WO2014118519A1

WO2014118519A1 - Localised surface plasmon resonance in an optical fiber comprising a surface plasmon supporting layer and a nanomaterial layer

Info

Publication number: WO2014118519A1
Application number: PCT/GB2014/050213
Authority: WO
Inventors: Oleksiy Rozhin; Thomas David Paul Allsop; David John Webb; Ronald Neal
Original assignee: Aston University
Priority date: 2013-01-29
Filing date: 2014-01-28
Publication date: 2014-08-07
Also published as: GB201301543D0

Abstract

A localised surface plasmon generator in which nanoscale structures are used at an surface plasmon supporting interface to enhance the sensitivity of the apparatus to certain materials. The surface plasmon generator comprises an optical waveguide (34) having a surface plasmon supporting layer (18) formed on an outer surface thereof. The optical waveguide has refractive index modulations, and the surface plasmon supporting layer includes a layer of nanomaterial (650) at its outermost surface.

Description

LOCALISED SURFACE PLASMON RESONANCE IN AN OPTICAL FIBER COMPRISING A SURFACE PLASMON SUPPORTING LAYER AND A NANOMATERIAL LAYER

FIELD OF THE INVENTION

The present invention relates to sensing methods and apparatus using surface plasmons.

BACKGROUND TO THE INVENTION

Free electrons of a metal can be treated as an electron liquid of high density. At the surface of a metal or

semiconductor, longitudinal electron density fluctuations, or plasma oscillations, may occur and will propagate along the surface.

These coherent fluctuations are accompanied by an electromagnetic field comprising a component transverse to (i.e. away from) the surface, and a component or components parallel to the surface. The transverse electromagnetic field falls rapidly with increasing distance from the metal or semiconductor surface, having its maximum at the surface, and is sensitive to the properties of the metal or semiconductor surface and the properties of the dielectric substance (e.g. air, aqueous solution) immediately at and above the surface and into which the transverse electric field component extends .

This propagating free electron surface charge

fluctuation, and its attendant electromagnetic field, is a surface plasmon.

A surface plasmon can propagate along a metallic or semiconductor surface with a broad spectrum of

eigenfrequencies from CO = 0 up to a maximum value depending upon its wave vector k . The dispersion relation co(k) of a surface plasmon, which relates the eigenfrequency to the wave vector, shows that surface plasmons have a longer wave vector than light of the same energy propagating along the surface. Surface plasmons are, as a consequence, non-radiative and are characterised as surface waves having an electromagnetic field which decays exponentially with increasing distance from, and transverse to, the surface upon which they propagate. Due to the differing dispersion relations of photons (in air) and surface plasmons, and the non-radiative nature of surface plasmons, photons in air cannot couple to surface plasmons. This is schematically illustrated in Fig. 1 which shows the dispersion relation of photons (in air) and surface plasmons graphically. The dispersion curve of the photon (in air) never crosses the dispersion curve of the surface plasmon. Consequently, the two cannot couple or "transform" between each other due to being unable to satisfy the requirements of both energy and momentum conservation during "transformation".

Excitation of surface plasmons is not possible using photons (in air) unless a means is used to transfer additional momentum (Ak_x) to the photon such that, for a given photon frequency, the photon momentum is equal to the momentum permitted for a surface plasmon at the same frequency.

One means of achieving this is to form the metal or semiconductor surface 2 upon a diffraction grating surface 1 (e.g. by forming corrugations in the surface) . When light 3 strikes the metal or semiconductor grating surface, having a grating constant a, at an angle Θ, the component (k_x) of the wave vector of the li ht along the surface becomes

where n is an integer and c is the speed of light in a vacuum. Thus, the metal or semiconductor surface grating may

. , 2m

impart the extra momentum ( Δκ_χ = ) needed by the photon to a

enable it to reach the surface plasmon dispersion curve to "transform" into (i.e. excite) a surface plasmon. Fig. 2 graphically illustrates this. The reflected light intensity attenuates when excitation of surface plasmons is greatest and photons "transform" into surface plasmons resonantly.

Another means for photon-plasmon coupling is the use of "attenuated total reflection" (ATR) such as exemplified by the so-called Kretschmann-Raether prism arrangement schematically illustrated in Fig. 3. Light 3 is directed towards an interface with a metal or semiconductor surface 2 using a prism 4 made of a material having a refractive index n_p (e.g. quartz) , at which it is totally reflected. The dispersion relation of photons in the prism, and reaching the interface, ck

is CO =— . Thus, the extra momentum ( Ak ) required by the photon to couple to surface plasmons arises from the optical properties of the coupling prism 5. Photons may excite plasmons when the component (k_x) of the wave vector of the reflected light (in-prism) matches that permitted by surface plasmons of the same frequency, i.e.: c

where Θ is the angle of incidence at which light is totally reflected. Fig. 3 graphically illustrates this. This resonant "transformation" of photons into surface plasmons results in an attenuation of the totally reflected light exiting the prism, hence the appellation "attenuated total reflection" .

In general, both long-range and short-range surface plasmons that exist at an interface between a metal (having a relative dielectric function s_m) and dielectric sample (having a refractive index n_s) obey the following dispersion relation two homogeneous semi-infinite media: k.

where k_x is the free space wave number.

Another means of generating surface plasmons is disclosed in WO 2009/063168. Here surface plasmons are generated on a metal or semiconductor layer arranged upon an outer surface of an optical waveguide, using light from inside the optical waveguide. As shown in Fig. 5, a surface plasmon generator according to this arrangement includes a length of optical fibre 11 (operating as a single-mode fibre for optical wavelengths) having an optical signal input part 19 comprising an open end of the optical fibre length arranged for receiving optical signals into the optical fibre, and an optical output part 20 comprising an open end of the optical fibre from which output optical signals can be received from the optical fibre.

The optical fibre has an optical fibre core part 13 clad by an optical fibre cladding 12. The cladding 12 includes a lapped region 16 that defines a proximal outer surface area 17 which is closer to the core part 13 than the adjacent

(unlapped) outer surface areas of the cladding 12. The proximal outer surface area 17 formed by lapping the cladding part defines a substantially flat outer surface area of the cladding part nearmost, but not exposing, a length of the underlying core part 13 of the optical fibre. The fibre thus has a D-shaped cross-section in the lapped region 16.

A multi-layer stack 18 is deposited on the substantially flat proximal outer surface area 17 in the lapped region 16. The multi-layer stack is of substantially uniform thickness, e.g. of about 150 nm when composed of three layers, and is substantially flat. It is in direct contact with, and forms an interface with, the flat proximal surface area of the fibre cladding. At its outward surface 18 opposite the interface, the multi-layer stack outwardly presents from the optical fibre a substantially flat and exposed surface which extends over the interface in question.

Periodically spaced regions 200 of compaction are photo- induced in the multi-layer stack 18 by irradiating the stack with ultraviolet radiation through a uniform phase mask to bathe the stack 18 in ultraviolet light having an intensity distribution which varied periodically (increasing and decreasing) along the stack in a direction parallel to the transmission axis of the optical fibre 11.

The regions of compaction 200 induce local strain in the material of the stack 18. The strain induced in the surface of the stack induces a strain gradient into the optical fibre 11 to form a spatially quasi-periodic or periodic strain field 250 therein. This can be seen as an asymmetric refractive index profile, similar to what is generated when an optical fibre is bent. Multiple strain-induced refractive index modulations 15 in the core part 13 result from this strain field and produce a periodic or quasi-periodic refractive index modulation region 14 in the core. One result of the strain-induced asymmetric refractive index profile is that the E-field of a guided mode is brought closer to the interface between the stack and the optical fibre.

The core part 13 of the optical fibre includes an extended region of uninscribed (i.e. strain-induced)

refractive index modulations 14 comprising a sequence of refractive index modulations 15 each of which extends across the optical fibre core part to form an area of (modulated) refractive index. The result is to render the interface 17 between the proximal surface of the lapped cladding, and the overlying multi-layer stack 18, simultaneously in optical communication with the input end 19 of the optical fibre by reflection or scattering 22 of at least a part of an input optical signal directed into the surface plasmon generator via the input part 19 of the optical fibre 11. The reflected or scattered part 22 of the input optical signal may be employed in generating surface plasmons at the outwardly presented surface 18 of the multi-layer stack arranged upon the proximal outer surface of the fibre cladding. In this way, the scattering or reflection of input optical signals incident upon the refractive index modulations 15 assists the first optical waveguide to generate coupled radiative optical modes which impinge upon the multi-layer stack 18 of the surface plasmon generator 10 and thereupon resonantly generate surface plasmons when the wave vector component of the radiative modes which is parallel to the fibre axis, matches the wave vector of surface plasmons excitable at that multi-layer stack. As a result of this resonant coupling between radiative modes and surface

plasmons, and in part in consequence of the optical coupling, by the refractive index modulations, between the radiative modes and the guided core modes of optical signals within the optical fibre 11, resonant coupling of surface plasmons and radiative optical modes influences the intensity of guided core optical modes 23 transmitted through the first optical waveguide and ultimately output from the output part 20 of the surface plasmon generator.

The above mentioned arrangements for resonantly coupling photons to surface plasmons result in "surface plasmon resonances" (SPR) indicated by a resonant drop in reflected light from the plasmon-bearing metal or semiconductor

interface. Since the surface plasmon propagates at the outwardly presented surface of the metal or semiconductor in question, the optical properties of the dielectric material

(e.g. air, aqueous solution etc) to which the metal or semiconductor surface is outwardly presented (e.g. exposed) , become highly influential upon the nature and degree of the resonant attenuation of reflected light used to resonantly excite the surface plasmons. This fact is exploited in sensor devices which measure properties of dielectric sample

substances using surface plasmons generated as discussed above .

If the relative dielectric constants of the metal or semiconductor surface and the dielectric material at the outwardly presented (e.g. exposed) surface of the metal or semiconductor, are e_m and s_d respectively, then the wave vector k_sp of a surface plasmon propagating at the outwardly presented (e.g. exposed) metal or semiconductor surface, and extending transversely thereto into the dielectric material is :

Thus, the value of S_d determines the value of k_sp and thus the angle of incidence ( Θ ) upon the plasmon-bearing surface at which a photon can resonantly excite surface plasmons. Thus, by monitoring the intensity of reflected light to determine the position of resonant attenuation of reflected light, one may determine a measure of ε_ά . Changes in 8_d may also be monitored as changes in the angular position of the reflected light attenuation resonance. Fig. 4

schematically illustrates an example of two attenuation resonances occurring at different reflection angles {θ and θ₂) each corresponding with the presence of a dielectric material of a different respective s_d at the outwardly presented (e.g. exposed) metal or semiconductor plasmon-bearing surface.

The value of ε_ά is intimately related to the properties

(e.g. optical properties) of the dielectric substance which can, in this way, be sensed and probed using surface plasmons.

For example, the value of the refractive index ( n_d ) of the dielectric is equal to the square root of its dielectric constant ( n_d = s_d ) . SUMMARY OF THE INVENTION

At its most general, the present invention proposes using nanoscale structures at the interface supporting the surface plasmon in order to enhance the sensitivity of the apparatus to certain materials. As referred to herein, a nanoscale structure, or "nanomaterial" , is a material having a physical structure with one or more dimensions in the order of

nanometres, i.e. less than 100 nm, preferably less than 10 nm. For example, "nanomaterial" may include nanotubes (e.g. carbon nanotubes), nanofibres, nanoparticles , or structures formed from graphene. For example, a layer of carbon nanotubes or a graphene layer may be formed (e.g. deposited or grown) on the surface supporting the surface plasmon.

The invention is based on the understanding that the nanomaterial presents a developed surface that may be more attractive to certain substances, either naturally or through the use of some kind of functional alteration or addition, such that those substances are concentrated in the region where the surface plasmon propagates. As a result, a greater effect on the surface plasmon resonance can be observed for a given concentration of material, i.e. the apparatus

incorporating the nanomaterial exhibits greater sensitivity. The invention may be particularly useful where the medium to be detected is gaseous, e.g. the substance that is attracted to the nanomaterial is in gas or vapour form, e.g. mixed with air. For example, nanoscale structures may comprises

nanowires which a seat in a surface relief structure inscribed in the surface supporting the surface plasmon. The nanowires may form a nanoscale sandwich between two dielectric

materials, i.e. the surrounding environment and a dielectric material beneath the surface supporting the surface plasmon. Thus, the developed surface of the invention may be thought of as having a corrugated structure, with the apex of the corrugations formed by conductive material between each nanowire. The nanowires may thus facilitate the creation of localised surface plasmons.

According to one aspect of the invention, there may be provided a localised surface plasmon generator for sensing a substance, the surface plasmon generator comprising: an optical waveguide for guiding optical radiation, and a surface plasmon supporting layer formed on an outer surface of the optical waveguide in the sensing region, wherein the optical waveguide has refractive index modulations formed in the sensing region thereof, and the surface plasmon supporting layer includes a layer of nanomaterial at its outermost surface. The refractive index modulations in the optical waveguide cause optical energy to be transferred (e.g.

coupled) from optical waveguide (e.g. by reflection,

scattering or interference processes such as cavity-type resonances between successive refractive index modulations) to the surface plasmon supporting layer. The coupled optical energy generates an localised surface plasmon at the surface plasmon supporting layer.

In use, the outermost surface of the surface plasmon supporting layer is exposed to the medium to be sensed, i.e. not itself embedded, or encased in any holding substrate or material (such as epoxy) . The layer of nanomaterial may be selected or arranged to exhibit an affinity or attraction to the substance to be sensed, whereby if the substance is present in the medium at the exposed surface plasmon

supporting layer, it is drawn to the layer of nanomaterial. The effect of the substance on the localised surface plasmon, which is caused by the change in dielectric properties of the medium at the exposed surface, is therefore enhanced.

The layer of nanomaterial may consist of any one of more of nanotubes (e.g. carbon nanotubes), nanofibres,

nanoparticles , or structures formed from graphene sheets, arranged to attract the substance to be sensed. The surface plasmon supporting layer may have a periodic or quasi-periodic surface relief structure inscribed into its outermost surface. The layer of nanomaterial may comprise nanoscale structures capable of aligning with the surface relief structure, e.g. being seated within recesses inscribed in the surface.

The surface plasmon supporting layer may comprise a dielectric sub-layer and a thin film of suitably conductive material on the dielectric sub-layer. Inscribing the surface relief structure may expose the dielectric sub-layer, e.g. along grooves or recesses formed in the surface. The

nanoscale structures may comprise nanowire elements in contact with the dielectric sub-layer, i.e. seated in the grooves or recesses. The thin film of conductive material may support localised surface plasmons between the nanowire elements. The nanowire elements may enhance the formation of such localised surface plasmons.

The suitably conductive material may be selected to support a surface plasmon. For example, the suitably

conductive material may be selected from germanium, gold, silver, platinum, copper, palladium, aluminium, a vanadium oxide or vanadium oxides. Preferably the material is chosen such that the optical skin depth of the material in question, at the operating optical wavelength of the surface plasmon generator, is greater than the thickness of the layer in question. The term "thin film" may thus correspond to a layer having a thickness less than the optical skin depth of the conductive material from which it is formed. For example, the thickness of the conductive material may be between 10 nm and 60 nm, but preferably 50 nm or less.

In one embodiment, the suitably conductive material may be a layer of graphene, whereby the functions of the surface plasmon supporting layer and the layer of nanomaterial are combined in a single layer. In other embodiments, the layer of nanomaterial may be deposited or grown directly on (i.e. in contact with) the outermost surface of the conductive material of the surface plasmon supporting layer to form an outwardly exposed surface of the device. As mentioned above, the nanomaterial may comprise any nanoscale material that is capable of attracting the substance to be sensed. For example, the nanomaterial may include or consist of nanotubes (e.g. carbon nanotubes), nanofibres, nanoparticles , or structures formed from graphene sheets. For example, it has been found that a layer of carbon nanotubes deposited on the surface plasmon supporting layer may naturally attract carbon dioxide. The sensitivity of the device to carbon dioxide is markedly increased. It is believed that similar effects may enable untreated

nanomaterial to be used to enhance the detection of amines and certain solvents.

The nanomaterial may be functionalized or otherwise adapted to be attractive to the substance to be sensed. Both covalent and non-covalent functionalization may be used. For example, the nanomaterial (e.g. carbon nanotubes) may be coated or mixed with a suitable surfactant. Alternatively, defect atoms may be introduced into the nanomaterial (e.g. in the wall of the carbon nanotube or sheet of graphene) during fabrication .

In one embodiment, the layer of nanomaterial is formed by dipping the device in a solution of carbon nanotubes followed by heat treatment to leave a layer of randomly orientated nanotubes on the surface plasmon supporting layer. A

preferred technique is to deposit carbon nanotubes using an inkjet printing technique. This technique may give control over the surface topology of the device, e.g. it may allow the layer of nanomaterial to comprise single or bundled carbon nanotubes. In addition to controlling the volume and position of deposited ink using a suitable unit, the topology of the layer of the deposited nanotubes may be controlled through the content of the ink itself and optionally through post- deposition steps. For example, various parameters of the ink (e.g. nanotube concentration, nanotube length, etc.) can be varied. These parameters may be relevant to the nature of the tubes that are deposited. These parameters may have an effect on the ability to repeatably deposit either bundles of nanotubes or isolated nanotubes. After deposition, the alignment of the carbon nanotubes may be controlled, e.g. by irradiating plasmonic structures on the substrate to generate electric fields to act on the deposited nanotubes. As mentioned below, aligning the deposited nanotubes may improve the sensitivity of the device for certain polarisations of light .

The layer of nanomaterial may be a single layer e.g. having a thickness of 1 to 5 nm covering the outermost surface of the conductive layer in the surface plasmon supporting layer. However, the layer may be made thicker, e.g. up to 100 nm, e.g. by performing multiple deposition or print steps.

In one embodiment, the surface plasmon supporting layer may comprise a multi-layer structure having two conductive layers (formed of metal or semiconductor) separated by a coupling optical waveguide. This multi-layer structure may adhered to (e.g. bonded to, or formed upon) the outer surface of the optical waveguide and optically coupled thereto. The coupling optical waveguide may be bonded or adhered to the two conductive layers it separates. In this embodiment, the layer of nanomaterial is formed on the outermost surface of the multi-layer structure.

With the multi-layer structure, optical radiation input to the optical waveguide may be used to generate concurrent surface plasmons on the surfaces of the two separate

conductive layers of the surface plasmon supporting layer. For example, the evanescent wave of optical radiation guided along the optical waveguide may couple, or extend to, the coupling optical waveguide to enable surface plasmons to be generated there. The separation between the two conductive layers is preferably substantially uniform and constant along the coupling optical waveguide. The thickness of the coupling optical waveguide is preferably of the same order of magnitude as the wavelength of optical radiation with which the surface plasmon generator is operated or arranged to generate surface plasmon (e.g. in one embodiment at or around 1500 nm) . This arrangement has been found to have the beneficial effect of allowing concurrently generated surface plasmons on opposite surfaces of the coupling optical waveguide to couple together or "cross-talk" such that the surface plasmon nearmost the optical waveguide, and the surface plasmon generating

radiation within it, may positively reinforce or support the surface plasmon furthest from the optical waveguide. The coupling optical waveguide acts to guide an enhanced surface plasmon mode in this way.

Herein, optical radiation means electromagnetic radiation having a wavelength in free space between about 100 nm and 1 mm, i.e. including ultraviolet to infrared radiation.

The optical waveguides referred to herein are formed from optically transparent material, such as silica. The optical waveguide may be an optical fibre comprising a core region and a cladding region. The optical fibre may be a clad single mode optical waveguide constructed and arranged to support single mode transmission of optical radiation of wavelengths above 1000 nm. The refractive index modulations may extend across at least a part of the core region. The cladding region may be lapped (i.e. have a portion of reduced

thickness) in the sensing region. The thickness of cladding at the lapped region may be between 15 μπι and 5 um. The optical fibre may therefore have a D-shaped cross-section in the sensing region, whereby the lapped cladding portion presents an outward flat surface. The surface plasmon supporting layer may be formed on the outwardly presented flat surface. The coupling optical waveguide used in the multilayer structure may be a planar optical waveguide.

The refractive index modulations may extend in a

direction transverse to an optical transmission axis of the optical waveguide. They may be directly optically inscribed into the optical waveguide (e.g. in the form of a grating structure, such as a Bragg grating (reflective) or a long- period grating (transmissive) or the like) using known optical inscription techniques (e.g. direct pulsed laser writing or by holographic or phase-mask processes) .

Alternatively, the refractive index modulation may be formed by photo-inducing changes (e.g. regions of material compaction) in the material of the optical waveguide using ultraviolet (UV) radiation. The UV radiation may be

sufficient to inscribe a surface-relief structure, i.e. the surface plasmon supporting layer may exhibit an undulating surface relief profile. This may assist in generating surface plasmons and/or in spatially localising the surface plasmons. In particular, such a surface relief structure may assist in coupling guided light in the optical waveguide to surface plasmon modes.

The refractive index modulations may be formed before the layer of nanomaterial is deposited on the surface plasmon supporting layer.

In another aspect, the invention provides a sensor (e.g. a gas sensor or a bio-molecule sensor) for detecting the presence of a substance, the sensor comprising a surface plasmon generator as set out above whose layer of nanomaterial is attractive to the substance to be sensed, an optical radiation source in optical communication with an optical input part of the surface plasmon generator, and an optical radiation detector arranged to detect optical radiation having passed through the surface plasmon generator from the input part . In use, the refractive index modulations enable part of the guided light to form a radiative optical mode(s) which is used to excite surface plasmons and which is also coupled to the remaining guided mode(s) of the light from which it derives. This coupling of the radiation mode(s) and the guided mode(s) enables changes in the radiation mode(s) to cause consequential changes in the guided mode(s) of light. Such changes in the radiation mode(s) may occur due to the coupling of the out-coupled mode(s) to the surface plasmons they excite at the metal or semiconductor layer. Thus, the greater the degree of coupling between the radiative optical mode(s) and the surface plasmons in question, the greater the consequential change in the remaining guided mode(s) to which the radiative mode(s) are coupled. In this way, the extent of surface plasmon generation is imprinted upon, or leaves a signature within, the properties of the remaining guided mode(s) of the light used to excite the surface plasmons.

The optical input part may be a first end of the optical waveguide of the surface plasmon generator. The optical radiation detector may be in optical communication with a second end of the optical waveguide, i.e. to receive optical radiation that is transmitted through the surface plasmon generator .

The layer of nanomaterial includes an outwardly exposed region which is the sensing region, i.e. the region to be presented to the medium in which sensing is required.

The optical radiation detector may be an optical spectrum analyser responsive to optical radiation generated by the optical radiation source.

The sensor may include a polarisation control means in optical communication with the optical radiation source and the input part of the surface plasmon generator, arranged for controlling the state of polarisation of optical radiation from the optical radiation source for input to the surface plasmon generator. The optical radiation source may be operable to generate Infra-Red (IR) optical radiation. The optical radiation source may be arranged to generate broadband optical radiation comprising a range of optical wavelengths.

The sensor may include a signal processor arranged to identify resonances in the spectrum of an optical radiation received thereby from the optical radiation source via the surface plasmon generator.

The signal processor means may be arranged to determine one or more of the position, the depth, the width of an identified the resonance.

The sensor may include sample control means for placing the sample in contact with the sensing area of the surface plasmon generator.

The optical signal detector may be an optical spectrum analyser or spectrometer responsive to optical radiation generated by the optical signal source. The optical signal source (laser, laser diode, monochromated or filtered lamp) may be operable to generate a signal in the UV, visible or Infra-Red (IR) spectral range. Lamps or super continuum light source may be used to generate broadband optical signals comprising a range of optical wavelengths, such as only within the range 300 nm to 800 nm, or such as only within the range 1000 nm to 2000 nm, or such as only the range 1100 nm to 1700 nm, or such as only the range 2000 nm to 3000 nm.

The degree of surface plasmon generation and/or the sensitivity of the sensor may be dependent upon the state of polarisation of the guided optical signal modes input to the optical waveguide. The polarisation control means, being of a type and structure such as would be readily apparent to the skilled person, may be employed to tune the sensor's

sensitivity accordingly. The instrument's sensitivity may be improved further by aligning carbon nanotube in the layer of nanomaterial in a manner that enhances the device's

performance. As has been discussed above, the degree of surface plasmon excitation, and the wavelength of optical signal used to resonantly excite surface plasmons, is detectable in the spectrum of the guided modes of the optical signal output by surface plasmon generator, as an output signal intensity attenuation resonance.

These and/or other properties of the spectrum may be monitored or measured in analysing the sample substance in question. The signal processor may include a computer suitably programmed to effect such monitoring and/or

measurement. Changes over a period of time, in any of the aforesaid properties, may be so monitored and/or measured and correlated to dynamic (or otherwise) properties of the sample in question. The signal processor may be arranged to

determine the refractive index of a sample substance according to the spectral position (e.g. signal wavelength) and/or strength, depth or amplitude of identified output signal intensity attenuation resonance, and may be arranged to determine a change in the refractive index according to a change in the spectral position.

In another aspect, the invention may provide use of nanomaterial , e.g. carbon nanotubes, as a sample interface layer in a sensor incorporating a surface plasmon generator as set out above.

In another aspect, the invention may provide a method of fabricating a surface plasmon generator as set out above, which includes the steps of forming the surface plasmon supporting layer on an optical waveguide, forming the

refractive index modulations in the optical waveguide, and, after forming the refractive index modulations, depositing or forming the layer of nanomaterial on the outermost surface of the surface plasmon supporting layer. The refractive index modulations may be formed (e.g. by UV irradiation) after the surface plasmon supporting layer is formed on the optical waveguide. This may cause the surface plasmon supporting layer to have an undulating surface relief. The method may include lapping the cladding region of an optical fibre before forming the surface plasmon supporting layer thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention is described below with reference to the accompanying drawings, in which:

Fig. 1 schematically illustrates the dispersion relations of a photon in air a surface plasmon and is discussed above;

Fig. 2 schematically illustrates a surface grating coupler for generating surface plasmons, together with a graphical dispersion relation illustrating the resonant excitation of a surface plasmon using a photon in air coupled to the surface plasmon via the grating and is discussed above;

Fig. 3 schematically illustrates a Kretschmann-Raether prism coupler for generating surface plasmons, together with a graphical dispersion relation illustrating the resonant excitation of a surface plasmon using photons in the prism coupled to the surface plasmon and is discussed above;

Fig. 4 schematically illustrates optical signal

attenuation resonances in the spectrum of light reflected from a coupler of Fig. 2 or Fig. 3 in exciting surface plasmons and is discussed above;

Fig. 5 schematically illustrates a cross-sectional view of a known surface plasmon generator and is discussed above;

Fig. 6 schematically illustrates a sensor employing a surface plasmon generator that is an embodiment of the invention;

Fig. 7 illustrates the spectral behaviour of the surface plasmon generator of Fig. 6 as a function of the refractive index of various sample materials; and

Fig. 8 is an atomic force microscopy (AFM) image of a deposited by ink jet printing layer of single wall carbon nanotubes on the surface of device. DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

Fig. 6 graphically illustrates a sensor device comprising a broadband infra-red optical signal source 31 arranged to generate optical signals within the range 1000 nm to 2000 nm and to output such optical signals to an optical signal polariser unit 33 placed in optical communication with broadband optical signal source via a linking optical

fibre 32. The polariser unit 33 is arranged to produce from input optical signals received thereby from the optical signal source 31, output optical signals of a pre-determined state of polarisation, and to output the polarised optical signals to a polarisation controller 35 with which the polariser unit 33 is in optical communication via an intermediate length of optical fibre 34. The polariser controller 35 includes a length of optical fibre mechanically twistable, or twisted, by a predetermined amount to induce a birefringence in the material of the fibre and a corresponding change in the polarisation state of the optical radiation transmitted through it.

The optical output of the polarisation controller 35 is in optical communication with the input part 19 of the surface plasmon generator 10 via an intermediate length of optical fibre 36 and a bare-fibre connector portion 37. The output part 20 of the surface plasmon generator 10 is in optical communication with the optical input of an optical spectrum analyser 41 via an intermediate bare-fibre connector 39 and length of optical fibre 40. Ends of both of the aforementioned bare-fibre connectors (37, 39) are optically coupled directly to the input and output parts of the surface plasmon

generator.

In use optical signals generated by the optical signal source 31 are output thereby to the polariser unit 33 which produces therefrom a polarised optical signal for input to the polarisation controller 35 which is operable to adjust to the state of polarisation of the received polarised optical signal as required, and to subsequently output the polarised optical signal to the optical input part 19 of the surface plasmon generator 10 for use in generating surface plasmons as discussed above with reference to Fig. 5. Those parts of the polarised optical signal input to the surface plasmon

generator which are transmitted through the strain-induced refractive index modulations 14 thereof are subsequently output at the output part 20 of the surface plasmon generator and are input to an optical input of the optical spectrum analyser 41 whereat the intensity and wavelength of the transmitted optical signal is measured. Subjecting the surface plasmon generator to optical signals of a wide range of differing wavelengths within the spectrum of the broadband optical signal source 31, enables a transmitted optical signal spectrum to be generated in respect of the transmitted optical signal 23 output by the surface plasmon generator.

The sensor device 30, illustrated in Fig. 6, also includes a sample control unit 38 in the form of a vessel containing a sample substance (e.g. a gas or an aqueous solution) within which the surface plasmon generator 10 is immersed and to which the outwardly presented surface of the multi-layer stack 18 of the surface plasmon generator is exposed.

Fig. 6 also shows an expanded view of the multi-layer stack 18. It includes a first layer of germanium 600

deposited on the lapped surface of the first optical waveguide having a uniform thickness of either 48 nm or 24 nm. Ά first layer of silica 610 is deposited upon the first germanium layer having a uniform thickness of 48 nm. A second germanium layer 620 is deposited on the first silica layer having a uniform thickness of 48 nm. Both the first and second germanium layers are arranged to, or are able to, support concurrent surface plasmons on the respective surface thereof to support cross-talk therebetween to generate an enhanced surface plasmon mode. In this way, the first silica layer 610 and the first and second germanium layers it separates, collectively define a second optical waveguide coupled to the first optical waveguide 12. A second silica layer 630 of 48 nm in maximum thickness is deposited upon the second germanium layer to protect it. A layer of platinum 640 is deposited upon the second silica layer to support outermost surface plasmon fields.

A periodic or quasi-periodic surface relief structure is inscribed into the outermost surface of the multi-layer stack by ultraviolet photo-inscription to produce material

compaction and strain fields within the multi-layer stack (and thereby the refractive index modulation in the optical fibre) as discussed above. Deposition may be carried out using conventional techniques, e.g. sputtering or the like. The deposition conditions may be controlled to provide a rough surface. This may be advantageous in broadening the surface plasmon resonance response in the spectra, i.e. so that the apparatus is operable or sensitive over a range of

wavelengths .

In an alternative embodiment, all but the first germanium layer 600 of the multi-layer stack 18 may be dispensed with, or the first silica layer and the second germanium layer may be dispensed with. In such a case, the surface relief structure (and compactions) would be formed in the remaining layer (s) of material (s) . Multi-layered stack with a plurality of coating may be used to tailor the spectral response of the device .

In the present invention, a layer of nanomaterial 650 is deposited as the outermost layer of the multi-layer stack 18. In the embodiment shown, the layer of nanomaterial 650 is formed on the layer of platinum 640. It is always formed in contact with the outermost layer on which a surface plasmon can be excited. The germanium layer or layers in the multilayer stack assist in coupling optical radiation between the layers . The layer of nanomaterial 650 in this embodiment is a carbon nanotube layer. The carbon nanotubes provide nanowire structures which lie on the platinum layer 640 to give it a quasi-corrugated profile. The underlying germanium layers may assist in the formation of the corrugated appearance, which may be related to the lattice constant of the underlying stack. The presence of the nanowires in this manner enables the generation of localised surface plasmons.

Corrugated nanostructures can be considered as apertures in the supporting surface plasmon material, which can thus be considered as an array of apertures [1] . A momentum component k_x of light parallel to the surface of such a structure can be expressed as:

where P is a lattice constant (the distance between the apertures), and and j are nonzero integer numbers

representing the scattering orders from the two dimensional aperture arrays. The resonant condition for the localised surface plasmons that needs to be satisfied for a lattice structure is known [2].

The surface plasmon resonance (SPR) fibre sensor device illustrated Fig. 6 can be constructed in four stages.

Firstly, a standard single-mode silica fibre (SMF) 12 is mechanically lapped down to provide a flat lapped surface 17 within 10 mm from the core-cladding interface.

Secondly, using an RF sputtering technique, such as would be readily apparent to the skilled person, a series of coatings (600, 610, 620, 630, 640 of Fig. 6) are deposited upon the flat of the lapped fibre with materials and average thicknesses of;

(600) : First germanium (Ge) layer = 48 nm thick,

(610) : First silica (Si0₂) layer = 48 nm thick,

(620) : Second Ge layer = 48 nm thick,

(630) : Second Si0₂ = 48 nm thick, (640) : Coating of Pt = 32 nm thick.

Thirdly, the coated lapped fibre was exposed to the diffracted pattern of UV light passed through a uniform phase mask. A UV laser beam was employed for this purpose and caused to scan the phase mask multiple times to effect and multiple exposures of the coated lapped fibre. For example, the phase mask may have a uniform period of 1 μπι and the UV source may be an Argon ion continuous wave laser operating at a

wavelength of 244 nm and an output power of 120 mW. The output UV beam was passed through an aperture to improve the beam profile (minimise diffraction pattern) and then through a plano-convex lens having a focal point coincident with the phase mask and multi-stack layer to be irradiated. The UV light was passed through the phase mask to produce a

diffraction pattern of UV light which impinged upon the multi- stack layer of the surface plasmon generator device. The focussed UV light was scanned over the phase mask and multi- stack layer, which remained static, at a speed of 0.1 mm/sec. Multiple scans may be performed.

Fourthly, i.e. (after UV photo-inscription), a layer of carbon nanotubes 650 is deposited on outermost surface of the device by deep coating the optical fibre in carbon nanotube N- methylpyrrolidone solution. The solvent for the carbon nanotubes may evaporate without further assistance. Heat may be applied to assist evaporation.

In an alternative embodiment, this step may be formed by inkjet printing the nanomaterial (e.g. carbon nanotubes) .

This step can be performed using a microdispensing unit such as the Microdrop Autodrop Professional Positioning System MD- P-802. This unit operates by moving the substrate relative to the dispensing head to enable controlled deposition of an array or pattern of droplets. The volume and position of each deposited drop can be controlled to arrange the carbon nanotubes in a desired configuration. Fig. 8 is an image of a bundle of carbon nanotubes deposited using this technique obtaining using atomic force microscopy.

The ink for the printing may be made using purified single wall carbon nanotubes (CNTs) purchased from Southwest NanoTechnologies , Inc. or Unidym, Inc. dispersed in N-methyl-

2-pyrrolidone (NMP) at concentrations 0.001 mg/ml to 1 mg/ml by ultrasonication using a Diagenode Nanoruptor processor for times between 0.2 and 10 hours at 21 kHz and power levels between 100-550 W. To control the size of CNT bundles in the ink, the dispersion may be subjected to ultracentrifugation during 2.5 hours at 47 000 rpm at 17°C, e.g. using a Beckman Coulter Optima ax-XP with MLS 50 rotor. Alternatively for bundle control, pressure filtration through paper-glass filters with retention diameters between 300 nm and 1200 nm may be used. CNT dispersions can be monitored by (i)

measuring optical absorption by Perkin Elmer Lambda 1050 UV- NIR spectrometer and/or (ii) measuring photoluminescence spectra by Horiba NanoLog excitation-emission

spectrofluorometer equipped with the nitrogen cooled InGaAs array detector. A photoluminescent map may be obtained using an excitation wavelength range of 300-810 nm with 5 nm step, an emission spectral range of 820-1600 nm, and an integration time (exposition) of 30 seconds.

Light from a broadband light source, is passed through a polariser, and a polarisation controller before illumination of the sample, with the transmission spectra being monitored using an optical spectrum analyser (accuracy of 0.005 nm) , see Fig. 6.

Fig. 7 shows two graphs which illustrate the enhanced sensitivity of the device shown in Fig. 6 to sensing carbon dioxide. The transmission spectra of the sensor was monitored to identify surface plasmon resonances when the sample control unit presented five substances to the exposed layer of nanomaterial 650. The five substances consisted of four gaseous alkanes (Methane, Ethane, Propane, Butane) and gaseous carbon dioxide. These substances were chosen to exhibit a refractive index range close to 1.

Graph (a) illustrates the wavelength of the primary detected surface plasmon resonance (SPR) identified in the transmission spectra for each sample. It shows that the SPR for carbon dioxide is noticeably distinct in wavelength from the position expected of a material having the same refractive index .

Graph (b) illustrates the optical strength of the detected SPR identified in the transmission spectra for each sample. The strength of the SPR for carbon dioxide was surprisingly higher than what would have been expected for a material having the same refractive index.

In combination, graphs (a) and (b) suggest that the sensor having the layer of nanomaterial (in this case carbon nanotubes) gives a specific optical functionality to carbon dioxide. Equivalent effects may be found for other substances to be sensed, e.g. through suitable configuration or

functionalization of the layer of nanomaterial.

REFERENCES

[1] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, "Extraordinary optical transmission through sub-wavelength hole arrays", Nature, Vol. 391, pp667, 1998

[2] A. G. Brolo, R. Gordon, B. Leathern, and K . L.

Kavanagh, "Surface plasmon sensor based on the enhanced light transmission through arrays of nanoholes in gold films,"

Langmuir Vol. 20, pp.4813, 2004

Claims

1. A localised surface plasmon generator for sensing a substance, the surface plasmon generator comprising:

an optical waveguide for guiding optical radiation, and a surface plasmon supporting layer formed on an outer surface of the optical waveguide in the sensing region,

wherein the optical waveguide has refractive index modulations formed in the sensing region thereof, and

wherein the surface plasmon supporting layer includes a layer of nanomaterial at its outermost surface.

2. A localised surface plasmon generator according to claim 1, wherein the layer of nanomaterial consists of any one of more of nanotubes (e.g. carbon nanotubes), nanofibres, nanoparticles , or structures formed from graphene sheets, arranged to attract the substance to be sensed.

3. A localised surface plasmon generator according to claim 1 or 2, wherein the surface plasmon supporting layer has a periodic or quasi-periodic surface relief structure

inscribed into its outermost surface, and wherein the layer of nanomaterial comprises nanoscale structures substantially aligned with the surface relief structure.

4. A localised surface plasmon generator according to claim 3,

wherein the surface plasmon supporting layer comprises a dielectric sub-layer, and a thin film of conductive material on the dielectric sub-layer,

wherein the nanoscale structures comprise nanowire elements in contact with the dielectric sub-layer, and

wherein the thin film of conductive material is for supporting localised surface plasmons between the nanowire elements.

5. A localised surface plasmon generator according to claim 4, wherein the thin film of conductive material is selected from germanium, gold, silver, platinum, copper, palladium, aluminium, a vanadium oxide or vanadium oxides.

6. A localised surface plasmon generator according to claim 4 or 5, wherein the thin film has a thickness less than the optical skin depth of the conductive material from which it is formed.

7. A localised surface plasmon generator according to claim 4, wherein the thin film of conductive material is a layer of graphene .

8. A localised surface plasmon generator according to any preceding claim, wherein the nanomaterial is

functionali zed.

9. A localised surface plasmon generator according to claim 1, wherein the surface plasmon supporting layer

comprises a multi-layer structure having two conductive layers (formed of metal or semiconductor) separated by a coupling optical waveguide, the layer of nanomaterial being formed at the outermost surface of the multi-layer structure.

10. A localised surface plasmon generator according to any preceding claim, wherein the optical waveguide is an optical fibre comprising a core region and a cladding region, and wherein the refractive index modulations extend across the core region and the cladding region is lapped in the sensing region .

11. A sensor for detecting the presence of a substance, the sensor comprising: a localised surface plasmon generator according to any preceding claim;

an optical radiation source in optical communication with an optical input part of the surface plasmon generator; and an optical radiation detector arranged to detect optical radiation having passed through the surface plasmon generator from the input part.

A sensor according to claim 11 for detecting carbon wherein the nanomaterial is carbon nanotubes

13. Use of nanomaterial as a sample interface layer the sensing region of a localised surface plasmon generator according to any one of claim 1 to 10.

14. Use according to claim 13, wherein the nanomaterial is carbon nanotubes.

15. A method of fabricating a localised surface plasmon generator for sensing a substance, the method comprising:

forming a surface plasmon supporting layer on an outer surface of an optical waveguide;

generating refractive index modulations in the optical waveguide; and

after generating the refractive index modulations, forming a layer of nanomaterial on the outermost surface of the surface plasmon supporting layer.

16. A method according to claim 15, wherein forming a layer of nanomaterial comprises inkjet printing carbon nanotubes on the outermost surface of the surface plasmon supporting layer.