WO2024223226A1 - Optical security components, manufacture of such components and secure objects provided with such components - Google Patents

Abstract

The invention relates in particular to an optical security component (101) comprising a first layer (113), at least one first diffractive structure (S) etched into the first layer, a reflective second layer (114) at least partially covering the first diffractive structure. The first diffractive structure consists of a set of optical elements (OE_i) adjacent to one another. Each optical element has a maximum dimension of less than about 300 μm and comprises a microstructure (M_i) modulated by a sub-wavelength diffraction grating (G_i). Each microstructure is configured to deflect an incident light beam and is substantially invariant by azimuthal rotation with an angle of less than 90° about a first axis. The diffraction grating is substantially invariant by azimuthal rotation with an angle of less than 90° about a second axis substantially coinciding with the first axis and is configured to produce, after depositing the second layer, a coloured effect.

Description

Security optical components, manufacture of such components and secure objects equipped with such components

Technical field of the invention

The present description relates to the field of security marking. More particularly, it relates to optical security components for verifying the authenticity of an object, for example a product or a valuable document, for example an identity document or a banknote, to a method for manufacturing such a component and to a secure object equipped with such a document. In particular, the present description relates to optical security components for verifying the authenticity of an object with the naked eye or by means of an optical imaging device.

State of the art

There are many known technologies for authenticating documents or products, and in particular for securing documents such as valuable documents, documents such as bank notes, passports or other identification documents. These technologies aim at producing optical security components whose optical effects, depending on the observation parameters (orientation of the component relative to the observation axis, position and dimensions of the light source, etc.), take on very characteristic and verifiable configurations. The general aim of these optical components is to provide new and differentiated optical effects, from physical configurations that are difficult to reproduce. Among these components, DOVID stands for "Diffractive Optical Variable Image Device", the optical components producing diffractive and variable images that are commonly called holograms.

The present description concerns security optical components exhibiting remarkable colored optical effects in reflection as well as security optical components exhibiting remarkable colored optical effects in both reflection and transmission.

Transmission control is used in particular in valuable documents, for example banknotes having for this purpose a recessed and/or partially transparent area or passports of which a page of data relating to the bearer is provided with a transparent window. The optical security component may be in the form, for example, of a security thread, a security track, or a "patch", intended to be seen from above, at least partially superimposed on the transparency area, and which can be positioned in surface or in the thickness of the document.

For example, optical security components are known that can be observed in reflection and transmission, which involve resonance mechanisms in a layer of dielectric material. Such optical security components are described, for example, in patent application EP 2264491 [Ref. 1] or in the article by M.T. Gale et al. [Ref. 2] and are known as zero-order diffractive filters (ZOFs) or guided mode resonant filters. The physical mechanism is based on a resonant reflection of a guided mode in the dielectric layer, for example a high-index layer. Remarkable colour effects are visible in reflection and transmission. In particular, when illuminated, for example, with unpolarised polychromatic light, zero-order diffractive filters (ZOFs) can exhibit characteristic and differentiated colour effects when they undergo an azimuthal rotation and are therefore clearly identifiable.

Plasmon resonance security optical components are also known which exhibit, in reflection or transmission, remarkable colored optical effects. The published patent application WO2013060817 [Ref. 3] describes for example an optical security component intended to be observed in the visible, in direct reflection. The optical security component described in [Ref. 3] comprises a continuous metal layer forming with a layer of dielectric material a metal-dielectric interface, the metal layer having a thickness sufficient to allow the reflection of the incident light on the interface. The metal layer is structured at the interface to form, in a first coupling zone, two sets of undulations which extend in two directions and form a two-dimensional coupling network, of subwavelength periods in each of the directions. In such an optical security component, original and contrasting color variation effects are observed, in particular by azimuthal rotation of the component. These remarkable effects can be explained by plasmonic resonance effects at the metal-dielectric interface, which allow the creation of a band-cut reflection filter, variable depending on the observation conditions.

The published patent application WO2020229415 [ref. 4] also describes a security optical component intended to be observed in the visible, in direct reflection. In this example, a first layer of dielectric material, a second layer of dielectric material and a metal layer form a double dielectric-dielectric-metal interface, comprising a dielectric-dielectric interface and a dielectric-metal interface, and structured to form a two-dimensional coupling network with subwavelength periods. This results in a plasmonic resonance in a first spectral band resonance and a hybrid plasmonic resonance, in a second resonance spectral band different from said first spectral band.

The optical security components described in the aforementioned references, implementing guided mode resonance effects or plasmonic resonance effects, are zero-order colored filters, i.e. they are observed in direct reflection.

It should be noted that plasmon resonance security optical components are also known which act as colour filters and exhibit, in transmission, remarkable coloured optical effects. Such components are described for example in patent application US2010/0307705 [Ref. 5] or in published patent application WO2012136777 [Ref. 6],

It is known to generate more complex visual effects from the color filters thus described, in particular dynamic visual effects, in order to generate an additional authentication check.

Thus, by way of example, the published patent application WO2015154943 [Ref. 7] describes an optical security component comprising a diffractive structure etched on a layer of dielectric material. The structure has a first pattern comprising a bas-relief with a first set of facets whose shapes are determined to simulate a series of concave or convex cylindrical optical elements, visible in reflection. The first pattern is modulated by a second pattern forming a subwavelength grating and configured to generate, after deposition of a layer of dielectric material and encapsulation of the structure, a guided mode resonant filter (ZOF). Such an optical security component has a dynamic visual effect of light bands of different colors and scrolling in opposite directions when it undergoes a tilt rotation around an axis parallel to one of the main directions of the cylindrical elements. Furthermore, color changes can be observed during an azimuthal rotation of the component.

Thus, resonance mechanisms in security optical components, some of which are described as examples in the aforementioned references, are known for the remarkable optical effects they produce. Added to the fact that such security optical components comprise structures that are difficult to reproduce, it follows that they constitute excellent candidates in the field of security marking.

The present application describes an optical security component which also implements such resonance mechanisms and whose original structure makes it possible to obtain original visual effects compared to those known from the state of the art, allowing even more secure authentication. Summary of the invention

In this description, the term “include” means the same as “include”, “contain”, and is inclusive or open and does not exclude other elements not described or shown. Furthermore, in this description, the term “approximately” or “substantially” means the same as “having a margin less than and/or more than 10%, for example 5%”, of the respective value.

According to a first aspect, the invention relates to an optical security component for securing an object, for example a valuable document, for example an identity document or a bank note.

The optical security component according to the first aspect comprises: a first layer of dielectric material, transparent in the visible, having a first refractive index; at least one first diffractive structure etched on said first layer; and a second layer, at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible; and wherein: said first diffractive structure is made up of a set of optical elements adjacent to each other, each optical element having a maximum dimension less than approximately 300 pm and comprising a microstructure modulated by a subwavelength diffraction grating with one or more steps between approximately 150 nm and approximately 500 nm, such that: said microstructure is configured to deflect an incident light beam and is substantially invariant by azimuthal rotation of an angle less than 90° around a first axis substantially perpendicular to a plane of the component; said subwavelength diffraction grating is substantially invariant by azimuthal rotation of an angle less than 90° around a second axis substantially coincident with said first axis and is configured to produce, after deposition of the second layer, a colored effect.

In the present description, a layer transparent in the visible is defined as a layer having a transmission of at least 70%, preferably at least 80% for a wavelength included in the visible, that is to say a wavelength between approximately 400 nm and approximately 800 nm. A layer thus transparent makes it possible to observe with the naked eye the layers located under the transparent layer.

In the present description, said microstructure is substantially invariant by azimuthal rotation of a given angle less than 90° around a first substantially perpendicular axis. to a plane of the component; this means that a rotation of said microstructure about said first axis with said angle generates a microstructure which substantially superimposes on the microstructure before azimuthal rotation. According to one or more exemplary embodiments, said angle is less than approximately 60°, advantageously less than approximately 45°. In exemplary embodiments, said microstructure is substantially rotationally symmetrical about said first axis, which constitutes an axis of symmetry.

As is well known, in the case where a physical object or system is invariant under any arbitrary rotation of any angle around an axis, we speak of continuous symmetry. In this case we simply say that the system admits an axis of symmetry, without further precision. There are also objects or systems in which there is symmetry around the axis only for certain values of the angle of rotation. This type of rotational symmetry is called discrete in contrast to the previous case. More precisely, an axis of symmetry of order n is such that the system is invariant under any rotation of an angle 27t/n, with n a positive integer. Of course, when a physical object or structure is invariant under a rotation of an angle 27t/n, it is invariant under rotation of any angle multiple of 27t/n. Thus, in the present description, the microstructure can be generally described as a symmetrical microstructure of order n around said first axis, with n greater than or equal to 4. When n is very large, the microstructure approaches a microstructure with symmetry of revolution.

In the present description, said diffraction grating is substantially invariant by azimuthal rotation of an angle of less than 90° about a second axis substantially coincident with said first axis; this means that an azimuthal rotation of a projection in the plane of the component of said diffraction grating about said second axis, with said angle, generates a diffraction grating which substantially superimposes on the diffraction grating before azimuthal rotation. According to one or more exemplary embodiments, said angle is less than approximately 60°, advantageously less than approximately 45°. In exemplary embodiments, said diffraction grating is substantially rotationally symmetrical about said second axis.

Thus, as explained above, the diffraction grating may be invariant to any arbitrary rotation of any angle about said second axis and the grating is rotationally symmetrical about said second axis. In the present description, the diffraction grating may be generally described as a symmetrical diffraction grating of order n about said second axis, with n greater than or equal to 4, i.e. an azimuthal rotation of a projection in the plane of the component of said diffraction grating about said second axis, with an angle equal to 27t/n or any multiple of this angle, generates a diffraction grating which substantially superimposes on the diffraction grating before azimuthal rotation. The optical component according to the first aspect is remarkable in particular in that it makes it possible to generate a texture (the optical elements are each invisible to the naked eye) which presents an original colored optical effect, namely a colored effect, for example a variable colored effect with one or more tilt movements, this colored effect being preserved under the effect of an azimuthal rotation.

For authentication with the naked eye, a first stable or variable color can thus be observed during a right/left and/or up/down tilt movement and/or a second stable or variable color during a right/left and/or up/down tilt movement, but whatever the colored effect observed during a right/left and/or up/down tilt movement, this same colored effect will be observed after azimuthal rotation. This conservation of the colored effect by azimuthal rotation makes it even easier to control. In practice, the conservation of the colored effect will be obtained for at least one azimuthal rotation angle of between approximately 30° and approximately 150°, advantageously between approximately 60° and approximately 120°.

In this description, azimuthal rotation of the optical security component is a rotational movement of the component around an axis substantially perpendicular to a plane of the component.

A tilt is a rotational movement of the component around an axis included in a plane of the component. From a user's point of view, a plane of incidence of the light can be defined which includes an axis of illumination of the component and an axis of observation. A right/left tilt is a rotational movement around an axis included in the plane of the component and in the plane of incidence and an up/down tilt is a rotational movement around an axis included in the plane of the component and perpendicular to the plane of incidence.

This technical effect results from the original structure of the optical security component in which a sub-wavelength grating, invariant by rotation, is centered on each of the microstructures, also invariant by rotation and modulated by said grating.

In contrast to state-of-the-art optical security components implementing resonant effects that produce colored effects that can only be observed in direct reflection, it is also possible to achieve, in exemplary embodiments of optical security components according to the first aspect, color stability in tilt over wide angular ranges.

Furthermore, as the security optical component forms a texture with microstructures invisible to the naked eye, it is possible to generate any pattern macroscopic visible to the naked eye and easily recognizable simply by giving an outline corresponding to the first diffractive structure.

Thus, according to one or more exemplary embodiments, said first diffractive structure has an outline configured to generate a macroscopic pattern recognizable to the naked eye by an observer.

Said macroscopic pattern has a minimum dimension greater than 500 pm, preferably greater than 1 mm, preferably greater than 2 mm, preferably greater than 5 mm. Such a minimum dimension makes it possible to generate a pattern recognizable to the naked eye. The optical component according to the first aspect also has many advantages for machine authentication, i.e. by means of a fixed or portable optical imaging device, compared to the components of the state of the art. On the one hand, it will be possible to simply note the conservation by azimuthal rotation of a variable colored effect in tilt. On the other hand, the conservation by azimuthal rotation of the variable colored effect makes it possible to limit the positioning imprecision during an authentication check of the colored effect. The conditions of the authentication check are thus facilitated.

In a security optical component according to the present description, said first diffractive structure consists of a set of optical elements adjacent to each other, each optical element comprising a microstructure modulated by a diffraction grating. According to one or more exemplary embodiments, a number of optical elements of said first diffractive structure is between approximately 300 and approximately 40,000. This number is for example suitable for optical elements with a maximum dimension of between approximately 50 pm and approximately 200 pm, for example between approximately 70 pm and approximately 150 pm, to make a diffractive structure whose surface area is between approximately 200 mm ² and approximately 800 mm ² , which corresponds for example to surfaces of security wires, tracks or patches.

According to one or more exemplary embodiments, each optical element has a contour of a given shape chosen from: a circular shape, a rectangular shape, for example a square shape, a hexagonal shape. Such shapes can allow an arrangement of the optical elements which optimizes the number of optical elements on a given surface.

According to one or more exemplary embodiments, the optical elements are arranged in a regular arrangement having a hexagonal mesh, which makes it possible to maximize the number of optical elements on a given surface. Other meshes are possible, for example a rectangular mesh or a square mesh.

According to one or more exemplary embodiments, the optical elements of said first diffractive structure are identical. In other exemplary embodiments, they may be different. For example, they may comprise microstructures modulated by different diffraction gratings. In exemplary embodiments, they may comprise different microstructures.

According to one or more exemplary embodiments, the microstructures comprise microlenses, for example spherical or aspherical microlenses.

According to one or more exemplary embodiments, the microstructures comprise subsets of facets each comprising a plurality of facets.

In exemplary embodiments, the facets are substantially rotationally symmetrical and are arranged concentrically or pseudo-concentrically (for example in a spiral shape) around said first axis. In other exemplary embodiments, the facets are similar to straight line segments whose quantity and dimensions are linked to the angle of invariance by azimuthal rotation of the microstructure; for example, when viewed from above, the facets could have a regular polygonal shape, for example a hexagonal or octagonal shape.

The plurality of facets are configured to form a microstructure for deflecting an incident beam. Each facet has a given height and width.

In this description, the "height" of a facet is a distance between a lowest level of the facet and a highest level, the distance being measured along an axis perpendicular to a plane parallel to the plane of the component.

The "width" of a facet is the width of the crown resulting from the projection of the facet in a plane parallel to the plane of the component.

A width of the facets is advantageously greater than or equal to approximately 4 times, advantageously greater than or equal to approximately 8 times a step of the diffraction grating. The minimum dimension can therefore be chosen according to the period of the grating. For example, a minimum dimension of the width of the facets is equal to approximately 2 pm.

According to one or more examples, the facets have a width of between approximately 2 pm and approximately 100 pm, advantageously between approximately 2 pm and approximately 80 pm, advantageously approximately 4 pm and approximately 40 pm.

According to one or more exemplary embodiments, all of the facets have a substantially identical height. The height of the facets is for example less than approximately 2 microns, advantageously less than approximately 1 micron. According to one or more examples, the facets of all of the facets have different heights. In this case, however, the facets have a maximum height. Said maximum height is for example less than about 2 microns, preferably less than about 1 micron.

According to one or more exemplary embodiments, all of the facets have a low level located in the same plane. In other exemplary embodiments, the low levels of the facets are not located in the same plane. In the case of facets of different heights, midpoints of the facets located between the low level and the high level may for example be located in the same plane.

According to one or more exemplary embodiments, said minimum angular value of the slope of a facet (in absolute value) is equal to approximately 1°. According to one or more exemplary embodiments, said maximum angular value of the slope of a facet (in absolute value) is equal to approximately 45°. According to one or more examples, the angular value of the slope of a facet (in absolute value) is between approximately 1° and approximately 30°, advantageously between approximately 2° and approximately 15°.

According to one or more embodiments, each subset of facets comprises a plurality of facets, for example between 2 facets and approximately 150 facets, advantageously between 2 facets and approximately 50 facets, advantageously between 3 and approximately 50 facets, for example between 3 and 10 facets.

According to one or more exemplary embodiments, each subset of facets comprises a plurality of facets arranged concentrically and the arrangement of the facets in at least part of the subset of facets is such that the facets are arranged with variable slopes and the variation of which is increasing in absolute value from the center of the subset of facets towards the edge of the subset of facets, alternately decreasing in absolute value from the center of the subset of facets towards the edge of the subset of facets, the subset of facets forming a “Fresnel lens” type structure. Each Fresnel lens has a diameter of less than 300 pm such that it is not visible to the naked eye.

According to one or more exemplary embodiments, each subset of facets comprises a plurality of facets arranged with alternately positive and negative slopes, which are variable, and whose variation is increasing in absolute value from the center of the subset of facets towards the edge of the subset of facets, alternately decreasing in absolute value from the center of the subset of facets towards the edge of the subset of facets, the subset of facets forming a structure which is called an “axicon” in the present description. Each axicon has a diameter of less than 300 μm such that it is not visible to the naked eye.

Of course, other arrangements are possible for the facets of the subsets of facets configured to produce microstructures for deflecting light. An advantage of such microstructures over microlens-type microstructures for example, is the limited height of such a microstructure, typically a height of less than about 2 pm.

In an optical security component according to the first aspect, the diffraction grating which modulates each microstructure is sub-wavelength. In practice, said diffraction grating comprises one or more steps between approximately 150 nm and approximately 400 nm, advantageously between approximately 200 nm and approximately 400 nm.

In a security optical component according to the first aspect, a projection, in the plane of the component, of the diffraction grating which modulates each microstructure is substantially invariant by azimuthal rotation of a given angle around a second axis substantially coincident with the first axis of said microstructure. Such a diffraction grating may be a concentric or pseudo-concentric grating (for example in the form of a spiral), a radial or pseudo-radial grating, a two-dimensional grating with a concentric or pseudo-concentric dimension and a radial or pseudo-radial dimension.

In the present description, a concentric diffraction grating is a diffraction grating comprising concentric grating lines, centered on said second axis. Such a diffraction grating may have a constant pitch, a plurality of pitches or a variable pitch.

In the present description, a pseudo-concentric diffraction grating is a spiral-shaped diffraction grating; for example, a pseudo-concentric diffraction grating comprises a line of substantially increasing radius so as to describe a spiral whose origin is the center of the grating defined by the intersection of the plane of the component with said second axis, and which is centered on said center of the grating. Such a diffraction grating may have a constant pitch (for example, an Archimedes spiral type spiral), a plurality of pitches or a variable pitch (for example, a Fermat spiral type spiral).

A radial diffraction grating is a diffraction grating comprising radial grating lines extending from the center of the grating defined by the intersection of the plane of the component with said second axis. In a radial diffraction grating, the pitch is for example a mean pitch, defined for example at a distance from the center equal to half a radius.

A pseudo-radial diffraction grating is a diffraction grating comprising radial grating lines extending from the center of the grating defined by the intersection of the plane of the component with said second axis and pseudo-radial grating lines parallel to the network lines extending radially from the center of the network. A pseudo-radial network allows defining angular sectors within which the network lines are parallel, which allows for a substantially constant pitch.

Angular sectors have an apex angle ranging from, for example, about 2° to about 60°, for example, about 10° to about 60°.

More generally, in exemplary embodiments, the diffraction grating may comprise angular sectors within which the grating lines are parallel, which makes it possible to have a substantially constant pitch within each angular sector. In the present description, we refer to an “angularly sectorized” diffraction grating. The angular sectors have an apex angle of, for example, between approximately 2° and approximately 60°, for example between approximately 10° and approximately 60°. Such an angularly sectorized diffraction grating is invariant by azimuthal rotation from one sector to another. The azimuthal invariance is discrete for an angular deviation equal to 360° divided by the number of angular sectors. If the number of angular sectors is large, the invariance effect obtained is substantially continuous. A two-dimensional diffraction grating is a diffraction grating comprising both concentric or pseudo-concentric grating lines and radial or pseudo-radial grating lines.

According to one or more exemplary embodiments, in the case of a two-dimensional diffraction grating, at least one parameter of the diffraction grating is different between the concentric or pseudo-concentric grating lines and the radial or pseudoradial grating lines (depth and/or period and/or profile) to obtain a color difference between an up/down tilt and a right/left tilt.

In an optical security component according to the first aspect, the subwavelength diffraction grating which modulates each microstructure is configured to produce, after deposition of the second layer, a colored effect. Generally, a colored effect results from an optical resonance making it possible to generate band-cut or band-pass filters.

According to one or more exemplary embodiments, the subwavelength diffraction grating is configured to produce a guided mode optical resonance as described in [Ref. 1] or [Ref. 2],

In these exemplary embodiments, the second layer is a thin layer of transparent dielectric material having a second refractive index and encapsulated between said first layer of electrical material and a third layer of dielectric material having a third refractive index. The second refractive index has a difference with the first refractive index on the one hand, and with the third refractive index on the other hand, greater than or equal to approximately 0.3, advantageously greater than or equal to approximately 0.5 in absolute value.

According to one or more exemplary embodiments, said thin layer of dielectric material is a layer of so-called “high refractive index” material (or “HRI” for “High Refractive Index”), having a refractive index of for example between 1.8 and 2.9, advantageously between 2.0 and 2.4, and the neighboring layers (first and third layers) are so-called “low refractive index” layers, having refractive indices of for example between 1.3 and 1.8, advantageously between 1.4 and 1.7. For example, said second layer comprises a material chosen from: zinc sulfide (ZnS), titanium dioxide (TiCL) silicon nitride (SisN^.

A thin layer within the meaning of the present description is a layer with a thickness of between approximately 5 nm and approximately 250 nm, preferably between approximately 10 nm and approximately 150 nm. The thin layer of transparent dielectric material has a thickness of, for example, between approximately 20 nm and approximately 200 nm and preferably between approximately 60 nm and approximately 150 nm.

The subwavelength diffraction grating configured to produce a guided mode optical resonance is for example a one-dimensional grating, for example concentric or pseudo-concentric, and comprises one or more pitches between approximately 150 nm and approximately 400 nm, advantageously between approximately 200 nm and approximately 400 nm. The subwavelength diffraction grating may also be a radial, pseudo-radial grating with a fixed pitch between approximately 150 nm and approximately 400 nm, advantageously between approximately 200 nm and approximately 400 nm. The subwavelength diffraction grating may also be a two-dimensional grating, with a concentric or pseudo-concentric dimension of fixed or variable pitch and a radial or pseudo-radial dimension of constant pitch. The depth of the grating is for example between approximately 30 nm and approximately 250 nm. The profile of the grating is for example sinusoidal or pseudo-sinusoidal or rectangular. In operation, such a diffraction grating is configured for the excitation of guided modes within said thin layer of transparent dielectric material, forming a band-pass resonant filter in reflection, the resonance spectral band of which is centered on a wavelength determined in a known manner depending on the characteristics of the grating and the nature of the layers.

This results in a colored effect which depends in particular on the type of diffraction grating and the pitch(es) of the diffraction grating, or on the refractive index of the layers.

In a first example of realization, with a concentric diffraction grating (a dimension), periodic with a constant step, an observer will thus be able to observe in reflection a stable colored effect in an up/down tilt with a first color and a change of color when the component is tilted to the left or to the right to reveal a second color. Such a security optical component will exhibit a conservation of the colored effect during an azimuthal rotation of a given angle, for example an angle between 30° and 150°, that is to say that the same colors will be visible during the up/down and/or left/right tilts.

In a second exemplary embodiment, with a concentric diffraction grating (one dimension), periodic with variable pitch, an observer will be able to observe in reflection a variable colored effect during an up/down tilt around a first color, and this due to the variation of the period, and will be able to observe in reflection a different variable colored effect during a left/right tilt. Such a security optical component will also exhibit conservation of the colored effect during an azimuthal rotation.

In a third embodiment, with a radial or pseudoradial diffraction grating (one dimension), of constant pitch, an observer will be able to observe in reflection a stable colored effect in a left/right tilt with a first color and a color change when the component is tilted up or down to reveal a second color. Such a security optical component will exhibit a conservation of the colored effect during an azimuthal rotation.

In a fourth embodiment, with a two-dimensional diffraction grating (radial or pseudo-radial and concentric), periodic with a constant pitch in each of the dimensions, and in which at least one parameter of the diffraction grating differs between the radial or pseudo-radial grating lines and the concentric grating lines, an observer will be able to observe in reflection a stable colored effect in a left/right tilt with a first color and a stable colored effect in a left/right tilt but with a second color different from the first color. Such a security optical component will exhibit conservation of the colored effect during an azimuthal rotation.

According to one or more exemplary embodiments, the subwavelength diffraction grating is configured to produce a plasmonic resonance effect in reflection as described for example in [Ref. 3] or [Ref. 4],

In these examples, the second layer is metallic and comprises a thin layer of metallic material, for example silver, aluminum, gold, chromium, copper, advantageously with a thickness greater than approximately 40 nm. Advantageously, said layer metallic is thick enough to exhibit a maximum residual transmission as a function of wavelength of 2%.

As in the previous examples, the subwavelength diffraction grating may comprise one or more pitches between approximately 150 nm and approximately 400 nm, advantageously between approximately 200 nm and approximately 400 nm; the subwavelength diffraction grating is for example a one-dimensional grating, for example concentric or radial or pseudo-radial, or a two-dimensional grating, with a concentric dimension and a radial or pseudo-radial dimension. The depth of the grating is for example between 30 nm and 200 nm. The profile of the grating is for example sinusoidal or pseudo-sinusoidal or rectangular. The diffraction grating may be periodic with a constant pitch in one direction, or may have a plurality of pitches or a variable pitch in the case of a concentric grating.

In operation, such a diffraction grating is configured for the excitation of plasmonic modes at the metal-dielectric interface, forming a resonant notch filter in reflection, whose resonance spectral band is centered on a wavelength determined in a known manner depending on the characteristics of the grating and the natures of the layers.

This results in a colored effect that depends on the type of diffraction grating and the pitch(es) of the diffraction grating and the refractive indices of the layers.

The colored effects are then substantially identical to those described above, for diffraction gratings configured for the excitation of guided modes within a thin layer of transparent dielectric material.

According to one or more exemplary embodiments, the subwavelength diffraction grating is configured to produce a plasmonic resonance effect in transmission as described for example in [Ref. 6],

In these examples, the second layer comprises a thin layer of metallic material, for example silver or aluminum, advantageously with a thickness of between 10 nm and 50 nm.

The subwavelength diffraction grating comprises one or more steps between about 150 nm and about 400 nm, advantageously between about 200 nm and about 400 nm; the subwavelength diffraction grating is for example a one-dimensional grating, for example concentric or radial or pseudo-radial, or a two-dimensional grating, with a concentric dimension and a radial or pseudo-radial dimension. The depth of the grating is for example between 50 nm and 200 nm. The profile of the grating is for example sinusoidal or pseudo sinusoidal. The diffraction grating can be periodic with a constant pitch in one direction, or can have a plurality of pitches or a variable pitch (case of concentric gratings).

In operation, such a diffraction grating is configured for the excitation and coupling of surface plasmon modes at both metal/dielectric interfaces on either side of the metal layer, thus enabling a resonant transmission effect. The resulting effect is a resonant transmission band-stop filter, whose resonance spectral band is centered on a wavelength determined in a known manner depending on the characteristics of the grating and the natures of the layers.

This results in a colored effect that depends on the type of diffraction grating and the pitch(es) of the diffraction grating and the refractive indices of the layers.

According to a first example of implementation, with a concentric or pseudo-concentric diffraction grating (one dimension), periodic with a constant pitch, an observer will thus be able to observe in transmission a stable color, for example green along the up/down tilt axis and blue along the right/left tilt axis.

According to one or more exemplary embodiments, the optical security component according to the first aspect comprises at least one second structure etched on said first layer, said second layer at least partially covering said second structure. The second structure is configured to form, for example and in a non-limiting manner, another structure according to the present description, a diffusing structure, a holographic structure, a diffractive structure, for example a structure making it possible to produce a so-called Alphagram® effect developed by the applicant.

According to one or more exemplary embodiments, the security optical component according to the first aspect comprises said at least one first diffractive structure and at least one second diffractive structure, in which said at least one first diffractive structure and said at least one second diffractive structure have contours configured to generate macroscopic patterns recognizable to the naked eye by an observer and are made up of optical elements comprising identical microstructures modulated by different diffraction gratings.

It is thus possible to generate colored scenarios that are easily recognizable by an observer.

In exemplary embodiments, the diffraction grating of said at least one first diffractive structure is circular, pseudo-circular or pseudo-radial with constant pitch and the diffraction grating of said at least one second diffractive structure is linear with pitch constant, with a pitch identical to that of the diffraction grating of said at least one first diffractive structure.

In exemplary embodiments, the diffraction grating of said at least one first diffractive structure is circular or pseudo-circular with a constant pitch and the diffraction grating of said at least one second diffractive structure is pseudo-radial, with a pitch identical to that of the diffraction grating of said at least one first diffractive structure. According to one or more exemplary embodiments, the optical security component according to the first aspect comprises one or more additional layers depending on the needs of the application, these additional layers being able or not to contribute to the desired visual effect. Thus, according to one or more exemplary embodiments, the optical security component is configured for securing an object, for example a document or a product, and further comprises, on the face opposite the observation face, a layer suitable for transferring the component onto the document or the product, for example an adhesive layer or a reactivatable adhesive layer.

According to one or more exemplary embodiments, the optical security component further comprises, on the side of the first observation face, a support film intended to be detached after transfer of the component to the document or product.

According to one or more exemplary embodiments, the optical security component is configured for the manufacture of a security track for securing banknotes, and comprises on the side of the first observation face and/or on the face opposite the first observation face, one or more protective layers.

According to a second aspect, the present description relates to a secure object, for example a secure valuable document, comprising a substrate and an optical security component according to the first aspect, deposited on said substrate or on one of the layers of said substrate in the case of a multilayer substrate.

Such a secure object is, for example, and without limitation: a banknote, an identity or travel document, on a paper or polymer substrate.

According to a third aspect, the present description relates to methods of manufacturing security optical components according to the first aspect.

Thus, the present description relates to a method of manufacturing an optical security component for securing an object, for example a valuable document, for example an identity document or a bank note, the method comprising: depositing on a support film a first layer of dielectric material, transparent in the visible; forming on said first layer at least one first diffractive structure, depositing a second layer at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible, and wherein: said first diffractive structure consists of a set of optical elements adjacent to each other, each optical element having a maximum dimension less than approximately 300 pm and comprising a microstructure modulated by a subwavelength diffraction grating with one or more steps between approximately 150 nm and approximately 500 nm, such that: said microstructure is configured to deflect an incident light beam and is substantially invariant by azimuthal rotation of an angle less than 90° around a first axis substantially perpendicular to a plane of the component; said subwavelength diffraction grating is substantially invariant by azimuthal rotation of an angle less than 90° around a second axis substantially coincident with said first axis and is configured to produce, after deposition of the second layer, a colored effect.

According to a fourth aspect, the present description relates to a method for authenticating a secure object with the naked eye according to the second aspect, the authentication method comprising: observing the optical security component of said secure object along an observation axis forming a given observation angle with the lighting axis; a left/right and/or down/left tilt movement allowing a given variable colored effect to be observed; an azimuthal rotation movement of a predetermined angle allowing the preservation of said variable colored effect to be noted.

The azimuthal rotation angle is advantageously between approximately 30° and approximately 150°, for example between approximately 60° and approximately 120°.

For authentication by the naked eye, said first diffractive structure may have an outline configured to generate a recognizable macroscopic pattern with a minimum dimension greater than about 500 pm, preferably greater than about 1 mm, preferably greater than about 2 mm, preferably greater than about 5 mm.

According to a fifth aspect, the present description relates to a method of authenticating a secure object according to the second aspect by means of an optical imaging device, said authentication method comprising: the formation of an image of said optical security component, by means of the optical imaging device; a left/right and/or down/left tilt movement allowing a given variable colored effect to be observed; an azimuthal rotation movement of a predetermined angle allowing the preservation of said variable colored effect to be noted.

For authentication by means of an optical imaging device, said first diffractive structure may, in exemplary embodiments, be configured to generate a non-figurative pattern, for example a QR code. Furthermore, due to the possible magnification of the optical imaging device, the dimensions of the pattern thus generated may have dimensions smaller than the dimensions required for ocular perception.

Brief description of the figures

Other features and advantages of the invention will become apparent upon reading the description which follows, illustrated by the following figures:

FIG. 1A schematically illustrates a (partial) sectional view of an exemplary embodiment of a component according to the present description.

FIG. 1B schematically illustrates a (partial) sectional view of another exemplary embodiment of a component according to the present description.

FIG. 2A schematically illustrates a three-dimensional (partial) view of microstructures in a component according to the present disclosure, each microstructure being formed of a microlens, the microlenses each being modulated, in this example, by a concentric diffraction grating.

FIG. 2B schematically illustrates a three-dimensional (partial) view of microstructures in a component according to the present disclosure, each microstructure being formed of a subset of facets.

FIG. 2C schematically illustrates a (partial) cross-sectional view of a subset of facets as shown in FIG. 2B, modulated by a subwavelength diffraction grating.

FIG. 3A illustrates a first example of a subwavelength diffraction grating in a component according to the present disclosure, the diffraction grating being concentric and of constant pitch.

FIG. 3B illustrates a second example of a subwavelength diffraction grating in a component according to the present disclosure, the diffraction grating being concentric and of not variable.

FIG. 3C illustrates a third example of a subwavelength diffraction grating in a component according to the present disclosure, the diffraction grating being pseudo-radial.

FIG. 3D illustrates a fourth example of a subwavelength diffraction grating in a component according to the present disclosure, the diffraction grating being two-dimensional, with a concentric component and a pseudo-radial component, the pitches in each dimension being constant and different from each other.

FIG. 4A illustrates a colored visual effect of a first example of a security optical component according to the present disclosure, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions.

FIG. 4B illustrates a colored visual effect of the security optical component illustrated in FIG. 4A, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions, after azimuthal rotation of the component by 90°.

FIG. 5 illustrates a colored visual effect of a second example of a security optical component according to the present description, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions.

FIG. 6A illustrates a colored visual effect of a third example of a security optical component according to the present disclosure, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions.

FIG. 6B illustrates a colored visual effect of the security optical component shown in FIG. 6A, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions, after azimuthal rotation of the component by 90°.

FIG. 7A illustrates a first example of an arrangement of optical elements in a security optical component according to the present disclosure, the optical elements having a square-shaped outline.

FIG. 7B illustrates a second example of an arrangement of optical elements in a security optical component according to the present disclosure, the optical elements having a hexagonal shaped outline.

FIG. 7C illustrates a third example of an arrangement of optical elements in a security optical component according to the present disclosure, the optical elements having a rectangular shaped outline.

FIG. 8 illustrates an example of an arrangement of optical elements in a security optical component according to the present disclosure, with different optical elements.

FIG. 9 illustrates another example of an arrangement of optical elements in a component safety optics according to the present description, with different optical elements.

Detailed description

In the figures, the elements are not shown to scale for better visibility. FIG. 1A and FIG. 1B schematically represent and in (partial) sectional views two examples of optical security components according to the present description. The optical security component 101 shown in FIG. 1A represents for example an optical security component intended to be transferred in the form of a track or patch on a document or a product in order to secure it. According to this example, it comprises a support film 111, for example a film made of polymer material, for example a polyethylene terephthalate (PET) film of a few tens of micrometers, typically 10 to 100 μm, as well as a detachment layer 112, for example made of natural or synthetic wax. The detachment layer makes it possible to remove the polymer support film 111 after transfer of the optical component onto the product or document to be secured. The optical security component 101 further comprises a first layer 113 made of dielectric material, having a first refractive index ni and at least one first diffractive structure S consisting of optical elements OEi and stamped on said first layer 113, and which will be described in more detail below.

In the example of FIG. 1 A, the optical security component 101 also comprises a second reflective layer 114, at least partly covering said first structure S, and having a spectral band of reflection in the visible. The second layer 114 is for example a metal layer or a so-called refractive index variation layer having a refractive index n2 different from the neighboring layers, the difference in refractive index with that of the neighboring layers having a value at least equal to 0.3, advantageously a value at least equal to 0.5.

As will be described in more detail hereinafter, the first diffractive structure S consists of a set of optical elements, having a maximum dimension less than about 300 pm. Each optical element comprises a microstructure Mi configured to deflect incident light, modulated by a subwavelength diffraction grating G, i.e. a diffraction grating comprising one or more steps between about 150 nm and about 400 nm.

The microstructure Mi is substantially invariant by azimuthal rotation of an angle less than 90° around a first axis Ai substantially perpendicular to a plane of the component. The microstructure has, for example, a symmetry of revolution. As illustrated in the example of Fig. 1 A, the microstructure is for example a microlens.

The sub-wavelength diffraction grating Gi is substantially invariant by azimuthal rotation of an angle less than 90° around a second axis substantially coincident with said first axis Ai. The sub-wavelength diffraction grating Gi is configured to produce, after deposition of the second layer 114, a colored effect. Examples of gratings will be described in more detail below. The colored effect may result from a resonance of guided modes, when the second layer 114 is made of dielectric material and has a difference in refractive index at least equal to 0.3 with the neighboring layers, advantageously at least equal to approximately 0.5. In exemplary embodiments, the layer 114 made of dielectric material has a refractive index n2 greater than the refractive indices ni, ns, of the neighboring layers and is referred to as a high index layer or HRI layer (for “High Refractive Index”). The colored effect can also result from plasmonic resonance when the second layer 114 is metallic.

The optical security component may further comprise one or more additional layers, optically functional or non-functional, adapted to the application. For example, in the example of FIG. 1 A, the optical security component further comprises an adhesive layer 117, for example a heat-reactivatable adhesive layer, for transferring the optical security component onto the product or document.

In practice, as will be detailed later, the optical security component can be manufactured by stacking the layers on the support film 111, then the component is transferred onto a document/product to be secured using the adhesive layer 117. Optionally, the support film 111 can then be detached, for example by means of the detachment layer 112. The main observation face 100 of the optical security component is thus on the side of the first layer 113 opposite the structured face of the layer 113.

The optical security component 102 shown in FIG. 1B represents for example an optical security component intended for securing banknotes; it is for example a part of a security thread intended to be integrated into the paper during the manufacture of the note or a laminated track covering a window in the paper. In this example, the component 102 comprises as previously a support film 111 (12 to 50 pm) which will also serve as a protective film for the security thread, and, as in the example of FIG. 1A, a first layer 113 of dielectric material having a first refractive index ni, at least one first diffractive structure S consisting of a set of optical elements OEi. A second layer 114 at least partly covers said first diffractive structure S, and has a spectral band of reflection in the visible. As described with reference to FIG. 1 A, each optical element comprises a microstructure Mi modulated by a subwavelength diffraction grating G.

As illustrated in the example of FIG. 1B, and as will be described in more detail hereinafter, each optical element OEi may comprise a microstructure Mi consisting of a subset of facets. The subset of facets comprises for example, but not limited to, one or more facets with rotational symmetry arranged concentrically around said first axis Ai. A subset of facets may have a similar optical effect of deflecting light to that of a microstructure as illustrated in FIG. 1A. A subset of facets is advantageous in that it comprises a height A, defined by a distance between a lowest level of the facet and a highest level, and measured along an axis perpendicular to a plane parallel to the plane of the component, less than the height of a microstructure as illustrated in FIG. 1A.

The optical security component 102 further comprises, in the example of FIG. 1B, a set of optional layers 115, 116, 118. The layer 115 (optional) is for example a layer of dielectric material, for example a transparent layer or an opaque colored layer which makes it possible to increase the contrast; the layer 116 (optional) is for example a security layer, for example a discontinuous layer with a specific pattern printed locally with a UV ink to produce a complementary marking that can be checked by eye or by machine; and the layer 118 (optional) is for example a protective layer, for example a second polymer film or a varnish. In the case of a laminated track, the layer 118 may be an adhesive layer. As in the previous example, the manufacturing may be carried out by stacking the layers on the support film 111. The dielectric layer 115 and the security layer 116 may form only one layer. The protective layer (or adhesive layer) 118 and the layer 115 may also form only one and the same layer.

It will be apparent to those skilled in the art that other optically or non-optically functional layers may be added depending on the needs of the application in each of the examples shown in FIG. 1A and FIG. 1B and that the embodiment variants shown in FIG. 1A and FIG. 1B may be combined.

Note that if the additional layers, for example layer 117, or layers 115, 116, 118, are transparent, as well as the destination medium, the optical security component can be observed from both sides for its authentication.

FIG. 2A schematically illustrates a three-dimensional (partial) view of an exemplary diffractive structure S consisting of optical elements OEi. In this example, each optical element OEi comprises a microstructure Mi formed of a microlens, modulated by a diffraction grating G, in this example a concentric diffraction grating with constant pitch.

In this example, the microlenses Mi are for example microlenses with rotational symmetry, for example spherical or aspherical microlenses. However, other examples are possible.

Furthermore, the microlenses Mi may be modulated by a constant-pitch pseudoconcentric, concentric or variable-pitch pseudoconcentric diffraction grating, a radial diffraction grating, an angularly sectored diffraction grating, for example a pseudoradial diffraction grating, or a two-dimensional diffraction grating. Examples will be described with reference to FIG. 3A to FIG. 3D.

FIG. 2B schematically illustrates a three-dimensional (partial) view of microstructures Mi in a component according to the present disclosure. In this example, each microstructure comprises a subset of facets.

FIG. 2C schematically and exemplarily illustrates a (partial) cross-sectional view of a subset of facets as illustrated in FIG. 2B, modulated by a subwavelength diffraction grating.

Again, the facet sets illustrated in FIG. 2B may be modulated by a constant-pitch pseudo-concentric, concentric, or variable-pitch pseudo-concentric diffraction grating, a radial diffraction grating, an angularly sectored diffraction grating, e.g., a pseudo-radial diffraction grating, or a two-dimensional diffraction grating.

FIG. 3 A illustrates a first example of a subwavelength diffraction grating 310 in a component according to the present description, the diffraction grating being concentric and of constant pitch Ai. Such a concentric grating comprises concentric lines 311. Such a diffraction grating makes it possible to observe a stable colored effect in an up/down tilt and a color change in a left/right tilt. The same colored effect will be observed with a pseudo-concentric grating.

FIG. 3B illustrates a second example of a subwavelength diffraction grating 320 in a component according to the present disclosure, the diffraction grating being concentric and of variable pitch. Such a diffraction grating will allow a variable color effect to be observed in an up/down tilt but this variable color effect will be preserved by azimuthal rotation.

Here again, the same colored effect will be observed with a pseudo-concentric network with variable pitch. FIG. 3C illustrates a third example of a subwavelength diffraction grating 330 in a component according to the present disclosure, the diffraction grating being pseudo-radial, with a pitch A2. As illustrated in FIG. 3C, the pseudo-radial diffraction grating 330 comprises radial grating lines 331 extending from the center of the grating 335 defined by the intersection of the plane of the component with said second axis and pseudo-radial grating lines 332, parallel to the grating lines extending radially from the center of the grating. A pseudo-radial grating makes it possible to define angular sectors 338 within which the grating lines are parallel, which makes it possible to have a substantially constant pitch A2. The angular sectors have an apex angle of, for example, between 2° and 60°. Although not shown, other angularly sectored diffraction gratings may be envisaged.

FIG. 3D illustrates a fourth example of a subwavelength diffraction grating 340 in a component according to the present disclosure, the diffraction grating being two-dimensional, with a concentric component and a pseudo-radial component, the pitches in each dimension being constant but different from each other. Such a diffraction grating comprises concentric lines 341 and radial and pseudo-radial lines 342. Such a diffraction grating will allow a stable colored effect to be observed in an up/down tilt and a different color during a left/right tilt. This variable colored effect will be preserved by azimuthal rotation. Here again, the same colored effect will be observed with a pseudo-concentric component of the grating in place of the concentric component.

FIG. 4A illustrates a colored visual effect of a first example of a security optical component according to the present disclosure, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions.

In this example, the first diffractive structure of the security optical component has an outline configured to generate a macroscopic pattern 401 recognizable to the naked eye by an observer, namely a salamander spitting fire.

The first diffractive structure is for example made up of a set of optical elements comprising microstructures each modulated by a two-dimensional subwavelength diffraction grating, with a first constant pitch in one dimension and a second constant pitch in the other dimension but different from the first pitch. The microstructures are for example microlenses. The diffraction grating is concentric or pseudo-concentric in one dimension, with a pitch between 280 nm and 320 nm and the diffraction grating is pseudo-radial in the other dimension, with a pitch between example between 230 nm and 270 nm. The second reflective layer is for example metallic, configured to generate a plasmonic resonance.

Image 410 illustrates an observation around direct reflection. The salamander is visible with a first color, for example a violet color. Positions 411 and 412 correspond to an observation of pattern 401 during an up/down tilt. We observe substantially the same first color, for example the violet color, in positions 411, 412, with a color stability during the up/down tilt that can go up to a few tens of degrees, for example at least ten degrees on either side of the central position.

Positions 413 and 414 correspond to an observation of the pattern during a left/right tilt. The salamander is then visible with a second color, for example a blue color. This is explained because positions 413/414 and 411/412 locally have a different 2D network step and therefore a different color.

FIG. 4B illustrates a colored visual effect of the security optical component illustrated in FIG. 4A, observed by an observer in specular reflection and in 4 left/right, up/down tilt positions, after azimuthal rotation of the component by 90°.

Remarkably, it is observed that after the azimuthal variation, the same colored effect is visible to an observer. Thus, the salamander appears with the first color (violet) in specular reflection (position 420) and in up/down tilt (positions 421, 422) and appears with a second color (blue) when the component undergoes a tilt to the left or to the right (positions 423, 424).

FIG. 5 illustrates a colored visual effect of a second example of a security optical component according to the present description, observed by an observer in specular reflection (510) and in 4 tilt positions left/right (513/514), up/down (511/512).

In this example, the security optical component comprises first diffractive structures in accordance with the present description and second diffractive structures. The first diffractive structures of the security optical component form a macroscopic pattern 501 recognizable to the naked eye by an observer, namely a vertical strip interrupted with oblique segments 502.

The first diffractive structures consist for example of a set of optical elements, each optical element comprising a microstructure modulated by a concentric or pseudo-concentric sub-wavelength diffraction grating, with a constant first pitch, for example between 260 nm and 320 nm. For example, the microstructures are subsets of facets, for example axicons with a number of facets between 4 and 12. The second diffractive structures of the security optical component generate said oblique segments 502.

The second diffractive structures comprise, for example, optical elements with microstructures similar to those of the first diffractive structures but each modulated by a linear subwavelength diffraction grating, with a second constant pitch, different from the first pitch. For example, the linear diffraction grating has a grating vector parallel to the main direction of the bands 501 and a pitch between 330 nm and 370 nm.

In this example, the second reflective layer is for example a high index layer configured for the observation of guided mode resonances.

Position 510 corresponds to an observation of the pattern around the direct reflection. The background 501 is visible with a first color, for example a purple color while the oblique segments are visible at another color, due to the difference in pitch, for example a red color.

Positions 511 and 512 correspond to an observation of the pattern during an up/down tilt. We observe a stability of the color of the whole.

Positions 513 and 514 correspond to an observation of the pattern during a left/right tilt. A variation of the color is observed only for the background 501: when the component is tilted from left to right, the background 501 changes color and becomes cyan while the oblique segments 502 remain red.

If we perform a 90° azimuthal rotation of the component, the variable colored effect is preserved for the first diffractive structures (background 501) but the oblique segments become green.

In an exemplary embodiment as illustrated in FIG. 5, it is also possible to display a message when, in a given position, the circular network has the same period as the linear network. This makes it possible to create optical scenarios presenting azimuthal metamerism.

FIG. 6A illustrates a colored visual effect of a third example of a security optical component according to the present disclosure, observed by an observer in reflection and in 4 left/right, up/down tilt positions.

In this example, a first diffractive structure of the security optical component has an outline configured to generate a macroscopic pattern 601 recognizable to the naked eye by an observer, namely a fire-breathing salamander, as in the example of FIG. 4 A, FIG. 4B. A second diffractive structure of the security optical component has an outline configured to generate a macroscopic pattern 602 recognizable to the naked eye by an observer, namely a seashell.

The first diffractive structure consists of a set of optical elements comprising, for example, microlenses modulated by a pseudoradial diffraction grating, with a constant first pitch, for example a pitch between approximately 330 nm and approximately 370 nm.

The second diffractive structure consists of a set of optical elements comprising, for example, microlenses identical to those of the first diffractive structure, modulated by a concentric sub-length diffraction grating, with a second constant pitch substantially identical to the first pitch.

In this example, the second reflective layer is for example a high index layer configured for the observation of guided mode resonances.

Image 610 illustrates an observation around direct reflection. The salamander is visible with a first color, for example a green color while the shell is visible with a second color, for example a red color.

Positions 611 and 612 correspond to an observation of the pattern 601 during an up/down tilt. We observe substantially the same first color, for example the green color for the salamander and the red color for the shell, in positions 611, 612, with a color stability during the up/down tilt that can go up to a few tens of degrees. Positions 613 and 614 correspond to an observation of the pattern during a left/right tilt. The salamander is then visible with the second color, for example a red color while the shell is visible with the first color, for example the green color.

FIG. 6B illustrates a colored visual effect of the safety optical component shown in FIG. 6A, observed by an observer in reflection and in 4 left/right, up/down tilt positions, after azimuthal rotation of the component by 90°.

Remarkably, it is observed that after the azimuthal variation, the same colored effect is visible to an observer. Thus, the salamander appears with the first color (green) in specular reflection (position 620) and in up/down tilt (positions 621, 622) and appears with a second color (red) when the component is tilted to the left or right (positions 623, 624). The shell appears with the second color (red) in specular reflection (position 620) and in up/down tilt (positions 621, 622) and appears with the first color (green) when the component is tilted to the left or right (positions 623, 624).

In addition to the azimuthal stability of the colored effects of each of the optical elements, this configuration allows to obtain a color permutation between the Up/Down and Right/Left tilt effect i.e. the colors of the salamander and the shell permute between the two tilt axes.

In the examples described above, authentication could also be performed by an imaging device.

FIG. 7A illustrates a first example of an arrangement of optical elements OEi in a security optical component according to the present description. In this example, the optical elements (OEi, OE2, etc.) are identical, and have a square-shaped outline to facilitate the arrangement. The optical elements each comprise a microstructure, respectively referenced Mi, M2, modulated by a subwavelength diffraction grating respectively referenced Gi, G2. In this example, each microstructure comprises a set of facets (in solid lines), only a few facets being represented in FIG. 7A. The set of facets is modulated by a concentric diffraction grating, the concentric grating lines being represented in dotted lines, only a few grating lines being represented.

FIG. 7B illustrates a second example of an arrangement of optical elements in a security optical component according to the present description. In this example, the optical elements comprise, as in the previous example, a microstructure comprising a set of concentric facets modulated by a concentric diffraction grating. In this example, however, the optical elements have a hexagonal-shaped outline. It is thus possible to have a greater number of optical elements per unit area compared to a square mesh.

Of course, other contour shapes are possible for the optical elements.

Thus, FIG. 7C illustrates a third example of an arrangement of optical elements in a security optical component according to the present description, the optical elements having a rectangular-shaped outline. Such an arrangement makes it possible to maximize the observation dynamics or the intensity of the color, for example in an up/down tilt.

FIG. 8 illustrates an example of an arrangement of optical elements in a security optical component according to the present disclosure, with different optical elements OEi and OE2. The optical elements OEi are for example identical to those illustrated in FIG. 7B while the optical elements OE2 comprise a similar microstructure (not shown in order not to clutter the drawing) but with a pseudoradial diffraction grating. Such an arrangement makes it possible to create composite colors by mixing several optical elements.

FIG. 9 illustrates another example of an arrangement of optical elements in a security optical component according to the present disclosure, with optical elements OEi and OE2 similar to the optical elements OEi and OE2 illustrated in FIG. 8 but with a particular arrangement. In this example, a gradual evolution of the density of OEi and OE2 makes it possible to create a component offering a gradual transition of colors.

Examples of methods for manufacturing optical security components according to the present description are now described.

A first step includes the design of said at least one first diffractive structure according to the methods described above, and any other structures. This is followed by a step of recording an original copy, also called an optical master. The optical master is, for example, an optical medium on which the structure(s) are formed.

The optical master can be formed by electronic or optical lithography methods known in the state of the art.

For example, according to a first embodiment, the optical master is produced by etching a resist sensitive to electromagnetic radiation using an electron beam. In this exemplary embodiment, the structure having the first pattern modulated by the second pattern can be etched in a single step.

According to another embodiment, an optical lithography (or photolithography) technique can be used. The optical master is in this example a photosensitive resin plate and the origination step is carried out by one or more exposures of the plate by projections of masks, of the phase mask type and/or of the amplitude mask type, followed by development in an appropriate chemical solution. For example, a first exposure is carried out by projection of amplitude masks whose transmission coefficients are adapted so that, after development, a relief corresponding to the microstructures is formed, in the regions in which the microstructures are provided. Then, a second exposure is carried out, for example according to optical lithography methods known to those skilled in the art, for example two-photon lithography, to record the diffraction grating which modulates each microstructure. Similar steps can be provided to generate other reliefs, such as for example a second diffraction grating in other regions. The order of formation of microstructures and diffraction gratings is arbitrary and can be modified. Subsequently, the development stage is carried out. In this way, an optical master comprising a first diffractive structure conforming to the present description is obtained after development.

The step of metal copying of the optical master can then be carried out, for example by electroplating, as mentioned above, in order to obtain the replication matrix or metal “master”. According to a variant, a step of matrix duplication of the metal master can be carried out to obtain a large-scale production tool suitable for replicating the structure in industrial quantities.

The manufacture of the optical security component then includes a replication step. For example, the replication can be carried out by stamping (by hot embossing of the dielectric material) of the first layer 113 (FIGS. 1 A, 1B) in dielectric material with a refractive index of ni, for example a low index layer, typically a stamping varnish a few microns thick. The layer 113 is advantageously carried by the support film 111, for example a film of 12 pm to 100 pm in polymer material, for example PET (polyethylene terephthalate). The replication can also be carried out by molding the stamping varnish layer before drying then UV crosslinking ("UV casting"). Replication by UV crosslinking makes it possible in particular to reproduce structures having a large depth range and makes it possible to obtain better fidelity in the replication. Generally, any other high-resolution replication method known from the prior art may be used in the replication step.

Next comes the deposition on the layer thus embossed of all the other layers, for example the reflective layer 114, the dielectric material layer 115 (optional), the security layer 116 (optional) which can be deposited uniformly or selectively to represent a new pattern and the glue or varnish type layer (117, 118) by a coating process.

Optional steps known to those skilled in the art are possible, such as partial demetallization of the reflective layer 114. It is also possible to introduce a continuous or discontinuous opaque layer to enhance the contrast.

Although described through a certain number of exemplary embodiments, the optical security component according to the invention and the method of manufacturing said component comprise different variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these different variants, modifications and improvements are part of the scope of the invention as defined by the claims which follow. References

Ref. 1: EP 2264491

Ref. 2: M.T. Gale, “Zero-Order Grating Microstructures” in R.L. van Renesse, Optical

Document Security, 2nd Ed., pp. 267 - 287 Ref. 3: WO2013060817

Ref. 4: WO2020229415

Ref. 5: US2010/0307705

Ref. 6: WO2012136777

Ref. 7: WO2015154943

Claims

1. Optical security component (101, 102) for securing an object, for example a valuable document, for example an identity document or a banknote, the component comprising: a first layer (113) of dielectric material, transparent in the visible, having a first refractive index (ni); at least one first diffractive structure (S) etched on said first layer; and a second layer (114), at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible; and wherein: said first diffractive structure consists of a set of optical elements (OEi) adjacent to each other, each optical element having a maximum dimension less than approximately 300 pm and comprising a microstructure (Mi) modulated by a sub-wavelength diffraction grating (Gi) with one or more pitches between approximately 150 nm and approximately 400 nm, such that: said microstructure (Mi) is configured to deflect an incident light beam and is substantially invariant by azimuthal rotation of an angle less than 90° about a first axis substantially perpendicular to a plane of the component; said sub-wavelength diffraction grating is substantially invariant by azimuthal rotation of an angle less than 90° about a second axis substantially coincident with said first axis and is configured to produce, after deposition of the second layer (114), a colored effect. 2. Optical security component according to claim 1, wherein said second layer (114) is a layer of dielectric material having a second refractive index (n2), encapsulated between said first layer (113) of electrical material and a third layer of dielectric material (117, 115) having a third refractive index (ns), the second refractive index (n2) having a difference with the first refractive index (ni) on the one hand, and with the third refractive index (ns) on the other hand, greater than or equal to approximately 0.3, advantageously greater than or equal to approximately 0.5 in absolute value. 3. The optical security component of claim 1, wherein the second layer (114) comprises a metallic material. 4. Optical security component according to any one of the claims previous, wherein said diffraction grating is selected from: a concentric (311) or pseudo-concentric diffraction grating, an angularly sectorized diffraction grating, for example a radial or pseudo-radial diffraction grating (313), a two-dimensional diffraction grating (314) with a concentric or pseudo-concentric dimension and a radial or pseudo-radial dimension. 5. Optical security component according to any one of the preceding claims, in which said diffraction grating comprises a variable pitch. 6. Optical security component according to any one of the preceding claims, in which each microstructure (Mi) consists of a subset of facets. 7. Optical security component according to any one of the preceding claims, in which each optical element (OEi) has an outline of a given shape chosen from: a circular shape, a rectangular shape, a hexagonal shape. 8. Optical security component according to any one of the preceding claims, in which the optical elements (OEi) of said first diffractive structure are identical. 9. Optical security component according to any one of the preceding claims, in which said first structure has an outline configured to generate a macroscopic pattern recognizable to the naked eye by an observer. 10. Optical security component according to any one of the preceding claims, further comprising at least one second structure etched in said first layer, said second structure being chosen for example from: a diffusing structure, a holographic structure, a diffracting structure of the Alphagram® type. 11. Security optical component according to any one of the preceding claims, comprising said at least one first diffractive structure and at least one second diffractive structure, in which said at least one first diffractive structure and said at least one second diffractive structure have contours configured to generate macroscopic patterns (501, 502) recognizable to the naked eye by an observer and are made up of optical elements comprising identical microstructures modulated by different diffraction gratings. 12. Secure object, for example secure valuable document, comprising a substrate and an optical security component according to any one of the preceding claims, deposited on said substrate. 13. Method of manufacturing an optical security component for securing a object, for example a valuable document, for example an identity document or a bank note, the method comprising: depositing on a support film a first layer of dielectric material (113), transparent in the visible; the formation on said first layer of at least one first diffractive structure (S), the deposition of a second layer (114) at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible, and in which: said first diffractive structure consists of a set of optical elements (OEi) adjacent to each other, each optical element having a maximum dimension less than approximately 300 pm and comprising a microstructure (Mi) modulated by a subwavelength diffraction grating (Gi) with one or more steps between approximately 150 nm and approximately 400 nm, such that: said microstructure (Mi) is configured to deflect an incident light beam and is substantially invariant by azimuthal rotation of an angle less than 90° around a first axis substantially perpendicular to a plane of the component; said subwavelength diffraction grating is substantially invariant by azimuthal rotation of an angle less than 90° around a second axis substantially coincident with said first axis and is configured to produce, after deposition of the second layer (114), a colored effect.