optimum response of

this device is around 750 nm, and can detect changes in refractive index as low as 5 x

References

1. Liedberg, B., Nylander, N., and Lundstrom, I., ‘Biosensing with surface plasmon resonance - how

it all began’, Biosensors and Bioelectronics, 10, pp. i-k, (1995).

2. Kretschmann, E. and Raether, H., ‘Radiative decay of non-radiative surface plasmons excited by

light’, Z. Naturforsch., (1968), 23a, pp 2135.

3. Jorgenson, R.C., and Yee, S. S., ‘A fibre-optic chemical sensor based on surface plasmon

resonance’, Sensors and Actuators B, (1993), 12, pp 213-220.

4. Lavers, C.R., and W w s o n , J.S., ‘A waveguide-coupled surface-plasmon sensor for an aqueous

environment’, Sensors and Actuators B, (1994), 22, pp 75-81.

5. Mackenzie, H.S., ‘Evanescent-Field Devices for Non-linear Optical Applications’ PhD

Dissertation, Cambridge University (1990).

1995 The Institution of Electrical Engineers. inted and published by the IEE. Savoy Place, London WC2R OBL, UK.

4 /5