Field of Invention

This invention relates to security devices which use the optical properties of certain metamaterials to produce optical effects. Such security devices find utility in, for example, security articles and security documents. The invention also extends to methods of forming such security devices, security articles and security documents.

Background to the Invention

Security devices are often used on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, in order to confirm their authenticity. Such documents may be referred to generally as "security documents". Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, Venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed. There exists an ever-present need to continue to develop novel security devices in order to combat the activities of counterfeiters and other parties wishing to defeat the security provided by existing devices. This has led to the development of security devices using new technologies such as photonic crystal materials together with the improvement of existing techniques (for example in terms of spatial resolution) as well as new combinations of existing technologies utilised in particular security devices. The increasing complexity of such devices has a number of associated problems. One particular problem is the physical thickness of the security device which is typically the result of the superposition of a number of different layers of materials. Optical" security features are particularly problematical in this regard. These are features which produce optical effects based upon the physics of refraction. Examples of such features include microlenses and prismatic surfaces. In each case these features may be tens of micrometres in thickness which represents a large fraction of the overall thicknesses of some types of security document or security article. Where there are practical upper limits set upon the physical thickness dimension of a security device, security article or security document, the use of optical features is understood to be necessarily limited, particularly to the extent that these can be combined directly with any overlying or underlying security devices of different types, so as to form a hybrid security device. It is in this context that the present invention has been conceived in order to provide new practical possibilities in terms of the security device applications within which optical security features can be used.

Summary of the Invention

In accordance with a first aspect of the invention there is provided a security device comprising a metastructured optical element, the metastructured optical element being formed from a support structure and a predefined array of discrete phase- shifting optical waveguide structures that are connected to or located within the support structure, wherein the predefined array is arranged to interact with an incident beam of light having a target wavelength in free space, wherein each phase-shifting optical waveguide structure has a predefined geometry for causing a respective phase-shift in the incident beam of light, said predefined geometry being such that at least one maximum dimension of the respective phase-shifting optical waveguide structure in a cross-array direction is less than the said target wavelength, whereby the metastructured optical element is transmissive to the incident beam of light and the combination of the phase-shifts in the light resulting from the interaction with the phase-shifting optical waveguide structure causes the focusing of the light. We have realised that recent developments in condensed matter physics and materials science enable a new type of security device to be formed using a particular class of metamaterials which we refer here to "metastructured optical elements". Conventional optical elements such as lenses rely on the chemical properties of the materials from which they are formed, particularly in relation to the way in which light propagates through the materials, in order to create desired optical effects, such as focusing of the light. Our reference herein to the term "optical" should be understood to mean light having wavelengths within the wavelength range including only the infra-red, human visible and ultraviolet parts of the electromagnetic spectrum. The metastructured optical elements discussed herein influence the propagation of light according to entirely different physics. In contrast to conventional optical elements, the metastructured optical elements influence the propagation of light principally due to their structure at the sub- wavelength scale (with respect to the wavelength of incident light in question). It is therefore the physical structure (rather than the chemical structure) of the metastructured optical elements, in terms of the geometries of the component objects and their relative spacings, which governs the propagation of light. As such, for the sake of clarity, the term "waveguide" is not being used here to imply that the material confines the light over an extended length scale compared to the wavelength. Rather, the waveguide aspect of the phase-shifting optical waveguide structures is to act as an entity to support the propagation of the light whilst also modifying its phase in a controlled manner.

Whilst the materials from which these metastructured optical elements are formed are the subject of certain boundary conditions, the materials which meet these boundary conditions provide only a minor role in the optical effect provided by the structure.

The metastructured optical elements influence the propagation of light passing through them. More specifically the phase-shifting optical waveguides each produce a phase shifting effect upon the respective part of the light beam which is incident upon them. By careful design of the individual phase shifts produced by the waveguide structures on the respective parts of the incident beam of light, the resultant superposed phase-shifts can be designed to cause the beam to be focused. This enables a lensing effect to be produced using a much thinner structure than that which would be required to produce the equivalent spherical lens. Furthermore, the nature of the underlying physics involving photonic modes of propagation in the waveguides means that novel optical effects may be produced which are not physically possible using conventional lenses. We have realised that the present technical field of security devices is one such field in which the differences between optical elements and those based upon metamaterials provides unexpected advantages. We have realised that such structures can be used as an alternative to or an addition to the optical structures discussed in many security device applications for security articles or security documents. Such optical structures include lenses which rely upon the refraction of light and also diffractive optical elements (including Fresnel lenses) which rely upon the diffraction of light to cause a focusing effect. The principal benefit of using metamaterials in this regard is the much reduced "thickness" of the resultant structure. Having made such a realisation there remains a challenge in developing the fabrication techniques which are capable of producing the required waveguide structures with dimensions that are generally less than that of the wavelength of the respective light, and therefore generally less than 1 micrometre in each dimension for example.

In order to minimise the thickness of the metastructured optical elements, and also to ease the manner in which they are fabricated, preferably the predefined array of discrete phase-shifting optical waveguide structures is formed in a generally planar geometry. The support structure may likewise take such a generally planar form. However, other geometries for one or each of the array and support structure, other than a planar geometry, are also envisaged, such as an arrangement on a non- planar three dimensional surface. This may be useful in particular applications such as when the metastructured optical elements are to conform with the geometry of a particular surface such as part of a cylinder, sphere or other shape typically used as the surface of a lens or other optical component. The phase-shifting optical waveguide structures may cause the focusing of the light by converging or concentrating the light into one or more focal regions. Such regions may exhibit an intensity of light which is at least two times, more preferably at least three times that of the background intensity. Such focal regions may be discrete or connected and may take the general form of an optical caustic. The focal regions may be arranged as one or more instances of geometrical patterns, including spots, lines, or other shapes. The focal regions may also be arranged as indicia, text or images.

We do not offer a detailed discussion of the related physics here since it is available elsewhere. However, example papers providing some further details in this regard are: "Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging", Khorasaninejad et al, Science, 3 June 2016, Vol. 352, Issue 6290, pages 1 190-1 195; "Achromatic Metalens over 60nm Bandwidth in the Visible and Metalens with Reverse Chromatic Dispersion", Khorasaninejad et al, Nano Letters, American Chemical Society; "Holey-Metal Lenses: Sieving Single Modes with Proper Phases"; Ishii et al, Nano Letters, 17 December 2013, Vol.13, p159-164.

Typically the predefined geometry of the array is such that the maximum dimensions of the respective waveguide structures in the cross-array direction are less than the target wavelength in free space of the light in question, preferably less than half of that target wavelength. The cross-array direction dimensions can be thought of as the two dimensional axes that lie within a plane which passes through the waveguide structures. Preferably the spacing between the centres of adjacent phase-shifting waveguide structures in the predefined array is less than the target wavelength in free space of the incident light, more preferably less than half the said target wavelength. It further preferred that the geometry of the waveguide structures is such that the dimension of each waveguide structure in a direction normal to the plane of the array is less than the target wavelength in free space of the incident light, preferably less than half the said target wavelength. Each of these dimensions is important and has a magnitude within the range stated owing to the influence of the waveguide on the photonic wave propagation in the waveguide, specifically in terms of the phase-shift that results in such small dimension waveguides. As will be understood, typically the waveguide structures have sub-wavelength dimensions in all three dimensions and this allows the thickness of the waveguide structures to be less than two micrometres and, more preferably, even less than 1 micrometre. This in turn enables the thickness of the security device as a whole (which includes the metastructured optical element) to be less than 20 micrometres, preferably less than 10 micrometres, more preferably less than 5 micrometres, more preferably less than 2 micrometres and even more preferably less than 1 micrometre. In practice the waveguide structures are supported by a support structure. In many cases this provides a connecting member to hold the waveguide structures in relative position with respect to each other. However, in addition it may define the waveguide structures themselves when the waveguides are provided as holes within the support structure.

The phase-shifting optical waveguide structures are typically formed from a dielectric material. Such a material preferably has a high refractive index to maximise the waveguiding function of the structure. Preferably such a material is optically transparent on a macroscopic scale, such that for example light may readily propagate through the material for distances of the order of millimetres or metres. However, the very small dimensions of the waveguide structures means that materials which on a macroscopic level are opaque can nevertheless support a photonic waveguided mode of propagation, these including metals (such as Au, Al, Ni, etc). Thus the phase-shifting optical waveguides may be formed from semiconducting materials or metals.

It is preferred that the support structure exhibits a different refractive index from that of the waveguide structures. The phase-shifting optical waveguide structures act together to cause the focusing of light. In practice the difference between the refractive index of the phase-shifting optical waveguide structures and the surrounding medium (which may be air or in some circumstances the support structure) should be sufficient to promote the waveguiding modes needed in the phase-shifting optical waveguide structures so as to focus the light rather than merely perturb its path. A refractive index difference of 0.6 or more, preferably 0.7 or more, more preferably 1 .0 or more, may be needed in most practical light focusing applications for security devices. It is preferred that the materials used to produce the phase-shifting optical waveguide structures exhibit high refractive indices such as 1 .6 or more, preferably 1 .7 or more and more preferably 2.0 or more. Example inorganic dielectric materials for use as the medium of the waveguide structures are selected from the group comprising: Ti0 ₂ , ZnO, GaP, SiN, BaTi0 ₃ , MgTi0 ₃ , Zr, Mica, Al ₂ 0 ₃ , MgO, BN, BeO, Si0 ₂ , metal oxide doped polymers. Where the waveguides are provided as holes or voids within the support structure then these could be filled with other materials such as those mentioned above. The materials may be crystalline or amorphous. Certain polymers may also be used as the dielectric structures. Examples of these polymers include the class of polymers known in the art as high refractive index polymers. Examples of such polymers include various brominated aryl polymers, sulfonated polymers with aromatic groups in the back bone (sulphur containing polyimide) and poly(phosphonate). They also assist in enhancing the waveguiding properties of such holes when such materials are used to fill the holes.

The support structure may be formed from one of a number of different materials including polymer materials which are convenient for processing, certain dielectric materials which may be formed using similar processing apparatus to the waveguide structures, or metallic materials, preferably with a difference in refractive index between the support structure material and that used to form the waveguide structures.

The support structure is in many cases formed from a single type of material and is preferably a unitary object. However, multi-layer support structures are contemplated. Furthermore an additional substrate may be provided to which one or more of the phase-shifting optical waveguide structures and the support structure may be attached.

The optical wavelength range for which these metastructured optical elements provide advantage includes the (human) visible and non-visible wavelengths. The human visible wavelength range is from around 390 nanometres to around 700 nanometres. The non-visible wavelengths where the present structures also provide particular benefit include part of the ultra-violet wavelength band from 250 nm to 390 nm and part of the infra-red wavelength band from 700 to 1000 nm.

The metastructured optical elements are designed in relation to a target wavelength. This can be thought of as the wavelength of light for which they are "tuned" and therefore it is for light at this wavelength that the structures provide the most significant effect. This does not mean that no such effect will occur for other wavelengths, merely that the effect in question is not as strong. The metastructured optical elements may be designed for broader band functionality at the expense of some optical efficiency. Thus, the security devices may therefore be used with broadband, narrowband or monochromatic light, provided each includes the target wavelength (or wavelengths in the event that sets of structures tuned to different wavelengths are employed). The waveguide structures may take a number of different geometrical forms. Typically they are geometrically prismatic, preferably such that they have opposed upper and lower surfaces arranged on a common axis normal to the surfaces and have a constant geometrical cross section along the axis between these surfaces. The structures may therefore take the form of columns with regular cross sections as circles, or rectangles including squares. In many cases the waveguide structures are conveniently arranged as rectangular prisms, particularly right rectangular prisms (using the geometrical definition thereof). They may also take the form of rods. It is to be expected as a result of the array design that the waveguide structures may take different geometries at different positions in the array, according to the underlying design of the device. If possible according to the underlying physics, the geometry of each phase-shifting optical waveguide structure is preferred to be substantially the same (although their orientation may be different). The waveguide structure may be provided as sets of one or more identically oriented structures, and wherein the sets are related to each other by a rotational transformation by a predetermined angle of rotation about the plane normal of the support structure. For example, such an arrangement of right rectangular prisms can be used to effect a spherical lens in that light passing through the metastructured optical element is focused to a focal point in space.

As will be understood the geometries of the waveguide structures are defined within the design parameters. In addition the relative orientation of the structures is also preferably controlled, particularly in terms of rotation about axes normal to the substrate. The structures can be thought to act jointly upon a wavefront and for this reason the spacing between the waveguide structures is an important parameter. In some examples this spacing is set as a constant.

The security devices are preferably provided in a manner such that they can be treated as unitary items for use in a variety of different applications. They therefore may be incorporated into other security articles such as security threads (for further use in security applications, including but not limited to security documents) or directly into security documents themselves. In order to protect the fine structures of the device a cover layer may be applied to protect the device from damage. This should of course be transmissive to light at the target wavelength. A polymeric film material can be advantageously used for this purpose. It will be appreciated that the waveguide structures discussed herein are typically "sub-micrometre scale" meaning generally that at least one of their dimensions is less than one micrometre. In order to provide an observable effect to the naked eye of a human observer the lateral dimensions of the metastructured optical elements may need to extend over a distance of a number of millimetres and typically in two dimensions. This may be achieved by the production of a large single array pattern of structures. However, in practice it may be preferred to provide multiple repeated instances of a smaller dimensioned pattern upon the substrate, for example following the principles of repeated unit cells with or without small gap regions therebetween. Each metastructured optical element may be arranged to focus light towards a respective region.

The metastructured optical elements may be arranged to provide an optical effect which is equivalent, at least in terms of propagation direction, to known optical elements which use refractive effects. Thus, for example, the metastructured optical elements may be arranged as an array of parallel cylindrical lenses. The metastructured optical elements may have a different effect upon the polarisation of the light than their "refractive" counterpart. In the event that it is important to the particular application in question, a coupling device may be provided on the support structure or the substrate (if present) so as to provide phase compensation and improved wave coupling.

One advantage of the use of the metastructured optical elements is the ability to use these with conventional optical elements. Thus the security device may comprise at least one further (conventional) optical element, which interacts with the light by virtue of a refractive or diffractive effect (such as in a classical rather than metamaterial sense). Various example applications are envisaged for such a hybrid system. In many such cases the at least one further optical element is a microlens which is in optical registration with the metastructured optical element. The metastructured and at least one further optical elements therefore act together on the light beam so as to produce a desired effect. Typically this is achieved sequentially, mostly by the use of a common optical axis. In some examples the metastructured optical element is arranged to correct a chromatic aberration caused by the at least one further optical element. Other effects are also envisaged such as where the metastructured optical element and at least one further optical element are arranged to act as a retroreflector. The multiple instances of the metastructured optical element and at least one further optical element may be provided as an array, each "instance" being designed to reflect a respective narrowband wavelength such that the multiple instances of the metastructured optical element and at least one further optical element act together to produce a colour image.

When used in combination with a classical optical element the metastructured optical element may be arranged to conform to the surface of the further optical element (which may be a planar surface). Such conformance may not necessarily include specific contact with the further optical element and may instead be achieved using a spacer to ensure that a constant finite distance is provided between the two elements at all points.

One particularly important concept is effected when the metastructured optical element is provided as a first metastructured optical element and the device comprises at least one further metastructured optical elements, wherein the first and at least one further metastructured optical elements are arranged to act upon a common light beam. The combination of two or more metastructured optical elements is particularly important in security devices due to the very stringent dimensional requirements placed upon such security devices (most particularly their thickness). The use of multiple elements enables entirely new optical effects to be produced which could not be produced in practice with classical optical elements. For example such elements allow the formation of a high numerical aperture doublet lens with aberration correction across a wide angle of view or enable wavelength specific focusing. In some cases a spacer layer may be located between the first and at least one of the further metastructured optical elements. Such a spacer layer may provide practical advantages in terms of ease of fabrication of the device or allow additional degrees of freedom optically in handling effects such as aberration. A spacer can be useful in the manufacturing process, for example in providing a planar base surface for the second lens forming process. An additional substrate is typically provided for use with devices having multiple metastructured optical elements. At least one of the first and at least one further metastructured optical elements may be positioned between the spacer layer and the substrate for example. Whilst the discussion herein is generally in relation to devices which operate upon transmitted light, it should be understood that "reflective" equivalents may be readily produced as desired by the use of optically reflective layers (either specularly or diffusely reflective) to reflect light received back through the assembly. In such cases an effective doubling of the phase shifts occurs.

The invention extends to security articles comprising such security devices, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label or patch. Such articles may be provided with a plurality of such security devices upon different surfaces thereof (such as opposed surfaces, for example to effect a two-sided lensing security thread). Thus a first set of metastructured optical elements may be provided upon a first face of an article substrate and a second set of metastructured optical elements is provided upon a second face of the article substrate, these faces typically being opposite and generally parallel. Furthermore the invention extends to security documents, wherein the security document comprises the security devices or security articles discussed herein. The security document is preferably a banknote, cheque, passport, identity card, driver's licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity. The security document may in particular be formed from a polymer or paper substrate. Two-sided arrangements in this context are also contemplated such that a first set of metastructured optical elements is provided upon a first face of the document substrate and wherein a second set of metastructured optical elements is provided upon a second face of the document substrate. In accordance with a second aspect of the invention there is provided a method of forming a security device comprising:

obtaining an array design for defining the material, shape and relative position of a metastructured optical element of a security device according to the first aspect; and forming the security device according to the first aspect comprising the metastructured optical element according to the obtained array design.

A number of different techniques may be used to form the metastructured optical elements, each being a technique capable of forming sub-micrometre scale structures. For example this may be performed by the deposition of the dielectric material using a resist material. This enables the use of techniques based upon atomic layer deposition (ALD) and chemical vapour deposition (CVD) to deposit the material within shaped apertures in the resist. Such shapes may be formed by photolithography or electron beam lithography methods. For example the forming of the metastructured material typically comprises:

a. Depositing the resist material on the substrate;

b. Patterning the resist material using a power beam;

c. Developing the resist;

d. Depositing the dielectric material into regions of the resist resulting from the developing; and,

e. Removing one or each of any surplus resist material or any surplus dielectric material.

The use of a power beam for the patterning is particularly preferred where the power beam is an electron beam. Other power beams are contemplated including laser beams provided these have a sufficiently small wavelength to produce the structures required. The resist used is dependent upon the type of power beam to be used to provide the patterning. Similarly, different methods for developing the resist are available in the art. Suitable e-beam developers can be obtained from ALLRESIST GmbH. A helpful discussion in this regard can be found at: http://www.ayresist.com/faq-ebeamresists-deveioper/. The AR-600 series from ALLRESIST GmbH are suitable in the present case, with the specific choice depending upon the speed and the material used. The method may be used to deposit the dielectric material onto substrates which include dielectric materials such as silica (Si0 ₂ ) or titania (Ti0 ₂ ) as well as polymeric substrates including PMMA. In the latter case there is advantage in that the polymer may be provided as a film which enables the security device to be handled more readily by automated processes and to conform with the surface of a security article or document to which it is adhered.

In another example the phase-shifting waveguide structures of the metastructured optical element are produced by forming holes within the support structure. The holes may be provided as either unfilled holes or are provided as holes filled with a dielectric material.

In yet another example, the process of forming the metastructured optical elements comprises:

i) Forming a first master structure with a surface texture representative of the metastructured optical elements desired to be formed;

ii) Replicating the first master structure as one or more second master structures;

iii) Mounting the second master structures to a stamping device; and

iv) Using the stamping device to produce the predefined array of dielectric structures in a target substrate of the security device.

The second master structures are therefore used repeatedly in a production process. As an alternative a number of first master structures could be produced by a process exhibiting high reproducibility (so as to provide identical structures) and these could then be used directly to generate the metastructured optical elements of the security device (thereby dispensing with the need for the second master structures). In either case the target substrate is preferably formed from a curable polymer. Such a polymer material is preferably one with a high refractive index (at least when cured) and has a low viscosity in the uncured state so as to infiltrate the structures of the master with which is it brought into contact.

A cover layer may be then provided over the metastructured optical elements, regardless how these are formed, this being typically by use of a polymer film. Thus the security devices may be provided with such a cover layer, prior to incorporation into a security article or document, whereas in some cases it may be preferred to provide the cover layer after the incorporation of the security device, as might be the case when other security devices are to be provided with the protective cover layer at the same time.

The methods may be provided as continuous processes (for example allowing the production of numerous batches of security devices within a production run). Reel- to-reel film processing techniques may be used to achieve this and these lend themselves particularly to industrial scale production.

The inventive method extends to a method of forming a security document comprising, forming a security device as discussed; and, attaching the security device to the security document. The attachment is generally performed by the use of an adhesive. For example, when the substrate of the security device is a polymer film, the method further comprises adhering the polymer film to the surface of the security document. In alternative approaches, localised heating of a polymer material may be used to provide the adhesive effect, or a cover layer may be used to provide such a function. The physical bringing together of the security device and a security article or document may be achieved by various automated processes, one of which that is particularly versatile is the use of a pick and place system.

Prior to the forming of the metastructured optical element there is a need to provide the design for the pre-defined array. The step of obtaining the array design is best performed by computer-implemented modelling of the interaction of an electromagnetic wavefield, representing an incident beam of light, with the metastructured optical elements.

Brief Description of the Drawings

Some embodiments of security devices according to the invention are now described with reference to the accompanying drawings, in which:

Figures 1 to 9 show process steps in a pick and place method of a security device according to a first embodiment;

Figure 10 is a flow diagram of the steps according to the first embodiment;

Figures 1 1 to 15 show process steps in a direct formation polymer method of a security device according to a second embodiment; Figure 16 is a flow diagram of the steps according to the second embodiment;

Figures 17 to 23 show process steps in an imprint and copy formation method of a security device according to a third embodiment;

Figure 24 is a flow diagram of the steps according to the third embodiment;

Figure 25 is a perspective view of a lenticular device according to a fourth embodiment;

Figure 26 shows a schematic cross section of the lenticular device of the fourth embodiment;

Figure 27 shows a more representative cross section of the lenticular device of the fourth embodiment;

Figure 28 shows the view of the lenticular device from a first direction;

Figure 29 shows the view of the lenticular device from a second direction;

Figure 30 shows a view of a moire magnifier device according to a fifth embodiment;

Figure 31 shows a magnified view according to the fifth embodiment;

Figure 32 shows a view of a hybrid moire magnifier device according to a sixth embodiment;

Figure 33 shows a magnified view according to the sixth embodiment;

Figure 34 shows a cross-section through a two sided security article according to a seventh embodiment;

Figure 35 shows a perspective view of (upper) unfocused light and (lower) an eighth embodiment using a projection image;

Figure 36 shows the effect of chromatic aberration when using a conventional refractive microlens;

Figure 37 shows a ninth embodiment in which a planar metastructured optical element is used to correct chromatic aberration;

Figure 38 shows a tenth embodiment in which a spherical lens shaped metastructured optical element is used to correct chromatic aberration;

Figure 39 shows the principle of operation of a retroreflector;

Figure 40 shows an eleventh embodiment using a metastructured optical element and hemispherical ball lens in combination as a retroreflector;

Figure 41 shows an twelfth embodiment based upon the eleventh embodiment and using narrowband metastructured optical elements; and

Figure 42 shows an assembly according to a thirteenth embodiment using two metastructured optical elements separated by a spacer layer. Description of Embodiments

Security devices within the scope of the invention may exhibit a range of different optical effects depending in particular upon the type of metastructured optical element in the device, together with other component parts which contribute to the optical effects. Fundamental to these is the existence of a number of precision manufactured and located phase-shifting waveguide structures which typically have relatively simple three-dimensional geometries and are of "sub-wavelength dimensions" such that their physical structure, as well as their material properties, influence the propagation of light.

We firstly describe a number of possible fabrication techniques, the selection of which may be dependent in part upon the desired array of phase-shifting waveguide structures to be produced, but also practical considerations such as throughput, unit cost, longevity in use, environmental impact and specific application. In the description now given we only include the most important steps. Other practical steps, including cleaning/washing processes for example are present but not described since these are well within the skills of those in the art.

Pick and Place Technique

With this technique the device is formed separately from the security document in which it is to be used. The device is then positioned and secured within or upon the security document surface so that its optical effect may be used within the security document. In the present case the security document takes the form of an identity card with a window in which the device is placed and secured. The fundamental steps of this process are shown in Figures 1 to 9 and the accompanying flow diagram of Figure 10.

At step 1001 a target array structure is generated, this including information concerning the material(s), geometry(ies) and relative locations of the sub- wavelength structures. This step may involve a number of different possibilities, including computer modelling of the resultant optical effect caused by the interaction of light (having a given wavelength) with the phase-shifting waveguide structures. For example the target array may have a desired optical effect which is analogous to a known optical component. The most readily understood would be a spherical lens. The underlying physics of metamaterials does however enable the production of more exotic effects, such as those resulting from a negative refractive index for example.

At step 1002 a silica (silicon dioxide) support structure 100 with a high surface flatness is coated with a layer of resist material 101 using a suitable technique such as spin coating (see Figure 1 ). The silicon dioxide support structure 100 may, in turn, be supported by a silicon substrate beneath it. In the present case the material chosen is an electron beam resist in the form of a nano imprint resist (either UV or thermal). An example is the mr-l 8000R series (such as mr-l 8020R) available from MICRO RESIST TECHNOLOGY GmbH. The thickness of the resist layer is chosen carefully since this is closely related to the height that the resultant phase-shifting waveguide structures will project away from the support structure surface. In the present case a resist layer thickness of about 600 nm is chosen. The lateral dimensions of the support structure may be extensive (such as a few millimetres or centimetres) allowing the simultaneous generation of arrays for multiple devices, these being separated in a later cutting process. In the case of the security device to be formed being an array of "microlenses" the support structure 100 could be common to all of the "microlenses" and the individual lenses may be produced by the pattern of the phase-shifting waveguide structures.

At step 1003 the target array structure is converted into a pattern for the resist and an electron beam lithography system is used to pattern the resist accordingly using a focused electron beam 102 (Figure 2). The pattern may be a "positive" or "negative" one depending upon the type of resist used. As will be understood, electron beam lithography is an extremely high precision technique with which materials can be treated with an electron beam with a positional accuracy of a few nanometres. As such the pattern relating to the designed array of step 1001 can be exposed into the resist material. This process is extremely beneficial from the perspective of generating a high precision set of structures. Nevertheless, other techniques may be considered in particular when the desire is to produce a large number of security devices quickly. It will be understood that the patterns of multiple different optical structures for different devices may be positioned laterally in the overall pattern to allow multiple device structures to be formed in parallel. At step 1004 the electron beam-patterned resist 101 is "developed" using a standard process, this resulting in the desired patterning for the array being existent within the resist. The patterning is schematically illustrated in Figure 3 with dotted lines indicating the existence of through-thickness apertures within the resist 101 .

At step 1005 an atomic layer deposition (ALD) process is performed which fills the apertures with the dielectric material to be used to produce the optical effect desired. In the present case the material is titanium dioxide (preferably amorphous) which can be readily deposited using ALD. The process is controlled to ensure that the apertures within the resist are filled fully. Since the deposition also occurs in the locations where the resist material remains, these locations 104 are coated with a layer of Ti0 ₂ which is of a thickness equal to the depth of the resist layer (about 600 nm). A schematic representation of this is illustrated in Figure 4. A blanket reactive ion etching procedure is then used to remove the material above the layer of resist.

At step 1006 an etching process is applied to remove the remaining resist, thereby leaving the intended pattern of raised Ti0 ₂ phase-shifting waveguide structures 105 projecting from the surface of the silicon dioxide support structure 100 upon which they have been deposited (Figure 5).

At step 1007 a protective polymer coating (cover layer) is applied to the structures 105. In the present case this is provided by a slot die process using PMMA as the polymer material. This layer is a few micrometres thick and, in a similar manner to the support structure, since it is optically transparent at human visible wavelengths and has no structural modulations within the electromagnetic wavelength range of interest (in this case less than about 700 nm), the visible light is substantially unaffected as it passes through the PMMA layer 105 (Figure 6).

At step 1008 the assembly is divided into the individual elements for use in the different final devices. A laser cutting process can be used to achieve this (Figure 7).

At this stage the metastructured optical element 1 10 has now been generated and is ready for incorporation into the identity case (security document). At step 1009 (which in practice will be performed independently at any earlier time) a security document substrate is prepared for the receipt of the security device. As will be appreciated the formation of a finished security document includes a significant number of process stages, beginning with a document substrate. Depending upon the type of security device, the security device is applied to the document during one of these processing stages. In the present case a security document substrate 200 for the identity card is provided in the form of a pre-printed polycarbonate substrate of the type known in the art for such applications. All prior printing and any other surface modification processes are represented by the layer 201 in Figure 8. A window region 202 is printed with a suitable image (in a location beneath where the metastructured optical element 1 10 is to be applied) to produce an optical effect in conjunction with the metastructured optical element 1 10. The image is printed upon a similarly sized region of reflective material (such as an Al layer). During step 1009, the window region 202 is coated with a thin layer of clear adhesive 203 using a suitable printing process. We note here that, for an identity card application, it is beneficial to take the advantage of the thickness of the card material and have the associated printed image (that is being focussed by the metastructured optical element) on the reverse side of the polycarbonate card. At step 1010 the security document substrate 200 and metastructured optical element 1 10 are brought into close physical proximity within a "pick and place" system (also referred to as Surface Mount Technology). The use of "pick and place" technology is widespread, particularly within other technical fields such as the electronics industry. Such systems are able to pick up, move, and deposit components at very high speed and with great precision. For example an acceptable lateral positional accuracy for the placement of such a device on a security document would be within 50 micrometres and this is significantly lower than the precision used in many pick and place applications for electronics fabrication. It is also important to use such apparatus with a high degree of rotational (skew) accuracy given the intended optical interaction between the metastructured optical element 1 10 and the image (and reflective region) beneath with which it is intended to optically generate an effect. The metastructured optical element 1 10 is placed upon the surface of the adhesive 203 during this step (Figure 9). If required, the adhesive is then cured using an optical or heating step. At step 101 1 one or more further downstream processing steps are performed. These may include the application of further security features to the document substrate 200. Such features may include elements which interact with the metastructured optical element 1 10 to produce further specific optical effects.

As will be appreciated from the above discussion, in particular due to the height of the Ti0 ₂ structures 105 being in this case less than 1 micrometre, the resultant structure is significantly thinner than a conventional microlens (which might conventionally have a typical thickness of 30 micrometres).

The resultant identity card of the present example is therefore provided with a security feature in which one or more thin metalenses overlie one or more corresponding images and these images are viewed in reflection by virtue of the reflective layer underlying the image within the window region 202.

It will be appreciated that a similar approach could be used in the production of a security article.

Further examples of how such structures may be used to produce optical effects are discussed later.

Form Directly onto a Polymer Substrate

A second technique for the application of the metastructured security device to a security document substrate is described in association with Figures 1 1 to 15 and the flow diagram of Figure 16. Here the approach is based upon forming techniques in which the metamaterial is formed onto a polymer substrate as the support structure, and that substrate is then incorporated into a security document. The deposition technique to provide the phase-shifting optical waveguide structures can be ALD (in an analogous manner to the above embodiment) or another process such as chemical vapour deposition (CVD). Of course providing localisation of the process using a mask is central to the success of the process. In the present case zinc oxide structures are deposited using a resist-based mask with electron beam patterning. Particularly for larger dimension structures (such as those for use in the infra-red part of the spectrum) an optical patterning approach (using a laser) can be employed. At step 2001 in Figure 16 a polymer film 300 is prepared, for example made of PMMA, which will act as the support structure (Figure 1 1 ). This may have dimensions of 10 mm in width and be provided on a spool.

At step 2002 a (pre-designed) patterned mask 301 is provided using a resist in a similar manner to the embodiment above and patterned using an electron beam 302 (Figure 12). At step 2003 the ZnO material is deposited into the patterned resist and the resist (together with any surplus ZnO) is removed chemically leaving a pattern of ZnO dielectric structures 305 upon the polymer film (Figure 13).

At step 2004 this assembly is then provided with a cover film 306 of similar dimensions to the support structure so as to "sandwich" the ZnO structures between the two films of similar polymer material (Figure 14). The resulting composite strip 310 is then ready for transfer to a security document (which might be a bank note, passport, security stamp or similar) or a security article. For example the security document may be provided as a pre-printed substrate. Different security document substrate materials may be used, including paper and polymer (such as biaxially oriented polypropylene (BOPP). For example the security document may be a "plastic" banknote which has a BOPP security document substrate. The security document substrate in this case is provided with an opacifying layer upon each face and is then printed with other suitable images and indicia, generally prior to the attachment of the presently described security device.

At step 2005 the composite strip 310 is adhered onto the surface of a security document 400 so as to be inseparable from the document structure without destroying the ZnO structured pattern (Figure 15).

At step 2006 the security document is ready for further processing steps including addition of other security features 401 , a cutting, a finishing process and so on. Imprint and Copy Technique

The above processes provide high quality metastructures formed to very highly precisioned dimensions. In the generation of larger production runs of security documents or security articles there is a desire to produce a "high throughput" process where large numbers of such structures can be formed quickly. In order to address this issue, with the present embodiment a structural copying technique is used which allows the replication of a "master" metastructure. Here in order to maximise the productivity of the process the master structure is replicated a number of times to produce effectively second master structures representing a number of repeated instances of the original master structure (with the structure inverted as a negative of the original). This second master structure is then used to produce the individual metastructures for the documents, these, due to a further inversion, then representing the original master structure. The process is now described in association with the flow diagram of Figure 24 and the schematic process diagrams of Figure 17 to 23.

At step 3001 the process begins with the formation of a first master structure 500 according to a pre-designed pattern representing the phase-shifting optical waveguide structures to be formed. The master structure may be formed from any suitable material substrate 502 which is capable of providing highly defined structures on a scale of less than one micrometre. This could be a metallic material such as gallium or an inorganic material such as silica, although in the present case a metallic structure formed from nickel is used (Figure 17). The patterned structure is cut into the metal using a focussed ion beam (FIB) system 501 . This process of forming the structure according to the pattern can be computer-implemented once a suitably prepared metallic surface is arranged in the FIB system (Figure 18).

At step 3002 the patterned first master structure 500 is removed from the FIB system and is then coated with a silicone resin 505. A number of different types of curable silicon resins are available, these being curable by a number of processes (including chemical curing using peroxide, UV illumination, elevated temperature and moisture). Here we select a very low viscosity resin (less than 10 mm ² s ^"1 ) to ensure full penetration of the surface morphology of the first master structure 500. Having coated the first master structure 500, the silicone resin is cured by exposure to UV illumination for a few minutes (Figure 19). Alternatively a thermal cure process could be used.

At step 3003 the silicone copy 501 of the master structure 500 is carefully separated from the master structure 500 and the rear (unstructured) surface is adhered to a stamping drum 520 (Figure 20). Further silicone copies of the first master structure

500 are then produced and adhered around the circumference of the stamping drum 520. At step 3004 the stamping drum 520 is rotated so as to bring one of the silicone copies 501 into contact with a high refractive index polymer (HRIP) 503 material which is present as a coating upon a polymer film support structure 504 (Figure 21 ). The HRIP polymer 503 and stamping drum 520 are held stationary and in physical contact as the HRIP material is cured using ultra-violet light.

At step 3005, following the curing, the support structure 504 with its cured HRIP coating 503, including a structural copy of the original master structure 500 (that is a negative of the silicone copy 501 ), is separated from the stamping drum 520 by translation of the substrate 504 and the rotation of the stamping drum 520. This brings a further uncured region of the HRIP coating 503 into contact with another silicone copy 501 attached to the stamping drum 520 and the process is then repeated for that region. An air knife can be used to clean the used silicone copy

501 at a location of the drum circumference distal from the contact point between the drum 520 and the HRIP coating 503 thus preparing it for re-use.

At step 3006, the support structure 504 is adhered to the surface of a security document substrate (or that of a security article), in a similar manner to the previous embodiment (Figure 22). In this case a further security feature 51 1 is then added such as a dichroic feature which partially overlies the HRIP coating 503 and therefore changes the appearance of the coating 503.

At step 3007 a protective cover film 515 is laminated onto the security document so as to cover the security feature 51 1 and HRIP coating 503 (Figure 23). At step 3008 further processes are performed, including any further printing, application of other security devices and a cutting step so as to separate the materials into individual documents. Some modifications of the above process can be used in alternative embodiments. For example the stamping drum 520 could be used alternatively to stamp a layer of resist material which, in turn could be used to etch an underlying dielectric material to produce the metastructures. We now describe some examples of the use of the structures described in the examples above as security devices in security documents and security articles.

Lenticular Device Embodiment

A lenticular device 601 having an array of lenticular lenses formed from the metamaterial structures (produced using any of the above embodiments), is shown schematically in Figure 25. Figure 25 shows the device 601 in a perspective view and it will be seen that an array 602 of focussing element structures, here represented schematically in the form of cylindrical lenses 603, is arranged on a transparent substrate 604. An image array 605 is provided on the opposite side of substrate 604 underlying (and overlapping with) the cylindrical lens array 602. Alternatively the image element array 605 could be located on the same surface of the substrate 604 as the lenses, directly under the lenses. Each cylindrical lens 603 has a corresponding optical footprint which is the area of the image element array 605 which can be viewed via the corresponding lens 603. In this example, the image array 605 is an interlaced image array comprising a series of image slices, of which two slices 606, 607 are provided in (and fill) each optical footprint.

The image slices 606 each correspond to strips taken from a first image IA whilst the image slices 607 each correspond to strips of a second image IB. Thus, the size and shape of each first image slice 606 is substantially identical (being elongate and of width equal to half the optical footprint), but their information content will likely differ from one first image slice 606 to the next (unless the first image IA is a uniform, solid colour block). The same applies to the second image slices 607. The overall pattern of image slices is a line pattern, the elongate direction of the lines lying substantially parallel to the axial direction of the cylindrical lenses 603, which here is along the y-axis. The lenses 603 and the image slices 606, 607 are periodic in the orthogonal direction (x-axis) which may be referred to below as the first direction of the device. As shown best in the cross-section of Figure 26, the image element array 605 and the focussing element array 602 have substantially the same periodicity as one another in the x-axis direction, such that one first image slice 606 and one second image slice 607 lie under each lens 603. The pitch S of the lens array 602 and of the image element array 605 is substantially equal and is constant across the whole device. In this example, the image array 605 is registered to the lens array 602 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element 606 lies under the left half of each lens and a second pattern element 607 lies under the right half. However, registration between the lens array 602 and the image array 605 in the periodic dimension is not essential.

In Figure 26 the focusing elements are represented schematically as cylindrical lenses with semi-circular cross section. This is to illustrate their optical properties to those familiar with optics. However, in practice a similar optical effect is produced by the metamaterial structures 610 formed using the above described techniques. The physical relative arrangement of the individual sub-wavelength metastructures in this case will have a two-fold rotational symmetry in a similar manner to the optical effect which is desired to be produced. The desired effect is achieved using multiple instances of a similar pattern of structures forming the metamaterial structures 610 (arranged in a cylindrical lensing configuration). This is illustrated in Figure 27. What is striking about Figure 27 in comparison with Figure 26 is the low profile provided by the metamaterial structures, enabling the overall structure to be significantly thinner. As has been explained, the metamaterial structures (metastructured optical elements) have a height of typically less than one micrometre which is substantially smaller than the equivalent microlens structures known in the prior art (around 30 or so micrometres thick). In Figure 27 the substrate 604 can in practice be that of the security document substrate itself, in which case it should be of high optical transparency such as is provided by biaxially-oriented polypropylene (BOPP). Depending upon the focal length of the lenses it is conceivable that the substrate 604 may also represent the security device substrate such as 100, 300, 504 in the earlier examples. In this latter case the image element array 605 may be printed upon the surface of the document substrate to which the security device is attached.

Returning now to the "classical optics" representation of Figure 26, when the lenticular device 601 is viewed by a first observer 01 from a first viewing angle, as shown in Figure 26 each lens 603 (understanding that this in practice is the metamaterial structure 610) will direct light from the underlying first image slice 606 to the observer, with the result that the device as a whole appears to display the appearance of the first image IA, which in this case is a uniform block colour as shown in Figure 28. The full image IA is reconstructed by the observer 01 from the first image slices directed to him by the lens array 602. When the device is tilted so that it is viewed by second observer 02 from a second viewing angle, now each lens 603 directs light from the second image slices 607 to the observer. As such the whole device will now appear to display a second image IB, which in this example is blank, as shown in Figure 29, although it could comprise any alternative image. Hence, as the security device is tilted back and forth between the positions of observer 01 and observer 02, the appearance of the whole device switches between image IA and image IB. In this example the first image elements 606 are provided by material forming the image array 605 whilst the second image elements 607 are provided by gaps therebetween. However in other cases as illustrated below the second image elements 607 could also be coloured, e.g. by providing a coloured background such as that described with reference to Figure 25 above. It is also possible to interleave three of more images by extending the above principles accordingly. As also noted in passing, the images need not be uniform blocks of colour (or lack thereof) but could each carry any desirable graphic, such as indicia or the like, by arranging each image slice to be provided only in accordance with the desired graphic rather than in a continuous form along its length, as shown.

The above principles can be used to provide numerous optically variable effects. Moire Magnifier Embodiment

Moire magnifier devices (examples of which are described in EP-A-1695121 , WO-A- 94/27254, WO-A-201 1/107782 and WO201 1/107783) make use of an array of focusing elements (such as lenses) and a corresponding array of microimages, wherein the pitches of the focusing elements and the array of microimages and/or their relative locations are mismatched with the array of focusing elements such that a magnified version of the microimages is generated due to the moire effect. Each microimage is a complete, miniature version of the image which is ultimately observed, and the array of focusing elements acts to select and magnify a small portion of each underlying microimage, which portions are combined by the human eye such that the whole, magnified image is visualised. This mechanism is sometimes referred to as "synthetic magnification". The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device itself. The degree of magnification depends, amongst other things, on the degree of pitch mismatch and/or angular mismatch between the focusing element array and the microimage array. Figure 30 depicts an exemplary moire magnifier security device 700, comprising an image element array 701 defining an array of microimages 703 and an overlapping focussing element array 702 with a pitch or rotational mismatch as necessary to achieve the moire effect. Figure 30 depicts part of the image element array 701 as it would appear without the overlapping focusing element array, i.e. the non-magnified array 701 of microimages 703 (but shown at a greatly increased scale for clarity). In contrast, Figure 31 depicts the appearance of the same portion of the completed security device, i.e. the magnified microimages 704, seen when viewed with the overlapping focussing element array, at one viewing angle. It will be seen from Figure 30 that the image array 701 here forms a regular array of microimages 703 which here each convey the digit "5". In this case all of the microimages 703 are of identical shape and size. The microimages 703 may be coloured or achromatic, formed of ink for example. Surrounding the microimages 703 is a contiguous, uniform background 705 which is preferably colourless but could be of a second contrasting colour. Alternatively, the arrangement could be reversed with the microimages 703 formed as negative, colourless gaps in a coloured background 705.

Figure 31 shows the completed security device 1 , i.e. the image element array 701 shown in Figure 30 plus an overlapping focusing element array 702, from a first viewing angle which here is approximately normal to the plane of the device 700. It should be noted that the security device is depicted at the same scale as used in Figure 30: the apparent enlargement is the effect of the focusing element array 702 now included. The moire effect acts to magnify the microimage array such that magnified versions 704 of the microimages 703 are displayed. In this example just two of the magnified microimages are shown. In practice, the size of the enlarged images and their orientation relative to the device will depend on the degree of mismatch between the focusing element array. This will be fixed once the focusing element array is joined to the image element array. The magnified microimages will appear to move laterally relative to the device upon tilting and depending on the magnification level may be visualised above or below the surface plane of the device 700.

The focusing element array 702 is formed using the techniques described earlier. Here, unlike in the example above where an array of cylindrical lenses 602 were fabricated using the metastructured optical elements, in this case each lens in the array 702 is configured as a metastructured optical element acting as a spherical lens. These are carefully positioned over the image array to produce the optical effect described. "Hybrid" Moire Magnifier / Integral Imaging Device Embodiment

Integral imaging devices are similar to moire magnifier devices in that an array of microimages is provided under a corresponding array of lenses, each microimage being a miniature version of the image to be displayed. However here there is no mismatch between the lenses and the microimages. Instead a visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three-dimensional image is given. "Hybrid" devices also exist which combine features of moire magnification devices with those of integral imaging devices. In a "pure" moire magnification device, the microimages forming the array will generally be identical to one another. Likewise in a "pure" integral imaging device there will be no mismatch between the arrays, as described above. A "hybrid" moire magnification / integral imaging device utilises an array of microimages which differ slightly from one another, showing different views of an object, as in an integral imaging device. However, as in a moire magnification device there is a mismatch between the focusing element array and the microimage array, resulting in a synthetically magnified version of the microimage array, due to the moire effect, the magnified microimages having a three-dimensional appearance. Since the visual effect is a result of the moire effect, such hybrid devices are considered a subset of moire magnification devices for the purposes of the present disclosure. In general, therefore, the microimages provided in a moire magnification device should be substantially identical in the sense that they are either exactly the same as one another (pure moire magnifiers) or show the same object/scene but from different viewpoints (hybrid devices).

Moire magnifiers, integral imaging devices and hybrid devices can all be configured to operate in just one dimension (e.g. utilising cylindrical lenses) or in two dimensions (e.g. comprising a 2D array of spherical or aspherical lenses).

In the above example security device, the microimages of the number "5" are all identical to one another, such that the devices can be considered "pure" moire magnifiers. However, the same principles can be applied to "hybrid" moire magnifier / integral imaging devices, in which the microimages depict an object or scene from different viewpoints. Such microimages are considered substantially identical to one another for the purposes of the present invention.

An example of such a device is shown schematically in Figures 32 and 33. Figure 32 shows the unmagnified microimage array, without the effect of the array of metastructured focusing elements 802, and Figure 33 shows the appearance of the finished device, i.e. the magnified image. As shown in Figure 32, the microimages 803 show an object, here a cube, from different angles. It should be noted that the microimages are formed as lines of one colour corresponding to the black lines of the cubes in the figure, the remainder of the image array providing a background thereto which may be coloured or contrasting. Again this arrangement could be reversed with the lines formed as colourless gaps in a coloured background layer. In the magnified image (Figure 33), the moire effect generates magnified, 3D versions of the cube labelled 804. As the device is tilted the magnified cubes 804 will appear to move across the device and change their orientation accordingly with respect to the viewer, amounting to an effect with significant visual impact. Double Sided Security Features

Whilst it has been proposed to provide microlens materials in registry on opposing sides of a substrate, in practice this involves a number of challenges, particularly in banknote production due to the overall thickness of the resultant structure. This issue is overcome with the metastructured materials described here.

Figure 34 shows an example two-sided security feature. In this case two sets of metastructured lenses are placed on opposite sides of a BOPP banknote 900. The thickness of the banknote BOPP substrate is about 75 micrometres. Each metastructured lens is identical. A first set 901 of lenses 910 is provided on the upper surface of the banknote 900, whilst a second set 902 of lenses 920 is provided on a lower surface. Each lens 910,920 has a lateral dimension of about 40 micrometres. In each set the lenses are spaced apart by a distance equal to the lateral width of the lenses. The two sets of lenses 901 ,902 are laterally interleaved such that no lens is opposite another on the correspondingly opposite side of the banknote 900. Instead, in the locations directly opposite the lenses are provided with two printed regions of coloured printing, positioned side by side. From beneath (as in the orientation in Figure 34 and from left to right) the two regions are arranged as red and green printing respectively. Viewed from above and viewed left to right, they are arranged green and red respectively. As will be understood, typically a BOPP banknote is provided with an opacifying layer upon each of its faces, this being about 10 micrometres thick. In this embodiment a window is provided in the opacifying layer which is immediately beneath the lenses 910,920, whereas the red (R) and green (G) printed regions are printed upon the substrate prior to the application of the opactifying layer (or are printed in a window in the layer and are then overprinted with an opacifying material).

The lenses 910 and 920 are designed to act as cylindrical lenses. They extend into the plane of the figure, as so the R and G regions. The R and G regions which are positioned upon the opposite side of the banknote 900 to any given lens 910, 920 are located in the focal plane of the respective lens.

The effect of this structure is that, when viewed from beneath (with reference to Figure 34) as the observer passes the banknote 900 from right to left in front of them, when illuminated with diffuse white light (such as from behind), the note will firstly appear green in the region which contains the metalenses 920 and as the transition of the note continues it will then appear red. When performing the same procedure and viewing the upper surface in a similar orientation, the metalenses 910 will firstly appear red and will transition to green as the note is passed from right to left.

Projection Image

The metastructured optical element may be arranged to provide a significantly more complicated focussing effect upon the light than that of replicating "classical" optical elements such as spherical lenses. As is understood by those skilled in the art, "focussing" in its purest (or narrowest) form means directing all of the light to a single point in space. However, the term focussing also includes the directing of the light so as to concentrate into multiple spots or regions. These regions may extend in a plane normal to the general incident beam direction. Alternatively or additionally, they may extend in a direction parallel to the beam direction. As is also known, such focusing may be used to arrange those spots or regions as an image and so to form a projected image from the incident light. The detail within the image is therefore provided by spatially dependent light intensity in the focal plane (assuming the intention is to focus upon a plane in order to make the image readily observable to a user). To form a perceptible image distinct from the background illumination the light is preferably concentrated by the focussing region so that local light intensity is increased by at least a factor of 2 or 3 with respect to the intensity that would otherwise occur. In order to produce the focusing effect, the phase of the light transmitted by the device is modulated such that it is either locally refracted to the desired focussed regions or that each bright image pixel will, on balance, receive constructive interference when considering all the light emerging from the device coupled with the focussing profile. This will cause the image to be formed in a particular plane. In the most general sense the focusing of the light by the metastructured optical element can be thought of as a "caustic" in optics. The ability to provide the controlled spatially dependent focusing of light by the metastructured optical element allows for the generation of improved security devices.

Referring now to Figure 35 which illustrates an eighth embodiment, the upper part of the figure illustrates how a homogeneous parallel beam of light will, if neither focused nor otherwise modified at an optical element position 900 in an element plane 901 , cause a corresponding homogeneous region of light 902 to fall on a focal plane 903. In contrast, when a metastructured optical element 905 is placed in the element plane 901 , this can selectively focus the beam of light into an image 906 in the image plane 903.

The design of the metastructured optical element 905 in this case is, again, achieved using appropriate modelling software. The design methodology can be achieved in a number of different ways. In a first approach the metastructured optical element 905 can be theoretically modelled as a grid of pixels (or voxels). Each pixel is then designed to project a corresponding "sub-beam" onto the image plane 903 at a specific location in the desired image (which in the present case is an image of the founder of the applicant organisation). The projections of the sub- beams on the focal plane can be formed into an intensity pattern which forms the visible image 906. The spatial mapping between the positions within the metastructured optical element 905 and the image 906 is taken into account in the software when designing the structure of each of the pixels within the metastructured optical element 905. The approach can be thought of as analogous to providing an array of individual lenses which direct respective beams of light onto the image plane 903 so as to form the image.

An alternative approach, somewhat analogous to diffractive optical elements (DOE) in terms of phase considerations, is a computer optimisation technique where the light reaching all the bright pixels in the image plane needs to be in phase whilst the dark pixels need to average so that no phase value dominates and therefore the phase "cancels out". In this approach the metatructured optical element is divided into cells (corresponding to the substructure) each of which gives a local phase shift to the wavefront emerging from the element. The computer optimisation then seeks to balance the requirements of the bright and dark pixels to generate the phase profile which gives the most efficient rendition of the image.

A metastructured optical element 905 made using these approaches allows for intricate images to be generated in a manner which is not presently possible with the conventional equivalent microlenses since such microlenses are physically too thick and too numerous to stack upon each other within a practical security device. With appropriate modelling to reproduce the optical effects of known structures, the metamaterial structures may be used to provide similar effects optically to a very wide range of optical structures (including of course microlenses and microprisms). In each case they provide the advantage of much smaller thickness dimensions enabling the use of such structures in locations where the equivalent optical structure would not be feasible to use. By stacking such structures it is possible to produce new combinations of optical effects which would also be impractical as stacked optical structures.

As will be appreciated the interaction between the metastructured optical elements and electromagnetic radiation is entirely reversible along any given wave path. As such these materials are useful to provide both transmissive effects, in which the observer looks through the security device in question, as well as reflective effects where the light passes through the metamaterial structure having been reflected from a suitable specularly or diffusely reflecting layer beneath (either directly bonded to the support structure or positioned upon a substrate to which the support structure is mounted). In most cases the array of image elements or microimages is located approximately in the focal plane of the focusing structures. Typical thicknesses of security devices according to the invention are 5 to 200 micrometres, more preferably 10 to 70 micrometres, more preferably 5 to 25 micrometres with phase-shifting optical waveguide structure heights of 0.1 to 2 micrometres, preferably 0.2 to 0.7 micrometres. For example, devices with thicknesses in the range 50 to 200 micrometres may be suitable for use in structures such as over-laminates in cards such as drivers licences and other forms of identity document, as well as in other structures such as high security labels. Suitable maximum image element widths (related to the device thickness) are accordingly 25 to 50 micrometres respectively. Devices with thicknesses in the range 65 to 75 micrometres may be suitable for devices located across windowed and half-windowed areas of polymer banknotes for example. The corresponding maximum image element widths are accordingly circa 30 to 37 micrometres respectively. Devices with thicknesses of up to 35 micrometres may be suitable for application to documents such as paper banknotes in the form of slices, patches or security threads, and also devices applied on to polymer banknotes where both the lenses and the image elements are located on the same side of the document substrate.

Conventional and Metastructured Composite System Embodiment

In conventional optical systems (cameras, microscopes, telescopes), it is normal to sequentially combine optical elements to minimise aberrations that would otherwise occur with a single focussing element. With metastructured optical elements this is also possible as well as making a hybrid metastructured and conventional optical sequence. The advantage of the metastructured optical element is that it is possible to engineer the properties of the optical propagation (for example optical dispersion) by varying the structure. As a result, it is possible to use the metastructured optical element to correct the aberration of a conventional focussing element. For example this can be achieved by sequencing the elements on two sides of a film or by putting the structure conformally into the surface of the conventional focussing element.

Figure 36 shows an example of a conventional microlens 1 101 positioned upon an optically transparent substrate 1 102 in a security document application. Three different wavelengths of light, namely Red (R), Green (G) and Blue (B) are provided to the microlens 1 101 as a parallel beam of incident white light. A chromatic aberration effect is produced due to the microlens 1 101 material causing a different degree of refraction upon each of the components of the light (R,G,B). Figure 37 shows how such an effect can be corrected, according to a ninth embodiment, by providing an optical device with a microlens 1 104 in combination with a planar metastructured optical element 1 105. The planar metastructured optical element 1 105 is located at the interface between the microlens 1 104 and the transparent substrate 1 106. The metastructured optical element 1 105 is designed by the modelling software to cause the opposite wavelength dispersive effect to that generated by the microlens 1 104, taking into account the material and lens shape of the microlens 1 104. In so doing the chromatic aberration of the microlens 1 104 is corrected without the need for a second relatively large classical optical lens.

Figure 38 shows a tenth embodiment where the planar metastructured optical element 1 105 is replaced by a curved metastructured optical element 1 107 which conforms with the exterior convex surface of the refractive microlens 1 108. In this case the metastructured optical element is no longer planar. The techniques described earlier may be used to fabricate the metastructured optical element as appropriately modified for the curved geometry. The support structure of the metastructured optical element in this case may provide the required geometry and may be used in a similar manner as a contact lens for the human eye. In principle the metastructured optical element could be either directly fabricated on the surface of the microlens 1 108 or alternatively prepared separately and adhered to the surface.

Retroreflector Embodiment

A retroreflector is another application of metastrutured optical elements in the context of security devices. Conventional retroreflectors are often formed of ball lenses (typically referred to as glass beads) where the back surface is reflective and the front surface focuses light onto the back surface. This can be seen in Figure 39 where an array of ball lenses 1 1 10 is positioned embedded into the surface of a reflective substrate 1 1 1 1 . As can be seen by the rays drawn in the figure, the retroreflector will reflect an incident ray back towards the ray source, regardless of the angle of incidence. In the eleventh embodiment shown in Figure 40 the ball lens has been divided into two hemispheres and the front surface hemisphere of the ball lens has been replaced with a metastructured optical element 1 1 12 configured to provide the "equivalent" optical effect of such a hemisphere with the advantage that the structure is significantly thinner. The metastructured optical element 1 1 12 is attached to (or formed upon) the planar surface of the remaining (rear) hemisphere 1 1 13. In addition because metastructured optical elements can be designed for use with particular wavelengths of light (that is they can effectively be "tuned" to a specific wavelength) it is possible to incorporate coloured images into the retroreflective device by having an array of retroreflectors, each acting as a pixel of an image, where each pixel is wavelength selective and reflects particular wavelength ranges. In this case the colour effect is optimised if the metastructured optical elements are relatively "narrowband" in nature, each band relating to a specific colour such as red, green and blue.

Figure 41 shows an example of this with a similar general structure to the assemblies of metastructured optical elements and hemispheres shown in Figure 40, with, in this case and according to this twelfth embodiment, a narrowband wavelength selectivity built into the metastructured optical elements. In Figure 41 the upper illustrated assembly 1 1 14 reflects green light, whereas the lower assembly 1 1 15 reflects red light.

Whilst the retroreflectors discussed above use a combination of classical lenses and metastructured optical elements, it is also possible to make a combined metasurface system where two metastructured optical elements are used in "sequence" which can act as a retroreflector.

Sequential Metastructured Optical Elements Embodiment

One of the powerful advantages of metastructured optical elements is their very thin geometry and their very low optical losses in terms of absorption of the incident light. These facts make such structures ideal to combine in sequences (which can be thought of as stacks) where each individual element typically is formed upon a common optical axis. In the context of security devices only a very small number of thin lenses can be combined before the thickness of the resulting structure becomes impractical to use. These problems are generally avoided with metastructured optical elements. It is therefore the combination of multiple metastructured optical elements which opens up a new range of potential security device applications. Figure 42 illustrates a thirteenth embodiment in the form of a security device including an assembly in which two metastructured optical elements are provided. In this case the assembly is designed to focus a parallel beam of white light which is incident on a front surface of the device into a single focus on an opposing rear surface of the device whilst correcting any chromatic aberration effects. A first metastructured optical element 1 1 16 is located as a planar structure and receives the light under normal incidence. The first metastructured optical element 1 1 16 is located on a front planar surface of a spacer layer 1 1 18. The spacer layer 1 1 18 can be any dielectric (eg. polymers such as PET, BOPP, Polycarbonate or an inorganic material such as a Si0 ₂ wafer). The thickness of the spacer layer 1 1 18 may be a few micrometres all the way up to the thickness of the security document (700μηι for a polycarbonate identity card application). A second metastructured optical element

1 1 17 is located as a planar structure on a rear planar surface of the spacer layer

1 1 18 and is sandwiched between the spacer layer 1 1 18 and a substrate 1 1 19 forming the remainder of the security device in this example. The rearward external face of the substrate 1 1 19 is formed as a plane which coincides with the focal plane of the assembly comprising the element 1 1 16, spacer layer 1 1 18 and element 1 1 17. There is a need in general to provide spatial register between the elements 1 1 18 and 1 1 17, just like there would be between two conventional lenses. The lensing mechanisms lend themselves to providing feedback to an inspection system to optimise the alignment/registration of the two (or more) lens elements. When there are just two elements the construction might most easily be achieved by replication on the two opposing sides of the dielectric. The degree of register is generally proportionate to the lens diameter with the registration tolerance being at least no worse than 10% of the lens diameter for example.

There are of course many other applications of security devices which can be effected using the techniques described here.