AU2017270014B2

AU2017270014B2 - Optical security component and method for manufacturing such a component

Info

Publication number: AU2017270014B2
Application number: AU2017270014A
Authority: AU
Inventors: Khalil CHIKHA; Kevin John HEGGARTY; Valéry PETITON; Jean SAUVAGE-VINCENT; Vincent TOLLET
Original assignee: Surys SA; Institut Mines Telecom IMT
Current assignee: Surys SA; Institut Mines Telecom IMT
Priority date: 2016-05-23
Filing date: 2017-05-23
Publication date: 2022-05-05
Anticipated expiration: 2037-05-23
Also published as: EP3465352A1; FR3051565B1; FR3051565A1; WO2017202866A1; AU2017270014A1

Abstract

According to one aspect, the present description relates to an optical security component (20) comprising a first layer (23) made of dielectric material at least partially structured on one side, a second layer (22) deposited on said at least partially structured side in at least one first region and having a spectral band of reflection in the visible, and a third layer (21) made of dielectric material, this third layer being deposited on said second layer. The first layer (23) has, in the first region, at least one first structure (S) formed by a first pattern (S1) that is modulated by a second pattern (S2), said patterns being such that the first pattern forms a first diffractive element of the computer-synthesized-hologram type generating a first recognisable image in at least one first reconstruction plane, and the second pattern is a periodic grating of period comprised between 100 nm and 700 nm producing a resonant filter in a first spectral band.

Description

OPTICAL SECURITY COMPONENT AND METHOD FOR MANUFACTURING SUCH A COMPONENT

Technical Field

This description relates to an optical security component and a process for manufacturing such a component. The optical security component according to this description applies in particular to security marking for the security and authentication of valuables.

State of the art

Many technologies are known to authenticate valuables, including the authentication of valuable documents, such as banknotes or travel documents (passports, identity cards or other identification documents), or the authentication of products by means of marking labels. These technologies aim at the production of optical security components whose optical effects as a function of observation parameters (orientation with respect to the observation axis, position and dimensions of the light source, etc.) take characteristic and verifiable configurations. The general purpose of these optical components is to provide new and differentiated visual effects, based on physical configurations that are difficult to reproduce or imitate by a counterfeiter. Among these components, the French patent application published under number FR 2 983 318 describes a diffracting optical element (or DOE according to the Anglo-Saxon abbreviation "Diffractive Optical Element"), also called computer synthesized hologram (or CGH). Such a diffracting optical element comprises complex phase and amplitude data physically encoded in a DOE microstructure, which are calculated so that, when the DOE is illuminated by a substantially collimated light beam, a desired light intensity diagram is generated in the far field or in a reconstruction plane. In practice, DOE data can be determined by calculating a reverse fast Fourier Transform (or reverse FFT, FFT being the Anglo-Saxon abbreviation for Fast Fourier Transform) of the desired reconstruction, i.e. the desired intensity diagram in the far field or reconstruction plane. Such a diffracting optical element can operate in transmission (transmissive DOE) or in reflection (reflective DOE). In the case of reflective DOE, a reflective layer can be applied to the microstructure encoding complex phase and amplitude data.

Figures 1A and 1B thus represent a document of value 1, for example an identity document, comprising an optical security component 2 with a diffractive optical element of (respectively reflective and transmissive) DOE type, as described in the prior art. In these figures, beam 3 represents the lighting beam; for example, it comes from a punctual source or a laser. Beam 4 represents the beam as reflected (FIG. 1A) or transmitted (FIG. 1B) by the optical security component 2. In these examples, data are calculated so that, when the DOE is illuminated by a beam of light from a punctual source, a light intensity diagram 6 appears in a reconstruction plane 5.

However, one difficulty arising when using this type of optical security component is the difficulty in producing elaborate color images. As a matter of fact, when this type of diffractive optical element is illuminated with polychromatic light, for example with a punctual white light, a white image with colored iridescences is usually obtained even if a DOE is calculated for a maximum efficiency with a given wavelength.

This problem is solved in the patent application FR 2 983 318 of the prior art by interlacing diffracting structures calculated for specific wavelengths in a diffractive optical element (DOE) and illuminating the DOE with monochromatic light beams, for wavelengths which the diffracting structures are sensitive to. Monochromatic beams at specific wavelengths are sent simultaneously, sequentially or cyclically.

However, for the authentication of a valuable object, this solution requires working with a set of monochromatic sources or with a set of spectral filters arranged in front of a monochromatic source. On the other hand, the quality of the colored image is not entirely satisfactory because each diffractive structure of the DOE, even if it reacts mainly to a specific wavelength, is also sensitive to a lesser extent to other wavelengths, causing unwanted color mixtures in the image.

This description presents an optical security component with a computer generated hologram type element, allowing the reconstruction of very good quality colored images under spatially coherent polychromatic lighting, of the punctual or near-punctual white source type and therefore without requiring a specific lighting device using multiple sources or filters.

Object of the invention

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

Summary

According to a first aspect, the present invention provides an optical security component intended to be authenticated in the visible spectrum in spatially coherent polychromatic light, comprising:

- a first layer made of a dielectric material, at least partially structured on one face, and having a first refractive index; - a second layer, deposited on at least one partially structured face of said first layer in at least a first region of the first layer, and having a visible reflection spectral band; - a third layer made of a dielectric material deposited on said second layer and having a third refractive index; and wherein said first layer has, in the first region, at least one first structure formed by a first pattern modulated by a second pattern, such that: o the first pattern is adapted to form a first diffractive element of the computer synthesized hologram type, calculated to generate, under spatially coherent lighting, at least a first image recognizable in at least one first reconstruction plane, o the second pattern is a periodic grating with a period between 100 nm and 700 nm, determined to produce, after deposition of the second layer and encapsulation of said first structure by the third layer, a resonant filter in a first spectral band. The resonant filter is for example a DID type resonant filter (for "Diffractive identification Device") which allows the excitation of guided modes within a region of the second layer of dielectric material, or a plasmonic type resonant filter obtained thanks to a region of the second layer of metallic material which forms, with the first and/or third layer of dielectric material, a metal/dielectric interface or dielectric/metal/dielectric interfaces, or any other resonant filter having a visible resonant spectral band.

For the purposes of this description, "visible" means the spectral band between 380 nm and 780 nm.

According to one or more exemplary embodiment(s), one and/or the other of the first and third layers of dielectric material is transparent in the visible spectral band.

A layer is called transparent in a spectral band within the meaning of this description if, for each wavelength of said spectral band, at least 70 % of a radiation at said wavelength is transmitted, preferably at least 80% and more preferably at least 90 %.

In this description, a thin film is defined as a layer with a thickness between 10 nm and 200 nm.

According to one or more exemplary embodiment(s), the period of the periodic grating ranges from 200 nm to 500 nm. According to one or more exemplary embodiment(s), the grating is a sub wavelength grating, i.e. having a wavelength shorter than the minimum wavelength of the wavelength range of a polychromatic light source intended to authenticate the component or the wavelength range in which the authentication control is performed, for example the range of wavelengths visible by the eye. In this description, CGH (for "Computer Synthesized Hologram") refers to a diffracting optical element comprising complex phase and amplitude data physically encoded in a microstructure, which data are calculated so that, when the CGH is illuminated by a substantially collimated light beam, a two-dimensional light intensity diagram (e. g. a recognizable image) is generated in the far field or in afinite distance reconstruction plane of the component. In practice, CGH data can be determined in a known way by calculating a reverse Fast Fourier Transform (or FFT, FFT being the Anglo-Saxon abbreviation for Fast Fourier Transform) of the desired reconstruction, i.e. the desired two-dimensional intensity diagram in the far field or reconstruction plane. CGHs are also known in the state of the art as "DOE" for "Diffractive Optical Element" or "CHG" for "Computer-Generated Holograms". For the purposes of this description, an CGH should therefore not be confused with a diffractive structure whose spatial contour is the only way to form a recognizable image under illumination.

The Applicant demonstrated that the modulation of the diffractive optical element with a sub-wavelength grating adapted to form a resonant filter significantly increases the wavelength selectivity of the diffractive optical element. According to one or more exemplary embodiment(s), the periodic grating is a unidimensional grating, defined by a grating vector. This type of grating makes it possible to show variable colored effects according to the azimuth, for non-zero incidence angles, and according to the incidence angle for a given azimuth (for example in so-called "colinear" lighting). According to one or more exemplary embodiment(s), the periodic grating is a two-dimensional grating, defined by two grating vectors of substantially perpendicular directions. The periods in each direction may be identical, in which case the first image has a stable azimuth color and a low variation with incidence or the periods may be different, in which case color effects that vary with azimuth can be observed for non-zero incidence angles. According to one or more exemplary embodiment(s), the periodic grating has a structural depth between 10 nm and 350 nm. According to one or more exemplary embodiment(s), the second layer comprises, in at least a first region, a thin layer of dielectric material, preferably between 20 nm and 200 nm thick and preferably between 60 nm and 150 nm, having a second refractive index such that the second refractive index differs from the first refractive index and the third refractive index by at least 0.3. The second pattern of at least one structure of said first region is adapted to produce a subtractive wavelength filter of the DID type, forming a resonant band-pass filter in reflection. According to one or more exemplary embodiment(s), the second layer comprises, in at least a second region, a thin layer of continuous metallic material with a thickness advantageously greater than 40 nm, for example between 40 nm and 200 nm; the second pattern of at least one structure of said second region forms a notch resonant filter in reflection, of the plasmon filter in reflection type, called "R' plasmon" in the following description. According to one or more exemplary embodiment(s), said thin layer of metallic material is thick enough to have a maximum residual transmission in the visible spectral band, as a function of the wavelength, of 2 %. According to one or more exemplary embodiment(s), the second pattern forms a sinusoidal or quasi-sinusoidal profile grating, i.e. with continuous variation, this type of profile allowing a better propagation of plasmonic modes while being compatible with photolithography manufacturing methods. Advantageously, the depth of the grating is between 10 % and 50 % of the period, and preferably between 10 % and 40 % of the period. According to one or more exemplary embodiment(s), the second pattern forms a two dimensional grating. According to one or more exemplary embodiment(s), the second layer comprises, in at least a third region, a thin layer of a metallic material, having a thickness advantageously between 10 nm and 60 nm; the second pattern of at least one structure of said third region is adapted to produce a band-pass resonant filter in transmission, of the plasmonic filter in transmission type, called "T'plasmon" in the following description. According to one or more exemplary embodiment(s), the thin layer of metallic material is continuous; the second pattern forms a grating of sinusoidal or quasi-sinusoidal profile, i.e. with continuous variation, this type of profile allowing a better propagation of plasmonic modes while being compatible with photolithography manufacturing methods. Advantageously, the grating depth is between 10 % and 53 % of the period, which increases the efficiency of plasmonic transmission. According to one or more exemplary embodiment(s), the second pattern forms a two-dimensional grating. According to one or more exemplary embodiment(s), the second layer is discontinuous; the second layer may also have, in one or more first region(s), a thin layer of dielectric material, to form DID-type filters, and/or in one or more second region(s), a thin layer of metallic material, for example, of a thickness greater than 40 nm to form R'Plasmon-type filters (metal/dielectric interface) and/or in one or more third regions a thin layer of metallic material, for example having a thickness between 10 nm and 60 nm to form T'Plasmon-type filters (dielectric/metal/dielectric interface). According to one or more exemplary embodiment(s), for at least a first structure, the first pattern is adapted to form a first diffractive element of the reflective computer synthesized hologram (CGH) type. The first pattern has a depth between 100 nm and 500 nm, according to one example. According to one or more exemplary embodiment(s), for at least a second structure, the first pattern is adapted to form a first diffractive element of the transmissive computer synthesized hologram (CGH) type. The refractive indices of the first and third layers of dielectric material then differ by at least 0.1, according to one example, and the first pattern has a depth between 100 nm and 1Ipm, according to an example. The first layer of dielectric material is transparent. According to one or more exemplary embodiment(s), the first diffractive element of the computer synthesized hologram (CGH) type is calculated to present an optimal efficiency for a wavelength within the resonance spectral band of the resonant filter. According to one or more exemplary embodiment(s), the first diffractive element of the computer synthesized hologram (CGH) type is calculated to present an optimal efficiency at a wavelength located outside said resonance spectral band of the resonant filter. According to one or more exemplary embodiment(s), the first layer also has, in the first region, at least one second structure formed by a first pattern modulated by a second pattern, such that: 0 the first pattern is adapted to form a second diffractive element of the computer synthesized hologram type, calculated to generate under spatially coherent lighting at least one second image recognizable in a given second reconstruction plane, o the second pattern is a periodic grating with a period between 100 nm and 700 nm, determined to produce, after deposition of the second layer and encapsulation of said second structure by the third layer, a resonant filter in a second spectral band. It is thus possible to generate two different images of different colors simultaneously, in confused or unmixed reconstruction planes, when the component is illuminated with spatially coherent polychromatic light in the visible spectrum. According to one or more examples of construction, the at least first and second structures are arranged in interlaced areas. This allows more complex images to be generated, including multi-color images in the same reconstruction plane. According to one or more exemplary embodiment(s), the at least first and second structures are arranged in adjacent areas, the successive lighting of each area allowing to simulate a movement effect of an image and/or transformation. This allows to generate a colorful animation. According to one or more exemplary embodiment(s), the optical security component includes other layers according to the needs required for the final application; for example, the optical security component may include, in addition to the active layers for the formation of CGH and resonant filter, a support film carrying one of said layers of dielectric material and/or an adhesive layer disposed on one of said layers of dielectric material. These layers are neutral for the CGH and/or resonant filter because they do not alter or influence the interfaces between the second layer and the first and third layers respectively. They facilitate adhesion to the object to be secured and/or implementation in an industrial way. According to one or more exemplary embodiment(s), the first layer includes, in addition to regions structured according to this description, flat regions and/or other structured regions, encapsulated between the first and second layers of dielectric material, the other structured regions being adapted to form other visual effects. In a second aspect, this description relates to a secured object comprising a support and an optical security component in the first aspect, the optical security component being fixed on the said support or integrated into the support. The secured object is for example a valuable document, such as a banknote, a travel document (passports, identity card or other identification document), a label intended for the authentication of a product. The secured object can be easily authenticated by observation in transmission or reflection, under coherent polychromatic lighting, using the optical security component according to this description; moreover, its resistance to counterfeiting is high due to the technology used. A third aspect is that this description relates to a method for manufacturing an optical security component according to the first aspect. The method comprises: - The formation on a carrier film of said first layer of dielectric material; - the deposition of the second layer on the first layer of dielectric material;

- the deposition on the second layer of the third layer of dielectric material. According to one or more exemplary embodiment(s), the first layer of dielectric material, partially structured, is obtained by moulding and UV crosslinking of a stamping varnish from a matrix carrying all the structures.

Brief Description of the Figures

Further characteristics and advantages of the invention will be clear from reading the following description, made in reference to the figures, which show: FIGS 1A and 1B (already described), two examples of secured objects integrating an optical security component with a diffractive element of the CGH type according to the prior art; FIG. 2 a cross-sectional view of an example of an optical security component according to this description; FIGS. 3A, 3B, 3C diagrams illustrating an example of a first pattern adapted to form a first diffractive element of the CGH type, an example of a second pattern adapted to form a resonant filter and a structural example resulting from the modulation of the first pattern with the second pattern; FIG. 4A to 4E, diagrams showing different exemplary embodiment(s) a resonant filter associated with a reflective or transmissive CGH; FIGS. 5A to 5D diagrams illustrating the dependence of the resonance wavelength of the resonant filter on the incidence angle, in colinear incidence (FIGS. 5A and 5C) and in conical incidence (FIGS. 5B, 5D); FIGS. 6A and 6B examples of secured objects integrating an example of an optical security component as described in this description with an off-axis CGH diffractive element in reflection and a ID resonant filter, for a non-zero incidence angle and for two azimuth values; FIG. 7A, a first example of an optical security component comprising a pixel matrix, a first portion of the pixels comprising a first structure formed of a first pattern adapted to form a first CGH-type diffractive element in reflection, modulated by a second pattern, and a second portion of the pixels comprising a second structure formed of a first pattern adapted to form a second CGH-type diffractive element in reflection, modulated by a second pattern; FIGS. 7B to 7D examples of secured objects incorporating an optical security component of the type in FIG. 7A for different incidence and azimuth values; FIG. 8, a second example of an optical security component comprising a matrixing of pixels as strips and images obtained by successive illumination of the different strips; FIG. 9, an example of a secured object integrating a first optical security component and a second optical security component, adapted for stereoscopic vision; FIGS. 1OA - 10B, 11A - 1IB, examples of secured objects integrating examples of optical security components with structures adapted for the formation of diffractive elements of the CGH type; On the figures, shown for illustrative purposes, the scales are not respected for greater clarity in the representation.

Detailed Description

FIG. 2 represents a cross-sectional view of an example of an optical security component 20 according to this description. The component 20 represented in FIG. 2 represents an example of an optical security component intended to be transferred to a document or a product for security purposes. It includes, according to a variant, a carrier film 24, for example a film made of a polymer material, for example a polyethylene terephthalate (PET) film of a few dozen

micrometers, typically 5 to 50pm, as well as an optional release layer 25, for example made of natural or synthetic wax. The release layer allows the polymer carrier film 24 to be removed after fixing the optical component to the product or document to be secured. The optical security component 20 also includes a set of layers 21 - 23 for the realization of the optical function of the component and which will be described in greater details later, as well as an adhesive layer 26, for example a hot re-activatable adhesive layer, for fixing the optical security component to the product or document. In practice, as disclosed in details hereunder the optical security component can be manufactured by stacking the layers on the carrier film 24, then the component is fixed on a document/product to be secured with the adhesive layer 26. If necessary for the application, the carrier film 24 can then be removed, for example using the release layer 25. The set of layers 21 - 23 include in the example of FIG. 2, a first layer 23 made of a dielectric material, structured at least partially on one face, having a first refractive index n3; a second layer 22, comprising, in at least a first region, a layer made of a dielectric material or of a metallic material, as will be disclosed in details hereunder, or comprising one or more region(s) with a layer made of a metallic material and one or more region(s) with a layer made of a dielectric material, coated on the face of the first layer which is at least partially structured and has in each of the regions a visible spectral reflection band; a third layer 21 made of a dielectric material, having a third refractive indexni, encapsulating the at least partially structured first layer and coated with the second layer. As shown in FIG. 2, the first layer 23 has, in at least onefirst region, at least onefirst structure S. FIG. 3C schematically shows an example of a first structure S, formed by a first Si pattern modulated by a second S2 pattern. The first pattern (FIG. 3A) is adapted to form a first diffractive element of the computer synthesized hologram (CGH) type, whether reflective or transmissive, calculated to generate, under spatially coherent polychromatic lighting, at least a first image recognizable in a given reconstruction plane. the second pattern (FIG. 3B) is a periodic grating with a period between 100 nm and 700 nm, for example sub-wavelength, determined to produce, after deposition of the second layer and encapsulation of said first structure, a resonant filter in a spectral band centred on a first visible wavelength. More precisely, the first pattern encodes, according to the known prior art, complex phase and amplitude data calculated so that, when the diffractive element is illuminated by a light beam, a diagram of desired light intensity appears either in the far field or in a finite distance reconstruction plane. The first pattern can form a multi-level structure (Si), as shown in FIG. 3A or a binary structure. A multi-level structure has at least 3 planes at different heights along the z axis perpendicular to the plane of the component, while a binary structure has only two level heights. A multi-level structure makes it possible to form images that are not symmetrical with respect to the zeroth order, i.e. different on either face of the zero order, the zero order designating the specular reflection. The depth of the first pattern determines the phase introduced and the proportion of "zero order", i.e. the amount of incident light reflected in direct reflection or transmitted without deviation (to the nearest refraction). In practice, the depth of the first chosen pattern is between 80 nm and 2 pm, preferably between 100 nm and 1I pm. The first pattern can be calculated to form an image on the axis (diffracted order(s) centered on zero order) or off-axis (diffracted order(s) around zero order). In addition, the first pattern can be calculated in a known way for image restitution in a finite distance reconstruction plane, or for image restitution known as "infinity", i.e. image restitution in the far field. To form an CGH allowing an image to be restored in the far field when the CGH is illuminated by a substantially collimated beam of light, we know for example the so called Fourier CGHs. In practice, a Fourier CGH can be made up of a single element with dimensions up to 30 mm x 30 mm, dimensions beyond which calculation times with current ECUs are very long, or from identical unit elements, for example, with dimensions between 2 pm x 2 pm and 2048 pm x 2048 pm which are then repeated in X and Y either mechanically at the time of engraving or electronically before engraving, in order to obtain a large CGH (or observation window) with dimensions up to 60 mm x 60 mm for example. To form an CGH allowing an image to be restored at a finite distance from the CGH when the CGH is illuminated by a substantially collimated beam of light, we know for example the so-called Fresnel CGH. Each point of the first pattern in the case of a Fresnel CGH contributes to forming the image in the reconstruction plane. Thus, in the case of a Fresnel CGH, an CGH with typical dimensions up to 30 mm x 30 mm is directly encoded. In both cases, an adapted optical projection system can be used to project the image onto a screen, for example. In practice, we start from an image containing recognizable information, we decide on the functions we want to have (symmetrical restored image or not, restitution plane close to the CGH or distant, etc...) to choose the type of CGH to be manufactured: binary or multi-level and Fourier or Fresnel, etc. Then, using CGH design software, for example IFTA (for "Iterative Fourier Transform Algorithm"), a first pattern or CGH is calculated with dimensions generally between 2 pm x 2 pm and 15 mm x 15 mm. It is considered that an CGH can act as an observation window if its lateral dimensions exceed the limit of the human ocular opening, i.e. 0.5 mm x 0.5 mm. The design rules of the first pattern to form an CGH with the characteristics described in the paragraphs above are described for example in "Applied Digital Optics: From Micro-optics to Nanophotonics", Bernard C. Kress, Patrick Meyrueis, 2009. In the rest of the description, it will be understood that all the examples described can be applied to all known diffractive elements, CGH restoring an image in the far field or at a finite distance from the CGH, CGH formed by a binary or multi-level structure, image restored symmetric or not symmetric with respect to the order zero, on or off axis, reflective or transmissive CGH etc. unless explicitly mentioned otherwise. FIG. 4A to 4E illustrate diagrams showing open-ended examples of how to make a resonant filter associated with a reflective or transmissive CGH to increase its spectral selectivity. In these examples, a polychromatic source PSused for component authentication has been symbolically represented. Polychromatic spatially coherent lighting can be obtained by a polychromatic punctual or quasi-punctual source, for example a white LED, a mobile phone flash, a flashlight, and generally a non-extended light source. More precisely, a source will be considered to be quasi-punctual or not extended if the ratio between the distance to the component and the largest dimension of the source is above 100. By convention, we note 41 the lighting face of the component and 42 the face of the component opposite the lighting face 41. The layer of dielectric material 23 arranged on the face of the lighting face 41 is transparent and preferably colorless. The layer of dielectric material 21 arranged on the face of the face 42 opposite the lighting face may or may not be transparent, depending on the applications. The first pattern is calculated to form an CGH that generates an image recognizable in a reconstruction plane 5, which is visible to an observer, either directly or by means of a projection device (not shown), or on the face of the lighting face (in the case of a reflective CGH) or on the opposite face (in the case of a transmissive CGH). In the examples of FIGS 4A and 4B, the second layer 22 comprises a layer of a dielectric material and the resonant filter is a subtractive wavelength filter of the DID type (Diffractive Identification Device). The second pattern forms a sub-wavelength grating, of one or two dimension(s), adapted to allow the excitation of guided modes within the second layer 22, forming a band-pass resonant filter in reflection, whose resonance spectral bandAX is centered on a first wavelength 1 . The second layer 22 comprises a thin layer, preferably between 20 nm and 200 nm and preferably between 60 nm and 150 nm, with a second refractive index n2 such that the second refractive index n2 differs from the first refractive index n3 and the third refractive index ni by at least 0.3, preferably by at least 0.5. According to one or more exemplary embodiment(s), said thin layer of dielectric material is a layer of a so-.called "high refractive index" (or "HRI") material, having a refractive index between 1.8 and 2.9, preferably between 2.0 and 2.4 and the first and third layers of dielectric material, on either face of the second layer, are called "low refractive index" layers, having refractive indices between 1.3 and 1.8, advantageously between 1.4 and 1.7. The first layer of dielectric material 23 arranged on the face of the lighting face of the component is transparent in the visible spectrum. In the example of FIG. 4A, the first pattern is adapted to form a first diffractive element of the reflective CGH type. For example, the first pattern is calculated to provide optimal efficiency for a wavelength within the spectral band AX of the reflective resonant filter, for example a wavelength close to X 1. Thus, the first image, observed by the observer located on the face of the lighting face 41, shows an increased spectral selectivity for the wavelength 1 for a given azimuth and angle of incidence and observation. In the example of FIG. 4B, the first pattern is adapted to form a first transmissive CGH type diffractive element. For example, the first pattern is calculated to provide optimal efficiency for a wavelength outside the spectral range of the DID type reflective resonant filter. As a matter of fact, in this example, the spectral band transmitted by the resonant filter corresponds to the spectral band of the illumination light X from which the resonant spectral AXband is subtracted. In this example, the first and third refractive indices have a difference of more than 0.1 for the formation of transmissive CGH. In this example, the first image, observed by the observer on the face of the component face 42 opposite the lighting face 41, has a lower selectivity than in the previous example; however, the color of the CGH is improved due to the rejection of a part of the spectral band. In the example of FIG. 4C, the second layer 22 comprises a thin layer of a metallic material, advantageously thicker than 40 nm. The second pattern forms a one or two dimension(s) sub-wavelength grating, adapted to allow the formation of a resonant band rejection filter in reflection, of the plasmonic filter in reflection "R'plasmon" type, as described for example in the patent application FR 2982038A1. Advantageously, the second metallic layer 22 is thick enough to have a maximum residual transmission as a function of the wavelength of 2 %. In this example, the first pattern is adapted to produce a first reflective CGH diffractive element, calculated to be optimally effective for a wavelength outside the spectral range of the reflecting resonant filter for a given azimuth and angle of incidence and observation. In the examples of FIGS 4D and 4E, the layer 22 comprises a thin layer of a metallic material, with a thickness advantageously between 10 nm and 60 nm and the second pattern is adapted to produce a band-pass resonant filter in transmission, of the "T'Plasmon" plasmonic filter in transmission type, as described for example in the patent application FR 2973917 and having a spectral resonance band AX centered on afirst wavelength X1 for a given azimuth and an incidence and observation angle. In the example of FIG. 4D, the first pattern is adapted to form a first diffractive element of the transmissive CGH type. For example, the first pattern is calculated to provide an optimal efficiency for a wavelength within the spectral band AX of the resonant filter in transmission, for example a wavelength close to X 1; The first and second reduction indices advantageously differ by more than 0.1. In this configuration, the user placed on the face 42 face will be able to observe, under spatially coherent polychromatic lighting EX, a colored image AX in transmission. In the example of FIG. 4E, the first pattern is adapted to form a first diffractive element of the reflective CGH type. For example, the first pattern is calculated to provide an optimal efficiency for a wavelength outside the spectral range of the T'Plasmon transmissive resonant filter. Indeed, in this example, the spectral band IX - AX reflected by the resonant filter corresponds to the spectral band of the illumination light IX from which the resonant spectral band AX is subtracted. Regardless of the nature of the resonant filter, the wavelengths of the excited resonances depend on the polarization and it can be shown that for a non-zero incidence of the light beam, the reflected (or transmitted) spectral band will be modified by changing the orientation of the component, obtained by azimuthal rotation of the component, except in the case of a two-dimensional grating of equal periods. FIGS. 5A to 5D illustrate these effects in the case where the first pattern forms a one-dimensional lattice characterized by a lattice vector kg, the direction of which is perpendicular to the lattice lines (symbolized by lines on FIGS. 5A and 5B) and the norm is inversely proportional to the period of the grating. The plane 1 corresponds to the plane in which the lighting beam is located.

FIG. 5C represents the spectra calculated in reflection for a so-called "colinear" incidence (configuration 5A: light beam in a plane 1 perpendicular to the surface of the component and parallel to the direction of the grating vector) and an angle of incidence of the light beam measured with respect to the component normal of 00 (curve 51) and 20 (curve 52). FIG. 5D represents the spectra calculated in reflection for a so-called "conical" incidence (configuration 5B): light beam in a plane 1 perpendicular to the surface of the component and perpendicular to the direction of the grating vector) and an angle of incidence of the light beam measured with respect to the component normal of 0 (curve 53) and 200 (curve 54). In both cases, the curves are calculated by assuming a DID with a 360 nm pitch, a 130 nm depth structuring a 100 nm thick layer of zinc sulfide (ZnS) and encapsulated between two identical layers of polystyrene. In colinear incidence, there is a rapid change in the resonance spectral band with the incidence angle, while the change is much slower in conical incidence. This effect further strengthens the authentication of the component by showing color effects that vary with the azimuth. The reflective curves in Figures 5C and 5D are calculated for a DID resonant filter, but similar transmissive curves would be observed for a T'Plasmon resonant filter. In the case of a R'Plasmon resonant filter, absorption curves would show the same dependence on the azimuth and the angle of incidence. Of course, in the case of a two-dimensional grating (not shown in the figures), for an approximately equal period in each direction, the color effect will be non-variable, or very slightly variable as a function of the azimuth. There will be a variation in the colored effect with the azimuth, for non-zero incidence angles, if the periods in each direction are different. FIGS. 6A and 6B show schematically a secured object 1, for example a valuable document of the identity document type, comprising an example of an optical security component 60 according to this description with a diffractive element of the off-axis CGH type in reflection and a resonant ID filter, for a non-zero angle of incidence and for two azimuth values; in these figures, beam 3 represents the illumination beam; it is generated for example from a polychromatic punctual or quasi-punctual source, for example a white LED source. Beam 4 represents the beam reflected by the optical security component 60. In these examples, data are calculated so that, when the CGH is illuminated by a divergent, converging or collimated beam of light, a light intensity diagram appears in a reconstruction plane 5 that can be infinite. In this example, the intensity diagram includes an image 66 formed by two off-axis objects 61, 62. A residue 63 of the beam transmitted on the axis is also visible. As it appears in FIG. 6B, the rotation of the component in azimuth reveals a color change in image 66 with a non-zero incidence of the illumination beam. In addition to the increased spectral selectivity of the CGH obtained by the resonant filter, which results in a much better image quality 66, in this example we obtain an additional means of authentication of the optical security component, thanks to a color effect that varies according to the azimuth. FIG. 7A schematically represents a first example of an optical security component 70 including pixel matrixing. A first part of the pixels 71 forms a first area. They comprise a first structure formed by a first pattern adapted to form a first diffractive element of the CGH type in reflection, modulated by a second pattern. A second part of the pixels 72 forms a second area. They comprise a second structure formed by a first pattern adapted to form a second diffractive element of the CGH type in reflection, modulated by a second pattern. The first and second zones, for example, occupy a comparable area. More precisely in this example, the first pattern of the first structure arranged in the first area (pixels 71) is a multi-level CGH to form the image "" in the far field off the axis. The second pattern of the first structure allows to form a one-dimensional grating adapted to form a resonant filter with a resonance spectral band. The grating is arranged so that with an azimuth of 00, the variability of the resonant spectrum is low as a function of the angle of incidence of the light beam (as in FIG. 5B). With the 90 azimuthal rotation of the component 70, the far-field variability of the resonance spectral band for the first region, as a function of the angle of incidence, becomes high. The first pattern of the second structure arranged in the second area (pixels 72) is a multi-level CGH for forming the image "a" in the far field off the axis. The second pattern of the second structure forms a one-dimensional grating similar to that of the second pattern of the first structure. But the grating is arranged substantially perpendicularly to that of the second pattern of the first structure so that, with an azimuth of 0, the variability of the resonant spectrum is high as a function of the angle of incidence of the illumination beam and that with the 90 azimuthal rotation of the component 70, the far-field variability of the resonant spectral band for the second region, as a function of the angle of incidence, is low. FIG. 7B shows an illumination of a secured object 1 equipped with a 70 optical security component as shown in FIG. 7A for an incidence of 0°. Thanks to pixel matrixing, both images "" and "a" are obtained simultaneously. The incidence of 0° does not allow to differentiate in color the 2 images. FIG. 7C illustrates a lighting of the secured object 1 with an incidence angle of 20. In this case, the incidence of 20° allows to differentiate in color the two images "" and

FIG. 7D shows the illumination of the secured object 1 with an incidence angle of 20° when the object 1 has undergone a 90° azimuthal rotation. In this case, we observe an inversion of the colors of the two images "n"and "a". Of course, the "colors" are presented here as examples. Second patterns could be obtained for the first and second structures such that the initial colors of the first and second images are different for an incidence of 0° and exhibit the same or different spectral variation behaviours as a function of the incidence angle. FIG. 8 schematically represents a second example of an optical security component 80 comprising a matrix of pixels as strips (81 - 84) and images obtained by successive illumination of the different strips. Each band, with a width L greater than 500 pm, for example, forms a region. In this example, the first patterns of each region are adapted to form off-axis CGHs with the same object (an arrow) but at a different position and orientation in the observation plane (see images Imi - Im4). The second pattern is for example identical for each region so that the object (the arrow) is the same color on each image. The coherent polychromatic illumination on each band then allows only one image to be revealed separately from the other bands. The successive illumination of the different regions allows to simulate movement effects. Similarly, it will be possible to create morphing effects (deformation/transformation) by scanning. For example, the first image of the first area may have a defined shape that evolves in each successive area to simulate a movement and/or transformation of the shape. In addition, each area of each first pattern may have a different second pattern in terms of period, grating orientation and/or grating depth to change the color of each area. FIG. 9 represents an example of a secured object integrating a first optical security component and a second optical security component, adapted for human stereoscopic vision or by video or stereoscopic image players; In this example, two optical security components according to this description

90L and 90R are arranged on a secured object 1 at a given distance from each other, so that for a given illumination 3, 3' respectively on the components 90L and 90R, each eye/sensor sees an image independently. In this example, the left eye can only see image 91, 92 generated by the security component 90L, while the right eye can only see image 93, 94 generated by the security component 90R. Of course, and as described above, each of the components 90L and 90R can have structures to generate CGHs with specific or identical colors related to resonant filtering. The mental reconstruction of the image allows the observation of the image on a virtual plane, the virtual plane can be positioned at a median distance from the two images generated by each of the components 90L and 90R. FIGS. 10A and 10B and 11A - 1lB show other examples of secured objects integrating examples of optical security components with structures adapted for the formation of diffractive elements of type CGH, for example in the nearfield. In the example of FIGS 10A and 10B, the optical security component 100 includes a first structure arranged on an area 101 and a second structure arranged on an area 102, the areas 101 and 102 being formed of pixels alternating with each other to form a checkerboard (interlaced areas). For example, the first structure 101 includes a first pattern modulated with a second pattern, so as to form a Fresnel CGH, allowing to generate an image 103 of a given color in a finite distance reconstruction plane 5 (near-field CGH), the reconstruction plane 5 being behind the secured object 1 with respect to the position of the lighting source. The second structure 102 comprises a first pattern modulated with a second pattern, so as to form a Fresnel CGH, allowing to generate an image 104 of a given color, for example different from the color of object 103, in a finite distance 5' reconstruction plane (near field CGH), the 5' reconstruction plane being in front of the secured object 1 with respect to the position of the lighting source. With a non-zero incident lighting, for example under incidence between 20° and 40, as shown in FIGS. 10A and 10B, a color inversion of images 103 and 104 can be generated by azimuthal rotation of the object 1, for example if the second patterns of each of the first and second structures are unidimensional, of the same period and oriented in substantially perpendicular directions. In the example of FIGS 11A and 11B, the optical security component 110 includes a first structure arranged on a zone 111, a second structure arranged on a zone 112 and a third structure arranged on a zone 113. Zones 111 - 113 are made up of pixels that alternate with each other to form a checkerboard (interlaced areas). For example, the first structure 111 includes a first pattern modulated with a second pattern, so as to form an CGH, allowing to generate an image 114 of a given color in a finite distance reconstruction plane 5. The second structure 112 comprises a first pattern modulated with a second pattern, so as to form an CGH, allowing to generate an image 115 of a given color, for example different from the color of the object 114, in the same reconstruction plane 5. The third structure 113 comprises a first pattern modulated with a second pattern, so as to form an CGH, allowing to generate an image 116 of a given color, for example different from the color of the object 114 and the color of the object 115, in the same reconstruction plane 5. In this example, images 114, 115, 116 complete each other to form a recognizable object, for example here a rectangular parallelepiped. With a non-zero incident lighting, for example under incidence between 20 and 40°, as shown in FIGS. 11A and I1B, a change in either of the colors of images 114 - 116 can be generated by azimuthal rotation of the object 1 by selecting the appropriate resonant filters, as described above. The security components as described above can be realized according to one or more exemplary embodiment(s) as follows. The different optical structures of the different regions are previously recorded by photolithography or electron beam lithography on a photosensitive medium or "photoresist" according to the Anglo-Saxon expression. A galvanoplasty step makes it possible to transfer these optical structures into a for example nickel-based resistant material, to create the matrix or "master", see for example the reference work " diffraction handbook grating " and more particularly chapter 5 " Replicated Grating " (Christopher Plainer, Sixth edition, Newport 2006). A stamping (or "embossing") can then be carried out from the matrix thus produced to form the first layer made of an at least partially structured dielectric material. Typically, the first layer of dielectric material 23 (FIG. 2) is a stamping varnish, a few microns thick carried by a film support 24 of 5 pm to 50 pm of polymeric material, for example PET (polyethylene terephthalate). Stamping can be done by hot pressing of the dielectric material ("hot embossing") or by molding and UV curing ("UV casting"), but preferably by molding and UV curing due to the depth of the structures (typically between 80 nm and 1 pm). The refractive index of the layer formed by the stamping varnish is typically close to 1.5 for visible light. Then comes the deposition of the second layer 22 on the thus stamped or moulded layer. The second layer can be a metallic layer, deposited by thermal evaporation, for example under a vacuum, with a perfectly controlled thickness, with at least one of the following metals for example: silver, aluminium, gold, chromium, copper, nickel etc. The second layer can be a layer of dielectric material, e. g. zinc sulfide, titanium oxide. In one or more exemplary embodiment(s), the second layer may include metal regions and dielectric material regions, as described below. A controlled refractive index clogging layer is then applied, for example by evaporation in the case of a thin film or by a coating process. For some applications, such as laminating or hot stamping products, this layer may be the adhesive layer. The closure layer, which forms the third layer of dielectric material 21, may have a refractive index approximately identical to that of layer 23, around 1.5, depending on the application, or may have a different refractive index than that of layer 23. The closure layer 21 can be one micron thick or thicker, for example a few microns. Depending on the final destination of the product, an adhesive layer 26 can be applied to the clogging layer. According to one or more exemplary embodiment(s), the second layer 22 may include in one or more regions a dielectric material and/or may include in one or more regions a metallic material and/or may be discontinuous. It is possible in either of these examples to deposit in a first step a metal layer on the first layer of dielectric material 23; then the metal layer is partially demetallized for specific patterns or in order to facilitate the readability of the effect, or to generate other regions in which a layer of dielectric material or another layer of metallic material can be locally deposited. For this purpose, a first partial demetallization method consists in applying a protective varnish to the regions where it is desired that the metal layer should be preserved. For example, this varnish has a refractive index that is essentially identical to that of the first layer 23, for example around 1.5, with a thickness of around one micron for example. Then a chemical bath step destroys the unprotected metal parts. Optionally, another metallic or dielectric material is deposited in one or more of the demetallized region(s) to form the second layer 22. Finally, the third layer or closure layer 21 is applied onto the entire component. Another partial demetallization process consists in applying a soluble ink onto the stamped or UV-crosslinked layer with a given pattern. When depositing the metal, it is applied uniformly onto the layer but only remains on areas where the ink is not present when the ink is removed. According to one or more exemplary embodiment(s), it is possible to form a second layer 22 with one or more regions of dielectric material and one or more regions of metallic material by applying one or more dielectric materials before the metallization step. For example, a soluble ink can be applied to the stamped or UV-crosslinked layer. A first thin film deposition allows a dielectric material to be applied uniformly over the entire stamped or UV-crosslinked layer and over the ink; the dielectric material remains only on areas where the ink is not present when the ink is removed. Then a metallization step, which can be selective, is carried out. If the metallization is selective, it will also include a step of preprinting soluble ink to select the areas of application of the metal or printing a protective ink after deposition of the metal. As shown by the example of the manufacturing method described above, the inclusion of an optical security component as described in this description in a secured document is fully compatible with the presence in the same document of grating-based structures usually used for the production of holographic components. In particular, it will be possible to produce an optical security component comprising one or more component(s) as described above and one or more other type(s) of optical security components, for example of the holographic type. For this purpose, a matrix is produced by recording the different patterns corresponding to the different optical security components on the photoresist support and then, as previously, a galvanoplasty step convert the optical structure of the photoresist to a solid support to form the matrix. Stamping or molding followed by UV crosslinking can then be performed from the matrix to transfer the different microstructures onto the polymeric film. Although described though a number of detailed exemplary embodiments, the optical security component and the method for manufacturing said component comprise different alternative embodiments, modifications and improvements which will be obvious to those skilled in the art, its being understood that these different alternative embodiments, modifications and improvements fall within the scope of the invention as defined in the following claims. In particular, the skilled person will be able to advantageously combine the optical properties of the numerous known optical security components with the properties of the optical security component according to the invention.

Claims

Claims:

1. An optical security component intended to be authenticated in the visible spectrum in spatially coherent polychromatic light, comprising:

- a first layer made of a dielectric material, at least partially structured on one face, and having a first refractive index; - a second layer, deposited on at least one partially structured face of said first layer in at least a first region of the first layer, and having a visible reflection spectral band; - a third layer made of a dielectric material deposited on said second layer and having a third refractive index; and wherein said first layer has, in the first region, at least one first structure formed by a first pattern modulated by a second pattern, such that: o the first pattern is adapted to form a first diffractive element of the computer synthesized hologram type, calculated to generate, under spatially coherent lighting, at least a first image recognizable in at least one first reconstruction plane, o the second pattern is a periodic grating with a period between 100 nm and 700 nm, determined to produce, after deposition of the second layer and encapsulation of said first structure by the third layer, a resonant filter in a first spectral band.

2. The optical security component according to claim 1, wherein the second pattern of at least a first structure is a one-dimensional grating.

3. The optical security component according to any one of the preceding claims, wherein:

- the first layer has, in the first region, at least one second structure formed by a first pattern modulated by a second pattern, such as: o the first pattern of the second structure is adapted to form a second diffractive element of the computer synthesized hologram type, calculated to generate, under spatially coherent lighting, at least a second image recognizable in a second reconstruction plane, o the second pattern is a periodic grating with a period between 100 nm and 700 nm, determined to produce, after deposition of the second layer and encapsulation of said second structure by the third layer, a resonant filter in a second spectral band.

4. The optical security component according to claim 3, wherein at least said first image and at least said second image are generated in the same reconstruction plane.

5. The optical security component according to any one of claims 3 or 4, wherein the at least first and second structures are arranged in interlaced zones.

6. The optical security component according to any one of claims 3 or 4, wherein the at least first and second structures are arranged in adjacent areas, the successive illumination of each area allowing a motion effect of an image and/or transformation effect to be simulated.

7. The optical security component according to any one of the preceding claims, wherein:

- in at least said first region, the second layer is a thin layer made of a dielectric material, having a second refractive index such that the second refractive index differs from the first refractive index and the third refractive index by at least 0.3; - for at least said first structure of said first region, the second pattern is adapted to produce a reflective band-pass resonant filter.

8. The optical security component according to any one of claims I to 6, wherein:

- in at least said first region, the second layer is a thin layer of a metallic material, with a thickness greater than 40 nm; - for at least said first structure of said first region, the second pattern forms a reflective band-stop resonant filter.

9. The optical security component according to any one of claims I to 6, wherein:

- in at least said first region, the second layer is a thin layer of a metallic material, with a thickness between 10 nm and 60 nm; - for at least said first structure of said first region, the second pattern is adapted to produce a transmissive band-pass resonant filter.

10. The optical security component according to any one of the preceding claims, wherein:

- the first pattern is adapted to form a first diffractive element of the reflective computer synthesized hologram type.

11. The optical security component according to any one of claims I to 9, wherein:

- the first pattern is adapted to form a first diffractive element of the transmissive computer synthesized hologram type; - the first refractive indices and third refractive indices have a difference of more than 0.1.

12. A secured object comprising a support and an optical security component according to any one of the preceding claims, arranged on said support.

13. A method for manufacturing an optical security component according to any one of claims I to 11, comprising:

- forming said first layer of dielectric material on a carrier film; - depositing the second layer on at least a first region of the first layer of dielectric material; - depositing the third layer of dielectric material on said second layer.