Optical security components visible in transmission, manufacture of such components and secure objects equipped with such components

Abstract

The invention relates to an optical security component enabling authentication in transmission, comprising: a first layer (113) made of dielectric material, which is transparent in the visible; a first diffractive structure (S) etched into said first layer; and a metal layer (114) at least partially covering said first diffractive structure. The first diffractive structure comprises a first pattern (M1) forming a first symmetric one-dimensional periodic undulation configured to form a diffraction grating producing in reflection a diffractive effect at the order 1 and at the order -1. For each period of said first pattern, defined between two extrema of the first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value strictly less than 10 nm and a maximum value between about 10 nm and about 100 nm, and the metal is distributed asymmetrically, in such a way as to produce a colour effect visible in transmission.

Description

[0001] Visible optical security components in transmission, manufacture of such components and secure objects equipped with such components

[0002] Technical field of the invention

[0003] This description relates to the field of security marking. More particularly, it relates to optical security components visible in transmission 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 of manufacturing such a component and to a secure object equipped with such a component.

[0004] State of the art

[0005] Many technologies are known for the authentication of documents or products, and in particular for the security of documents such as valuable documents, documents such as banknotes, passports or other identification documents. These technologies aim at the production of security optical 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.

[0006] In this description, we are more specifically interested in optical security components exhibiting, in transmission, remarkable colored optical effects which are differentiated from optical effects in reflection.

[0007] Transmission control is used in particular in valuable documents, for example banknotes having for this purpose a hollowed-out and / or partially transparent or diffusing area or passports of which a page of data relating to the bearer is provided with a transparent window. The optical security component can be presented for example in the form 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 on the surface or in the thickness of the document.

[0008] Plasmon resonance security optical components are known which exhibit, in transmission, remarkable colored optical effects. Such components are described for example in patent application US2010 / 0307705 [Ref. 1]. The security optical component described in [Ref. 1] comprises, on a transparent substrate, a metal structure forming a periodic pattern of sub-wavelength period, with a rectangular profile, integrated between a layer of a stamping varnish and a layer of a protective varnish. The metal structure is obtained by evaporation of a thin layer of metal on the varnish layer after stamping. By varying the evaporation angle of the metal, it is possible to obtain a sub-wavelength grating having an asymmetric profile.The spectral characteristics in transmission and reflection of the plasmon resonance component thus obtained depend on the evaporation angle of the metal, which makes it possible to design security optical components having distinct colored effects in transmission and in reflection. Also known (see application WO2012136777 [Ref. 2]), is a security optical component with a plasmonic effect having remarkable colored effects in transmission. The optical component described in [Ref. 2] comprises two layers of transparent dielectric material and a metal layer arranged between said layers of dielectric material to form two dielectric-metal interfaces and structured to form on at least part of its surface undulations capable of coupling surface plasmon modes supported by said dielectric-metal interfaces with an incident light wave.The corrugations are arranged in a first coupling zone along a first main direction and in at least one second coupling zone distinct from said first coupling zone along a second main direction substantially perpendicular to the first main direction, said metal layer being continuous on each of said coupling zones. Such a component has an extraordinary transmission effect in a spectral band centered on a wavelength defined by the characteristics of the corrugations of the coupling zones, and for an observer, color variation effects with the observation angle of the component which vary according to the coupling zones, allowing easy and reliable authentication of the security component.

[0009] In both of the above-mentioned references, the physical mechanism involved is a plasmonic resonance mechanism that is very sensitive to polarization. In particular, only the TM component of the incident electromagnetic field is coupled with the plasmonic mode. As a result, low-intensity colored effects are observed in these optical security components when they are illuminated with unpolarized light and observed in transmission.

[0010] We also know of optical security components observable in transmission that also involve resonance mechanisms but in a layer of dielectric material and no longer in a layer of metallic material. Such optical security components are described for example in patent application EP 2264491 [Ref. 3] or in the article by MT gale et al. [Ref. 4] and are known as zero-order diffractive filters (ZOF) or guided mode resonant filters. The physical mechanism is based on a resonant reflection of a guided mode in the high-index dielectric layer. In practice, the transmission spectrum of such a resonant filter is the complement of the reflection spectrum. However, the transmission in such a component remains significant for all wavelengths and the optical effect in transmission therefore appears little colored to an observer.The present application describes an optical security component with an original structure making it possible to obtain, in transmission, both remarkable colored optical effects and very good light intensity.

[0011] Summary of the invention

[0012] In this description, the term "comprise" 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.

[0013] 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 banknote, configured for authentication in transmission, with the naked eye.

[0014] The optical security component according to the first aspect comprises: a first layer of dielectric material, transparent in the visible; at least one first diffractive structure etched in said first layer; a metallic 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 comprises, in a first region, at least one first pattern forming a first periodic undulation in one dimension, symmetrical, comprising a first pitch between approximately 400 nm and approximately 900 nm, and a first depth, said first undulation being configured to form a diffractive grating producing, after deposition of the metallic layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm;for each period, defined between two extrema, of said first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, and the metal is distributed in a non-symmetrical manner, so as to produce a colored effect visible in transmission.;

[0015] 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 700 nm. A layer thus transparent makes it possible to observe with the naked eye the layers located under the transparent layer.

[0016] The "depth" of a corrugation is a distance between a lowest level of the structure forming the corrugation and a highest level, the distance being measured along an axis perpendicular to a plane of the component.

[0017] In the present description, a one-dimensional periodic undulation is a periodic structure with a profile continuously varying in a single direction, called the direction of variation of the profile, i.e. a structure having a height which varies according to a continuously varying and periodic function. A symmetric one-dimensional periodic undulation is a one-dimensional periodic undulation which comprises, for each period of the undulation defined between two extrema of the undulation, at least one plane of symmetry perpendicular to a plane of the component and perpendicular to the direction of variation of the profile.

[0018] According to one or more exemplary embodiments, the profile of the symmetrical periodic undulation is a sinusoidal or pseudo-sinusoidal profile, that is to say comprising a sum of sinusoids of different periods, without these examples being limiting.

[0019] According to the present description, said first periodic undulation has a first pitch and a first depth configured to form a diffractive grating determined to produce, after deposition of the metal layer, a diffractive effect in reflection at least at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm. The symmetry of the periodic undulation makes it possible to reinforce the efficiency of the diffractive effect at order 1 and at order -1. The first pitch is between approximately 400 nm and approximately 900 nm, advantageously between approximately 450 nm and approximately 750 nm, advantageously between approximately 500 nm and approximately 700 nm.

[0020] Furthermore, according to the present description, for each period, defined between two extrema, of the first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, and the metal is distributed in a non-symmetrical manner, so as to produce a colored effect visible in transmission.

[0021] A non-symmetrical distribution of the metal means that for each period defined between two extrema, there is no plane of symmetry of the metal layer that is perpendicular to a plane of the component and perpendicular to the direction of variation of the profile of the undulation. The applicant has shown remarkable colored effects with such a security optical component in which the profile of the diffractive structure is symmetrical but the metal distribution is asymmetrical with, within the same period defined between two extrema, a region with very little, or even no, metal, and a region with a greater thickness of metal. Such a security optical component exhibits, in transmission, remarkable and luminous colored effects.

[0022] Furthermore, the applicant has shown that during transmission observation, with variation of the tilt observation angle along an axis perpendicular to the direction of variation of the undulation profile, an observer will be able to observe an asymmetrical change in color relative to the direction normal to the plane of the component. Such a remarkable and differentiated effect allows for more secure authentication with a high technological barrier, due to the design and then the mass manufacturing steps of the component necessary to obtain the visual effect described above.

[0023] A rotation of the component along an axis contained in the plane of the component is generally called a "tilt movement" of the component, or "rocking".

[0024] Such an effect in transmission which couples color intensity and light intensity, original in the field of security, is explained in particular, without this explanation being limiting, by a resonant transmission resulting from a diffractive effect combined with the excitation of surface modes. The luminous efficiency results from an effect visible both in TE mode and in TM mode, unlike the plasmonic resonances implemented in optical security components known from the state of the art, by means of subwavelength diffractive structures, with periods generally between 200 nm and 400 nm. According to one or more exemplary embodiments, the ratio between the maximum value of the thickness of the metal layer and the minimum value of the thickness of the metal layer is greater than or equal to approximately 5, advantageously greater than or equal to approximately 10, advantageously greater than or equal to approximately 15.With a high ratio between the maximum value of the thickness of the metal layer and the minimum value of the thickness of the metal layer, the asymmetry in the distribution of the metal layer is strong and the colored optical effect even more marked. According to one or more exemplary embodiments, said metal layer results from a metal evaporation of said first structure with a non-zero evaporation angle, for example an evaporation angle of between approximately 25° and approximately 70°. For a known undulation profile, for example a sinusoidal or quasi-sinusoidal profile, it is possible to choose the evaporation angle of the metal to obtain the desired asymmetry in the distribution of the metal layer.

[0025] According to one or more exemplary embodiments, said first diffractive structure comprises, in a second region, at least one second pattern forming a second one-dimensional, symmetrical periodic undulation, comprising a direction of variation of the profile parallel to the direction of variation of the profile of the first undulation, comprising a depth identical to that of the first undulation and comprising a pitch different from that of the first undulation, the first pattern and the second pattern having contours recognizable when observed with the naked eye, in transmission.

[0026] In such a component, due to the different pitches of the undulations, during an observation in transmission at zero order, we will observe different colors for each pattern. Furthermore, by tilting the component along an axis perpendicular to the direction of variation of the profile of the undulations, we will observe a variation of the color for each pattern, with, remarkably, an asymmetry in the observation of the colors on either side of a position corresponding to an observation in a direction normal to the component.

[0027] According to one or more exemplary embodiments, said first diffractive structure comprises, in a second region, at least one second pattern forming a second one-dimensional, symmetrical periodic undulation, comprising a direction of variation of the profile parallel to the direction of variation of the profile of the first undulation, comprising a pitch identical to that of the first undulation, and comprising a second depth strictly less than the first depth of the first undulation, the first pattern and the second pattern having contours recognizable when observed with the naked eye, in transmission.

[0028] In such a component, due to the different depths of the corrugations, it will be possible to generate colored patterns in transmission and uncolored patterns. Indeed, especially when the metal layer results from vacuum evaporation with a non-zero evaporation angle, in the region where the corrugation has a smaller thickness, the asymmetry of the metal layer will not be as pronounced.

[0029] According to one or more exemplary embodiments, said first diffractive structure further comprises a microscopic pattern comprising a set of facets of microscopic dimensions, of variable slopes, the first pattern modulating the microscopic pattern.

[0030] According to one or more exemplary embodiments, the optical security component further comprises 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.

[0031] 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 any one of the preceding claims, deposited on said substrate.

[0032] According to a second aspect, the present description relates to a method for manufacturing an optical security component according to any of the embodiments previously described.

[0033] In general, 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 banknote, configured for authentication in transmission, to the naked eye, the method comprising: depositing on a support film a first layer of dielectric material, transparent in the visible;the formation on said first layer of at least one first diffractive structure, the deposition of a metallic layer at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible, in which: said first diffractive structure comprises, in a first region, at least one first pattern forming a first periodic undulation in one dimension, symmetrical, comprising a first pitch between approximately 400 nm and approximately 900 nm, and a first depth, said first undulation being configured to form a diffractive grating producing, after deposition of the metallic layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm;for each period of said first pattern, defined between two extrema of the first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, and the metal is distributed in a non-symmetrical manner, so as to produce a colored effect visible in transmission.;

[0034] According to one or more exemplary embodiments, said metallic layer results from metallic evaporation of said first structure with a non-zero evaporation angle, for example an evaporation angle of between approximately 25° and approximately 70°.

[0035] Brief description of the figures

[0036] Other characteristics and advantages of the invention will appear on reading the description which follows, illustrated by the following figures:

[0037] FIG. 1 A schematically illustrates a (partial) sectional view of an exemplary embodiment of a component according to the present description.

[0038] FIG. 1B schematically illustrates a (partial) sectional view of another exemplary embodiment of a component according to the present description.

[0039] FIG. 2 illustrates several examples of a first diffractive structure according to the present description with a first pattern Mi, the structure being covered with a metal layer arranged non-symmetrically, in a security component according to the present description.

[0040] FIG. 3 illustrates, according to one example, an animation visible in reflection during a tilt movement of a secure object equipped with an optical security component according to an example in accordance with the present description.

[0041] FIG. 4 illustrates, according to an example, an animation visible in transmission during a tilt movement of a secure object equipped with an optical security component similar to that illustrated in Fig. 3.

[0042] FIG. 5 A illustrates parameters in an optical security component according to the present disclosure;

[0043] FIG. 5B shows a calculated transmission curve as a function of wavelength in an example of a security optical component as illustrated in Fig. 5A illuminated at normal incidence with TE polarized light and comparative transmission curves.

[0044] FIG. 5C shows a calculated transmission curve as a function of wavelength in an example of a security optical component as illustrated in Fig.5A illuminated at normal incidence with TM polarized light and comparative transmission curves.

[0045] FIG. 5D shows a calculated transmission curve as a function of wavelength in an example of a security optical component as illustrated in Fig. 5A illuminated at normal incidence with unpolarized light and comparative transmission curves.

[0046] FIG. 6A schematically illustrates a (partial) sectional view of an optical security component with a diffractive structure comprising in first regions a first pattern in accordance with the present description and between the first regions, unstructured regions.

[0047] FIG. 6B illustrates a visible optical effect in transmission of a high-definition, colored image, with a variable color in tilt, obtained with a security optical component as shown schematically in Fig. 6A.

[0048] FIG. 7 shows transmission curves calculated as a function of wavelength in an example of a security optical component according to the present description, for different tilt values.

[0049] FIG. 8A schematically illustrates a (partial) sectional view of an optical security component with a diffractive structure comprising, in first regions, a first pattern in accordance with the present description, and in second regions, a second pattern formed of undulations identical to those of the first pattern but less deep, and illustrates a visual effect in reflection in such a component.

[0050] Fig. 8B illustrates a visual effect in transmission in the optical security component shown in Fig. 8A.

[0051] FIG. 8C illustrates an optical effect visible in the transmission of a high-definition, colored image, with a variable color in tilt, obtained with a security optical component as shown schematically in Fig. 8A.

[0052] FIG. 9A schematically illustrates a (partial) 3D view of an example of an optical security component in which a first pattern according to the present description modulates a second pattern formed of facets of microscopic dimensions. FIG. 9B illustrates an optical effect visible in transmission in an optical security component of the type described in Fig. 9A.

[0053] FIG. 9C shows diagrams illustrating a visible visual animation in transmission obtained by means of a security optical component of the type described in Fig. 9A.

[0054] Detailed description

[0055] In the figures, the elements are not shown to scale for better visibility.

[0056] FIG. 1 A and FIG. 1B schematically represent, in (partial) sectional views, two examples of optical security components according to the present description. The optical security component 100A shown in FIG. 1 A represents, for example, an optical security component intended to be transferred onto a document or a product for the purpose of securing 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 with a thickness of a few tens of micrometers, more generally between approximately 10 μm and approximately 50 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 security optical component 100A further comprises a first layer 113 of dielectric material, having a first refractive index ni and at least one first diffractive structure S, comprising at least one first pattern Mi, said first pattern Mi forming a one-dimensional periodic undulation, stamped in said first layer 113 and which will be described in more detail later.

[0057] In the example of FIG. 1 A, the optical security component 100A also comprises a metal layer 114 covering said first structure S, and having a spectral band of reflection in the visible. The metal layer comprises for example a metal chosen from the group comprising aluminum, silver, copper, chromium or an alloy of the aforementioned metals.

[0058] The optical security component also includes one or more optional layers, which are not optically functional but are adapted to the application.

[0059] 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 to the product or document.

[0060] 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. An optical security component according to the present description generally comprises a first face 101 for observation in reflection of the optical security component which is located in the example of Fig. 1A on the side of the first layer 113 opposite the etched face of the layer 113 and a second face 102 for observation in transmission of the optical security component which is located in the example of Fig. 1A on the same side as the etched face of the first layer 113.

[0061] The optical security component 100B 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 or a patch. In this example, the component 100B comprises, as previously, a support film 111 (thickness generally between approximately 10 μm and approximately 50 μm) which will also serve as a protective film for the security thread, and, as in the example of FIG. 1 A, a first layer 113 made of dielectric material having a first refractive index n1. at least one first diffractive structure S, stamped on said first layer 113, and a metal layer 114 covering said first structure S, and having a spectral band of reflection in the visible.

[0062] In the example of Fig. 1B, the first diffractive structure S comprises a first pattern Mi and a second pattern M2 each forming a one-dimensional periodic undulation, but of different depths, the first and second patterns being stamped in said first layer 113 to form specific visual effects which will be described in more detail later.

[0063] The optical security component 100B 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 transparent dielectric material 115; the layer 116 (optional) is for example a security layer 116, 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 transparent 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 a single layer.

[0064] It will be apparent to those skilled in the art that other optically non-functional layers may be added depending on the needs of the application in each of the examples shown in FIGS. 1 A and 1B and that the embodiment variants shown in FIGS. 1 A and 1B may be combined.

[0065] It should be noted that the additional, optically non-functional layers, for example layer 117, or layers 115, 116, 118, are transparent in the visible spectrum, as is the destination support, so that the optical security component can be authenticated according to its two faces 101, 102, and in particular in transmission.

[0066] FIG. 2 illustrates in more detail examples 201, 202, 203 of a first diffractive structure according to the present description with a first pattern Mi forming a one-dimensional, symmetrical periodic undulation, the first structure being covered with a metal layer 114. The diagrams illustrate partial sectional views of a component, the sectional views being taken in a plane (xz) perpendicular to a plane (xy) of the component and containing the direction (x) of variation of the profile of the undulation.

[0067] In the three examples illustrated in diagrams 201, 202, 203, the first pattern consists of a one-dimensional periodic undulation with a period (or not) di and a profile height h defined along the z axis perpendicular to the plane (xy) of the component. Such a periodic undulation is a periodic structure whose profile, described by the height h, is continuously variable. Furthermore, according to the present description, the profile is symmetrical, that is to say that it comprises, as illustrated by way of example in diagram 201, for each period of the undulation defined between two extrema, at least one plane of symmetry % perpendicular to the plane of the component (xy) and perpendicular to the direction (x) of variation of the profile of the undulation.

[0068] In the examples illustrated in diagrams 201 and 202, the profile of the ripple is substantially sinusoidal, that is to say it is engraved with as instruction a sinusoidal function h(x) of the form: [Math 1]

[0069] Where di is the period (or not), hi is the depth, x is the abscissa along the (x) axis of variation of the corrugation profile. The height h of the corrugation is measured in the z direction perpendicular to the plane of the component, relative to a base plane parallel to the plane of the component.

[0070] In practice, of course, during the production of the component, for example in an embossing step, there may be deformations of the profile compared to a perfect sinusoid. Furthermore, any other form of symmetrical periodic undulation is possible.

[0071] As illustrated in the example of diagram 203, the profile of the undulation can for example be substantially pseudo-sinusoidal, that is to say that it is engraved with as instruction a pseudo-sinusoidal function h(x) comprising a sum of sinusoidal functions, for example a sum of two sinusoidal functions, for example as illustrated in diagram 203, a function h(x) of the form: [Math 2]

[0072] In all cases, the periodic and symmetrical undulation has a pitch di of between approximately 400 nm and approximately 900 nm, advantageously between approximately 500 nm and approximately 700 nm, and a depth Ay, to form a diffractive grating determined to produce, after deposition of the metal layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range of between 400 nm and 700 nm. The symmetry of the periodic undulation makes it possible to reinforce the efficiency of the diffractive effect at order 1 and at order -1. In exemplary embodiments, a depth of the undulation is between approximately 60 nm and approximately 400 nm.

[0073] Furthermore, according to the present description and as illustrated in examples 201-203, over a period Pi of the first diffractive structure defined between two extrema, the thickness e of the metal layer, defined along a direction (z) perpendicular to the plane of the component, is variable between a minimum value strictly less than 10 nm and a maximum value between approximately 10 nm and approximately 100 nm, and the metal is distributed non-symmetrically. A non-symmetrical distribution of the metal means that for each period Pi defined between two extrema, the metal layer does not have any plane of symmetry perpendicular to the plane (xy) of the component and perpendicular to the direction (x) of variation of the profile of the undulation.

[0074] According to exemplary embodiments, such a non-symmetrical distribution of the metal to form the metal layer 114 is obtained by means of a metal evaporation of the first structure with a predetermined evaporation angle "which depends on the profile of the corrugation and the minimum and maximum values ​​sought for the thickness of the metal layer. The evaporation angle a is defined in the plane (xz), plane of Fig. 2, relative to the direction (z) normal to the plane of the component, the plane (xz) being perpendicular to the plane of the component (xy) and containing the direction (x) of variation of the profile of the corrugation.

[0075] Thus, for example, in the case of a sinusoidal or quasi-sinusoidal profile, with a period between approximately 400 nm and approximately 900 nm, advantageously between approximately 500 nm and approximately 700 nm, and a depth between approximately 60 nm and approximately 400 nm, an evaporation angle may be between approximately 25° and approximately 70°. Of course, this range of values ​​may be adjusted according to the parameters of the undulation to obtain the desired thicknesses of metal and the corresponding visual effects, as will be explained in more detail later.

[0076] Other methods than oblique evaporation of the metal can be used to obtain a non-symmetrical distribution of the metal on the first pattern, for example partial demetallization operations.

[0077] Fig. 3 and Fig. 4 schematically illustrate visual effects obtained respectively in reflection and in transmission with an optical security component according to the present description.

[0078] These figures show a secure object 300, for example a security document, on which an optical security component 301 is arranged, for example in the form of a security track, for example as illustrated in Fig. 1B, the optical security component comprising at least a first structure in accordance with the present description.

[0079] In this example, the first structure comprises in a first region a plurality of patterns whose contours are indicated by the references 311 - 315 in the figures. The patterns are formed respectively of one-dimensional, periodic, symmetrical undulations, arranged in the same direction (parallel undulations), but having for example a different pitch. For each of the patterns, the undulation is configured to form a diffractive grating determined to produce, after deposition of the metal layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm. Diffractive effects at higher orders can also be observed. However, since the pitch is different for each pattern, an observer will see in reflection at the first diffraction order, a different color for each of the patterns.According to the present description, the first structure is covered with a metal layer (114, Fig. 1B), with a non-symmetrical distribution of the metal.

[0080] As illustrated in Fig. 3, when an observer observes the secure object 300 in reflection in a given observation direction and exerts a tilt movement along an axis (A) included in the plane of the component and perpendicular to the direction of variation of the profile of the undulations (direction (y), Fig. 2), an iridescent colored animation is visible. This colored animation, shown diagrammatically in Fig. 3 between two tilt positions respectively indicated 30A and 30B of the secure object, includes a successive appearance of the patterns in different colors. Indeed, due to the different pitch of the undulations for each of the patterns, for a given observation angle, only one or more of the patterns will be visible in a diffraction at order +1 or -1 and with a color that depends on the pitch. By modifying the observation angle, other colors will appear for other patterns.The colors visible in reflection are the classical colors of diffraction at order +1 or -1 which result from the laws of angular dispersion of the gratings (Bragg formula).

[0081] Note that due to the symmetry of the undulations and the quasi-continuous presence of metal on the first structure, there is little impact on the observation of diffraction at order +1 or -1, even if the metal distribution is not symmetrical.

[0082] Fig. 4 schematically illustrates a second animation which results from the observation of the same secure object 300 but in transmission. Diagrams 30c, 30D and 30E illustrate three tilt positions of the secure object 300 when an observer observes the secure object in transmission, the observation direction being coincident with the illumination direction represented in Fig. 4 by an arrow (observation at zero diffraction order).

[0083] As explained previously, due to the non-symmetrical distribution of the metal, we observe intense and bright colors. Due to the different pitch for the waves of the different patterns, the colors are different for each pattern. Furthermore, by tilting the component along the axis (A) perpendicular to the direction of variation of the wave profile, we vary the color for each pattern.

[0084] Furthermore, as illustrated in Fig. 4, during a variation of the secure object in tilt, we observe an asymmetry in the observation of the colors on either side of a position (3c) corresponding to an observation in a direction normal to the component (observation position at zero order). In other words, the same pattern is not visible with the same color in the tilt positions 30D and 30E which correspond to tilts of the secure object symmetrical on either side of the normal to the component.

[0085] In the optical effect observed in transmission and illustrated by the diagrams in Fig. 4, it is no longer, as in an observation in reflection, an angular dispersion linked to diffraction in the orders + / - 1 since we observe at the zero order. The applicants attribute this original optical effect to a redistribution of the spectral energy density coupled to the different diffraction orders when the angle varies. In other words, the energy which is not coupled in the 0 order is absorbed or radiated in other diffraction orders. This phenomenon depends on the angle of incidence, and is by nature asymmetric. As will be explained with reference to Fig. 5A, Fig. 5B, this effect does not depend on the polarization. This constitutes an important difference with plasmonic colors.

[0086] Fig. 5B, Fig.5C, Fig.5D represent, for illustrative purposes, transmission curves as a function of wavelength calculated for an optical security component as shown in Fig. 5A (partial sectional representation) and comparative transmission curves.

[0087] The curves are calculated using a known calculation code using the RCWA (“Rigorous Coupled Wave Approximation”) method [Ref. 5],

[0088] More specifically, FIG. 5B represents transmission curves calculated in the case of security optical components illuminated at normal incidence with TE polarized light, Fig. 5C represents transmission curves calculated in the case of security optical components illuminated at normal incidence with TM polarized light, Fig. 5D represents transmission curves calculated in the case of security optical components illuminated at normal incidence with unpolarized light.

[0089] The optical security component C3 as illustrated in Fig. 5 A and in accordance with the present description comprises a first pattern with a one-dimensional undulation, of sinusoidal profile, a pitch di of 500 nm and a depth hi of 200 nm. The metal layer 114 results from an evaporation of aluminum on the structure with an angle a = 40° and a deposition rate such that an equivalent thickness on a flat surface with a normal incidence would be 30 nm. The metal layer 114 thus formed has a non-symmetrical distribution of metal with in particular sections of the sinusoid with a significant thickness of metal, typically a maximum thickness of approximately 20 nm, and on the opposite sections of the sinusoid, a very small thickness of metal, typically a minimum thickness of approximately 2 nm.

[0090] As illustrated in Fig. 5B on the curve referenced 503, if such a security optical component is observed at normal incidence with TE polarized light, a strong transmission response is observed which depends on the wavelength.

[0091] For comparison, curves 501 and 502 are calculated respectively with a security optical component Ci and with a security optical component C2 identical to component C3 in accordance with the present description but in which, for curve 501, the metal layer is uniform, with a thickness equal to 30 nm, and for curve 502, the metal layer has a non-symmetrical distribution but which results from an evaporation of aluminum with an evaporation angle a of 20°.

[0092] Curve 501 is a transmission curve of an optical security component as described in the prior art, for example as described in [ref. 2]. The evaporation angle being zero, the structure is very weakly transmissive.

[0093] Curve 502 is a transmission curve of a security optical component with a metal layer that has a non-symmetrical distribution of metal since the evaporation angle is not zero. However, the evaporation angle of 20° for a structure as described with reference to Fig. 5 A is not very important and the distribution of metal over a period of the undulation remains fairly uniform, so that the transmission effects are not as remarkable.

[0094] Curves 513, 511 and 512 shown in Fig. 5C are transmission curves calculated in TM polarization at normal incidence, respectively for the optical security component C3 in accordance with the present description and for the optical security components Ci and C2 described above and introduced for comparison.

[0095] It is remarkable to note that we again observe a strong transmission response as a function of wavelength for the C3 component in accordance with the present description (curve 513).

[0096] Components Cl and C2 have a very low transmission response in TM polarization (respectively curves 511, 512).

[0097] Fig. 5D shows transmission curves calculated for the components described above but this time in unpolarized light.

[0098] It is observed that the curve 523 calculated with the optical security component C3 in accordance with the present description has a strong transmission response and is also strongly dependent on the wavelength. This is explained by the response of such a component for both TE and TM polarizations. In practice, this results, if the component is illuminated with white light, in an intense and luminous colored effect.

[0099] For comparison, curves 521 and 522 illustrate the transmission curves calculated under the same conditions for the optical security components Ci and C2 described above. A very low transmission response is observed.

[0100] Also for comparison, curve 524 illustrates a transmission curve calculated under the same conditions for a security optical component C4 which comprises a first pattern identical to that of the security optical component C3 (one-dimensional periodic undulation, period 500 nm and depth 200 nm) but in which the metal layer is replaced by a layer of high-index material with a thickness of 80 nm. Such a security optical component is in accordance with the prior art as described for example in [ref. 3] or [ref. 4] (guided mode resonant filter). Such a component behaves in transmission like a subtractive filter. As can be seen on curve 524, the transmission is very strong, the resulting colored effect is therefore very bright but the color is not very pronounced because the transmission remains high for all wavelengths.

[0101] The figures described above thus show the remarkable effect in transmission for an optical security component conforming to the present description.

[0102] This remarkable effect can be used to generate more complex visual effects or animations, as will be described by means of the following figures.

[0103] Fig. 6A illustrates in a (partial) sectional view an optical security component according to the present description, in which the first diffractive structure S comprises, in first regions (“zone 1”) a first pattern Mi in accordance with the present description and as described for example by means of Fig. 2 and between these first regions, unstructured regions (“zone 2”).

[0104] Fig. 6B illustrates a transmission observation of a security optical component in which the diffractive structure is designed in the embossing phase of the embossing layer (113, Fig. 1 A and Fig. 1B) to generate a high-definition image, i.e. a resolution greater than 2500 dpi (“dots per inch”) (i.e. a pitch between two points less than approximately 10.1 pm), preferably a resolution greater than 3000 dpi, (i.e. a pitch between two points less than approximately 8.5 pm). The diffractive structure comprises, in the example of Fig. 6B, a structure of the type shown diagrammatically in Fig. 6A, with first regions comprising the first pattern Mi (zone 1) and unstructured regions (zone 2, zone 3). At the regions structured with the Mi pattern, the optical security component appears colored with a color that depends, as explained previously, on the first pattern, the metal layer and the tilt angle.On the other hand, in the unstructured regions, the metal layer is thick, there is no resonant effect and the transmission is almost zero. Thus, as illustrated in Fig. 6B, an observer sees, in transmission, a colored image with a bright and vivid color, and very good definition.

[0105] By varying the tilt angle around a direction A parallel to the direction of the undulations, we observe a variation in the color in transmission.

[0106] Fig. 7 shows, for illustration purposes, transmission curves calculated for a security optical component according to the present description. More precisely, for the simulation, the first pattern consists of a one-dimensional undulation, with a sinusoidal profile, a pitch equal to 500 nm and a depth equal to 200 nm. The metal layer results from an evaporation of aluminum on the structure with an angle a = 60° and a deposition rate such that an equivalent thickness on a flat surface with normal incidence would be 30 nm.

[0107] Curves 701, 702, 703, 704, 705, 706, 707 are transmission curves calculated as a function of the wavelength respectively for angles of incidence 0 = 0° (normal incidence), 0 = -5°, 0 = -10°, 0 = -15°, 0 = +5°, 0 = +10°, 0 = +15°. It is remarkable to note that not only does the spectrum vary significantly with the wavelength, which results in a change of color with the tilt, but also that the spectrum is not symmetrical on either side of the normal incidence. In other words, on either side of the normal incidence, an observer does not see the same colors, which reinforces the authentication of the document. If we return to the example of an optical security component as illustrated in Fig. 6A and in Fig.6B, an observer will thus be able to authenticate the component in transmission, thanks to the observation of an intense and unusual colored visual effect, variable depending on the tilt angle and not symmetrical on either side of the normal incidence.

[0108] Note that if the observer observes this same component in reflection, he will also be able to observe a colored image with a color varying with the tilt angle, the color resulting from diffraction at order +1 or -1. In an observation in reflection, the colored effect will be symmetrical on either side of an illumination at normal incidence. Note that at order zero, in reflection, we will be able to observe a colored effect, complementary to the colored effect in transmission.

[0109] Fig. 8A and Fig. 8B illustrate in a (partial) sectional view another example of an optical security component according to the present description, which has a remarkable visual effect. In this example, the optical security component comprises a first diffractive structure S with, in first regions ("zone 1") a first pattern Mi in accordance with the present description and as described for example by means of Fig. 2 and in second regions ("zone 2), a second pattern M2 forming a one-dimensional, symmetrical periodic undulation with a pitch between approximately 400 nm and approximately 900 nm, for example with a pitch identical to that of the undulations forming the first pattern Mi, but with a second depth smaller than the first depth of the first pattern Mi.As for the first pattern, the undulations of the second pattern are configured to form a diffractive grating determined to produce, after deposition of the metal layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm. However, in this example, due to the shallower depth of the undulations, over a period of said first diffractive structure defined between two peaks, the thickness of the metal layer, even if it remains variable, has a minimum thickness strictly greater than 10 nm, such that the asymmetry in the distribution of the metal layer is no longer sufficient to generate remarkable effects in transmission described in relation to the first pattern Mi.

[0110] Thus, as illustrated in Fig. 8A, colored effects resulting from diffraction, similar to those in the first regions ("zone 1") and in the second regions ("zone 2"), can be observed in reflection. On the other hand, as illustrated in Fig. 8B, the transmission will be zero or almost zero in the second regions ("zone 2") and highly dependent on the wavelength in the first regions ("zone 1") due to the remarkable effects resulting from the asymmetry of the metal layer at the level of the first pattern. Fig. 8C illustrates a transmission observation of a security optical component in which the diffractive structure is designed in the embossing phase of the embossing layer (113, Fig. 1A and Fig. 1B) to generate a high-definition image, similar to that illustrated in Fig. 6B.

[0111] The diffractive structure comprises in the example of Fig. 8C a structure of the type shown diagrammatically in Fig. 8A and Fig. 8B, with first regions (zone 1) comprising the first pattern Mi, second regions (zone 2) comprising the second pattern M2, and in this example, a third unstructured region (zone 3). At the level of the first regions structured with the pattern M1, the optical security component appears colored with a color which depends, as explained previously, on the first pattern, the metal layer and the tilt angle. On the other hand, at the level of the second regions comprising the second pattern M2, the metal layer does not have sufficient asymmetry to generate a remarkable effect and the transmission is zero or almost zero. The transmission is also zero or almost zero in the unstructured regions (zone 3). Thus, as illustrated in Fig.8C, an observer sees, in transmission, a colored image with a bright and vivid color, and very good definition. By varying the tilt angle around a direction A parallel to the direction of the undulations, we observe a variation of the color in transmission. As before, the color does not vary symmetrically on either side of the normal incidence.

[0112] In reflection, the image is not visible since, if the pitch of the undulations of the first pattern is identical to that of the undulations of the second pattern, the second regions diffract under the same conditions (wavelength function of the tilt) as the first regions. Only the contour (zone 3) stands out since this region is not structured, it does not diffract. The rapid variation of colors in a security optical component can be advantageously exploited to generate original colored animations.

[0113] Fig. 9A represents a three-dimensional (partial) view of an optical security component comprising a first structure S with a first pattern Mi in accordance with the present description, the first pattern Mi modulating a microscopic pattern M3 consisting of facets 110, 120 of microscopic dimensions. The facets are characterized by a slope (Pi, P2 in the example of Fig. 9A). The slope of the facets is for example in a direction parallel to the direction of variation of the profile of the undulations of the first pattern.

[0114] For example, the facets have a dimension in the direction of the slope (or “width”) greater than or equal to approximately 4 times, advantageously greater than or equal to approximately 8 times the period of the network formed by the undulations which constitute the first pattern Mi. The minimum dimension may therefore be chosen according to the period of the undulations. For example, a minimum dimension of the width of the facets is equal to approximately 2 pm. According to one or more examples, the widths of the facets are between approximately 2 pm and approximately 100 pm, advantageously between approximately 2 pm and approximately 80 pm, advantageously approximately 4 pm and approximately 80 pm.

[0115] According to exemplary embodiments, the facets have a substantially rectangular shape and have a “length” measured in a direction perpendicular to the direction of the slope. The length is for example less than approximately 100 μm.

[0116] According to one or more examples, all of the facets have a height measured in a direction perpendicular to the plane of the component, which is substantially identical. The height of the facets is, for example, less than 2 microns, advantageously less than 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 2 microns, advantageously less than 1 micron.

[0117] According to one or more exemplary embodiments, a maximum angular value of the slopes (in absolute value) is between approximately 7° and approximately 15°. By convention, in the present description, the positive direction for measuring the angular values ​​of the slopes of the facets is the clockwise (or anti-trigonometric) direction.

[0118] As illustrated in Fig. 9B, in exemplary embodiments, at least some of the facets of the set of facets are arranged with variable slopes, the variation of which is increasing, respectively decreasing, in order to simulate a reflective element with a convex, respectively concave region. In the present description, the visual effect resulting from such an arrangement of facets will be referred to as a "half-wave" type dynamic effect when the slopes of the facets have angular values ​​whose variation is increasing or decreasing, but which are of the same sign. The visual effect resulting from such an arrangement of facets will be referred to as a "wave" type dynamic effect when the slopes of the facets have angular values ​​whose variation is increasing or decreasing, and for which at least one change of sign is observed, as illustrated in Fig. 9B.In reflection, due to the modulation of the facets by the first pattern Mi in accordance with the present description, a dynamic effect of the “wave” or “half-wave” type may appear for an observer, during a tilt movement of the component, as a continuous scrolling of colored lines of light.

[0119] As illustrated in Fig. 9B, if we consider a set of facets 110, 120, 130, 140, 150, 160 which have different slopes (angles of the respective slopes (Pi, 02, (03, 04, (05, 0Ô), we can observe, in transmission, for incident light with a normal incidence (perpendicular to the plane of the component), as many different colors shown diagrammatically by the arrows 11, 12, 13, 14, 15, 16 respectively. Indeed, the angle of the facet translates for the incident light beam by a non-zero incidence on the first pattern consisting of the undulations.

[0120] Fig. 9C illustrates a color animation visible in transmission for a security optical component in which the first pattern Mi modulates a set of facets M3 as previously described.

[0121] When observing the optical security component in transmission, if we change the inclination of the component in tilt (rotation around an axis parallel to the direction of the undulations), we observe a modification of the color of each facet which will result in a displacement of a colored line. Note that due to the asymmetry of the colors depending on the angle of incidence, the colors will be different depending on whether we tilt (rock) the component in one direction (position 121) or in another (position 122).

[0122] It should be noted that the optical component according to the present description may comprise, in addition to the first structure described in particular by means of the preceding examples and in accordance with the present description, other structures (not shown in the figures). These may be, for example, diffusing structures, holographic structures or diffracting structures making it possible to produce so-called Alphagram® effects.

[0123] Furthermore, partial demetallization (or local removal of the metal layer) is also possible using known methods to reveal macroscopic patterns visible to the naked eye, providing an additional authentication possibility. Examples of methods for manufacturing optical security components according to the present description are now described.

[0124] A first step comprises the design of said at least one first diffractive structure according to the methods described above, and any other structures.

[0125] Next comes the 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.

[0126] The optical master can be formed by state-of-the-art electronic or optical lithography methods.

[0127] 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, when the first pattern modulates a second pattern, the structure having the second pattern modulated by the first pattern can be etched in a single step.

[0128] 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 first pattern is formed, in the regions in which the first pattern is provided.Then, a second global exposure is carried out, according to interference photolithography methods known to those skilled in the art, the undulations constituting the first pattern are recorded in at least first regions in which the first pattern is provided. Similar steps can be provided to generate other reliefs, such as for example a second pattern in other regions. The order of formation of the patterns is arbitrary and can be modified. Subsequently, the development step is carried out. In this way, an optical master comprising at least the first structure with the first pattern is obtained after development.

[0129] The step of metallic copying of the optical master can then be carried out, for example by electroplating, as mentioned above, in order to obtain the matrix or metallic "master". According to a variant, a matrix duplication step of the metallic master can be carried out to obtain a large-scale production tool suitable for replicating the structure in industrial quantities.

[0130] 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) the first layer 113 (FIGS. 1 A, 1B) in dielectric material with a refractive index of 11, 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 10 pm to 50 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.

[0131] Next comes the deposition on the layer thus embossed of all the other layers, in particular the metallic layer 114, then the layer of dielectric material 115 (optional), the security layer 116 (optional) which can be deposited uniformly or selectively to represent a new pattern and the layer of glue or varnish type (117, 118) by a coating process.

[0132] As explained previously, the metal layer is advantageously deposited by vacuum vaporization of metal with a non-zero angle to generate the desired asymmetry in the metal distribution. Alternatively, it is possible to carry out partial demetallization operations to obtain the desired asymmetry.

[0133] Furthermore, optional steps known to those skilled in the art are possible, such as partial demetallization of the reflective layer 114 to form transparent regions whose contours of macroscopic dimensions are visible to the naked eye.

[0134] Although described through a certain number of exemplary embodiments, the optical security component according to the invention and the method of manufacturing said component include 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 following claims.

[0135] References

[0136] Ref. 1: US2010 / 0307705

[0137] Ref. 2: WO2012136777

[0138] Réf. 3 : EP 2264491 Réf. 4: M.T. Gale, "Zero-Order Grating Microstructures" in R.L. van Renesse, Optical Document Security, 2nd Ed., pp. 267 - 287

[0139] Réf. 5: Popov, Evgeny (2001). "Maxwell equations in Fourier space: fast-converging formulation for diffraction by arbitrary shaped, periodic, anisotropic media" . Journal of the Optical Society of America A. 18 (11): 2886-94. Bibcode:2001 JOSAA.18.2886P. doi: 10.1364 / JOSAA.18.002886

Claims

CLAIMS 1. Optical security component (100A, 100B) for securing an object, for example a valuable document, for example an identity document or a banknote, configured for authentication in transmission, to the naked eye, the component comprising: a first layer (113) of dielectric material, transparent in the visible; at least one first diffractive structure (S) etched in said first layer; a metal 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 comprises, in a first region, at least a first pattern (Mi) forming a first symmetrical one-dimensional periodic undulation, comprising a first pitch (di) of between approximately 400 nm and approximately 900 nm, and a first depth (hi), said first undulation being configured to form a diffractive grating producing, after deposition of the metal layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range of between 400 nm and 700 nm; for each period of said first pattern, defined between two extrema of the first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value (e; m in) strictly less than 10 nm and a maximum value (e max) between about 10 nm and about 100 nm, and the metal is distributed non-symmetrically, so as to produce a colored effect visible in transmission.

2. Optical security component according to claim 1, wherein said first pitch (di) is between approximately 500 nm and approximately 700 nm.

3. Optical security component according to any one of the preceding claims, wherein said first diffractive structure comprises, in a second region, at least one second pattern (M2) forming a second one-dimensional, symmetrical periodic undulation, comprising a direction of variation of the profile parallel to the direction of variation of the profile of the first undulation, comprising a depth identical to that of the first undulation and comprising a pitch different from that of the first undulation, the first pattern and the second pattern having contours recognizable when observed with the naked eye, in transmission.

4. Optical security component according to any one of the preceding claims, wherein said first diffractive structure comprises, in a second region, at at least one second pattern (M2) forming a second one-dimensional, symmetrical periodic undulation, comprising a direction of variation of the profile parallel to the direction of variation of the profile of the first undulation, comprising a pitch identical to that of the first undulation, and comprising a second depth strictly less than the first depth of the first undulation, the first pattern and the second pattern having contours recognizable when observed with the naked eye, in transmission.

5. Optical security component according to any one of the preceding claims, wherein said metallic layer results from metallic evaporation of said first structure with a non-zero evaporation angle, for example an evaporation angle of between approximately 25° and approximately 70°.

6. Optical security component according to any one of the preceding claims, wherein said first diffractive structure further comprises a microscopic pattern comprising a set of facets of microscopic dimensions, of variable slopes, the first pattern modulating the microscopic pattern.

7. 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.

8. 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.

9. A method of manufacturing an optical security component for securing an object, for example a valuable document, for example an identity document or a banknote, configured for authentication in transmission, to the naked eye, the method comprising: depositing on a support film a first layer of dielectric material (113), transparent in the visible; forming on said first layer at least one first diffractive structure (S), depositing a metal layer (114) at least partially covering said first diffractive structure, and having a spectral band of reflection in the visible, in which: said first diffractive structure comprises, in a first region, at least one first pattern (Mi) forming a first one-dimensional periodic undulation, symmetrical, comprising a first pitch (di) between approximately 400 nm and approximately 900 nm, and a first depth (hi), said first undulation being configured to form a diffractive grating producing, after deposition of the metal layer, a diffractive effect in reflection at order 1 and at order -1 in a wavelength range between 400 nm and 700 nm; for each period of said first pattern, defined between two extrema of the first undulation, the thickness of the metal layer, defined in a direction perpendicular to the plane of the component, is variable between a minimum value (e m in) strictly less than 10 nm and a maximum value (e ma x) between about 10 nm and about 100 nm, and the metal is distributed non-symmetrically, so as to produce a colored effect visible in transmission.

10. Method of manufacturing an optical security component according to claim 9, wherein said metallic layer results from metallic evaporation of said first structure with a non-zero evaporation angle, for example an evaporation angle of between approximately 25° and approximately 70°.

Images