AU2020273504B2

AU2020273504B2 - Optical security component having a plasmonic effect, manufacture of such a component, and secure object provided with such a component

Info

Publication number: AU2020273504B2
Application number: AU2020273504A
Authority: AU
Inventors: Sylvain Blaize; Soukaina ES-SAIDI; Loïc Le Cunff; Demetrio MACIAS
Original assignee: Surys SA
Current assignee: Surys SA
Priority date: 2019-05-13
Filing date: 2020-05-11
Publication date: 2022-04-07
Anticipated expiration: 2040-05-11
Also published as: JP2022527666A; FR3095981A1; EP3969293A1; CA3140437A1; AU2020273504A1; CA3140437C; WO2020229415A1; FR3095981B1; EP3969293B1

Abstract

According to one aspect, the present invention relates to an optical security component (101) having a plasmonic effect, comprising at least a first transparent layer made of dielectric material (113), having a first refractive index (n

Description

Optical security component having a plasmonic effect, manufacture of such a component, and secure object provided with such a component

Technical field of the invention The present description relates to a plasmon-resonance optical security component and to a method for manufacturing such a component. The optical security component according to the present description is especially applicable to security markings permitting authentication of objects of value, and more precisely authentication with the naked eye via observation in reflection and/or in transmission. Prior art Many technologies are known that permit authentication of objects of value and especially authentication of documents of value, such as banknotes or travel documents (passports, identity cards or other identity documents), or that permit authentication of products by means of marking labels. These technologies aim to produce optical security components that have optical effects, as a function of observation parameters (orientation with respect to the observation axis, position and dimensions of the light source, etc.), that adopt characteristic and verifiable configurations. The general goal of these optical components is to generate new and differentiated effects, using physical configurations that are difficult for a forger to reproduce or imitate. Among these components, plasmon-resonance optical security components, i.e. optical security components that employ so-called "plasmon resonance", allow, in reflection or in transmission, color effects resulting from excitation of bulk or surface plasmons during interaction of incident light waves with nanometric metal patterns to be generated. The colors thus produced, which are said to be "structural", have the advantage of being variable as a function of observation parameters, this facilitating authentication. Moreover, unlike optical security components based on purely diffractive effects, such as holographic components for example, plasmon-resonance optical security components have the advantage of generating visual effects at zero order (specular reflection or direct transmission), which are more difficult to reproduce because of the very small dimensions of the structures but easy to authenticate. Published patent application EP 2 771 724 in the name of the applicant thus describes an optical security component that is intended to be observed in the visible, in direct reflection, and that comprises a transparent layer of dielectric material and a sufficiently thick continuous metal layer, forming with the layer of dielectric material a metal dielectric interface. The metal layer is structured at the interface to form two sets of corrugations extending in two directions and forming a two-dimensional coupling grating, of sub-wavelength periods in each of the directions. The periods in each direction are determined so as to optimize coupling of an incident light wave, in a given spectral band, to a plasmon mode that propagates at the metal-dielectric interface. The various physical mechanisms at work in the optical security component described in patent application EP 2 771 724 allow, in reflection, a band-stop filter to be formed, the coupled radiation being absorbed in the metal layer. The applicant has thus demonstrated obtention of characteristic yellow or purple color effects. Patent application EP 2 695 006 in the name of the applicant also describes a plasmon resonance optical security component, though this component is intended to be observed in transmission. In the optical security component thus described, the metal layer is arranged between two layers of dielectric material to form two dielectric-metal interfaces. The metal layer is, in this example, sufficiently thin and structured to form corrugations able to couple surface-plasmon modes supported by the two dielectric-metal interfaces to an incident light wave. When the coupling condition is met, the light energy is able to pass through the continuous metal layer and thus produce a transmission peak; it is thus a question of resonant transmission. Patent application EP 3 099 513 in the name of the applicant describes a plasmon resonance optical security component that generates, in reflection, different visual effects on its front side and on its back side. The optical security component comprises, as in the previous example, a metal layer arranged between two transparent layers of dielectric material to form two dielectric-metal interfaces. In this example, the metal layer is structured to form, in a first coupling region, a first coupling grating that has a dissymmetric profile in each of its directions, and, in a second coupling region, a second coupling grating that has a dissymmetric profile in each of its directions, and that is different from the first grating. The second coupling grating is for example the negative of the first coupling grating. Such a component has, in the first and second coupling regions, a resonant-transmission effect that remains stable in the first and second coupling regions and under observation of the component from each of its sides. In contrast, due to the dissymmetric nature of the profiles of the coupling gratings and to the difference between the dissymmetric profiles of the first and second coupling gratings in the first and second coupling regions, observation in reflection from one side of the component allows a region dependent variable color effect to be seen, the color effect being inverted between the two regions on observation of the component from each of its sides.

The same patent application, patent application EP 3 099 513, also describes depositing, on the metal layer and in a delineated zone, for example one taking the form of a recognizable pattern, a layer of dielectric material of lower or higher index. This results in a shift of the resonant transmission spectral band in said zone, and therefore in a different color effect.

The present description presents a plasmon-resonance optical security component that generates, especially in reflection, color effects that are original and distinctive, compared to those described for prior-art optical security components, allowing even easier and more reliable authentication, including with the naked eye, by an untrained user.

Object of the invention

It is an object of the invention to overcome or ameliorate one or more of the above disadvantages, or at least to provide a useful alternative.

Summary of the invention

According to a first aspect, the present description relates to a plasmonic-effect optical security component, said component being able to be inspected with the naked eye, in reflection, via at least a first observation face.

The optical security component according to the first aspect comprises:

- at least one first transparent layer of dielectric material, having a first refractive index,

- at least one second transparent layer of dielectric material, with a thickness comprised between 20 nm and 150 nm, having a second refractive index, the difference between the second refractive index and the first refractive index being larger than or equal to 0.5 and said second layer of dielectric material making contact with said first layer of dielectric material; and

- a metal layer making contact with said at least one second layer of dielectric material.

According to the invention, said first layer of dielectric material, said second layer of dielectric material and said metal layer form a first dielectric-dielectric-metal double interface that comprises a first dielectric-dielectric interface and a first dielectric-metal interface, and that is structured to form, in at least one first coupling region, a first two-dimensional coupling grating, with a first direction and a second direction, having a first period comprised between 150 nm and 350 nm in the first direction and a second period comprised between 150 nm and 350 nm in the second direction.

According to the invention, said first coupling grating is determined so as to generate a first plasmon-resonance effect at said at least one first dielectric-metal interface in a first resonance spectral band, and the thickness of said at least one second layer of dielectric material is determined so as to generate, by means of said first coupling grating, a hybrid plasmon-resonance effect, in a second resonance spectral band different from said first spectral band. In the present description, said first layer of dielectric material is also called the "low index" layer of dielectric material and said second layer of dielectric material is also called the "high-index" layer of dielectric material. More generally, a so-called "high-index" layer of dielectric material is defined solely in relation to a so-called "low-index" layer of dielectric material with which it makes contact, a difference between the refractive index of the high-index layer of dielectric material and the refractive index of the low-index layer of dielectric material being larger than or equal to 0.5. The applicant has shown that the choice of a sufficient thickness for the high-index layer of dielectric material not only results in a shift of the first spectral band of plasmon resonance at the dielectric-metal interface but also in the generation of a second plasmon-resonance effect, referred to as the "hybrid" plasmon-resonance effect, in a second spectral band different from the first spectral band. The applicant has shown that during generation of the first plasmon-resonance effect (described in the prior art) the electromagnetic field is confined to the surface of the metal. During generation of the second plasmon-resonance effect, the energy of the electromagnetic field remains maximum at the surface of the metal but spreads into the high-index layer of dielectric material, hence the term "hybrid" effect. Within the meaning of the present description, a resonance spectral band may be defined as a continuous range of wavelengths in which a trough is observed in the normalized reflectance curve of the optical security component, when the component is illuminated with white light at normal incidence, the trough being characterized by a local maximum and a local minimum that are consecutive and that have normalized-reflectance values that differ by an amount strictly larger than 0.2 (or 20%). For each resonance spectral band, it is possible to define a central resonance wavelength corresponding, in said spectral band, to the wavelength at which the normalized reflectance has a minimum value. By different resonance spectral bands, what is meant in the present description is two spectral bands respectively having resonant central wavelengths separated by at least 50 nm.

The presence of the two different spectral bands results, especially when the optical security component is observed in reflection, in original color effects, i.e. color effects that are original compared to those observed in the prior art, and especially in colors having hue angles comprised between 1200 and 320°, without addition of dyes or pigments. In the present description, a transparent layer of dielectric material is defined to be a layer having a transmittance of at least 70% and preferably of at least 80% at a wavelength comprised in the visible spectral band, i.e. comprised between 400 nm and 800 nm. In the present description, a two-dimensional grating or "crossed" grating is a grating defined by the superposition of two sets of periodic corrugations extending in two directions. It is possible to define, for each set of corrugations, a grating vector having a direction perpendicular to the corrugations and a norm inversely proportional to the period. According to one or more examples of embodiment, the profile of said at least one first two-dimensional grating is continuously variable in each direction, this type of profile especially allowing a better propagation of the plasmon modes. For example, the profile of said at least one first grating in each direction is sinusoidal or quasi-sinusoidal, i.e. with a duty cycle other than 0.5. Duty cycle is defined, in a period comprising one corrugation trough and one corrugation peak, as: the ratio of the length having a value higher than the mean value between the peak and trough, to the period. Generally, said at least one first two-dimensional grating according to the present description may be symmetric, asymmetric or dissymmetric. By symmetric, what is meant in the present description is that the appearance of said at least one first coupling grating when looked at via the observation face is identical to its appearance when looked at via the face opposite the observation face. By dissymmetric, what is meant is that the appearance of said at least one first coupling grating when looked at via the observation face is different from its appearance when looked at via the face opposite the observation face. Such a dissymmetric grating is for example described in patent EP 3 099 513 in the name of the applicant. It is characterized by a profile that does not have central symmetry (i.e. symmetry with respect to a point), in each of the directions. By asymmetric, what is meant is a grating that has, in one direction, in a period, a corrugation with a rising edge different from the falling edge (so-called "blazed" grating). According to one or more examples of embodiment, the first direction and the second direction of the coupling grating are substantially perpendicular. The two-dimensional grating thus forms a structure the shape of which is similar to that of an egg box. By substantially perpendicular, what is meant is that the first direction and the second direction make an angle of 90° 5°. In the case of a square unit cell, the color effect of such an optical security component remains stable during an azimuthal rotation between 0 and 90. In the case of a (non-square) rectangular unit cell, the color effect will be able to be observed to vary during an azimuthal rotation between 00 and 90. According to other examples of embodiment, the first direction and the second direction of the coupling grating may make an angle different from 90, and for example comprised between 300 and 60. According to one or more examples of embodiment, the difference between the refractive index of the high-index second layer of dielectric material and the refractive index of the low-index first layer of dielectric material is larger than or equal to 0.8. It is then possible to excite hybrid modes with high-index layer of dielectric material thicknesses that are smaller than is possible if the difference in index is 0.5. According to one or more examples of embodiment, the thickness of said at least one high index second layer of dielectric material is comprised between 20 nm and 100 nm. Specifically, beyond 100 nm, the high-index layer of dielectric material may generate unwanted hues in unstructured regions. In practice, the thickness of said at least one high index second layer of dielectric material may be comprised between 40 nm and 100 nm. According to one or more examples of embodiment, said at least one high-index second layer of dielectric material results from deposition, vacuum deposition for example, of a layer of zinc sulfide (ZnS), of titanium dioxide (TiO 2 ), or of silicon nitride (Si 3 N 4 ). Other examples of materials from which the high-index layer of dielectric material may be formed are known and have been disclosed, for example in patent US4856857. According to one or more examples of embodiment, said at least one low-index first layer of dielectric material results, for example, from deposition, vacuum deposition for example, of an organic layer, of refractive index generally comprised between 1.4 and 1.6, an adhesive or a resin for example. Examples of materials from which the low-index layer of dielectric material may be formed are known and have been disclosed, for example in patent US4856857. According to one or more examples of embodiment, the thickness of said at least one high index second layer of dielectric material is sufficient to allow an effect of resonance of guided modes in the high-index layer of dielectric material, in a third resonance band separate from the first and second resonance bands. In some examples of embodiment, the presence of the third resonance band may be sought with a view to generating color effects with other hue angles. In contrast, in other examples of embodiment, the thickness of said at least one high-index second layer of dielectric material will be limited to avoid the effect of resonance of guided modes and obtain the color effect which results solely from the first plasmon-resonance effect and the hybrid plasmon-resonance effect. According to one or more examples of embodiment, the optical security component further comprises a third transparent layer of dielectric material that has a third refractive index, and that makes contact with said metal layer via a face opposite the face of the metal layer making contact with the high-index second layer of dielectric material, so as to form a second dielectric-metal interface, said second dielectric-metal interface being structured, in said at least one first coupling region, as said first coupling grating. It is then possible to observe in reflection, and via a second observation face opposite the first observation face, a color effect resulting from a plasmon-resonance effect at the second dielectric-metal interface. According to one or more examples of embodiment, the optical security component further comprises a third transparent layer of dielectric material having a third refractive index, and a fourth transparent layer of dielectric material, with a thickness comprised between 20 nm and 150 nm, having a fourth refractive index, the difference between the fourth refractive index and the third refractive index being larger than or equal to 0.5. Said (high index) fourth layer of dielectric material makes contact with the metal layer via a face opposite the face of the metal layer making contact with the second layer of dielectric material, and said (low-index) third layer of dielectric material makes contact with said fourth layer of dielectric material. Thus, said low-index third layer of dielectric material, said high-index fourth layer of dielectric material and said metal layer form a second dielectric-dielectric-metal double interface that comprises a second dielectric-dielectric interface and a second dielectric-metal interface, and that is structured, in said at least one first coupling region, as the first coupling grating. It is then possible to observe in reflection, and via a second observation face opposite the first observation face, a color effect resulting from a plasmon-resonance effect at the second dielectric-metal interface, and a hybrid plasmon-resonance effect, in the case of a sufficiently thick high-index fourth layer of dielectric material. According to one or more examples of embodiment, the thickness of the high-index fourth layer of dielectric material is different from the thickness of the high-index second layer of dielectric material. It is thus possible to make the colors seen change depending on whether the component is looked at via the first observation face or via the second observation face, opposite the first observation face. These colors may be determined depending on the profile of the gratings as seen from the second observation face, when the gratings are dissymmetric. According to one or more examples of embodiment, the metal layer is "thick", i.e. of sufficient thickness to allow the incident light to be reflected from the dielectric-dielectric metal double interface with a maximum wavelength-dependent residual transmittance of 2%. In this case, the optical security component cannot be observed in transmission. It may however be observed in reflection via each of the observation faces if all of the layers other than the metal layer are transparent. According to one or more examples of embodiment, the metal layer is sufficiently thin to allow surface-plasmon modes supported by the first and second dielectric-metal interfaces on either side of the metal layer to be coupled to. In addition to color effects in reflection, a color effect in transmission is then observed that is identical regardless of the observation face, this color effect resulting from a resonant-transmission effect. According to one or more examples of embodiment, the metal layer comprises a metal chosen from aluminum, silver, gold, copper, chromium, and nickel. Other examples of materials from which the metal layer may be formed are known and have been disclosed, for example in patent US4856857. According to one or more examples of embodiment, said first period and said second period of said at least one first coupling grating are different. In the case of grating directions that are perpendicular, the unit cell is then rectangular, and not square. The applicant has shown that the hybrid resonance effect is strongly dependent on the period in the direction such that the grating vector is perpendicular to the plane of incidence. Different periods in the first and second directions therefore allow colors that are not observable with a square unit cell to be achieved. Moreover, with a (non-square) rectangular unit cell, it is possible to observe a change in color via azimuthal rotation of the component between 0° and 90°. Thus, according to one or more examples of embodiment, the optical security component has, in reflection from said first observation face and at a given observation angle, and for example at normal incidence, a first color effect with a first hue angle at a given first azimuthal angle, and a second color effect with a second hue angle at a given second azimuthal angle, the second hue angle being different from the first hue angle by a given minimum value, for example a value at least equal to 20, advantageously 30°. The first azimuthal angle is for example 0° and the second azimuthal angle is for example 90°. The azimuthal angle is 0° for a given direction of the coupling grating when the plane of incidence coincides with the plane comprising said direction and the direction normal to the component. According to one or more examples of embodiment, the ratio between the depth of said first coupling grating and said first period or said second period (aspect ratio in one of the first and second directions) is comprised between 10% and 80%, and for example between 10% and 50%. According to one or more examples of embodiment, said at least one first dielectric dielectric-metal double interface is structured to form, in at least one second region, a structure different from said first coupling grating, said at least one first double interface remaining continuous through all of said regions. According to one or more examples of embodiment, said at least one second region is contiguous with said first coupling region. By "continuous" double interface, what is meant in the present description is that all of the layers forming said at least one first dielectric-dielectric-metal interface remain continuous through all of said regions, i.e. have no interruptions. According to one or more examples of embodiment, said structure of the second region is formed of a second coupling grating that is different from the first coupling grating. The second coupling grating is different from the first coupling grating in that at least one of the parameters of the second grating comprising: the profile of the grating, the azimuthal orientation, the depth of the grating, and the period in a first direction or in a second direction, is different from the corresponding parameter of the first coupling grating. For example, the second coupling grating has, just like the first coupling grating, a first direction and a second direction, with a first period comprised between 150 nm and 350 nm in the first direction and a second period comprised between 150 nm and 350 nm in the second direction, and grating parameters optimized to generate, in reflection, a second color effect that is different from the first color effect produced by the first coupling grating, for example a second color having a hue angle that differs by at least 30° from the hue angle of the first color generated in the first coupling region.

According to one or more examples of embodiment, said structure of the second region is formed of a structure configured to scatter incident light or of a diffractive structure that diffracts at zero order or at a higher order. It is thus possible to observe, via at least the first observation face, especially in reflection, different color effects in the two regions. It is remarkable to note that an observer will be able to observe the two regions with a perfect correspondence between the regions, because the difference in color effect between the regions is a result of a difference in structure and not in printing. Thus, according to one or more examples of embodiment, the optical security component has, in reflection from said first observation face and at a given observation angle, and for example at normal incidence, a first color effect with a first hue angle in the first coupling region, and a second color effect with a second hue angle in said second region, the second hue angle being different from the first hue angle by a given minimum value, for example a value at least equal to 20, advantageously 30°. According to one or more examples of embodiment, said at least one first dielectric dielectric-metal double interface is not structured in at least one region contiguous with said first coupling region, said at least one first double interface remaining continuous through all of said regions. Similarly to above, an observer will be able to observe a colored region and a colorless reflecting region (colorless because it is unstructured) with a perfect correspondence between the regions, because the difference between the regions is a result of a difference in structure and not in printing. According to one or more examples of embodiment, said at least one first dielectric dielectric-metal double interface is structured to form a plurality of contiguous regions, including said first coupling region, said at least one first double interface remaining continuous through all of said regions. The regions of the plurality of regions for example form a recognizable pattern. The one or more regions other than said first coupling region may comprise at least one structured region and/or at least one unstructured region. Generally, with a plurality of different regions, including said first coupling region, it is possible to make information appear only at one given azimuthal angle and/or different information to appear on the front side (observation via the first observation face) and on the back side (observation via the second observation face, opposite said first observation face). For example, it will be possible to make information appear only when the document is observed via one face. According to one or more examples of embodiment, said at least one first dielectric dielectric-metal double interface is structured to form, in said at least one first coupling region, a first microscopic structure modulated by said first coupling grating. According to one or more examples of embodiment, said first microscopic structure is a diffractive structure. For example, the diffractive first microscopic structure comprises a bas-relief configured to simulate an image in relief of an object in relief, as for example described in patent EP2567270 in the name of the applicant, or generate a dynamic visual effect, for example one that makes a graphic visual object appear to move when the optical security component is tilted, as for example described in patent EP3129238 in the name of the applicant or in patent FR3066954 in the name of the applicant. The diffractive first microscopic structure may also comprise a diffractive element such as a computer synthesized hologram, as for example described in patent FR 3051565 in the name of the applicant, or more generally any diffractive microscopic structure. According to one or more examples of embodiment, said first microscopic structure comprises microstructures that are randomly distributed so as to produce an optical scattering effect. Such microstructures are for example described in patent EP 2836371 in the name of the applicant. The cone of visibility of the optical effect will be enlarged thereby. According to one or more examples of embodiment, the optical security component is suitable for increasing the security of a document or of a product, and further comprises, on the face opposite the observation face, a layer suitable for transferring the component to the document or product, for example a layer of permanent adhesive or a layer of reactive adhesive. According to one or more examples of embodiment, the optical security component further comprises, on the side of the observation face, a support film intended to be detached after the component has been transferred to the document or product. According to one or more examples of embodiment, the optical security component is suitable for manufacture of a security thread for increasing the security of banknotes, and comprises, on the side of the observation face and/or on the face opposite the observation face, one or more protective layers.

The optical security component according to the first aspect may moreover comprise one or more additional transparent layers depending on the needs of the application, this or these additional layers not contributing to the sought-after visual effect. According to a second aspect, the present description relates to an optical security element intended to increase the security of an object, for example a document of value, and comprising at least one optical security component according to the first aspect. According to a third aspect, the present description relates to a security object, for example a security document of value, comprising a substrate and, arranged on said backing, the optical security component according to the first aspect or said security element according to the second aspect. The document of value is for example a banknote, a passport, a visa, a driving license, an identity card or any identity document. The substrate for example comprises polycarbonate, PVC, PET, paper, a cardboard sheet, etc. Advantageously, the substrate comprises a region of transparency such that the 2 faces of the component may be observed in turn. The optical security component according to the first aspect may also be affixed to a product liable to be the subject of an attempt at forgery or counterfeiting, such as a branded product, an alcoholic drink, or an electrical component. In this case, the optical component will possibly be supplied in the form of a potentially destructible adhesive label such as known to those skilled in the art. According to one or more examples of embodiment, the substrate has a rectangular shape with two perpendicular axes, the axes being collinear with the directions of said at least one first coupling grating. According to other examples of embodiment, the axes are not collinear with the directions of said at least one first coupling grating. When the directions of the grating are not aligned with the axes of the document, this amounts to observation with an azimuthal angle different from zero. Color effects are then observed with a larger color variation under up/down tilting than is the case if the directions of the coupling grating are collinear with the axes of the document. According to a fourth aspect, the present description relates to methods for manufacturing optical security components according to the first aspect. According to one or more examples of embodiment, the method for manufacturing an optical security component according to the first aspect comprises: - forming said first layer of dielectric material;

- depositing, on said first layer of dielectric material, said second layer of dielectric material; - depositing, on said second layer of dielectric material, said metal layer, so as to form said first dielectric-dielectric-metal double interface, said first dielectric dielectric-metal double interface being structured to form, in said at least first coupling region, said first coupling grating. According to one or more examples of embodiment, the method comprises structuring said first layer of dielectric material, for example by stamping (for example by hot pressing or by cold molding then UV casting), then depositing said second layer of dielectric material and said metal layer. In all cases, structuring the double interface allows the one or more different regions exhibiting the different color effects to be formed, enabling a perfect correspondence to be obtained between said regions. The applicant has thus demonstrated that it is possible to produce, without printing, an image that is multi-color at zero order, using a stack of layers that remain continuous through all of the component. According to one or more examples of embodiment, the method further comprises: - depositing a protective layer of dielectric material. According to one or more examples of embodiment, said protective layer is a third transparent layer of dielectric material deposited in contact with the metal layer so as to form a second dielectric-metal interface, said second dielectric-metal interface being structured, in said at least one first coupling region, as said first coupling grating. According to one or more examples of embodiment, the method further comprises: - depositing a fourth transparent layer of dielectric material in contact with said metal layer; and - said protective layer forms a third transparent layer of dielectric material making contact with said fourth layer of dielectric material, a difference between a refractive index of the fourth layer of dielectric material and a refractive index of the third layer of dielectric material being larger than or equal to 0.5. Said (low-index) third layer of dielectric material, said (high-index) fourth layer of dielectric material and said metal layer form a second dielectric-dielectric-metal double interface that comprises a second dielectric-dielectric interface and a second dielectric metal interface, and that is structured, in said at least one first coupling region, as said first coupling grating.

According to a third aspect, the present description relates to a method for authenticating an optical security component according to the first aspect, comprising: - a step of illuminating said optical security component with natural light and observing, through a linear polarizer, a change in the color of the color effect as a function of polarization direction; or - a step of illuminating said optical security component with linearly polarized light and observing a change in the color of the color effect as a function of polarization direction. According to one or more examples of embodiment, said at least one first coupling grating has identical periods in the first direction and in the second direction and authentication is performed at an angle different from normal incidence, for example at an observation angle comprised between 150 and 60°. Brief description of the figures Other advantages and features of the invention will become apparent on reading the description, which is illustrated by the following figures:

[Fig. 1A] shows a schematic illustrating a cross-sectional view of one example of an optical security component according to the present description;

[Fig. IB] shows a schematic illustrating a (partial) 3D view of a structured double interface of one example of an optical security component according to the present description;

[Fig. 2A] is a graph showing curves of normalized reflectance as a function of wavelength, for various thicknesses of the (high-index) second layer of dielectric material, the reflectance curves being obtained at normal incidence with one example of an optical security component according to the present description;

[Fig. 2B] is a graph showing curves of normalized transmittance as a function of wavelength, for various thicknesses of the (high-index) second layer of dielectric material, the transmittance curves being obtained at normal incidence with an example of an optical security component that is the same as that of figure 2A;

[Fig. 2C] is a graph showing curves of normalized reflectance as a function of wavelength, for TE and TM polarizations, the reflectance curves being obtained with an incidence of 40°, with an example of an optical security component that is the same as that of figure 2A and with a thickness of the high-index layer of dielectric material of 80 nm;

[Fig. 3A] shows an image illustrating the electromagnetic-field distribution in one example of an optical security component according to the present description, at a wavelength comprised in a first spectral band corresponding to the first plasmon-resonance effect;

[Fig. 3B] shows an image illustrating the electromagnetic-field distribution in an example of an optical security component that is the same as that of figure 3A, at a wavelength comprised in a second spectral band corresponding to a hybrid plasmon-resonance effect;

[Fig. 3C] shows an image illustrating the electromagnetic-field distribution in an example of an optical security component that is the same as that of figure 3A, at a wavelength comprised in a third spectral band corresponding to a guided-mode resonance effect;

[Fig. 4A] shows a schematic illustrating a view from above of a double interface of one example of an optical security component, said double interface being structured to form a plurality of contiguous regions forming a recognizable pattern;

[Fig. 4B] shows a schematic illustrating a first color effect visible in reflection from the first observation face, with an optical security component, according to the present description, in which the double interface is structured according to the pattern illustrated in figure 4A;

[Fig. 4C] shows a schematic illustrating a second color effect visible in reflection from the face opposite the first observation face, with an optical security component identical to that illustrated in figure 4B;

[Fig. 4D] shows a schematic illustrating a third color effect visible in transmission, with an optical security component identical to that illustrated in figure 4B;

[Fig. 5A] is a graph showing curves illustrating the spectral position of the hybrid mode as a function of the thickness of the (high-index) second layer of dielectric material, in the case of a coupling grating having a square unit cell and for various values of the period, in one example of an optical security component according to the present description;

[Fig. 5B] is a graph illustrating the colors that may be obtained, by varying the period of the coupling grating, in an example of an optical security component that is the same as that used for figure 5A, for different values of the thickness of the (high-index) second layer of dielectric material;

[Fig. 6A] shows a schematic illustrating a (partial) view in 3D of a first double interface and of a second double interface of one example of an optical security component according to the present description;

[Fig. 6B] shows a schematic illustrating a first color effect visible in reflection from the first observation face, with an optical security component, according to the present description, with first and second double interfaces, and in which the double interfaces are structured according to the pattern illustrated in figure 4A;

[Fig. 6C] shows a schematic illustrating a second color effect visible in reflection from the face opposite the first observation face, with an optical security component identical to that illustrated in figure 6B;

[Fig. 6D] shows a schematic illustrating a third color effect visible in transmission, with an optical security component identical to that illustrated in figure 6B;

[Fig. 7A] shows a schematic illustrating a view from above of a coupling grating with different periods in the first and second directions, in one example of an optical security component according to the present description;

[Fig. 7B] is a graph showing curves illustrating normalized reflectance as a function of wavelength, for a given value of the period of the coupling grating in one direction and various values of the period of the coupling grating in another direction, in one example of an optical security component according to the present description;

[Fig. 8] shows a schematic illustrating histograms of hue angles in an optical security component according to the present description, and in an optical security component according to the prior art;

[Fig. 9] shows a schematic illustrating a security document with a security element incorporating one example of an optical security component according to the present description;

[Fig. 10] shows a schematic illustrating the CIE Lab sphere adopted by the International Committee for Illumination (CIE) in 1976, (or "1976 CIE L*a *b*" sphere), and defined according to the standard ISO 11664-4. Detailed description of the invention In the figures, the elements have not been shown to scale for the sake of legibility. Figure 1A is a schematic illustrating a (partial) cross-sectional view of one example of an optical security component according to the present description. The optical security component 101 shown in figure 1A is, for example, an optical security component intended to be transferred to a document or product with a view to increasing its security. In this example, it comprises a support film 111, for example a polymer film, for example a film made of polyethylene terephthalate (PET) of a thickness of a few tens of microns, typically 15 to 100 [m, and an (optional) detachment layer 112, which is for example made of natural or synthetic wax. The detachment layer allows the polymer support film 111 to be removed after the optical component has been transferred to the product or document the security of which is to be increased. The optical security component 101 moreover comprises a first layer 113 of dielectric material that is said to be "low-index", that is transparent in the visible range, and that has a first refractive index ni, and a second layer 114 of dielectric material that is said to be "high-index", that is transparent in the visible range, that makes contact with the low-index first layer 113, and that has a second refractive index n2. The difference between the second refractive index and the first refractive index is larger than or equal to 0.5, and hence the first layer 113 of dielectric material is called the "low-index" layer and the second layer 114 of dielectric material is called the "high-index" layer. The optical security component 101 further comprises a metal layer 115 making contact with the high-index second layer 114 of dielectric material. In figure 1A, the low-index first layer 113 of dielectric material, the high-index second layer 114 of dielectric material and the metal layer 115 form a first continuous dielectric dielectric-metal double interface I, which comprises a first dielectric-dielectric interface and a first dielectric-metal interface. The double interface I is structured to form, in at least one first coupling region Z1, a first two-dimensional coupling grating C1 , one example of which is illustrated in figure lB and which will be described in more detail below. By continuous double interface, what is meant is that all the constituent layers 113, 114, 115 of the double interface I are deposited continuously, i.e. without interruptions, in at least one zone comprising said first coupling region. More precisely, in the case of figure 1A, as will be described in more detail below, the double interface I is structured to form a plurality of contiguous regions Zi, Z 2 , Z 3 , including the first coupling region Z1, and all of the layers 113, 114, 115 remain continuous through the plurality of said regions. In the example of figure 1A, the optical security component 101 also comprises a protective layer 116 of dielectric material making contact with the metal layer 115. The protective layer 116 may moreover form an adhesive layer or a protective layer. As will be described in more detail below, the protective layer 116 may also form a third layer of dielectric material that is transparent in the visible range, that has a third refractive index n3 and that forms a second dielectric-metal interface, said second dielectric-metal interface being continuous and structured in the same way as the double interface I. The optical security component may moreover comprise one or more layers (not shown in figure 1A) that are optically non-functional but that are tailored to the application, for example an adhesive layer if the layer 116 does not already form an adhesive layer, for example a heat-reactive adhesive layer, for transferring the optical security component to the product or document. According to one particular example of embodiment, the detachment layer 112 may be discontinuous, for example with a view to forming a label. Furthermore, the adhesive layer may have a permanent adhesive power, an intermediate backing thus being used to allow the component to be handled before it is applied to the document or product to be protected. In practice, as will be detailed below, the optical security component may be manufactured by stacking the layers on the support film 111; the component is then transferred, by means of the adhesive layer, to a document/product the security of which is to be increased. Optionally, the support film 111 may then be detached, for example by means of the detachment layer 112. The main observation face 1OOA of the optical security component (or first observation face) is thus located on the side of the first layer 113, i.e. on the side opposite the etched face of the layer 113. In other examples, the optical security component may be intended to increase the security of banknotes; it is for example a question of a track applied by hot-melt adhesive bonding, or of one portion of a security thread intended to be incorporated into the paper during manufacture of the note. In these other examples, the optical security component comprises, as above, a support film 111 (12 to 25 m in thickness) that will also serve as a protective film for the security thread, but no detachment layer. The optical security component may also comprise in these other examples, in addition to the protective layer 116, optional layers, such as a protective layer, a second polymer film or a varnish for example. As in the previous example, manufacture may be carried out by stacking the layers on the support film 111. It will be obvious to anyone skilled in the art that other layers that are optically non functional in the visible spectral range may be added, depending on the needs of the application, in each of the examples described above. It will be noted that if the additional, optically non-functional layers, the adhesive layer for example, or the layers that provide contrast and/or protection, are transparent in the visible, and if the same goes for the destination substrate, the optical security component will possibly be visible from both sides.

According to the present description, said at least one first coupling grating C1 , one example of which is illustrated in a 3-dimensional view in figure IB, is a two-dimensional grating, with a first direction X and a second direction Y, that has a first sub-wavelength period px, comprised between 150 nm and 350 nm, in the first direction, and a second sub wavelength period py, comprised between 150 nm and 350 nm, in the second direction. According to the present description, the first coupling grating C 1 is determined so as to generate a first plasmon-resonance effect at said at least one first dielectric-metal interface in a first resonance spectral band. In figure IB, only the high-index dielectric layer 114 and the metal layer 115 have been shown. The coupling grating is formed in this example by two sets of corrugations extending in the two directions X, Y to form a two-dimensional structure. In this example, the two directions are perpendicular. The grating is characterized by the pitch (or period) of each set of corrugations in each of the X and Y directions, by the depth or amplitude of the corrugation (defined as the height between peak and trough), by the profile of the grating in each direction and by the duty cycle in each direction. The depth of the grating is advantageously comprised between 10% and 80% of the pitch of the grating, and advantageously comprised between 10% and 50%. The profile of the corrugations is for example sinusoidal or quasi-sinusoidal, or more generally continuously variable as illustrated in figure IB, this type of profile allowing a better propagation of the plasmon modes and being compatible with manufacturing methods employing photolithography. Of course, other two-dimensional-coupling-grating profiles are possible, irrespectively of whether the gratings are symmetric, dissymmetric or asymmetric. Generally, it is known that, at the interface between a conductive material, a metal for example, and a dielectric material of refractive index n, an electromagnetic surface wave is able to propagate, this wave being associated with an oscillation in charge density at the surface, which oscillation is called a surface plasmon. This effect is for example described in the textbook by H. Raether ("Surface Plasmons", Springer-Verlag, Berlin Heidelberg). An incident light wave may be coupled to a plasmon mode in various ways, and especially by structuring the interface to form a coupling grating, for example a coupling grating such as described with reference to figure lB. In each direction of the coupling grating, a grating vector kgx, kgy of direction perpendicular to the lines of the grating and of norm defined by kgx = 27c/px and kgy= 2;c/py, respectively, may be defined.

As described in patent EP 2 771 724 in the name of the applicant, such a structure has a response that differs depending on the polarization of the incident wave. Thus, to start with let the following be considered: an incident wave of wavelength X and of TM polarization (transverse-magnetic wave), i.e. an incident wave the magnetic field H of which is perpendicular to the plane of incidence XZ, which is defined as the plane comprising the wave vector ko of the incident wave and the direction Z normal to the component (plane of the figure in figure 1A). The wave of TM polarization is moreover incident on the grating with an azimuth of 0° with respect to the grating defined by the grating vector kgxand with an angle of incidence on the layer 113 of 0 with respect to the axis Z normal to the plane of the grating. For there to be coupling, i.e. transfer of energy between the wave incident on a dielectric medium of relative permittivity ed and the plasmon mode, it has been shown that the following equality must be satisfied (see H. Raether, ibid.):

[Math 1] ksp = nako sinG+ m.kgx where m is the evanescent diffracted order, ko is the wavenumber defined by ko = 2 /l,ksp

is defined by k,= n, ko, where nsp is the effective plasmon index, given by:

[Math 2] ns, = EmEd/(Em + Ed) in the case of a metal layer of infinite thickness withcm and Ed being the permittivities of the metal and of the dielectric material, respectively. It is thus possible to define a central coupling wavelength Xox for an observation of the component at a given angle of incidence Oo:

[Math 3] Xox= (px/m)*(nsp-nd*sin(0o)). In a spectral band centered on the central coupling wavelength, the light energy incident on the dielectric medium is coupled to the plasmon mode, resulting in the absorption of this energy in the metal layer. This results in a modification of the spectrum of the reflected light energy. For incident radiation in TM mode, the optical security component thus behaves, with respect to color, like a band-stop filter. Let the following now be considered: an incident wave with the same angle of incidence but with a TE polarization (transverse-electric wave, i.e. a wave the electric field E of which is perpendicular to the plane of incidence XZ, which is the plane of the figure in figure 1A), the grating having a vector grating kgy (figure 1B) at 900 to the grating vector kgx. Plasmon excitation will then be possible if the incident wave meets the coupling conditions,i.e.:

[Math 4]

k, = ((ko*nd*sin(6o)) 2 +(m*kgy) 2)1/ 2

s A new central coupling wavelength Aoy, which is independent of the first because of its opposite polarization, may then be defined as:

[Math 5]

Aoy = (py/Im|)*(nsp 2 - nd2*sin28O)1/ 2

Thus for an unpolarized incident wave, one portion of the incident radiation will be coupled to a plasmon mode by virtue of a set of corrugations in one direction, and another portion of the radiation will be coupled to a plasmon mode by virtue of the set of corrugations in another direction, this resulting in absorption, in the metal layer, in a first spectral band as a result of coupling at the various coupling wavelengths described above.

As moreover described in patent EP 2 695 006 in the name of the applicant, when the metal layer is of finite thickness and, in addition, its thickness is of the same order of magnitude as the depth of penetration of the electromagnetic field of the plasmon mode into the metal (which is about 1/(ko(nsp2+Re(|Em|)) 1/2)), the electromagnetic field of the plasmon mode at the upper interface of the metal layer also "sees" the lower interface and must therefore also meet the field boundary conditions at this lower interface. It follows that there are then two plasmon modes able to propagate along the metal layer, both of which have a maximum field at the upper and lower interfaces of the metal layer: one plasmon mode (called the "long range" plasmon mode) the transverse magnetic field H of which is even (the longitudinal electric field that is responsible for the longitudinal oscillation in charge density is therefore odd, with a zero crossing in the metal layer), and one plasmon mode (called the "short range" plasmon mode) the field H of which is odd, and which is more strongly absorbed by the metal. Their effective indices are similar when the thickness of the metal layer is not too small (larger than 15 nm, for example) and these modes are both coupled to in the presence of a grating when the incident wave is generated by a light source that is not very spatially and temporally coherent, such as an incandescent lamp or natural sunlight. Thus, when the coupling condition is met, the field of the two plasmon modes coupled to (or "excited") also possesses a maximum at the lower interface of the metal layer and may therefore, by virtue of the presence of the grating, radiate into the transmission medium (layer 116, figure 1A) and thus allow light energy to pass through the continuous metal layer and thus produce a transmission peak - hence the term resonant transmission. In the optical security component according to the present description, the effective index of the plasmon (equation (2) above) is determined by the refractive index ni of the low index first layer 113 of dielectric material but also by the refractive index n2 of the high index second layer 114 of dielectric material. The presence of the high-index layer of dielectric material thus causes a shift of the first spectral band with respect to a single dielectric-metal interface. Compared to the plasmonic effects described above, the applicant has further demonstrated, for the first time in the security field, an additional effect that is due to the high-index layer of dielectric material and that results in original color effects, and especially hue angles that are not observed in prior-art optical security components. More precisely, the thickness of the high-index second layer 114 of dielectric material, which is generally comprised between 20 nm and 150 nm, is determined, especially depending on the nature of the materials and on the characteristics of the coupling grating, so as to generate a hybrid plasmon-resonance effect, in a second resonance spectral band that is different from said first spectral band. This effect is demonstrated, for example, by means of the curves shown in figure 2A. These curves show normalized reflectance Roas a function of wavelength for a wave of unpolarized light incident, at normal incidence (Oo = 0) and zero azimuth, on the coupling region Zi of an optical security component of the type in figure 1A. The reflection was observed from the side of the first observation face 1OOA. The coupling grating C1 was a two-dimensional grating with a period of 280 nm in each direction, and with a sinusoidal profile with a depth of 42 nm (aspect ratio of 0.15). The refractive index of the transparent layers 113, 116 of dielectric material was 1.5 and the metal layer was a layer of 25 rn of aluminum allowing plasmon modes at the two metal-dielectric interfaces 114/115 and 115/116 to be coupled to; this resulted in resonant-transmission effects. The curves were respectively computed for the following cases: no high-index layer of dielectric material (curve 21R), high-index layer of dielectric material (114, figure 1A) with a refractive index of 2.4 (ZnS) and a thickness of 30 nm (curve 2 2 R), high-index layer of dielectric material (114, figure 1A) with a refractive index of 2.4 and a thickness of 60 nm (curve 2 3 R), high- index layer of dielectric material (114, figure 1A) with a refractive index of 2.4 and a thickness of 90 nm (curve 2 4 R).

The curves were computed using the commercial software package MC Grating Software. As may be seen in figure 2A, curve 21R exhibits a single first resonance spectral band corresponding to a trough (referenced 210) in the normalized-reflectance curve. This is a conventional normalized-reflection curve resulting from a plasmonic effect at a single dielectric-metal interface. When a high-index layer of dielectric material is introduced (curve 2 2 R), a shift in the first resonance spectral band is observed as a result of the variation in the effective index of the plasmon mode (trough 220). When the high-index 2 3 layer of dielectric material is sufficiently thick (curve R), in addition to the shift in the first spectral band (trough 231), a second trough (232) in the normalized-reflectance curve, corresponding to a second resonance spectral band, is observed to appear. When the 2 4 thickness of the high-index layer of dielectric material is increased further (curve R), in addition to the shift in the first spectral band (trough 241) and to the second trough (242) corresponding to the second resonance spectral band, a third trough (243) corresponding to a third resonance spectral band is observed to appear. The applicant has shown that the second resonance spectral band results from a "hybrid" plasmon mode in which the energy of the electromagnetic field remains maximum at the surface of the metal but spreads into the high-index layer of dielectric material. The third resonance spectral band results from resonance of guided modes in the high-index layer of dielectric material. These effects are demonstrated in the images shown in figures 3A - 3C, which show the intensity of the electromagnetic field in the high-index layer of dielectric material at wavelengths corresponding to the central wavelengths of the resonance spectral bands evidenced by the troughs 241, 242, 243 of normalized-reflection curve 24R in figure 2A, respectively. Figures 3A - 3C show images of the modulus of the electric field determined in the near field, and more precisely the component Ey of the field, i.e. the projection of the electric field onto the axis Y. The images were computed with the software package MC Grating Software. The near-field computation allows the particular characteristics of the various resonances present in the system to be revealed. This allows where the energy of the field is concentrated to be known. In particular, it may be seen that the energy of the hybrid mode (figure 3B) is less concentrated at the surface of the metal than the energy of the pure plasmon mode (figure 3A). This amounts to saying that the hybrid mode has a lower effective index than the pure plasmon mode. As explained above, the hybrid mode is dependent on the high-index second layer of dielectric material and may exist and be efficiently coupled to only if the thickness of the high-index second layer of dielectric material is sufficiently large. In addition, if the high-index second layer of dielectric material is very thick (figure 3C), in addition to the hybrid mode, a guided mode is observed to appear in the thickness of the high-index second layer of dielectric material (and not at the surface of the metal). The effective index of the guided mode is even lower than the effective indices of the hybrid and plasmon modes. There are no analytical formulae allowing the effective indices of the modes to be determined, but electromagnetic-field simulations carried out by the applicant have allowed the various modes that appear as the thickness of the high-index second layer of dielectric material increases to be revealed. In particular, the appearance of the second resonance spectral band allows, in reflection and at zero order, color effects with original colors having hue angles that cannot be obtained with the plasmonic structures of the prior art to be made to appear. Thus, let it be assumed for example that it is desired to obtain, in reflection and at normal incidence, a color that resembles cyan and that is characterized by two central resonance wavelengths, X1 = 430 nm and X2 = 580 nm, of two resonance spectral bands corresponding to two troughs in the normalized-reflectance curve, respectively. It will further be assumed that the grating is a two-dimensional coupling grating with a square unit cell (identical periods in the two directions). For example, the commercial software package MC Grating Software ©, which allows diffraction efficiencies to computed for various types of structures, is used. There are other software packages that may also be used, such as the software package S4 or S4 (Stanford Stratified Structure Solver) or the software package developed by the company Lumerical©. The profile of the grating in each of its directions is defined, for example by means of Fourier harmonics. Next, two semi-infinite media corresponding to the low-index layers of dielectric material 113, 116 (figure 1A), the refractive index of which is for example 1.5, are defined. The period of the grating and the thickness of the high-index layer of dielectric material (114, figure 1A) are then chosen so as to adjust the spectral positions of the plasmon and hybrid resonances to bring them closer to the initially set wavelengthsX1 and X2 . These spectral positions are determined based on computation of the diffraction efficiency reflected at order 0. Finally, the depth of the grating and the thickness of the metal (aluminum for example) layer are chosen so as to adjust the amplitudes of resonance and obtain the most efficient coupling possible, i.e. a maximum trough amplitude and a minimum of troughs corresponding to resonance spectral bands tending toward 0. Figure 2B illustrates curves showing normalized transmittance To as a function of wavelength for a wave of unpolarized light incident, at normal incidence (Oo = 0) and zero azimuth, on the coupling region Zi of an optical security component of the type in figure 1A. The transmission was observed either from the side of the second observation face 1OOB when the component was illuminated from the side of the face 1OOA, or from the side of the observation face 1OOA when the component was illuminated from the side of the face 1OOB. The conditions were identical to the conditions used to determine the curves shown in figure 2A. The curves 21T, 2 2T, 2 3T, 2 4 T correspond to the curves computed in the case of no high-index layer of dielectric material, of a high-index layer of dielectric material with a refractive index of 2.4 (ZnS) and a thickness of 30 nm, of a high-index layer of dielectric material with a refractive index of 2.4 and a thickness of 60 nm, and of a high index layer of dielectric material with a refractive index of 2.4 and a thickness of 90 nm, respectively. It may be seen from these curves that the addition of the high-index second layer of dielectric material also has an impact on transmittance. In the presence of the dielectric dielectric-metal double interface and if the high-index second layer of dielectric material is of sufficient thickness, transmitted colors different from those observed with a single dielectric-metal interface as in the prior art (curve 21T) are observed. Figure 2C illustrates the influence of polarization on normalized reflectance. The parameters were identical to those used to compute the curves shown in figures 2A and 2B, with the exception that the incident wave is polarized (TM or TE polarization) and incident with an angle of incidence Oo = 40. The thickness of the layer of dielectric material was 80 nm. As illustrated in figure 2C, in TM polarization a normalized-reflectance curve (curve 27) that is different from the normalized-reflectance curve in TE polarization (curve 28) is observed. This polarization dependency allows the optical security component to be authenticated in a manner other than authentication with the naked eye, in natural light. For example, provision might be made to illuminate the optical security component with natural light and to observe, through a polarizer, a change in the color of the color effect as a function of polarization direction; or to illuminate the optical security component with linearly polarized light and to observe a change in the color of the color effect as a function of polarization direction. In the example of figure 2C, the coupling grating was chosen to have a square unit cell (identical periods in the two directions), and thus the effect of polarization will be visible with a non-zero incidence. In the case of a rectangular unit cell, or more generally of a unit cell having different periods in the two directions, the authentication will also be able to be done at normal incidence. An optical security component could thus be designed with at least one first double interface structured to have at least one first coupling region comprising a first coupling grating with a square unit cell, and at least one second coupling region comprising a second coupling grating with a (non-square) rectangular unit cell. In this configuration, a polarization-based inspection at normal incidence will allow a change in color with the change of polarization to be observed only in the second coupling region. It will also be possible to carry out a polarization-based inspection of twofold nature, with the color changing or not changing, in the first coupling region, depending on observation angle. It will be noted that authentication via polarization analysis may be achieved with the naked eye as described above, or with equipment such as an automatic inspecting machine (such as a document reader, for example of the type used to check passports or inspect banknotes) or any other inspecting equipment incorporating a source of white light, a polarizing filter, a sensor, and a data-processing unit. Figures 4A - 4C illustrate the authentication of an optical security component 42 according to one example of embodiment, said component 42 being arranged on a substrate 41 of a security document 40. Authentication is performed with the naked eye, in unpolarized white light. In this example the optical security component is a component such as described with reference to figure 1A, in which component the double interface I comprises 3 contiguous regions Z1, Z 2 , Z 3 . The first region Zi is structured with a first coupling grating C 1 , the second region Z 2 is structured with a second coupling grating C 2 different from the first coupling grating, and the region Z 3 is not structured. The three regions are contiguous and arranged to form a recognizable pattern, for example an "R" as shown in figure 4A. The metal layer 115 is assumed to be sufficiently thin to allow a resonant-transmission effect, as illustrated in figure 2B. Figures 4B, 4C and 4D show observation in reflection from the first observation face OOA, observation in reflection from the second observation face 1OOB opposite the first observation face, and observation in transmission, respectively. In the figures, the position of the light source has been referenced SRC and the position of the observer relative to the light source has been symbolized by an eye OBS. On the side of the main observation face (face OOA, figure 1A) two different colors corresponding respectively to the coupling regions Zi and Z 2 may be observed, as schematically shown in figure 4B. In these coupling regions, the low-index, high-index, and metal layers and the coupling gratings are determined so as to obtain hybrid plasmon modes in separate resonance spectral bands. In the regions Z Iand Z 2 , original and intense colors with different hue angles are thus observed in reflection. By way of reminder, figure 10 shows a schematic illustrating the CIE Lab sphere adopted by the International Committee for Illumination (CIE) in 1976 and defined according to the standard ISO 11664-4. The hue angle hab indicates the color; it changes from red (angle 0) to yellow (90°) then to green (180) then to blue (270°). On the sphere, lightness L is represented on the y-axis, the colors varying from lightest (L = 100) to darkest (L = 0). The distance C measured from the center of the sphere indicates the chroma (or saturation) and is equal to:

[Math 6]

C = a 2 + b2 In practice, the applicant has shown that it is possible to obtain, by means of the optical security component, colors with any hue angle, and especially with hue angles between 1200and 320°, i.e. between green and violet, and therefore shades of blue close to cyan, which colors were not obtainable with the plasmonic components of the prior art. As shown in figure 4B, region Z 3 will appear reflective because it is unstructured. It is remarkable to note that the 3 regions Z, Z 2 and Z 3 and the corresponding visual effects will be able to be identified very precisely (inter-region junctions referenced 118 in figure 1A) because these color effects result from the structures of the various regions and not from local deposition of an additional layer of a material (a pigment or dye for example). Specifically, the local deposition of an additional layer, for example by printing, requires a minimum distance to be provided between two zones of different colors. It will thus be possible to form multicolor images, for example images formed from pixels, each pixel corresponding to one region and having, for example, dimensions smaller than 300 m so as to be invisible to the naked eye, or, in contrast, larger regions that are identifiable with the naked eye, as in the example of figures 4A - 4C. Figure 4C schematically shows the same document, the security of which has been increased by means of the same optical security component 42, but seen in reflection from the side of the second observation face 1OOB. Seen from this side, the color effect results from a plasmonic effect at the single dielectric-metal interface (115/116, figure 1A). The colors observed in the coupling regions Zi, Z 2 are therefore different from those observed via the main observation face 1OOA. Again, the region Z 3 appears reflective because it is unstructured. Figure 4D schematically shows the same document, the security of which has been increased by means of the same optical security component 42, but seen in transmission. The color effects in the regions Z, Z 2 result from a resonant-transmission effect as illustrated by means of the curves of figure 2B. The region Z 3 appears black and not transparent, because it is unstructured. It will be noted that when a thin metal layer is used, as is the case in the example of figures 4B - 4D, to observe a resonant-transmission effect, the effect in reflection is substantially the same as the effect in reflection in the case where a thick metal layer is used. In particular, hue does not change but lightness is modified, i.e. the light or dark aspect of the color. As explained above, a person skilled in the art will be able to use known software packages to determine the characteristics of the structure (choice of materials, thicknesses of the layers, parameters of the coupling grating or gratings in the various regions) in order to choose the desired color effects. Figure 5A is a graph showing curves illustrating the spectral position of the hybrid mode as a function of the thickness of the (high-index) second layer of dielectric material, in the case of a coupling grating having a square unit cell and for various values of the period, in one example of an optical security component according to the present description. Observation was in reflection, at normal incidence.

To compute these curves, the material chosen was ZnS (refractive index 2.4), the metal layer was an aluminum layer of 25 nm thickness, the low-index layers (113, 116, figure 1A) had a refractive index of 1.5, and the coupling grating had a sinusoidal profile in each direction, a constant aspect ratio equal to 0.15, and a square unit cell with a period in each direction of 200 nm (curve 51), 240 nm (curve 52), 280 nm (curve 53), and 320 nm (curve

54). As may be seen from the various curves, the choice of the period, for various values of the thickness of the high-index layer of dielectric material (114, figure 1A), allows the spectral position of the hybrid mode (troughs in the normalized reflectance) to be influenced. It may be seen from this example that a minimum thickness of 45 nm of ZnS is required to observe the hybrid mode. Figure 5B is a graph illustrating the colors that may be obtained (ab representation, see figure 10), by varying the period of the coupling grating between 220 nm and 350 nm, in an example of an optical security component that is the same as that used for figure 5A, for a thickness of ZnS of 60 nm (curve 57). This curve is compared with a curve obtained without ZnS (curve 55) and with a thickness of ZnS of 20 nm (curve 56). As may be seen in figure 5B, the hue angles that may be obtained with an optical security component according to the present description (curve 57), in which component the hybrid plasmon mode is present, are different from those obtained with an optical security component according to the prior art (curve 55) or with an optical security component in which the thickness of the high-index layer is too small to obtain the hybrid mode (curve 56). In the examples described by means of the preceding figures, the optical security component comprises a single first dielectric-dielectric-metal double interface. In other examples of embodiment, the optical security component comprises a second dielectric-dielectric-metal double interface, as illustrated in a partial 3D view in figure 6A. In this example, the optical security component comprises, in addition to the layers 113, 114, 116 of dielectric material described above, a fourth layer 117 of dielectric material, making contact with the metal layer 115 on the side opposite the side making contact with the second layer 114 of dielectric material. In this example, the third layer 116 of dielectric material (not shown in figure 6A) makes contact with said layer 117 of dielectric material. For example, the third layer 116 of dielectric material has a third refractive index n3 and the fourth layer 117 of dielectric material has a fourth refractive index n4 such that the index difference between n4 and n3 is larger than 0.5. The third layer 116 of dielectric material then forms a second "low-index" layer and the fourth layer 117 of dielectric material forms a second "high-index" layer. In this example, it is possible, especially by suitably choosing the thickness of the "high index" fourth layer 117 of dielectric material, to also obtain, from a second observation face (1OOB, figure 1A) opposite the first observation face (1OOA, figure 1A), a hybrid plasmon-resonance effect as described above and therefore noteworthy color effects. Figure 6B thus shows a schematic illustrating a first color effect visible in reflection from the first observation face, with an optical security component 62 arranged on a substrate 61 of a security document 60, the optical security component having first and second double interfaces, the double interfaces being structured with the same pattern as that illustrated in figure 4A. As above, the metal layer is assumed to be sufficiently thin for plasmon resonance effects to be observed in transmission. Thus, as illustrated in figure 6B, on the side of the main observation face (face OOA, figure 1A), two different colors corresponding to the coupling regions Zi and Z 2 are observed, respectively. In these coupling regions, the low-index layer (113, figure 1A), the high index layer (114), the metal layer (115) and the coupling gratings are determined so as to obtain hybrid plasmon modes in separate resonance spectral bands. In the regions Zi and Z 2 , original and intense colors with different hue angles are thus observed in reflection. As above, the region Z 3 will appear reflective because it is unstructured. Figure 6C shows schematically the same document, the security of which has been increased by means of the same optical security component 62, but seen in reflection from the side of the second observation face 1OOB (back side). Seen from this side, in this example, the color effect is identical to that observed on the side of the main observation face (front side) because, for example, the thicknesses and indices of the high-index dielectric layers 114, 117 have been chosen to be identical. However, it is also possible to choose different high-index dielectric layers 114, 117, in which case the colors observed in the coupling regions Zi, Z 2 will be different from those observed via the main observation face 1OOA. In contrast, the unstructured region Z 3 always appears reflective and colorless. Figure 6D schematically shows the same document, the security of which has been increased by means of the same optical security component 62, but seen in transmission. The color effects in the regions Z, Z 2 result from a resonant-transmission effect, as illustrated by means of the curves of figure 2B, but are influenced by the second high- index layer 117. The region Z 3 appears black and not transparent, because it is unstructured. As explained previously, it is also possible to choose, for said at least one first coupling grating, a coupling grating having different periods in each of the directions. Figure 7A thus shows a schematic illustrating a view from above of a coupling grating C 3 with different periods px, py in the first and second directions X, Y, in one example of an optical security component according to the present description. In the example of figure 7A, the directions are perpendicular to each other, but it is also possible to have two directions that make between them an angle different from a right angle, an angle comprised between 300 and 60° for example. Figure 7B is a graph showing curves illustrating normalized reflectance Ro as a function of wavelength, at normal incidence, from the main observation face (front side) for a double interface structured with a two-dimensional coupling grating having a coupling-grating period of given value in one direction (px = 280 nm) and a period py of various values in the other direction. The axis X is in this example comprised in the plane of incidence and the axis Y is perpendicular to the plane of incidence. The curves were computed for a two dimensional grating of the type shown in figure 7A, i.e. one having a sinusoidal profile in each of the directions, and an aspect ratio (depth over period) along the axis Y equal to 0.15. The double interface comprised an aluminum metal layer of 25 nm thickness and a high-index ZnS (refractive index 2.4) layer of dielectric material of 80 nm thickness. The low-index layers 113, 116 (figure 1A) had a refractive index of 1.5. More precisely, the curves 74, 75, 76 were computed for a period py = 240 nm (curve 74), py = 280 nm (curve 75), and py = 320 nm (curve 76), respectively. It is remarkable to note, from these curves, that the trough in the normalized-reflectance curve corresponding to the first spectral band (troughs at longer wavelengths, which troughs are referenced 741, 751, and 761, respectively) and resulting from the plasmon resonance effect remains constant with the period py, whereas the trough in the normalized reflectance curve corresponding to the second spectral band (troughs at shorter wavelengths, which troughs are referenced 742, 752, and 762, respectively) and resulting from the hybrid plasmon-resonance effect varies with the period py. These curves reveal the dependency of the hybrid mode on the period py. It is thus possible to obtain colors that cannot be achieved with a square unit cell (obviously only if the thickness of the high-index layer allows the hybrid mode to be excited).

Figure 8 shows a schematic illustrating histograms of hue angles in an optical security component according to the present description (histogram 82), and in an optical security component according to the prior art (histogram 81). More precisely, the histogram 82 shows the occurrence of structures for each hue angle between 0° and 350, for a structure of the type shown in figure 1A (two-dimensional coupling grating of square unit cell). More precisely, the histogram 82 was obtained by varying the period between 230 nm and 300 nm, the depth of the grating between 40 nm and 120 nm, the thickness of the (aluminum) metal layer between 10 nm and 35 nm and the thickness of the high-index layer of dielectric material between 50 and 140 nm. The profile of the grating was sinusoidal in each of the directions, the refractive index of the low-index layer of dielectric material was 1.5 and the refractive index of the high-index layer of dielectric material was 2.4. By way of comparison, the histogram 81 shows the occurrence of structures for each hue angle, for a structure similar to that used to obtain the histogram 81, but in which the thickness of the high-index dielectric layer is comprised between 0 and 40 nm. The other parameters were varied in the same way as for the histogram 82. In the histogram 81 (prior art), only hue angles comprised between 0-120° and 320-350° are observed. In contrast, in the histogram 82 obtained with an optical security component according to the present description, a uniform distribution of the hue angles is observed, and in particular it may be seen that certain structures allow hue angles comprised between 120° and 320° to be obtained. A method for manufacturing optical security components according to the present description advantageously comprises the following steps. The optical structure formed of said at least one first coupling grating or of a microscopic structure modulated by said at least one first coupling grating is recorded by photolithography or electron-beam lithography in a photoresist. An electroplating step allows the optical structure to be transferred to a resistant material, one based on nickel for example, so as to produce a metal master comprising the optical structure. The manufacture of the optical security component then comprises a replication step. For example, the replication may be carried out by stamping (by hot embossing the dielectric of) the first layer 113 of dielectric material (figure 1A) of refractive index ni (low-index layer), which is typically a stamping varnish of a few microns thickness. The layer 113 is advantageously borne by the support film 111, which is for example a film of 12 m to 100 m thickness made of a polymer material, of PET (polyethylene terephthalate) for example. Replication may also be achieved by molding the layer of stamping varnish before drying and then UV casting. Replication by UV casting especially allows structures having a depth of large amplitude to be reproduced, and allows a higher fidelity replication to be obtained. Generally, any other high-resolution replication method known in the prior art may be used in the replication step. Next, all the other layers, and especially the second layer 114 of dielectric material (high-index layer), the metal layer 115, and the optional other layers, including for example the protective layer 116, are deposited on the layer thus embossed. As should be clear from the example of a manufacturing process described above, the inclusion of an optical security component according to the present description in a security document is perfectly compatible with the presence in the same document of grating-based structures conventionally used to produce holographic components. In particular, it will be possible to produce an optical security element comprising one or more plasmonic components such as described above and one or more other types of optical security components known in the prior art (holographic optical security components for example). To this end, a master will possibly be produced by recording the various patterns corresponding to the various optical security components in the photoresist, this step being followed by an electroplating step. Stamping may then be carried out using the master in order to transfer the various microstructures to the polymer film intended to be embossed. The metal layer and/or the high-index layer of dielectric material, the thicknesses of which are set to achieve plasmonic-effect optical security components according to the present description, will possibly be deposited on the whole of the film, or selectively on the optical security component according to the present description. Selective deposition of the reflective (metal and high-index) layers for example allows the optical effects of said components according to the present description to be further enhanced and/or the substrate of the document or object the security of which is to be increased to be uncovered, the uncovered areas forming non-reflective patterns. Selective metallization (selective deposition of the high-index layer, respectively) may be obtained via a first step of depositing a metal layer (high-index layer, respectively) on the whole of the structured film, then partial demetallization (partial removal of the high-index layer, respectively) to form said non-metallized regions (regions without a high-index layer, respectively).

Figure 9 shows a security document 90, for example a document of value of the banknote type, thus equipped with a security element 92 comprising an optical security component 93 according to the present description. The security element 92 takes the form of a strip, typically of 15 mm width, that is affixed to a substrate 91 of the document 90. The security element 92 is affixed to the substrate by known means. For example, the security element may be affixed via hot-melt transfer, causing a transparent adhesive layer previously applied to the protective layer 116 to react. In this case, an (e.g. wax) detachment layer 112 may be applied between the stamping varnish 113 and the PET support film 111 (figure 1A). The security element is transferred to the document by hot pressing the security element on the document, the plasmonic component being located facing the transparent region. During the transfer, the adhesive film bonds to the substrate 91 of the document and the detachment layer and the support film may be removed. In the substrate 91, a see-through window may be provided level with the plasmonic component according to the present description, if said component is intended to be visible in transmission. It will be noted that if the see-through window corresponds to a transparent substrate, then the support film may actually be removed, as described above. If the see-through window corresponds to a hole in the paper, then the security element is adhesively bonded and the support film retained. The security document thus obtained may be inspected with the greatest ease with the naked eye, in white light, by an inexperienced user, and with high reliability. Visual authentication of the security document is therefore possible, including in natural light. This authentication, which is based on visual effects in reflection and possibly in transmission, is particularly easy to achieve. Due to the dependency of the described effects on polarization, authentication may also be achieved by illuminating the optical security component with linearly polarized light, or by illuminating the optical security component with natural light and observing it through a linear polarizer. The change in polarization will cause a change in color, as explained above with reference to figure 2C. A machine inspection is also possible, inspection by smartphone for example. Specifically, due to the very different hue angles that may be obtained by means of the optical security component, it will be possible, for example, to determine whether the angular difference between two hue angles corresponding to two coupling regions is as it should be.

Although described through a certain number of examples of embodiment, the optical security component according to the present description comprises various variants, modifications and improvements that will appear obvious to those skilled in the art, and it will be understood that these various variants, modifications and improvements fall within the scope of the invention such as defined by the following claims.

Claims

CLAIMS:

1. A plasmonic-effect optical security component, said component being able to be inspected with the naked eye, in reflection, via at least a first observation face and comprising:

- at least one second transparent layer of dielectric material, with a thickness comprised between nm and 150 nm, having a second refractive index, the difference between the second refractive index and the first refractive index being larger than or equal to 0.5 and said second layer of dielectric material making contact with said first layer of dielectric material;

- a metal layer making contact with said at least one second layer of dielectric material; wherein:

- said first layer of dielectric material, said second layer of dielectric material and said metal layer form a first dielectric-dielectric-metal double interface that comprises a first dielectric dielectric interface and a first dielectric-metal interface, and that is structured to form, in at least one first coupling region, a first two-dimensional coupling grating, with a first direction and a second direction , having a first period comprised between 150 nm and 350 nm in the first direction and a second period comprised between 150 nm and 350 nm in the second direction,

- said first coupling grating is determined so as to generate a first plasmon-resonance effect at said at least one first dielectric-metal interface in a first resonance spectral band;

- the thickness of said at least one second layer of dielectric material is determined so as to generate, by means of said first coupling grating , a hybrid plasmon-resonance effect, in a second resonance spectral band different from said first spectral band.

2. The optical security component as claimed in claim 1, further comprising a third transparent layer of dielectric material making contact with said metal layer so as to form a second dielectric-metal interface, said second dielectric-metal interface being structured, in said at least one first coupling region, as said first coupling grating.

3. The optical security component as claimed in claim 1, further comprising a third transparent layer of dielectric material having a third refractive index, and a fourth transparent layer of dielectric material, with a thickness comprised between 20 nm and 150 nm, having a fourth refractive index, the difference between the fourth refractive index and the third refractive index being larger than or equal to 0.5, and wherein:

- said fourth layer of dielectric material makes contact with the metal layer and said third layer of dielectric material makes contact with said fourth layer of dielectric material; and

- said third layer of dielectric material, said fourth layer of dielectric material and said metal layer form a second dielectric-dielectric-metal double interface that comprises a second dielectric dielectric interface and a second dielectric-metalinterface, and that is structured, in said at least one first coupling region, as the first coupling grating.

4. The optical security component as claimed in either one of claims 2 and 3, wherein the metal layer is sufficiently thin to allow surface-plasmon modes supported by said first dielectric metal interface and said second dielectric-metal interface on either side of the metal layer to be coupled to, resulting in a resonant-transmission effect.

5. The optical security component as claimed in any one of the preceding claims, wherein said at least one first coupling grating is dissymmetric.

6. The optical security component as claimed in any one of the preceding claims, wherein said first period and said second period of said at least one first coupling grating are different.

7. The optical security component as claimed in any one of the preceding claims, wherein said at least one first dielectric-dielectric-metal double interface is structured to form, in at least one second region, a structure different from said first coupling grating, said at least one first double interface remaining continuous through all said regions.

8. The optical security component as claimed in claim 7, wherein said structure of said at least one second region comprises a second coupling grating that is different from the first coupling grating.

9. The optical security component as claimed in either one of claims 7 and 8, wherein the optical security component has, in reflection from said first observation face and at a given observation angle, a first color effect with a first hue angle in the first coupling region, and a second color effect with a second hue angle in said second region, the second hue angle being different from the first hue angle by a value at least equal to 20.

10. The optical security component as claimed in any one of the preceding claims, wherein said at least one first dielectric-dielectric-metal double interface is not structured in at least one region contiguous with said first coupling region, said at least one first double interface remaining continuous through all of said regions.

11. The optical security component as claimed in any one of the preceding claims, wherein said at least one first dielectric-dielectric-metal double interface is structured to form a plurality of contiguous regions, including said first coupling region, said at least one first double interface remaining continuous through all of said regions.

12. The optical security component as claimed in claim 11, wherein all of said regions of the plurality of regions form a recognizable pattern.

13. The optical security component as claimed in any one of the preceding claims, wherein said at least one first dielectric-dielectric-metal double interface is structured to form, in said at least one first coupling region, a diffractive first structure modulated by said first coupling grating.

14. An optical security element intended to secure an object, for example a document of value, comprising at least one optical security component as claimed in any one of the preceding claims.

15. A security object, for example a security document of value, comprising a substrate and, deposited on said substrate, an optical security element as claimed in claim 14 or an optical security component as claimed in any one of claims I to 13.

16. The security object as claimed in claim 15, wherein the substrate has a rectangular shape with two perpendicular axes, the axes being non-collinear with the directions of said at least one first coupling grating of the optical security component.

17. The security object as claimed in claim 15, wherein the substrate has a rectangular shape with two perpendicular axes, the axes being collinear with the directions of said at least one first coupling grating of the optical security component.

18. A method for manufacturing an optical security component as claimed in any one of claims I to 13, comprising:

- forming said first layer of dielectric material;

- depositing, on said first layer of dielectric material, said second layer of dielectric material;

- depositing, on said second layer of dielectric material, said metal layer, so as to form said first dielectric-dielectric-metaldouble interface, said first dielectric-dielectric-metal double interface being structured to form, in said at least first coupling region, said first coupling grating.

19. A method for authenticating an optical security component as claimed in any one of claims I to 13, comprising:

- a step of illuminating said optical security component with natural light and observing, through a linear polarizer, a change in the color of the color effect as a function of polarization direction; or

- a step of illuminating said optical security component with linearly polarized light and observing a change in the color of the color effect as a function of polarization direction.