WO2010124320A1 - A differential charge oscillation device - Google Patents

Abstract

A differential charge oscillation device (100), including at least three mutually spaced but electromagnetically coupled electrically conductive bodies, the bodies including: a first electrically conductive body ( 102) for generating first charge oscillations therein; a second electrically conductive body (104) for generating second charge oscillations therein; and a third electrically conductive body (106) electromagnetically coupled to the first conductive body and to the second conductive body such that the first charge oscillations generated in the first conductive body and the second charge oscillations generated in the second conductive generate third charge oscillations in the third conductive body. A particular use of the device includes sensing or detecting biological or chemical species of interest, via coupled surface plasmon resonances (SPR) in the device.

Description

A DIFFERENTIAL CHARGE OSCILLATION DEVICE

TECHNICAL FIELD

The present invention relates to a differential charge oscillation device for responding to charge oscillations, and in particular changes — such as phase changes caused by changes in the morphology of the device and/or in the permittivity around the device — allowing the device to be used, e.g., as a chemical or biological sensor.

BACKGROUND The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

In recent years there has been much interest in the optical properties of nano-scale particles, often referred to as 'nanoparticles'. For example, a metallic nanoparticle scatters light over a range of wavelengths, but its optical reflectivity has a maximum at a specific wavelength that is determined by the particle's shape and by the optical electric permittivity of the medium surrounding the particle. This maximum optical reflectivity corresponds to a surface plasmon resonance of the nanoparticle, and its dependence on the permittivity of the particle's environment allows the nanoparticle to be used as a chemical sensor by coating the nanoparticle with a material that selectively binds to the chemical that is to be detected. Once coated, any chemical binding to the coated surface of the nanoparticle changes the local permittivity, and therefore changes the wavelength of maximum optical reflectivity of the nanoparticle. The degree of presence of the target chemical can in principle be assessed by quantifying this wavelength shift. However, the sensitivity of this method is therefore limited by the natural width of the resonance and the resolution of the spectrometer used to measure it. It is desired to address or ameliorate one or more difficulties or limitations of the prior art, or to at least provide a useful alternative.

SUMMARY

In accordance with the present invention, there is provided a differential charge oscillation device, including at least three mutually spaced but electromagnetically coupled electrically conductive bodies, the bodies including: a first electrically conductive body for generating first charge oscillations therein; a second electrically conductive body for generating second charge oscillations therein; and a third electrically conductive body electromagnetically coupled to the first conductive body and to the second conductive body such that the first charge oscillations generated in the first conductive body and the second charge oscillations generated in the second conductive body generate third charge oscillations in the third conductive body.

The third charge oscillation may be based on at least one difference between the first and second charge oscillations. The first and second oscillations may be coupled to respective and mutually spaced portions of the third conductive body such that, when the first and second oscillations are of equal amplitude, frequency and phase, the third oscillation is not generated, and that the at least one difference includes at least one difference of amplitude, frequency and/or phase between the first and second oscillations, which causes the third oscillation to be generated, and wherein the amplitude, frequency and phase of the third oscillation is representative of the at least one difference.

The third charge oscillation may also or alternatively be based on a difference in electromagnetic coupling of the first body to the third body and/or electromagnetic coupling of the second body to the third body. The difference or differences in coupling may be due to a difference in the electromagnetic environment between the third body and one of the first and second bodies. The device may be arranged to allow substantial coupling between the first body and the third body, and between the second body and the third body, and to allow less coupling, or substantially no coupling, between the first body and the second body, except via the third body. The first and second conductive bodies may both be strongly coupled to the third body. The first and second conductive bodies may be weakly coupled to each other to provide that the charge oscillation in each of the first and second conductive bodies is substantially independent from the electromagnetic environment of the other of the first and second conductive bodies.

The first and second oscillations may have substantially the same amplitude and frequency, but may differ in phase, wherein the resulting third oscillation is representative of the phase difference between the first and second oscillations.

The first and second oscillations may be generated in the first and second conductive bodies by an optical input signal, wherein the wavelength of the optical input signal is similar to or substantially larger than dimensions of the first and second conductive bodies. The conductive bodies may be in a substantially spatially uniform electric field of the optical input signal that oscillates at the frequency of the optical input signal. Although it is not necessary that the conductive bodies are in a uniform electric field, it is desirable that the dimensions of the bodies are about equal to the wavelength of the optical input signal or smaller, because if the bodies are too large compared to that wavelength, then phase differences across the applied electric field may provide a background phase difference between the first and second oscillations.

The third oscillation may generate an optical output signal representative of the third oscillation.

The optical input signal may have an input frequency matched to one or more resonance frequencies of the first body and the second body. The input frequency may also match one or more resonance frequencies of the third body. - A -

The optical input signal may include input light with an input polarisation matched to a resonance of the charge oscillations in the input bodies. The configuration of the output body may be selected so that the resonance in the charge oscillation in the output body substantially does not match the polarisation of the input light. The charge oscillation in the output body may be substantially orthogonal to the charge oscillations in the input bodies so that the output polarisation is substantially orthogonal to the input polarisation. The input polarisation and the output polarisation may be respectively: x-polarisation and y-polarisation; s-polarisation and p-polarisation; or right-elliptical-polarisation and left- elliptical-polarisation.

The output optical signal may be representative of a change in one of the first and second charge oscillations. The output optical signal may be representative of a change in the coupling of the first body to the third body and coupling of the second body to the third body.

The device, or a system including the device, may further include an optical sensor to measure the optical output signal.

The first and second conductive bodies may be substantially identical in size and shape. The third conductive body may be disposed substantially between the first and second conductive bodies. The first and second conductive bodies may be of elongate form and mutually parallel, and the optical input signal may be linearly polarised and its axis of polarisation aligned with the longitudinal axes of the first and second conductive bodies such that the electronic oscillations generated therein are directed along the longitudinal axes of the first and second conductive bodies. The third conductive body may also be of elongate form but with its longitudinal axis orthogonal to the longitudinal axes of the first and second conductive bodies. The optical output signal generated by the third conductive body is substantially polarised in a direction orthogonal to the axis of polarisation of the optical input signal, thereby allowing the output signal to be readily discriminated from the optical input signal. All three conductive bodies may be of parallelepiped or elongate parallelepiped form. All three conductive bodies may be substantially thinner in a direction orthogonal to both polarisation directions so that the bodies are in the form of thin rectangular films or sheets.

The device may be used as a sensor, where the first conductive body is subjected to a change in electromagnetic environment that modifies the first oscillation, and the second conductive body is substantially isolated from that change, and thus provides a reference so that the optical output signal is representative of the change in the electromagnetic environment of the first conductive body.

The change in electromagnetic environment may be effected by binding of a target or target species to a corresponding coating on the first conductive body. The target may be a molecule or body or cell, such as a protein, or a functional group on a larger molecule, including: enzymes, antibody-antigen conjugates and oligonucleotides. The coating may include a selective material with chemical selectivity for the target. The binding may be hybridisation of a DNA, RNA, or oligonucleotide sequence to a corresponding sequence of the coating. The targets may be nanoparticles of a selected composition, shape and/or geometry. The change in the electromagnetic environment may be due to electrical charge added to or subtracted from the first conductive body or changes to the magnetic fields at or near the first conductive body. The change in the electromagnetic environment may be due to one or more proximate conductive or electrically polarisable bodies.

Each body may be a nano-body or nano-particle, the dimensions or size of each body being less than about 100 nm. Having a smaller body relative to the target provides for a larger change in the electromagnetic environment of the sensor body when the target is present, and thus more sensitivity. Each body may include at least one of the following body materials: a metal (e.g., gold, silver, copper, aluminium, nickel, or platinum) or a semiconductor or a semimetal. The body material may include a substance that can support charge oscillations or an electrically polarisable material.

The bodies may be arranged on a substrate including at least one of the following substrate materials: glass, silicon, silicon nitride, insulator, or plastic. The substrate may support charge oscillations only weakly, or not at all, so as not to interfere with the operation of the device. The substrate may include an at least partially conducting material, e.g., as a substrate or surface, wherein the conducting material provides conduction that does not interfere with the operation of the device, such as tin-doped indium oxide (indium tin oxide, or ITO), which is not conductive at visible optical frequencies.

The charge oscillations may be electronic oscillations based on electron density. The charge oscillations may be surface plasmons or surface plasmon resonances.

A sensor system may include: an optical input system for generating the optical input signal; the device; and a detector system for detecting the third charge oscillation.

Also described herein is a charge oscillation device including at least two conductive first bodies coupled to a conductive second body such that charge oscillations generated in the first bodies give rise to a charge oscillation in the second body.

Also described herein is a sensor, including: a sensing conductive body for generating a sensing oscillation therein from an optical input signal coupled thereto; a reference conductive body for generating a reference oscillation therein from an optical input signal coupled thereto; and an output conductive body electromagnetically coupled to the sensing and reference conductive bodies to generate an output oscillation in the output conductive body from the sensing oscillation and the reference oscillation.

Also described herein is a method for sensing, including: generating a first charge oscillation in a first conductive body; generating a second charge oscillation in a second conductive body; generating a third charge oscillation in a third conductive body, electromagnetically coupled to the first conductive body and the second conductive body, based on the first charge oscillation and the second charge oscillation; and generating an optical output signal based on the third charge oscillation.

Also described herein is a differential charge oscillation device, including: a first conductive body for generating a first charge oscillation therein; and at least two second conductive bodies electromagnetically coupled to the first charge oscillation in the first conductive body so as to generate respective second charge oscillations in the at least two second conductive bodies, based on the first charge oscillation.

The present invention also provides a method for sensing, including: generating first charge oscillations in a first electrically conductive body; generating second charge oscillations in a second electrically conductive body; exposing at least one of said first electrically conductive body and said second electrically conductive body to an environment to be sensed; detecting at least one change in an electromagnetic signal emitted from a third electrically conductive body electromagnetically coupled to the first conductive body and to the second conductive body, said electromagnetic signal being generated by the first charge oscillations generated in the first conductive body and the second charge oscillations generated in the second conductive body, said at least one change in said electromagnetic signal being representative of the presence of at least one chemical and/or biological species in said environment.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, which are not to scale, wherein:

Figure l is a schematic diagram of an embodiment of a differential charge oscillation device having three mutually spaced electrically conductive bodies, including two input bodies electromagnetically coupled to an output body disposed therebetween;

Figure 2 is a further schematic diagram of the device with balanced electronic oscillations in its input bodies, the symmetry of the device producing no electronic oscillations in the output body;

Figure 3 is a further schematic diagram of the device with a phase mismatch between electronic oscillations in its input bodies, producing electronic oscillations in the output body;

Figure 4 is a schematic diagram of the device of Figures 1 to 3, but with the addition of a sensor region associated with one of the input bodies;

Figure 5 is a block diagram of an embodiment of a sensor system including the device of Figure 4;

Figure 6 is a schematic diagram of an embodiment of a fluid-based sensor system including the device of Figure 4; Figure 7 is a schematic diagram of the device including a balancer;

Figure 8 is a schematic diagram of an array of the devices; Figure 9A is a schematic diagram of inter-body coupling in the device; Figure 9B is a schematic diagram of an H layout of the device; Figure 9C is a schematic diagram of a rotated-H layout of the device; Figure 9D is a schematic diagram of a vertically separated layout of the device; Figure 10 is a pair of graphs showing a scattering cross section of an output body for an increasing number of target dipoles associated with the sensor region;

Figure 11 is a set of three graphs showing: (i) an increase in an output-polarised component of the scattering cross section of the output body due to an increasing number of target dipoles; (ii) a gradual decrease in an input-polarised component of the scattering cross section of the device due to the increasing number of the target dipoles; and (iii) an increase in an average output wavelength of the output light due to the increasing number of dipoles; and

Figure 12 is a graph showing an increase in normalised amplitude of output light from the output body as the phase mismatch between the electronic oscillations in the input increases.

DETAILED DESCRIPTION

A plasmonic bridge device 100 is a form of differential charge oscillation device. As shown in Figure 1, the device 100 includes three mutually spaced electrically conductive bodies 102, 104, 106, including first and second 'input' bodies 102, 104, each of which is electromagnetically coupled to mutually spaced portions of a third 'output' body 106 disposed therebetween. The symmetry of this arrangement means that, as described below, any difference (i.e., a differential or "imbalance") either between charge oscillations generated in the first or second conductive bodies 102, 104, and/or between electromagnetic inter-body coupling from the third conductive body 106 to the first and second conductive bodies 102, 104, will generate third charge oscillations in the third conductive body 106.

As described below, the third charge oscillation can be used to generate a signal representing the imbalance, from which an output signal representing changes in the imbalance can be generated. An imbalance change may be from zero imbalance (i.e., no imbalance) to a non-zero value. In general, imbalance changes can be caused by (i) one of the first and second oscillations changing while the other remains substantially unchanged, or (ii) by both the first and second oscillations changing differently, or (iii) by changes in inter-body coupling of the first body 102 to the third body 106 and/or inter-body coupling of the second body 104 to the third body 106.

The third body 106 is referred to herein as an "output body" because it provides a differential "output", such as the output signal, based on charge oscillations in the first and second bodies 102, 104, which are referred to herein as "input bodies".

The three bodies 102, 104, 106 are mutually spaced and separated by one or more non- conductive materials: the bodies 102, 104, 106 are arranged in, or on, a non-conductive substrate 108, and otherwise surrounded by other non-conductive materials, such as a fluid (e.g., a liquid or gas) or a vacuum.

In the described embodiments, the electronic oscillations include plasmons, which are collective oscillations of free electron gas density in a conductive body. Plasmons confined to surfaces are referred to as surface plasmons, which interact strongly with light. Due to the role of the plasmons, the device 100 can also be referred to as "plasmonic device".

The arrangement and morphology of the bodies 102, 104, 106 — and their surrounding materials — provide for strong inter-body electromagnetic coupling between each of the input bodies 102, 104 and the output body 106, but only weak inter-body electromagnetic coupling between the two input bodies 102, 104 themselves. "Strong" inter-body coupling refers to sufficient coupling for the charge oscillations in one body to substantially affect the charge oscillations in the other body, whereas "weak" inter-body coupling refers to coupling that allows the charge oscillations in one body to be substantially independent of the charge oscillations in the other body.

In the described embodiments, the charge oscillations are electronic oscillations, generated in the input bodies 102, 104 by propagating electromagnetic energy, which includes optical electromagnetic radiation, such as a beam of light from a laser, referred to herein as an "input signal". The input bodies 102, 104 therefore act as transducers of propagating electromagnetic energy to charge oscillation energy.

In the described embodiments, the charge oscillations in the third body 106 act as a source of electromagnetic radiation that propagates away from the device 100, providing an output signal in the form of an optical output signal. The third body 106 therefore acts as a transducer of charge oscillation energy to propagating electromagnetic energy.

The symmetrical arrangement of the plasmonic bridge device 100 can be considered analogous to the electronic circuit configuration known as an "AC Wheatstone bridge circuit". The first body 102 and the second body 104 effectively form two "arms" of the Wheatstone bridge circuit. Where the first body 102 and the second body 104 are substantially identical in morphology and composition and symmetrically disposed about (and equally coupled to) the third body 106, any imbalance between the charge oscillations (/.e., oscillating currents or voltages) in the first two arms of the circuit will generate corresponding charge oscillations in the third "arm" (i.e., the third body 106) that lies between the other two. The third body 106 can therefore be considered to be a "bridge body", bridging between the input bodies 102, 104. A phase mismatch between charge oscillations in the first two arms causes third charge oscillations to be generated in the third arm, and the third charge oscillations cause the third arm to radiate electromagnetic energy, thereby providing an output signal that can be detected and measured to provide a measure of the imbalance. For the device dimensions and configurations of the described embodiments, the third electronic oscillations are at optical frequencies, and the radiated electromagnetic energy can be measured as an optical output signal. The intensity, spectral and/or polarisation properties of the radiated output signal is determined by the phase mismatch. The device 100 is thus useful for measuring relative phase shifts of optical or plasmonic waves, and, as described below, for sensing changes in morphology and/or electrical permittivity local (i.e., substantially close) to the bodies 102, 104, 106. In the described embodiments, an imbalance includes a phase mismatch (i.e., a relative phase shift) between the electronic oscillations generated in the first body 102 and the electronic oscillations generated in the second body 104. The third body 106 is sensitive to these phase shifts. The three bodies 102, 104, 106 are arranged symmetrically so that, when plasmons in the first body 102 and the second body 104 oscillate substantially in phase (i.e., are balanced), and the coupling from each of the input bodies 102, 104 to the output body 106 is substantially identical, negligible electronic oscillations are generated in the third body 106. Conversely, when there is a phase imbalance between the oscillations in the first and second bodies 102, 104, the imbalance generates an electronic oscillation in the third body 106, and this oscillation causes the third body 106 to emit an output signal representative of the imbalance.

In the described embodiments, the first, second, and third bodies 102, 104, 106 are much smaller than the wavelength of the incident radiation, and the device 100 can therefore be assumed to be in a spatially uniform electric field that is oscillating at the frequency of the incident input light: this assumption is referred to as "the electrostatic approximation" and is described in the following papers by Mayergoyz et al. : I. D. Mayergoyz, D. R. Fredkin, and Z. Zhang, Phys. Rev. B 72 (15), 155412 (2005); and I. D. Mayergoyz, Z. Zhang, and G. Miano, Phys. Rev. Lett. 98 (14), 147401 (2007). The electrostatic approximation is a simplification of the boundary element method, which is described in the following papers by Garcia de Abajo et al. : F. J. Garcia de Abajo and A. Howie, Physical Review Letters 80 (23), 5180-5183 (1998); and F. J. Garcia de Abajo, A. Howie, Phys. Rev. B 65 (1 1), 115418 (2002).

In some embodiments, the first, second, and third bodies 102, 104, 106 have at least one dimension (orthogonal to the propagation direction) that is substantially less than the wavelength of the incident radiation so that the device 100 is in a substantially spatially uniform electric field. Otherwise, phase differences across the device may degrade its performance. In the embodiment shown in Figure 1, the third body 106 is disposed between and substantially at one end of each of the first and second bodies 102, 104 so that one end of the third body is predominantly electromagnetically coupled to one end only of the first body 102 and the other end of the third body is predominantly electromagnetically coupled to one end only of the second body 104. For convenience of reference, these ends of the first and second bodies 102, 104 are referred to as the 'coupled ends'. Consequently, when the electronic oscillations or plasmons in the first body 102 and the second body 104 are in phase, the first body 102 and the second body 104 have the same surface charge at their coupled ends at any moment in time, so the third body 106 has the same surface charge across it and therefore remains electrically unpolarised. In this state, there is no charge flow in the third body 106, the electrical voltage across it is zero, and no plasmons are generated. The surface dipole distribution of the third body 106 is substantially homogeneous in the balanced state, as shown in Figure 2. When there are substantially no charge oscillations in the output third body 106, light output from the third body 106 is substantially zero, or practically negligible.

Conversely, when the first and second oscillations changes are not substantially equal (which can be due to a change in the local environments of the first and second bodies 102, 104 and/or changes in one or both of the first and second bodies 102, 104 themselves), the charges at the coupled ends of the first body 102 and the second body 104, at any instant of time, are no longer the same. The different charges of the coupled ends of the first body 102 and the second body 104 electromagnetically coupled to respective ends of the third body 106 create a potential difference across the third body 106, and this voltage induces a current flow or electronic oscillation. The surface dipole distribution of the third body 106 varies substantially in the imbalanced state, as shown in Figure 3. This varying surface dipole distribution, or charge oscillation, generates an output beam of light having substantially the same wavelength(s) as the input beam.

Each of the first, second, and third bodies 102, 104, 106 scatters light over a range of wavelengths, but has a maximum reflectivity at a specific wavelength that is determined by the material composition of the body, the shape of the body, and by the electric permittivity of the surrounding medium or by the proximity of other electrically polarisable bodies (via inter-body coupling). This maximum reflectivity corresponds to a resonance of each body. The coupling efficiency of the input light to the first conductive body 102 and to the second conductive body 104 is improved by matching the frequency (and the wavelength) of the input light to the electronic resonances of the conductive bodies 102, 104.

In the described embodiment, the optical input signal has a input frequency substantially matched to resonance frequency of the first body 102, the second body 104 and the third body 106. Coupling between the input bodies 102, 104 and the output body 106 is most efficient when the output body 106's resonance frequency is substantially equal to the resonance frequency of the input bodies 102, 104. Using the input light and the output light corresponding to the resonances of the first body 102, the second body 104, and the third body 106 thus improves the efficiency and sensitivity of the overall device 100.

The proximity of one body to another modifies their resonances due to the coupling and an interaction between the induced electric fields and the surface charges: this interaction is referred to as a "capacitive coupling". The inter-body coupling is equivalent to capacitances c_s , between the third (output) body 106 and each of the two parallel input bodies 102, 104, and this inter-body coupling shifts the resonant frequency of the output body 106. The inter-body coupling varies with the spacing between the bodies, which, in terms of a circuit analogy, is equivalent to changing the capacitances c_s ■

When operating at resonance, the oscillating input field excites one or more resonant surface-dipole modes that are associated with the charge oscillations, or "surface-charge oscillations" (also referred to as "localised surface plasmon resonances"). The surface- charge oscillations are the eigenmodes of each body; these eigenmodes are associated with surface-dipole and surface-charge eigenfunctions, as described in the above-referenced papers by Mayergoyz et al.. The eigenfunctions have associated eigenvalues, which, together with the permittivity of the body and the permittivity surrounding the body, determine the resonant frequencies of the body. The surface-charge oscillations induce a dipole moment P in the output body 106, which leads in turn to the emission of dipole radiation forming the output optical signal.

Each body 102, 104, 106 may have many localised surface plasmon resonance modes. The amplitude of the uncoupled induced dipole moment P of the k-th mode of they-th body, a_j ^k , can be determined from the relation

where p* is the dipole moment of the k^th eigenmode of the /^h body, /* is the associated eigenvalue, η_} is a constant and E₀ is the applied electric field. A resonance occurs when the real part of the denominator is zero; given the electric permittivity ε(ω) of the body and the permittivity of the background medium ε_b , this determines the resonant frequency ω . For example, for the fundamental mode of a sphere, Ic=I, γ^ = 3 , and the zero in the denominator yields the condition for a Frδhlich resonance, as described in the book entitled "Absorption and Scattering of Light by Small Particles" by C. F. Bohren and D. R.

Huffman (Wiley, New York, 1983).

With reference to the coupling theory in the paper by T. J. Davis, K. C. Vernon, D. E.

Gomez, Physical Review B 79, 155423 (2009) (hereinafter referred to as "Davis"), the amplitude of the induced dipole moment of the fundamental mode of the third body 106 can be determined from the relation

where a] and

are the uncoupled amplitudes for the dipole moments of the fundamental modes of the first and second bodies 102, 104 respectively, and are determined according to Equation (1). The coupling factors C* describe the coupling of the k-Xh mode of body y to the /-th mode of body i, where / and j correspond to the first, second and third bodies 103, 104, 106. When the first and second bodies 102, 104 are substantially identical in shape and size, and are arranged symmetrically at either end of the third body 106, as shown in Figure 1, the coupling factor between the first body 102 and third body 106 is approximately equal and opposite to the coupling factor between the second body 104 and third body 106, i.e.,

-C]] . As shown in equation (2) above, the amplitude of the dipole moment induced in the third body 106 is proportional to the difference between the dipoles induced in first and second bodies 102, 104. In the balanced condition for the device 100, the uncoupled amplitudes are equal (i.e., ), thus — as shown in Equation (2) — the third body 106 has no induced dipole mome

= 0 which implies P = O , and the output light is zero.

The optimum coupling between the input bodies 102, 104 and the output body 106 is determined by an appropriate selection of the spacing between the input bodies 102, 104 and the output body 106, or by an appropriate selection of the geometries of the bodies 102, 104, 106. The condition for optimum coupling depends on the loss factor in the electric permittivity of the material used to construct the bodies 102, 104 and 106. The loss factor r can be determined from the known Drude model for the electric permittivity of a metal, given by the relation

where ω is the frequency at which the metal is excited, ω_F is the plasma frequency of the metal, and / = V-T represents a complex number. Since the coupling factors are substantially equal according to C]₁ ¹ = C₁" = -C]] = -Cj] , then at resonance the coupling factors can be written in the form C]₁ ¹ = - 2iG/T, where G has a value that depends on the spacing and the geometry of the bodies 102, 104, 106. The expressions for the coupling factors, when substituted into Equation (2), lead to a condition where the amplitude

has a maximum sensitivity; that is, the variation of

with variations in

) is the largest. The condition for maximum sensitivity is when G = r/2V2 . This condition provides a means by which the operation of a plasmonic bridge device can be optimised for sensitivity.

To couple the input light more efficiently to the first body 102 and the second body 104, the input light is selected to be substantially polarised parallel to the longitudinal axes of the elongate first and second bodies 102, 104, along a direction referred to herein as the "y- direction", as shown in Figure 4. Due to a different geometry and/or orientation of the third body 106, the applied electric field of the input light polarised in the ^-direction does not excite a dipole resonance of the third body 106 (or at least, any such excitation is insubstantial). The dipole resonances have a direction related to the oscillatory flow of charge. Due to the elongate form of the three bodies, a lower frequency resonance is due to charge flowing along the longitudinal axis of each body; a higher frequency resonance is associated with charge flowing transversely across the width of each body. The higher resonance is excited when an applied electric field is oriented transversely across the width and oscillating at the higher frequency. When the applied electric field is at the lower frequency, the transverse or "width" resonance is only very weakly excited.

Due to the orientation of the third body 106 substantially in a direction orthogonal to the y- direction, referred to herein as the "x-direction" as shown in Figure 4, the output light from the third body 106, is substantially polarised parallel to the longitudinal axis of the elongate third body 106, and is thus substantially orthogonal to the parallel longitudinal axes of the first and second bodies 102, 104, and therefore also to the polarisation (in they- direction) of the input light. The size and shape of the third body 106 are selected such that its major resonances substantially do not match the polarisation of the input light; e.g., the width of the third body 106 in the ^-direction (the input polarisation direction) is significantly shorter than the lengths of the first and second bodies 102, 104 in the ^-direction, and the input light efficiently couples to resonances of the first and second bodies 102, 104 but not to resonances of the third body 106. Although it is possible to select the size and shape of the third body 106 such that its resonance does at least partially match the polarisation and frequency of the input light, this selection may reduce the overall efficiency of the device.

The respective orthogonal polarisations of the input light and the output light allow the output light to be optically separated from any reflected input light using polarisation- sensitive techniques, such as polarisation splitting with standard optical components. The orthogonal polarisations may be referred to as "s-polarisation" and "p-polarisation" for linearly polarised light, or "right-elliptical-polarisation" and "left-elliptical-polarisation" for elliptically or circularly polarised light.

The actual or apparent phase mismatch in the device 100 can arise due to factors such as: a change in the relative coupling efficiencies of the input light to the first and second bodies 102, 104; a change in the shape or morphology of one of the three bodies 102, 104, 106 which leads to a change in its eigenmode (and eigenvalue); or by a change in background electric permittivity surrounding at least one of the three bodies 102, 104, 106. In general, any change that breaks the symmetry of the device or the electromagnetic couplings of the device will cause a phase mismatch in the device 100.

A change in the background permittivity of the first and second bodies 102, 104 can be caused by one or more additional electrically polarisable bodies interacting with one of the first or second bodies 102, 104, and this property can be exploited for sensing applications by using one of the input bodies as a sensing element whilst maintaining the other of the input bodies as a reference element. When each input body 102, 104 is at resonance, its phase is at 90 degrees relative to that of the input light. However, with a change in suirounding local permittivity of one of the input bodies 102, 104 — referred to herein as a "sensor body" — the resonant frequency of the sensor body changes and the plasmon phase shifts towards zero relative to that of the incident input light. The other input body — referred to herein as the "reference body" — is substantially unaffected by the change in the other (sensor) body's local permittivity. The phase shift of the charge oscillations in the sensor body, but not in the reference body, causes charge oscillations to be generated (or "excited") in the output body 106. This phase shift — more than any change in the magnitude of the input charge oscillations — results in the generation of the output light. The output light from the third body 106 is related to its induced dipole moment P₃ , which causes dipole radiation. The dipole moment P₃ is directly proportional to its amplitude

, thus the imbalance Δ between the two arms (the first and second bodies 102, 104) of the bridge circuit determines the output light radiated from the third body 106.

For sensing applications, the binding of a target to the sensor body can be produce the changes in the local background permittivity. The target provides one or more additional bodies, which can be considered to provide "point dipoles", that interact with the sensor body. When the target binds to the sensor body (but not to the reference body), the resonances of the two input bodies 102, 104 become mismatched, causing the third body

106 to generate an output signal representative of the binding. The target can include one or more molecules of a chemical species, or molecules associated with a biological marker.

In the arrangement shown in Figure 4, the sensor body 102 is prepared to sense a target chemical species by coating at least a portion of the sensor body with a chemically or biologically selective material to provide a sensor region 502. The sensor region 502 may be formed by coating a thin film of photo-resist or other masking material over the three bodies 102, 104, 106, except for an exposed area defining the sensor region 502 on the sensor body. In some embodiments, the sensor body is made from gold and the exposed gold region is shaped as a 40-nm diameter circle (or other shape of similar area), located at the non-coupled end of the sensor body away from the third body 106. The exposed gold region is coated with a thin film of chemically selective material to form the sensor region. In some embodiments, the chemically selective material is deposited on the sensor region 502 directly, without the masking steps.

The reference and sensor bodies 102, 104 are substantially uncoupled electromagnetically to the extent that the charge oscillations in the reference body 104 are substantially independent of the electromagnetic environment of the sensor body 102. In alternative embodiments, the coupling between the first and second bodies 102, 104 may be stronger; however, the device will have a lower sensitivity as a sensor because a change in the electromagnetic environment of the sensor body also affects the electromagnetic environment of the reference body, and thus the difference (e.g., the phase mismatch) between the first and second charge oscillations is not as great as when the reference body is unaffected by changes in the electromagnetic environment of the sensor body.

The use of the third body 106 to generate the intensity change in the output light representing the phase mismatch between the sensor and reference bodies provides a higher sensitivity to permittivity changes around the sensor body than do prior art methods requiring measurement of changes in the wavelength of a single body's maximum optical reflectivity, e.g., using a single nanoparticle resonator. Intensity (or amplitude) changes in the output light are simpler and more convenient to measure accurately than wavelength changes. The device 100 therefore allows sensing of very small concentrations of chemicals with higher sensitivity than conventional optical methods. The device 100 also allows simpler measurement systems, with a reduction in both complexity and cost of the measurement instruments, in particular because no spectrometer is required.

During sensing with the device 100, the sensor region 502 is exposed to fluids, including liquids or gases, containing the target chemical(s) to be sensed. For use as a biosensor, the selective material can be an antibody and the target a corresponding antigen, for example; or the selective material can include one of the biotin-avidin pair and the target the other. In some embodiments of the device 100, the three bodies 102, 104, 106 are stripes of gold on a substrate 108 of glass. The three gold stripes are defined lithographically by: (i) depositing a layer of photo-resist on the glass substrate; (ii) defining the body shapes and locations as openings in the layer of photo-resist using a corresponding mask created by an e-beam system; (iii) using a physical vapour deposition process to deposit a layer of gold over the photo-resist and the portions of the glass substrate exposed by the openings in the photo-resist; and (iv) using a lift-off method to remove the resist and the gold deposited on the resist, only leaving the desired gold stripes on the glass substrate.

A sensor system 400, as shown in Figure 5, includes the device 100. An optical input system 408 with an input source 410 (e.g., an infra-red, visible, or ultra-violet laser) generates input light 412 which is incident on at least the first conductive body 102 and the second conductive body 104. The resulting charge oscillations in the form of plasmons generated in the first and second conductive bodies 102, 104 are electromagnetically coupled to respective ends of the third conductive body 106 via respective inter-body couplings 414 A, 414B. Output charge oscillations are generated in the output or third conductive body 106 by an imbalance between the first and second charge oscillations, and/or between the inter-body couplings 414A, 414B. These output charge oscillations are a source of output electromagnetic radiation, including output light 416, that is detected by an opto-electronic detector system 420. The optical input system 408 includes optical components for controlling the polarisation and location of the input light 412. The optoelectronic detector system 420 includes: optical components for controlling the polarisation and location of the output light 416; opto-electronic components for detecting the output light 416; and electronic and digital components for processing, recording and displaying an output signal. Example optical components include focusing and polarisation optics, such as lenses and filters, as well as wavelength selective optics, such as gratings. The optical components may include fibre-optic components, such as fibres and wavelength division multiplexing (WDM) combiners and splitters. The device 100 can be mounted to the end of an optical fibre, and the optical input and output signals can be guided in that fibre. The opto-electronic components may include photo-detectors or detector arrays. The electronic and digital components may include low-noise amplifiers, signal processing filters, and general purpose computing devices for analysing and displaying the output signals. The sensor system 400 may also include a general purpose computer and/or electronics for monitoring and controlling the sensor system 400, in particular the input source 410.

A fluid-based sensor system 900, as shown in Figure 6, includes: (i) a chemically selective material, selective for the desired target chemical and/or biological species to be sensed, adjacent the sensor particle of the device 100; (ii) an input optical system 902 for illuminating the device 100 with polarised input light of an appropriate range of wavelengths; (iii) the device 100 for generating output light based on the phase mismatch between the charge oscillations generated in the sensor body and the reference body by the input light; (iv) an optical detector system 904 for detecting the polarised output light from the device 100; (v) an opto-electronic detector system 906 for generating a signal based on the output light; and (vi) a fluid handling system 908 for transporting the target to the sensor region between pump-sample chambers 909 A, 909B. The fluid-based sensor system 900 may also include a general purpose computer and/or electronics for monitoring and controlling the system 900, in particular the input optical system 902 and the fluid handling system 908.

The system 900 illuminates the device 100 with polarised input light from below through a transparent substrate 910. The sensor region is exposed to the target in a liquid or gas sample 912. If the target (e.g., a molecule or a chemical) is present in the sample 912, it binds to the chemically selective material, which in turn causes a phase mismatch between the plasmons in the sensor body and the plasmons in the reference body (i.e., in the first body 102 and the second body 104). Any resulting output light from the third body 106 is detected by the optical detector system 904, and the extent (e.g., number of molecules or amount of chemical) of target binding to the sensor region can be determined based on the intensity of the output light. The input optical system 902 includes a light source 920 for generating radiation of an appropriate wavelength for the device 100. The light source 920 may be a broadband source, such as a white light source or a light emitting diode (LED), or a narrow band light source such as a laser or laser diode (LD). The input optical system 902 also includes an input polariser 922 for polarising the input light to a polarisation that matches the input bodies, and an input lens system 924 for focussing the input light and directing it to the input bodies. The optical detector system 904 includes an output lens system 930 for collecting light scattered from the device 100, particularly the output light emitted by the output body, and an output polariser 932 for selecting light polarised parallel to the polarisation emitted by the third body. The output polariser 932 provides a polarisation filter that selectively blocks light scattered at the input polarisation, which does not reflect the phase mismatch that is being sensed. The output detector system 904 also includes, at least in some embodiments, an output spectral filter 934 for selecting only those wavelengths in the output light, i.e., only those wavelengths that are associated with the oscillation in the output body. The opto-electronic detector system 906 includes one or more optical detectors, such as photodetectors, for detecting the output light 416, converting it to an electronic signal representing intensity (and wavelength in some embodiments), and providing an electronic signal to record and/or display characteristics of the output light, and in particular the output signal. The fluid handling system 908 may include a fluid pump for distributing the fluid to the device 100, and in particular to and over the sensor body, such as a micro-fluidic channel in the substrate 108 or a jet for placing a small sample of fluid into contact with the sensor body. The fluid handling system 908 may be substantially sealed to prevent loss or contamination of the fluid with the environment (e.g. , the air).

The input optical system 902 and the optical detector system 904 may be part of a single optical system, and some optical components may be shared (e.g., the lens systems 924 and 930). In some embodiments, the device 100 and/or the systems 400, 900 includes a balancer that is used to resist or prevent the undesired generation of unwanted output light due to any unintentional asymmetry between the first body 102 and the second body 104 in the device 100's nominally "balanced" condition or state. For example, the asymmetry may be due to unintended differences in the morphologies of the first and second bodies 102, 104 due to limitations of the fabrication process, and/or due to the presence of the chemically selective coating itself prior to any binding of the target. In various embodiments, the balancer is provided by: changing the local electromagnetic environment of any one of the three bodies 102, 104, 106, e.g., by depositing additional material onto one of the first body 102 and the second body 104; by changing the inter-body coupling between any of the three bodies 102, 104, 106; by introducing a further conductive body, electromagnetically coupled to one or more of the three bodies 102, 104, 106, e.g., by additional introduction of material; by changing the permittivity of the liquid or gas containing the sample; or by changing the electric permittivity of the region adjacent to or nearby one or more of the three bodies 101, 104, 106; e.g., by using a liquid crystal material with optical properties that change with an applied voltage or by temperature. In some embodiments, the balancer changes the geometry or spacing between the first body 102 and the second body 104. In some embodiments, the balancer includes one or more micro-electro-mechanical system (MEMS) or nano-electro-mechanical system (NEMS) components, e.g., manufactured on the same substrate as the bodies 102, 104, 106, or attached thereto after separate fabrication.

In one embodiment, a balanceable device 1000 includes a MEMS balancer, as shown in Figure 7. The balanceable device 1000 includes a substrate 1002 into which is machined a cantilever 1004. The cantilever 1004 has an attachment 1006 that has a polarisable body 1008, such as a dielectric or a metal, which may be of similar size to the charge oscillation device 100. When a voltage is applied to electrodes 101 OA, B an electrostatic force causes the cantilever 1004 to bend generally parallel to the plane of the substrate 1002. As the cantilever 1004 bends, the polarisable body 1008 is moved towards or away from the device 100, which changes the electronic coupling to one of the input bodies. This change in the coupling alters the resonant frequency of a balanced body, i.e., the input body most affected by the polarisable body 1008. The voltage over the electrodes 101 OA, B is selected to move the cantilever 1004 such that generated output light is minimised, indicating that the device 100 is substantially balanced.

The structures and sensors described above can be provided— and indeed fabricated — in large arrays. As will be apparent to those skilled in the art, the structures can be readily fabricated using standard micro-scale and nano-scale batch fabrication methods. For example, conductive bodies having nano-scale lateral dimensions can be fabricated using standard surface patterning, deposition, and etching techniques such as contact, projection, or immersion lithography with masks produced by electron-beam lithography. Suitable insulating substrates include, but are not limited to, optically transparent substrates such as various glasses (e.g., soda-lime glass) or even sapphire, or optically transparent and conducting substrates such as tin-doped indium oxide (ITO).

Although arrays of devices formed by batch processing can be diced into individual or small groups of devices, there is also utility in using large arrays of differential charge oscillation devices together. For example, large arrays of identical devices can be used to provide a larger optical output signal, where that may be desired. Alternatively, where the devices are used as selective chemical sensors, many different chemically selective coatings can be provided throughout the array to enable a single aggregate sensor (which in actuality includes many individual sensors) to detect a variety of different chemical target entities, for example. The distribution of different chemically selective coatings and devices can be selected as desired. For example, there can be as many different coatings as there are devices, or each type of coating can be provided on a corresponding subset of the devices, according to the needs of the particular application. Such considerations will be apparent to those skilled in the art of chemical sensing and need not be discussed further here. In addition to the above, an array of devices 100 of differing resonant frequencies allows for different devices to be optically and selectively addressable: different devices can be excited and the consequent output light detected using optical multiplexing and demultiplexing systems. For example, as shown in Figure 8, a white light source 1 102 produces input light over a range of wavelengths that encompasses the resonances of the devices in the array 1 104. The input light is shaped (and collimated) using a first lens system 1106, and polarised using a first polarising filter 1108. The polarised input light is then incident on the array 1104 of differential charge oscillation devices through a shared transparent substrate 11 10. Each device is manufactured to respond to input light of a different wavelength. In other words, each device resonates at a different frequency. Using white light or other multi-wavelength light means that all devices in the array 1104 will resonate. Output light emitted from the array 1 104 is polarised, so a second polarising filter 1112 is used to select the output light and block light scattered with other polarisations (e.g., reflected input light). The output light is shaped (and focussed) by a second lens system 1 1 14. The output light is spectrally filtered using an optical demultiplexer 1116 (e.g., a diffraction grating or a variable optical filter) to allow small bands of wavelengths to be separately detected by an opto-electronic detector system 1118. Since only one or a small number of devices are resonant at each separated wavelength, the opto-electronic detector system 11 18 detects light only from this small number of devices and not from others. The other devices can be sensed (or "interrogated") in turn by changing the optical demultiplexer 1116 to select a different small band of wavelengths. In alternative embodiments, the diffraction grating or variable optical filter can be placed just after the white light source, or the white light source can be replaced by a light source with adjustable wavelength. Then only light over a small band of wavelengths is incident on the array of devices and only those devices tuned to that band of wavelengths will respond. In further alternative embodiments, the optical demultiplexer 11 16 can separate the output light for a plurality of detectors that simultaneously interrogate all of the devices in the array 1104. Although the three conductive bodies 102, 104, 106 of the device 100 have been described above as being of elongate parallelepiped form (i.e., as being rectangular in plan view with uniform thickness, as shown in Figure 1), it is not necessary that the bodies have this shape. The bodies can be of many different shapes. The preferred shapes are those that lead to charge oscillations associated with a preferred direction of polarisation to exploit the method of separating the incident light from the emitted output light using polarisation separation. The thickness of the bodies affects the resonant frequencies and changes the distributions of charge in the resonant modes. The thickness can be selected to obtain the desired surface charge distribution. If the thickness is too small, then the bodies do not interact strongly with the light and are not optimally energised by it. Conversely, if the bodies are too thick, then the charge oscillations are mainly confined to the top surface or the surface charges can form travelling waves over the surface which may lead to greater losses of resonance.

As shown in Figure 9A, in alternative embodiments, a differential charge oscillation device generally includes three conductive bodies, a sensor body 1202, a reference body 1204 and an output body 1206, separated by non-conductive material. The sensor body 1202 and the reference body 1204 are both input bodies, arranged in a common input field area 1208 of input electromagnetic energy. The input field generates charge oscillations in the sensor body 1202 and the reference body 1204, which are in communication via a weak electromagnetic coupling 1210. Both the sensor body 1202 and the reference body 1204 are, however, in communication with the output body 1206 via respective strong electromagnetic couplings 1212, 1214. The output body 1206 generates an output field when the imbalance occurs either between the input charge oscillations, or between the strong couplings 1212, 1214.

As shown in Figures 9B and 9C, in certain embodiments, the sensor, the reference and the output bodies 1202, 1204, 1206 are elongate and extend across the input field area 1208.

The sensor and reference bodies 1202, 1204 are generally aligned parallel to each other and orthogonal to the output body 1206 to allow polarisation separation between the input field and the output field. As shown in Figure 9D, in certain embodiments, the sensor and reference bodies 1202, 1204 are on one side of a non-conductive substrate 1216 in the input field area (represented by an input field direction 1218), while the output body 1206 is on the other side of the substrate 1216 but in communication with the input bodies 1202, 1204 via respective strong couplings 1212, 1214. An output field direction 1220 may be substantially the same as the input field direction 1218, as shown in Figure 9D; alternatively, the output field may be emitted in the opposite direction (e.g., reflected back through the substrate 1216) or transverse the input field direction 1218 (e.g., along the substrate 1216).

Although the embodiments described above and shown in Figures 1 to 5 and 9A to 9D have generally included only three conductive bodies 102, 104, 106, it will be apparent to those skilled in the art that other embodiments can include more than three conductive bodies. For example, three or more first conductive bodies could be coupled to a second body. In one embodiment, all but one of the three or more first conductive bodies have respective different chemical coatings disposed thereon that selectively bind to respective different chemical or biological target species, as generally described above. In the absence of any binding, equal charge oscillations are established in each of the three or more first conductive bodies as generally described above. When any one or more of the coatings binds to a target species, the second conductive body generates an output signal as described above.

Alternatively, all of the three or more first conductive bodies can be coated, and the device detects an imbalance in the selective binding - this also applies to the two-input-body structures described above, providing that the strengths of the electromagnetic effect of the different binding species are equal or are accounted for by modification of the chemically selecting coatings. This allows the sensors to be used to detect differences in the concentrations of two or more different species in a sample. In other embodiments, at least one of the three bodies 102, 104, 106 includes a plurality of conductive sub-bodies arranged so as to provide charge oscillations and coupling as described herein for one conductive body. Furthermore, the device can include further conductive bodies that affect the inter-body couplings and/or the electromagnetic environments of the main three bodies 102, 104, 106. Furthermore, one or more other bodies can act as additional input bodies for coupling the at least one input signal to the input bodies, i.e., rather than the input signals coupling directly to the input bodies 102, 104. Furthermore, one or more other bodies can act as additional output bodies for coupling the at least one optical output signals from the output body 106.

Although the differential charge oscillation devices have been largely described above in terms of sensing applications, in other embodiments, differential charge oscillation devices can be used for other applications, such as coupling differential signals into a plasmonic circuit, or detecting differences between at least two optical input signals. Additionally, the devices can be operated in the opposite manner to that described above, where the optical input signal is used to generate charge oscillations in the third body 106, and these charge oscillations couple to the first and second bodies 102, 104 to generate further charge oscillations therein, which act as sources of respective optical output signals. This mode of operation of course be extended to other embodiments including more than three conductive bodies.

SIMULATION

In a simulated differential charge oscillation device, the first body 102, the second body

104 and the third body 106 were simulated as three stripes of gold on a glass substrate 108, arranged as shown in Figures 1 to 3, including: a sensor stripe corresponding to one of the first body 102 or the second body 104, with a sensor region 502; a reference stripe corresponding to the other of the input bodies 102, 104; and an output stripe corresponding to the third body 106.. Each stripe was simulated as having approximately the same dimensions: the dimensions are shown in Table 1. The spacing between the output stripe and each input stripe was selected so that any shift in resonance due to inter-body (or "inter-stripe") coupling was small, but still strong enough to produce a significant output signal.

The three stripes were simulated as being made of gold, which has a complex, frequency- dependent permittivity as described in the CRC Handbook of Chemistry and Physics, 87th edition, 2006-2007. The stripes were simulated as formed on a glass substrate (ε_s = 2.31 ) and immersed in liquid water ( ε_b = 1.77 ).

The eigenmodes of charge oscillations in or on the stripes were simulated based on the methods described in the papers by Mayergoyz et ai, relating to the electrostatic approximation (listed above), and using the dimensions shown in Table 1.

Table 1 : Approximate simulated device dimensions and parameters

Width x Lengthy Thickness z

Stripes 43 nm 100 nm 29 nm Dipole sensor 28.6 nm 17.9 nm patch

Centre-centre distance

Input Stripes 257.1 nm radius permittivity

Dipole target r_mol = 0.7 nm ε_mo] = 2.13

A target was simulated using a sphere of radius r_mol , with an electric permittivity ε_mol and a polarisability of

+ 2ε_b ), as described in the book "Absorption and Scattering of Light by Small Particles", by Craig F. Bohren and Donald R. Huffman, Wiley, 1983. A simulated value of permittivity for biological materials was selected to be about ε_mo] ~ 2.13 , as described in S. Johnsen and E. A. Widder: J. Theor. Biol. 199, 181- 198 (1999). The target "molecule" was simulated as being bound to the sensor region 502 and the effect of the polarisation was averaged over the sensor region 502.

As described above, the change in the background permittivity of the first and second bodies 102, 104 can be due to one or more additional electrically polarisable bodies interacting with one of the first or second bodies 102, 104. The target was simulated as a plurality of the additional bodies using point dipoles. In simulation, the point dipoles provide additional terms in the denominator of Equation (1), and change the resonance condition. These additional terms relate to an effective background permittivity that includes the dipole polarisability. The first and second bodies 102, 104 were simulated as being substantially identical with respect to the input light, and having the same resonant mode (and thus the same eigenvalue and dipole moments), thus the amplitude of the third body 106's induced dipole moment,

, was proportional to the imbalance, or a small difference Δ = ε_M - ε_b2 , between the effective background permittivities associated with each body 102, 104, 106, as shown in Equation (3):

al -αj oc -Arl((rl +1>_A2 ² +ε{a>trl

(4)

to a first order in Δ .

The input light wavelength was chosen to be at the resonant frequency of each stripe.

The charge differences (or "charge differential") at the ends of the sensor stripe and the reference stripe adjacent the output stripe were simulated as driving (or "generating") the localised surface plasmon on the output stripe, and the optical radiation output from the output stripe depended on the strength of the output surface plasmon on the output stripe.

Operation of the simulated device was simulated using the coupling theory described in Davis (cited above), which describes relationships between the amplitudes αj , the induced dipole moments and the radiated light, expressed in terms of scattering cross sections C_s ^x _ca , C_s ^y _ca for each polarisation direction x and y. For input light polarised in the ^-direction, the output intensity spectrum of x-polarised light from the plasmonic bridge device was determined by the number N of dipoles distributed over the sensor region 502 (also referred to as the "dipole patch") on the sensor stripe, which caused an effective change in the electric permittivity around the sensor stripe.

As shown in the simulated spectrum in Figure 10 (expressed in terms of a scattering cross section C_sca normalised to the maximum of the output scattering cross section for the input polarisation direction Cζ_ca , with N-O), there was a substantial absence of an output optical signal when the system was balanced (i.e., no dipoles, N=O), and a rapid increase in the output signal as the number N of dipoles increased (i.e., as more dipoles are attached to the surface of the sensor stripe). The output signal C_s ^x _ca varied from about 0.01% of C_s ^y _ca for

N=I to 6% when N= 125. The response of the device 100 to the adsorption of a few molecules of the target to the sensor stripe caused an output signal with an intensity that was related to the phase difference between the charge oscillations in the two input bodies 102, 104.

Also as shown in Figure 10, the wavelength of the peak of the output intensity spectrum of x-polarised light C^* _ca shifted slightly higher for increasing values of N, due to an overlap of the reference stripe resonance and the shifted sensor stripe resonance. This output frequency change was increasingly observable as a frequency split for a high imbalances, such as for TV= 125. This shift in wavelength of the scattering cross section may provide a further measure of the refractive index change — -and thus the number of adhering dipoles — in addition to the shift in amplitude.

As shown in Figure 11 , (which shows normalised scattering cross sections for x and y- polarised light as a function of the number of attached dipoles), there was an increase in the x-polarised component of the scattering cross section of the output stripe with an increase in the number of dipoles; and a gradual decrease in the ^-polarised component of the scattering cross section of the device with an increase in the number of dipoles.

Also as shown in Figure 11 , there was a shift of the centre peak wavelength as a function of the number of dipoles, in particular an increase in the wavelength of the scattered output light due to an increase in the number of dipoles.

The normalised amplitude of the plasmon on the output stripe

varied linearly as a function of the phase difference φ between the oscillations in the sensor and reference stripes, as shown in Figure 12, up to about 30 degrees: the difference between two sinusoidal signals sm{ωt + φ)-sm ωt ∞ φcos ωt is proportional to a phase difference φ for sufficiently small phase differences. The scattering cross section, which is indicative of the optical output signal, depends on αj ² , thus the output signal intensity was a substantially direct measure of the phase difference between the oscillations in the two input stripes.

The typical radius of an organic molecule, such as a protein, is about 2 nm, as described in O. Tcherkasskaya, E. A. Davidson and V. N. Uversky: J. Proteome Research 2, 37-42 (2003). As the polarisation of the molecule is volume dependant, N=22 dipoles in the simulated device are generally equivalent to one protein of 2-nm radius (this corresponds to a molecule dipole radius of 0.71 nm). In terms of the scattering cross section, the optical output signal due to 22 dipoles (simulating one protein) was simulated to be about 1% of the scattering from the entire structure.

Light scattering from a single nanoparticle may be measured by either total internal reflection (TIR) or by dark-field illumination, thus it may be possible to observe a light signal of about 1% of the total scattered light. In the prior art, however, the measurements rely on spectral resolution, as described in C. Sonnichsen, S. Geier, N. E. Hecker, G. von Plessen, J. Feldmann, H. Ditlbacher, B. Lamprecht, J. R. Krenn, F. R. Aussenegg, V. Z. H. Chan, J. P. Spatz and M. Moller (Appl. Phys. Lett., 77, 2949) in 2000 for TIR, and S. Schultz, D. R. Smith, J. J. Mock and D. A. Schultz (Proc. Natl. Acad. Sci. USA, 97, 996) in 2000 for dark-field illumination. In contrast, there is no need to spectrally resolve the optical signal from the differential charge oscillation device, and since it is the signal amplitude that conveys the information, the detector integrates over each spectral peak, thus improving the performance of the system relative to prior art systems that rely on measuring spectral shift. The simulation results described above, e.g., with reference to Figure 10, indicate detection of the adsorption of a single protein molecule on the simulated device.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

CLAIMS:

1. A differential charge oscillation device, including at least three mutually spaced but electromagnetically coupled electrically conductive bodies, the bodies including: a first electrically conductive body for generating first charge oscillations therein; a second electrically conductive body for generating second charge oscillations therein; and a third electrically conductive body electromagnetically coupled to the first conductive body and to the second conductive body such that the first charge oscillations generated in the first conductive body and the second charge oscillations generated in the second conductive body generate third charge oscillations in the third conductive body.

2. The device of claim 1, wherein the first and second oscillations have substantially the same amplitude and frequency, but differ in phase, the third oscillations being representative of the phase difference between the first and second oscillations.

3. The device of claim 1 or 2, wherein the first and second oscillations are coupled to respective and mutually spaced portions of the third conductive body such that the output signal is representative of at least one difference of amplitude, frequency and/or phase between the first and second oscillations.

4. The device of any one of claims 1 to 3, wherein the third body is disposed substantially between the first and second conductive bodies.

5. The device of any one of claims 1 to 4, wherein the device is configured so that, when the first and second oscillations are of substantially equal amplitude, frequency and phase, the third oscillations are substantially not generated.

6. The device of any one of claims 1 to 5, wherein the first, second, and third bodies have nanoscale dimensions.

7. The device of any one of claims 1 to 6, wherein the charge oscillations are plasmon resonances.

8. The device of any one of claims 1 to 7, including at least one source of electromagnetic radiation for generating at least one input signal to generate the first charge oscillations and the second charge oscillations, and the third charge oscillations in the third conductive body generate an output signal.

9. The device of claim 8, including a detector of electromagnetic radiation to measure changes in the output signal to assess changes in at least one of: (i) the charge oscillations in the first body, (ii) the charge oscillations in the second body, (iii) the electromagnetic coupling between the first body and the third body, and (iv) the electromagnetic coupling between the second body and the third body.

10. The device of claim 8 or 9, wherein the at least one input signal and the output signal are optical signals.

11. The device of any one of claims 8 to 10, wherein the at least one input signal is selected to have at least one frequency that matches one or more resonance frequencies of the first body and/or the second body.

12. The device of any one of claims 8 to 11, wherein at least one input signal is selected to have a polarisation matched to a resonance of the charge oscillations in the first and/or second bodies.

13. The device of any one of claims 8 to 12, wherein the third body is configured so that the polarisation of the input signal substantially does not excite any resonances in the charge oscillations in the third body.

14. The device of any one of claims 8 to 13, wherein the first, second, and third bodies are of elongate form, the longitudinal axes of the first and second bodies being substantially aligned with the polarisation of the input signal, and the longitudinal axis of the third body being substantially orthogonal thereto.

15. The device of any one of claims 8 to 14, wherein the charge oscillations in the third body are substantially orthogonal to the charge oscillations in the first and second bodies so that a polarisation of the output signal is substantially orthogonal to a polarisation of the input signal, thereby facilitating discrimination of the output signal from the input signal.

16. The device of any one of claims 8 to 15, wherein the first body constitutes a sensing component configured for exposure to an environment to be sensed, and the second body constitutes a reference component substantially isolated from said environment, such that, during or following exposure of the sensing component to said environment, the output signal is representative of the presence of one or more chemical and/or biological species in said environment.

17. The device of claim 16, wherein at least a portion of the sensing component exposed to said environment includes a chemically and/or biologically selective coating for selective binding to at least one desired target chemical and/or biological species.

18. The device of claim 16 or 17, wherein the first body is one of a plurality of first sensing bodies for generating first charge oscillations therein and configured for exposure to an environment to be sensed, and the second body is one of a plurality of reference bodies for generating second charge oscillations therein and configured to be substantially isolated from said environment; the third body being electromagnetically coupled to the first conductive bodies and to the second conductive bodies such that the first charge oscillations generated in the first conductive bodies and the second charge oscillations generated in the second conductive bodies generate the third charge oscillations in the third conductive body; wherein the first sensing bodies are configured for sensing respective different chemical and/or biological targets such that, during or following exposure to said environment, the third charge oscillations in the third conductive body are representative of the presence of said chemical and/or biological species in said environment.

19. The device of any one of claims 1 to 18, including a balancing component that allows the charge oscillations in the third body to be substantially suppressed under a set of conditions so that the output signal is more directly representative of a change in said conditions.

20. A method for sensing, including: generating first charge oscillations in a first electrically conductive body; generating second charge oscillations in a second electrically conductive body; exposing at least one of said first electrically conductive body and said second electrically conductive body to an environment to be sensed; detecting at least one change in an electromagnetic signal emitted from a third electrically conductive body electromagnetically coupled to the first conductive body and to the second conductive body, said electromagnetic signal being generated by the first charge oscillations generated in the first conductive body and the second charge oscillations generated in the second conductive body, said at least one change in said electromagnetic signal being representative of the presence of at least one chemical and/or biological species in said environment.