WO2009068041A1

WO2009068041A1 - Three-dimensional optical structure

Info

Publication number: WO2009068041A1
Application number: PCT/DK2008/050281
Authority: WO
Inventors: Jörg HÜBNER; Lars Pleth Nielsen
Original assignee: Danmarks Tekniske Universitet; Teknologisk Institut
Priority date: 2007-11-29
Filing date: 2008-11-28
Publication date: 2009-06-04

Abstract

An optical structure (10, 100, 150) for surface-enhanced spectroscopy based on surface plasmons (SP). The structure comprises a plurality of layers (11) with SP-active sites, and the plurality of layers (11) is optically arranged so as to allow direct access for a target analyte (T) into and/or onto the plurality of layers. This optical structure is advantageous for obtaining optical sensitivity that may be increased by essentially going from a two-dimensional (2D) optical structure, as known in the prior art, to a three-dimensional (3D) optical structure.

Description

THREE-DIMENSIONAL OPTICAL STRUCTURE

Field of the invention

The present invention relates to an optical structure suitable for surface-enhanced spectroscopy based on a three-dimensional nanostructure. The invention also relates to an optical system, particularly suitable for high sensitivity detection, and a corresponding method for detection of a target analyte with surface- enhanced spectroscopy based on surface plasmons (SP), e.g. Surface-Enhanced Raman Scattering (SERS).

Background of the invention

Raman scattering is a unique technique for identification of molecular species since the re-emitted Raman photons correspond to a transition between a particular set of vibrational modes. Hence, by measuring the frequency of the emitted Raman photons it is in principle possible to identify the molecule causing the emission. Furthermore, the concentration of the specific molecule can by quantified by counting the number of photons with that specific frequency combined with a proper calibration. Unfortunately, the cross section for the

Raman process is very small and only about one out of every 10¹² incident photon will undergo Raman scattering. Due to this very low cross section it has so far only been possible to apply Raman spectroscopy in situations where there is a large concentration and/or numbers of molecules to be analyzed.

More than 30 years ago it was observed that the Raman signals could be enhanced extensively by placing the molecules in close proximity of a textured metal surface. Clearly these observations have intensified the research activity within the field of Surface-Enhanced Raman Scattering (SERS). As a consequence hereof, a broad variety of possible applications has appeared especially in the scientific literature as well as in numerous patents.

Both in the scientific literature and in several patents it has been well documented that the Raman intensity can be enhanced by joining together molecules and metal surfaces. Roughened metal substrates or na no/micro-structured metal surfaces have been shown to enhance the SERS signals even further.

US 2006/0197953 discloses a metallic nanoparticle-coated thermoplastic film and the process for preparing such a film. The plasmon resonance absorption spectrum of the film may be shifted by stretching or shrinking the coated film, i.e., the position of the absorption may be varied by mechanically influencing the coated film, cf. figures 2-4 for illustration of the effect. It is further contemplated that the optical structure can be multilayered and transparent to obtain an accumulated effect of the absorption. This can be used for instance in connection with a sensor or similar devices. The relevance or application for inelastic scattered light, e.g. SERS or the like, is however not disclosed. The present invention is not related to frequency shift or tunability as suggested by US 2006/0197953.

So far the applications seen are most often associated with gold (Au) and silver (Ag) surfaces. The main reason for this is that the SERS process has been thought to be correlated with surface plasmons (SP), and since the noble metals are known to be very plasmon-active, these metals are indeed good SERS candidates. However, candidates such as Pt, Cu, Al, Na, etc. have also been suggested as possible candidates in connection with SERS active surfaces.

Clearly not only flat surfaces, rough surfaces, and structured surfaces but also nano- and micro-particles of e.g. Au or Ag may by applied as SERS active scattering centers causing enhanced Raman intensities. This is indeed a very promising field, see for example K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, M.

S. FeId, "Surface-enhanced Raman scattering and biophysics", J. Phys. : Condens.

Matter, 2002, 14, R597-R624. When optical excitations are localized to small particles, so called "hot spots" are created. These "hot spots" show extremely intense electromagnetic antennas causing SERS enhancement factors of up to

10¹² - 10¹⁴. This has been predicted both theoretically and observed experimentally.

While an understanding of SERS may not yet be completely mature with respect to predicting optimised SERS-structures, it is generally recognised that upon light excitation with an appropriate light wavelength, the metal surface of a SERS active site provides conduction electrons that are excited so as to collectively oscillate by the incident oscillating electromagnetic radiation. This oscillation of the electrons results in an increased interaction between the metal surface and the electron cloud of an analyte molecule/atom that may give rise to a so-called surface plasmon resonance (SPR). Spectroscopy based on surface plasmons (SP) or a surface plasmon resonance (SPR) may drastically increase the optical response.

US 2006017918 provide an optical structure with dual- and/or multi-layer metal film-over-nanostructure (FON) substrates. These Dual-FON and Multi-FON SERS substrates comprise a rough nano-structured layer and two or more SERS-active metal film layers deposited thereon with a layer of dielectric material between the metal film layers. Thereby, there is provided a way of increasing the intensity of a Raman signal during surface-enhanced Raman spectroscopy using the SERS substrates. Unfortunately, the optical structure is not stable because the surface of the optical structure degrades over time, cf. figure 4 of US2006017918. Moreover, as the target analyte is bonded to the surface of the optical structure and because SERS enhancement only occurs on the active site, this optical structure has a relatively small effective surface area. This substrate also suffers from an inherently random structure and there are problems with reproducibility due to the demonstrated deterioration of the surface.

Hence, an improved optical structure would be advantageous, and in particular a more efficient and/or reliable optical structure would be advantageous.

Summary of the invention

Accordingly, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-mentioned disadvantages singly or in any combination. In particular, it may be seen as an object of the present invention to provide an optical structure that solves the above-mentioned problems of the prior art with surface enhanced spectroscopy based on surface plasmons (SP). This object and several other objects are obtained in a first aspect of the invention by providing an optical structure for surface-enhanced spectroscopy based on surface plasmons (SP), the structure comprising:

- a plurality of layers having surface plasmon (SP) active sites, the layers being at least partly transparent to radiation (R_in, R_out) for use in spectroscopy based on surface plasmons (SP), wherein the plurality of layers is optically arranged with respect to a surface of the optical structure so as to facilitate multiple surface plasmon (SP) enhancement due to inelastic scattering of radiation (R_in) entering the said surface, and wherein the plurality of layers is arranged so as to allow direct access for a target analyte (T) into and/or onto the plurality of layers.

The invention is particularly, but not exclusively, advantageous for obtaining an optical structure wherein the surface plasmon (SP) based optical sensitivity may be increased by essentially going from a two-dimensional (2D) optical structure, as known in prior art, to a three-dimensional (3D) optical structure. If a 2D optical structure enables a sensitivity factor of 10¹², preliminary tests and calculations indicate that sensitivity factors as high as 10¹⁸ may be reached. Furthermore, the sensitivity is expected to be even higher since the present invention enables an optical structure where the target analyte may be distributed on the plurality of layers with SP-active sites, i.e. there may be a build-up in concentration due to the layers functioning as a filtration mechanism.

It is appropriate to point out that the latter feature is quite different from US patent application 2006017918 (mentioned above, using film-over-nanostructure (FON) substrates), and US patent application 2006/0197953 (mentioned above, using a tunable plasmon resonance absorption spectrum). Neither reference disclose that the target analyte may have access to several layers of the optical stacked structure, which accordingly results in a more limited enhancement of the inelastic scattered light as compared to the present invention.

In the context of the present invention, it is to be understood that the term "partly transparent" means that, within a relevant optical region, e.g. ultraviolet (UV), visible (VIS), or infrared (IR), there is a transmission of at least 15%, preferably at least 30%, or more preferably at least 70%. In some embodiment, the transparency may be up to 90% or almost 100%, if the material of the layers is almost completely transparent in the relevant optical interval of wavelengths.

In one embodiment, the plurality of layers may be periodically arranged in a direction (A) towards the surface of the optical structure. Advantageous optical effects, e.g. resonances, may be obtained from this periodicity. As will be explained in more detail below, the periodicity may have several periods. The direction (A) may be substantially orthogonal to the surface of the optical structure so as to provide a simplified geometry of the optical structure itself and/or of the optical system where the optical structure is a component or a part, e.g. as a sensing or enhancing unit, or as a filter unit. The said orthogonality of the direction (A) may be globally for the entire optical structure, or alternatively the said orthogonality of the direction (A) may be locally, possibly several different orthogonal directions may be defined if for instance the optical structure has a non-plane surface, e.g. a curved surface.

The SP-active sites on the plurality of layers may be additionally prepared with dedicated bonding sites suitable for bonding a desired target analyte (T) for spectroscopy based on SP in the optical structure. These sites may include, but are not limited to, mechanical roughening, chemical ligands, biochemical probe- molecules, etc. The biochemical bonding sites may provide bonding that may be taken to include any kind of interaction between corresponding pairs of molecules that exhibit mutual affinity or binding capacity, typically specific or non-specific binding or interaction.

In preferred embodiments, the SP-active sites may be metallic nanoparticles, where the term "metallic" means elemental metal or compounds thereof. The term "nanoparticles" means particles with an average diameter of 5, 10, 15, 20, 25, 30, etc. up to 50 or 100 nm. In a special embodiment, one or more layers of the plurality of layers is a thin metal layer in combination with metallic nanoparticles. The thin metal layer should be sufficiently thin so as so allow at least some radiation to penetrate the layer. In another preferred embodiment, the SP-active sites on one or more layers may be arranged in an ordered array of sites. Some arrays structure may be quadratic web, where the particles are arranged in square-like relation to each other or other similar geometric shapes.

In one embodiment, the plurality of layers may have a porous structure for enhanced bonding of a target analyte (T) in the optical structure. Thus, the layers may effectively function as a filter; preferably with dedicated adsorption sites. Each layer may be described as a porous membrane with penetrating holes for the target analyte (T).

In one embodiment, the plurality of layers may be interconnected and curved to form a roll. This will be termed a so-called "nano-roll"; where the plurality of layers is connected so as to form a single layer. This may performed via stress- induced rolling or by rolling of a sufficiently flexible material, e.g. a polymer. This will be further explained in detail below.

In another embodiment, the layers may be substantially plane layers, the layers being arranged substantially parallel to each other. Possibly, this parallel arrangement is global for the optical structure, or alternatively limited to a local portion of the optical structure. Possibly, distances between layers may be comparable, preferably substantially equal, to an average diameter of the metallic nanoparticles on the different layers. The comparable size of particles and layer distance may be exploited in connection with resonances. Alternatively or additionally, distances between the nanoparticles in the array on a layer are comparable, preferably substantially equal, to an average distance to the two neighboring layers. Furthermore, distances of arrayed particles may be comparable, preferable substantially equal to a multiple of half the wavelength of the radiation (R_in) entering the surface, when said radiation is a substantially monochromatic radiation, e.g. a laser source.

The distances between layers may be below a distance chosen from the group of: 5, 10, 15, 20, 25, 30, 35, 40, or 50 nanometers. It is expected that layer distances is short due to fast decay of the SP-light coupling, i.e. related with the distance in powers of 3 or 6. In such a stacked configuration, there may also be embedded one or more fluid channels within several layers. In particular, the optical structure may comprise a fluid channel arranged for conveying a fluid with a possible target analyte (T) past and/or through the plurality of layers for deposit of the target analyte on the SP- active sites. The fluid may be a gas or a liquid. Preferably, the optical structure may be integrated together with the fluid channel so as to form a probe, preferably for on-site testing. More preferably, the probe may be a disposable probe. In one embodiment, the fluid channel may comprise a portion oriented along the plurality of layers. In another embodiment, the fluid channel may comprise a portion oriented substantially perpendicular to the plurality of layers, when the plurality of layers are substantially parallel to each other.

Preferably, the SP-active sites may comprise Au, Ag, Al, Na, Pt, or Cu. Alloys thereof and mixture/composites of these metals may also be applied. The layers may comprise poly-crystalline Si, single-crystalline Si, SiO₂, Si₃N₄, Al, AI₂O₃, TiO₂, SiON-compounds, or glass (Phosphor- and/or Boron-doped), or doped variations of these materials. Alternatively, the layers may comprise polymers, preferably porous polymers. It may be doped polymer or co-polymers. In one variant, the polymer may have embedded SP active sites.

In another aspect, the present invention relates to an optical system for detection of a target analyte (T) with surface enhanced spectroscopy based on surface plasmons (SP), the system comprising:

- an optical structure according to the first aspect, wherein the optical structure is optically connected to:

-a radiation unit capable of emitting radiation (R_in) suitable for spectroscopy based on surface plasmons (SP) so as to detect the target analyte (T), and/or

-a detector unit arranged for detecting emitted radiation (R_out) from the optical structure so as to detect the target analyte (T).

In particular, the optical structure and/or the radiation unit and/or the detector unit may be arranged for performing the following kind of spectroscopy: Raman, surface-enhanced Raman spectroscopy (SERS), surface-enhanced resonance Raman spectroscopy (SERRS), second harmonic generation (SGH), hyper Raman, or coherent anti-Stokes Raman scattering (CARS). Other kinds of spectroscopy available to the skilled person, now or in the future, may also be applied within the context of the present invention.

In another aspect, the present invention relates to a method for detection a target analyte (T) with surface-enhanced spectroscopy based on surface plasmons (SP), the method comprising: - emitting radiation (R_in) on an optical structure according to the first aspect, the radiation being suitable for surface enhanced spectroscopy based on surface plasmons (SP), and

- detecting emitted radiation (R_out) from the optical structure so as to detect the target analyte (T).

The various aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Brief description of the Figures

The present invention will now be explained, by way of example only, with reference to the accompanying Figures, where

Figure 1 schematically shows an optical system according to the present invention,

Figure 2 schematically shows a plurality of surface plasmon (SP) active layers according to the present invention,

Figure 3 is a schematic cross-sectional view of an optical structure according to the present invention,

Figures 4 and 5 are schematic cross-sectional views of two different embodiments of an optical structure according to the present invention, Figure 6 schematically shows an a cross-sectional view of optical structure having a fluid of target analyte flowing through according to the present invention,

Figure 7 is a step-by-step schematic description of how to construct a so-called hanging membrane embodiment according to the present invention,

Figure 8 is a stacked configuration of the hanging membrane embodiment according to the present invention, and

Figure 9 is a step-by-step schematic description of how to construct a so-called nano-roll embodiment according to the present invention.

Detailed description of embodiments

Figure 1 schematically shows an optical system according to the present invention for detection a target analyte T with surface-enhanced spectroscopy based on surface plasmons (SP). The target analyte T may be in a fluid, i.e. a gas or a liquid of any kind or type.

The system has a radiation unit 20 capable of emitting radiation R_in suitable for spectroscopy based on surface plasmons (SP) so as to detect the target analyte (T). Thus the radiation unit 20 may be arranged for performing the following kind of spectroscopy: Raman, surface enhanced Raman spectroscopy (SERS), surface enhanced resonance Raman spectroscopy (SERRS), second harmonic generation (SGH), hyper Raman, or coherent anti-Stokes Raman scattering (CARS).

The system also has an optical structure 10 for surface-enhanced spectroscopy based on surface plasmons SP. The structure comprises a plurality of layers 11a, lib, and lie, the layers having SP-active sites 12, cf. Figure 2. The layers are at least partly transparent to radiation R_in and R_out for use in spectroscopy based on surface plasmons (SP). The plurality of layers 11a, lib, and lie are optically arranged with respect to a surface 13 of the optical structure 10 so as to facilitate multiple surface plasmon (SP) enhancement due to inelastic scattering of radiation entering the said surface 13. The plurality of layers 11a, lib, and lie are arranged so as to allow direct access for a target analyte T into and/or onto the plurality of layers. This is schematically indicated with the entry 15 in the optical structure 10. This can however be obtained in a variety of different ways as it will be readily appreciated.

As schematically indicated in Figure 1, the optical structure 10 is optically connected to the radiation unit 20, and a detector unit 30 arranged for detecting emitted radiation R_out from the optical structure 10 so as to detect the presence and/or the concentration of the target analyte T on and/or within the layers 11a, lib, and lie. For merely illustrative purpose only three layers are drawn in Figure 1, but any number of layers can be applied within the context of the present invention on the condition that the incoming radiation R_in can penetrate trough a number of layers (not necessarily all the layers 11), and correspondingly that the emitted radiation R_out can penetrate out of the optical structure 10 to facilitate detection of the outgoing radiation R_out.

As indicated in the circular insert (dashed circle) next to R_in, the incoming radiation R_in will typically have a spectrum dominated by a single frequency f_0 with a certain intensity, for example emitted by a laser. Upon entering into the optical structure 10, the majority of the radiation R_in will typically be elastic scattered (no change of energy), and correspondingly there will be a relatively large peak in the spectrum of the outgoing radiation R_out at the same frequency; f_0. However, due to the inelastic scattered radiation there will also be a much smaller but significant peak in the spectrum at another frequency: f_l. For f_l < f_0, the inelastic scattered radiation is conventionally called Stokes lines, and for f_l > f_0, the inelastic scattered radiation is conventionally called anti-Stokes lines. In the right inserted spectrum (dashed circle) only a single Stoke line is indicated, but both Stokes and anti-Stokes lines can of course be utilised in connection with the present invention. It should be mentioned that the inelastic scattered radiation is generally orders of magnitude lower than the elastic scattered radiation, which therefore requires dedicated and highly sensitive detections means. The number of layers 11 in the plurality can be at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20. Alternatively, the number of layers 11 in the plurality can be at least 30, 40, 50, 60, 70, 80, 90 or 100. The detector unit 30 may, in one embodiment, be integrated with the radiation unit 20.

Figure 2 schematically shows a plurality of surface plasmon (SP) active layers 11 forming part of the optical structure 10 for surface enhanced spectroscopy based on surface plasmons SP. The structure comprises a plurality of layers 11 with surface plasmon (SP) active sites 12, i.e. the SP-active sites 12 may comprise Au, Ag, Al, Na, Pt, or Cu, or alloys thereof, or mixture/composites thereof.

The layers 11 are at least partly transparent to radiation R_in and R_out for use in spectroscopy based on surface plasmons (SP), i.e. the layers 11 may comprise poly-crystalline Si, single-crystalline Si, SiO₂, Si₃N₄, Al, AI₂O_{3 7} TiO₂, SiON- compounds, or glass (phosphor and/or Boron doped), or doped variations of these materials. In one embodiment, the layers may comprise polymers, preferably porous polymers. It should be mentioned that SU8 can advantageously be applied, but any doped polymer or co-polymers may be applied. In particular, the polymer can have embedded SP active sites in order to ease manufacturing. In a specific embodiment of the invention the substrate material may be a structured or pre-patterned thin film structured in such a way that the refractive index becomes negative.

The layers 11 can be stacked, i.e. arranged substantially parallel to each other with respect to a normal direction A. More particularly, the layers 11 can be periodically arranged with a distance D. The layers 11 may also be periodically arranged with several inter-layer distances, i.e. Dl, D2, D3, Dl, D2, D3 etc.

It is schematically indicated in Figure 2 how the target analyte T particles are attached to the layers 11. Additionally, the SP-active sites 12 on the plurality of layers 11 can be prepared with dedicated bonding sites suitable for bonding a desired target analyte (T) for spectroscopy based on surface plasmon (SP) in the optical structure 10. Such bonding sites can include, but are not limited to mechanical roughening, appropriate chemical ligands (preferably surface bonded), biochemical probe-molecules (specific or non-specific), etc., near or on the sites. Figure 3 is a schematic cross-sectional view of an optical structure 10 similar to Figure 2, where the plurality of layers 11 is periodically arranged in a direction A towards the surface 13 of the optical structure 10. Like with the embodiment of Figure 2, the distance between the layers 11 can be approximately distance D, possibly several distances, Dl, D2, D3, etc. Preferably, the direction A may be substantially orthogonal to the surface 13 of the optical structure 10. If the surface 13 is rough, the direction A may be approximately orthogonal to an average surface plane of the surface 13.

Figures 4 and 5 are schematic cross-sectional views of two different embodiments of an optical structure 10 corresponding to Figure 3.

The embodiment of Figure 4 differs in that the layers 11 are periodically arranged with respect to a direction A which is not orthogonal to the surface 13, but rather has a certain angle different from 90 degrees with respect to the surface 13.

The embodiment of Figure 5 differs in that the layers 11 are randomly oriented with respect to each other, and with respect to the surface 13. This may, for example, be the case when the manufacturing of the layers 11 in the optical structure 10 is not completely controllable, e.g. if the layers are grown as self- assembled layers, by electrochemical deposition, etc. As will be explained in more detail below in connection with Figure 9, it is also possible that the optical structure 10 may be provided by mechanical bending of microscopic structures.

Figure 6 schematically shows a cross-sectional view of optical structure 10 having a fluid of target analyte T flowing through from entry 15a to exit 15b. Thus, the optical structure 10 may inherently comprise a fluid channel arranged for conveying a fluid with a possible target analyte T past and/or through the plurality of layers 11 for deposit of the target analyte on the surface plasmon (SP) active sites. Thus, the optical structure 10 can be integrated together with the fluid channel so as to form a probe, preferably for on-site testing. Due to the significantly increased sensitivity of the surface-enhanced spectroscopy provided by the present invention, such an optical detection system may provide increased safety and reliability in for example security application for monitoring chemical and/or biological hazardous material. The fluid channel may convey gas (of any pressure, in particular also low pressure gas e.g. near vacuum conditions) or liquid.

Figure 7 is a step-by-step schematic description of how to construct a so-called hanging membrane embodiment according to the present invention. In this embodiment, an optical structure 100a for surface-enhanced spectroscopy based on surface plasmons (SP) comprises a fluid channel 500 formed in a substrate, and a porous layer with SP active sites, the layer being arranged so as to substantially close the fluid channel 500 from above. The porous layer functions as a filter during fluid conduction through the channel 500.

The hanging SP active membrane has the capability to combine e.g. the known Raman gain associated with thin metal films, nano-particles, colloidal particles etc. with the ability to filtrate air or liquid in an ongoing flow. This will allow for an up concentration of the molecular species to be identified by there associated RAMAN fingerprints increasing the sensitivity of the sensor unit even further. This filtration effect and the SP enhancing effect may be combined in an optical structure formed by an ordered array of holes in a hanging Au membrane with build in flow channel(s). Alternatively the SP active material, (ex. Au, Ag, Pt, Al, and Na etc.) may be deposited on a hanging membrane constructed from a suitable material (ex. polymer, Si3N₄, SiO₂, AI2O3, TiO₂ etc.). The air or the liquid fluid may be transported through the hanging membrane by a build in flow channel(s) formed by under-etching the hanging membrane. In a specific embodiment of the invention the substrate material may be a structured or pre-patterned thin film structured in such a way that the refractive index becomes negative. Though only one membrane is shown in the embodiment of Figure 7 described below, there may also be embodiments with a plurality of membrane layers, similar to the embodiment of Figure 8.

The hanging membrane may be manufactured by conventional wafer process technologies as schematically indicated in Figure 7. A vast variety of alternative manufacturing processes are of course available to the skilled person within the context of the present invention. Below, each of the steps A-F is briefly described: Step A: Standard lithography is applied to define the structure of the hanging SERS active membrane on a substrate 110 applying conventional resist 200. The structuring of the membrane (Step A) may involve the use of self assembled nano/micro particles to structure the array of holes in the membrane. Step B: The SP-active metal layer 300 is deposited by e-beam, sputtering or alternatively by electrochemical deposition.

Step C: The resist is dissolved whereby the overlying metal is removed. Step D: A second resist mask is added 400 defining the area where the membrane is to be under-etched during step E. Step E: Etching.

Step F: The second mask is removed.

The etching (Step E) depends critically on the used substrate 110. In case this is formed by SiO₂, it is possible to perform the under etch by a solution of hydrofluoric acid (HF). In case the substrate is Si it is possible to form the under- etch by Potassium hydroxide solution (KOH). Alternatively, designed plasma etches can be driven into regimes where they also can perform isotropic etches capable of making a free hanging membrane. In the case where the substrate is polymer, a plasma-containing oxygen can be optimized to create hanging membranes.

In particular, the optical structure 100a may have one or more photonic band gaps (PBG) known to be associated with periodic air holes in high index waveguides. These waveguides could be made of e.g. Si3l\l₄, SiON or other high index materials. The SP active metal can either be added before or after the holes have been created in the membrane. The SP active membrane may also be formed without an underlying support, i.e. as a free hanging metal membrane full of holes. The holes may have a random periodicity or alternatively they may form a well-ordered array. Instead of holes it would also be possible to add protrusions or SP active nano/micro particles onto a hanging membrane. This will enable a hanging membrane forming a SP active optical structure to be intertwined by air or liquid flow. Coupling of in- and outgoing light or radiation R_in and R_out may be enabled by constructing suitable waveguides therefore. Figure 8 is a stacked configuration of the hanging membrane embodiment according to the present invention. The optical structure based on a single hanging membrane 100a will be two-dimensional although the evanescent field will penetrate a few micrometers to each side of the SP active membrane. A three-dimensional optical structure 100 can be developed by adding 2 or more layers 100b of hanging SP active membranes onto the single layer configuration 100a. The SP enhancement factor of this optical unit composed of n membranes is expected to be at least n times more sensitive, particularly for SERS activity.

The hole in the membrane of the structure 100a may be very tiny (200-300 nm) and may be fabricated by lithographic techniques or by self-assembled monolayer technology (SAM), which has shown to be a feasible production technology for large scale production of nano structures beyond the limits of conventional contact lithography. The nano holes in the membrane will be combined with flow of air or a liquid through the holes enabling a combined particle filtration built into the optical structure 100. Hence, the holes will function as both SP spectroscopy gain centers, e.g. for Raman, forming "hot spots", and at the same time the holes will simultaneously function as a filtration unit causing an up concentration of particles in close proximity of the SP active membrane. This will increase the sensitivity of the SP detector/flow unit beyond that known previously from the literature in this field. The concepts can be developed into two or more stacked membranes 100b as indicated in Figure 8 in order to increase the gain even further.

Figure 9 is a step-by-step schematic description of how to construct a so-called nano-roll embodiment of an optical structure 150. It should be noted how the plurality of layers 11 in the final roll (lower part of Figure 9) is arranged so as to facilitate multiple SP enhancement and that a target analyte (not shown) may access the plurality of layers. Also, the layers 11 may be periodically arranged with respect to a direction A. The optical structure 150 will now be explained with reference to a SERS embodiment, but this nano-roll embodiment may also be applied for other kinds of surface plasmon (SP) based spectroscopy.

The optical structure 150 may be formed from a substrate 120 onto which is added a SERS active material 200. The bilayer structure is subsequently exposed to an anodic etch in an electrolyte by adding a positive bias voltage to the substrate and the negative pole to a working electrode of e.g. Pt. Hereby the substrate 120 is oxidized into material 300 and at the same time the process is forming and array of more or less well-ordered holes. The stress introduced by the anodic etch will cause the bilayer structure to curl into a nano-roll forming a cylindrical three-dimensional optical structure 150 suitable for SERS. It may be necessary to add spacers 400 in order to form open flow pathways into the optical structure 150.

By combining a SERS active material (e.g. Au) and Al deposition with conventional lift-off and/or self-assembled monolayer (SAM), it will be possible to construct an ordered array of holes in an SERS active metal layer supported on an underlying Al layer. Subsequently the Al layer is to be converted into transparent AI₂O₃ filled with holes by anodic etch. As a consequence of the anodic etch; Al will be transformed into AI2O3. The transformation of Al into anodic etched AI2O3 is associated with a lattice growth of a factor of 1.4 causing a strained AI2O3 layer. The stress is of the order of σ = 4χlO³ MPa, which will cause the metal-AI₂O₃ sandwich layer to curl up into a spiral. Thus, it is possible to construct a three- dimensional SERS active nano-spiral with built-in porous AI2O3 substrate thereby forming a unique three-dimensional SERS optical structure 150. By tuning the inlayer stress, it will be possible to make this spiral open enough allowing for inter channel flow of air or liquid whereby a porous SERS sensing unit can be constructed. Alternatively spacers 400 can be introduced by conventional lithography and lift-off processes. The suggested SERS optical structure 150 combines known SERS sensitivity associated with e.g. thin Au film with a porous substrate rolled into a spiral forming a three dimensional roll into which flow of air or liquid can be added preferential from the ends. For further details on how to create such a spiral structure, the reader is referred to Akiyama et al., "Ceramic microtubes self-formed at room temperature that exhibits a large bending stress", J. Appl. Phys., Vol. 88, 7, 4434-4436, 2000, which is hereby incorporated by reference in its entirety. Specifically, the reader may be guided on how to control the stress of the roll by that reference.

The anodic etch of Al into AI2O3 may also be substituted by other materials and combinations hereof. Alternative candidates may be Si or Ti which can be converted into SiC>2 and TiC>2. In an alternative embodiment of the nano-roll it may be formed on a substrate material that might be structured or pre-patterned in such a way that the refractive index becomes negative. Thus, the invention relates in a further aspect 5 to an optical structure comprising a metal part structured in such a way that a negative index of refraction (N) is obtained at optical frequencies, the structure comprising :

- a plurality of layers 11, the layers having a metal part structured in such a way that a negative index of refraction (N) is obtained at optical frequencies, the

10 layers being at least partly transparent to radiation, R_in and R_out cf. Figure 1, wherein the plurality of layers 11 is optically arranged with respect to a surface 13 of the optical structure so as to facilitate multiple interactions between incoming radiation R_in entering the said surface 13 and the plurality of layers and/or the structured metal part, and

15 wherein the plurality of layers 11 is arranged so as to allow direct access for a target analyte (T) into and/or onto the plurality of layers.

Accordingly, the metal part may constitute a so-called meta-material, preferably with a periodic nano-material, cf. Busch et al., Physics Reports, 444, (2007), 101-

20 202, which is hereby incorporated by reference in its entirety. In particular, various shapes of meta-materials like the ones presented in Figure 61- 62 of that reference may advantageously be implemented in connection with the present invention. The plurality of layers 11 may be interconnected and curved to form a roll, a so-called nano-roll. The rolling process may be controlled so that the

25 periodicity of the optical structure extends across the plurality of layers. The structure may have a cylindrical symmetry.

The metal part of the optical structure is structured or dimensioned in a way so as to obtain negative refractive index (N); N < 0, at optical frequencies, i.e. at light 30 with frequencies of 10-1500 THz (T for tera; 10¹²), 1-2000 THz, or 0.5-3000 THz. The structured metal part may be characterized by a negative dielectric constant

(ε < 0) and/or a negative permeability (μ < 0).

The form and shape of the nano-roll can be varied by careful preparation and/or 35 tailored growth of the layers to be rolled. It is contemplated that various shapes of the roll may be implemented. Thus, the roll may be shaped as a funnel, a cone, a roll with varying thickness along the length, etc. The roll may also have sharp edges, i.e. having a triangular cross-section, a square-shaped cross-section, a pentagon-shaped cross-section, etc.

EXAMPLES:

The present invention also relates to the following examples:

1. An optical structure (100a) for surface-enhanced spectroscopy based on surface plasmons (SP), the structure comprising:

- a fluid channel (500) formed in a substrate,

- a porous layer with SP (112) active sites, the layer being arranged so as substantially close the fluid channel from above,

wherein the porous layer functions as a filter during fluid conduction through the channel.

2. An optical structure comprising a metal part structured in such a way that a negative index of refraction (N) is obtained at optical frequencies, the structure comprising :

- a plurality of layers (11), the layers having a metal part structured in such a way that a negative index of refraction (N) is obtained at optical frequencies, the layers being at least partly transparent to radiation (R_in, R_out),

wherein the plurality of layers (11) is optically arranged with respect to a surface (13) of the optical structure so as to facilitate multiple interactions between incoming radiation (R_in) entering the said surface (13) and the plurality of layers and/or the structured metal part, and wherein the plurality of layers (11) is arranged so as to allow direct access for a target analyte (T) into and/or onto the plurality of layers. 3. The structure according to example 2, wherein the plurality of layers (11) are interconnected and curved to form a roll.

4. The structure according to example 3, where the rolling process can be controlled so the periodicity of the structure extends across the plurality of layers.

5. The structure according to any of examples 3-4, wherein the structure has a cylindrical symmetry.

6. The structure according to any of examples 2-5, wherein the plurality of layers (11) comprise poly-crystalline Si, single-crystalline Si, SiO₂, Si₃N₄, Al, AI₂O₃₇ TiO₂, SiON-compounds, or glass (phosphor- and/or boron- doped), or doped variations of these materials.

The above examples may be combined with the any of the above embodiments.

Although the present invention has been described in connection with the specified embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term "comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc. do not preclude a plurality. Furthermore, reference signs in the claims shall not be construed as limiting the scope.

Claims

1. An optical structure (10, 100, 150) for surface-enhanced spectroscopy based on surface plasmons (SP), the structure comprising:

- a plurality of layers (11) having SP-active sites (12), the layers being at least partly transparent to radiation (R_in, R_out) for use in spectroscopy based on surface plasmons (SP),

wherein the plurality of layers (11) is optically arranged with respect to a surface (13) of the optical structure (10, 100, 150) so as to facilitate multiple surface plasmon (SP) enhancement due to inelastic scattering of radiation (R_in) entering the said surface (13), and

wherein the plurality of layers (11) is arranged so as to allow direct access for a target analyte (T) into and/or onto the plurality of layers.

2. The structure according to claim 1, wherein the plurality of layers is periodically arranged in a direction (A) towards the surface (13) of the optical structure (10).

3. The structure according to claim 2, wherein the direction (A) is substantially orthogonal to the surface (13) of the optical structure (10).

4. The structure according to claim 1, wherein the SP-active sites (12) on the plurality of layers (11) are additionally prepared with dedicated bonding sites suitable for bonding a desired target analyte (T) for spectroscopy based on SP in the optical structure (10).

5. The structure according to claim 1, wherein the SP-active sites are metallic nanoparticles.

6. The structure according to claim 1 or 5, wherein the SP-active sites on one or more layers is arranged in an ordered array of sites.

7. The structure according to any of claims 1-6, wherein the plurality of layers (11) has a porous structure for enhanced bonding of a target analyte (T) in the optical structure (10).

8. The structure according to claim 1, wherein the plurality of layers (11) is interconnected and curved to form a roll.

9. The structure according to claim 1, wherein the layers (11) are substantially plane layers, the layers (11) being arranged substantially parallel to each other.

10. The structure according to claims 5 and 9, wherein distances between layers are comparable, preferably substantially equal, to an average diameter of the metallic nanoparticles on the different layers.

11. The structure according to claims 6 and 9, wherein distances between the nanoparticles in the array on a layer are comparable, preferably substantially equal, to an average distance to the two neighboring layers.

12. The structure according to claims 6 or 9, wherein distances of arrayed particles is comparable, preferable substantially equal to a multiple of half the wavelength of the radiation (R_in) entering the surface, when said radiation is a substantially monochromatic radiation.

13. The structure according to claim 9, wherein the distances between layers (11) are below a distance chosen from the group of: 5, 10, 15, 20, 25, 30, 35, 40, or

50 nanometers.

14. The structure according to claim 1 or claim 9, wherein the optical structure (10) comprises a fluid channel (15, 500) arranged for conveying a fluid with a possible target analyte (T) past and/or through the plurality of layers (11) for deposit of the target analyte on the SP-active sites (12).

15. The structure according to claim 14 in dependency of claim 9, wherein the fluid channel comprises a portion oriented along the plurality of layers.

16. The structure according to claim 14 in dependency of claim 9, wherein the fluid channel comprises a portion oriented substantially perpendicular to the plurality of layers.

17. The structure according to claim 14, wherein the optical structure is integrated together with the fluid channel (15, 500) so as to form a probe, preferably for on- site testing.

18. The structure according to claims 1 or 5, wherein the SP-active sites (12) comprise Au, Ag, Al, Na, Pt, or Cu.

19. The structure according to claim 1, wherein the layers (11) comprise poly- crystalline Si, single-crystalline Si, SiO₂, Si₃N₄, Al, AI₂O₃, TiO₂, SiON-compounds, or glass (phosphor- and/or boron-doped), or doped variations of these materials.

20. The structure according to claim 1, wherein the layers (11) comprise polymers, preferably porous polymers.

21. An optical system for detection of a target analyte (T) with surface-enhanced spectroscopy based on surface plasmons (SP), the system comprising:

- an optical structure (10, 100, 150) according to claim 1,

wherein the optical structure is optically connected to:

-a radiation unit (20) capable of emitting radiation (R_in) suitable for spectroscopy based on surface plasmons (SP) so as to detect the target analyte (T), and/or

-a detector unit (30) arranged for detecting emitted inelastic scattered radiation (R_out) from the optical structure (10) so as to detect the target analyte(T).

22. The optical system according to claim 21, wherein the optical structure (10, 100, 150) and/or the radiation unit (20) and/or the detector unit (30) are arranged for performing the following kind of spectroscopy: Raman, surface- enhanced Raman spectroscopy (SERS), surface-enhanced resonance Raman spectroscopy (SERRS), second harmonic generation (SGH), hyper Raman, or coherent anti-Stokes Raman scattering (CARS).

23. A method for detection a target analyte (T) with surface enhanced spectroscopy based on surface plasmons (SP), the method comprising :

- emitting radiation (R_in) on an optical structure according to claim 1, the radiation being suitable for surface enhanced spectroscopy based on surface plasmons (SP), and - detecting emitted inelastic scattered radiation (R_out) from the optical structure (10) so as to detect the target analyte (T).