Security element with nanostructures

Abstract

The invention relates to a security element for an article of value or document of value (2), comprising a sub-wavelength grating structure (24) on a substrate (20), which in a vertical top view (8) shows a colour generated on the basis of plasmon resonance, in which a plurality of sub-regions (16, 17) that are indistinguishable by the naked eye are formed, the invention relates to a security element (4) comprising a plurality of sub-regions (16, 17), in each case of which a sub-wavelength grating structure (24) is uniformly formed, the sub-wavelength grating structures (24) differing between the sub-regions (16, 17) with respect to at least one structure parameter influencing light diffraction, and a plurality of sub-regions which are indistinguishable by the naked eye cover a surface region (6, 32) of the security element (4), the partial regions (16, 17) differ with respect to at least one structural parameter such that, when the security element (4) is tilted about at least one tilt axis (10) lying in the base plane, an observer does not perceive a diffraction effect dependent on the angle of rotation in a surface region (6) occupied by the partial regions (16, 17).

Description

Technical Field

[0001] The present invention relates to a security element for valuable articles or valuable documents, the security element having a sub-wavelength grating structure on a substrate, which displays color by light diffraction and resonance effects, wherein a plurality of sub-regions that are indistinguishable with the naked eye are formed, the sub-wavelength grating structures are uniformly constructed in each of the sub-regions, wherein the sub-wavelength grating structures differ between the sub-regions in at least one structural parameter that affects light diffraction. The present invention also relates to a method for manufacturing such a security element. Background Art

[0002] Holographic gratings with a period of typically between 600 nm and 1500 nm have been used as anti-counterfeiting features in banknotes and documents since the 1980s. For decades, holographic gratings have influenced the optical appearance of these documents and ensured their anti-counterfeiting security.

[0003] For the term "diffractive structure", the following aspects are relevant. Diffraction is the deflection of a wave at an obstacle. By diffraction, a wave can propagate to areas of space that would be blocked by an obstacle on a straight path. Any form of physical wave can exhibit diffraction. Diffraction occurs by the formation of new waves along the wave front according to the Huygens-Fresnel principle. These waves can produce interference phenomena by superposition. An example is diffraction (of the first or higher order) on a periodic grating.

[0004] Periodic structures can be used to achieve diffraction. One-dimensional or two-dimensional periodic structures ("holographic gratings") with a period of about 400 nm to about 5 μm usually display colorful "rainbow colors".

[0005] Special optically variable effects can be produced by changing, for example, the grating period and / or the azimuth angle. Here, the change of parameters can be achieved continuously or at least in small step values ​​(for example to achieve a pumping effect). Even larger structures show diffraction effects, however, in practice they can only be observed in illumination that is as aligned as possible. Unlike gratings with symmetrical profiles (sinusoidal gratings, rectangular gratings), blazed gratings have an asymmetrical profile shape (especially a sawtooth profile). This can be achieved in a targeted manner, for example, by diffracting a greater intensity into the diffraction order +1 compared to the diffraction order -1, thereby enabling an asymmetric view to be produced. Non-periodic diffraction structures are used as "matte structures" and have structural dimensions similar to those of holographic gratings, but are arranged irregularly, so that in the diffracted light, the superposition of light of different wavelengths leads to "whitening". These structures can actually diffract isotropically in all directions in the same way or only in specific preferred directions. If different structures with different preferred directions are used, for example, an achromatic running effect can be achieved. Based on holographic gratings, these structures can be produced by “blurring” the grating lines, ie, for example, by varying the line distance or the azimuth angle of the individual grating lines.

[0006] However, in recent years, the anti-counterfeiting security of embossed holograms based on the physical diffraction effect has been significantly reduced due to the increasing number of devices for their production, so that embossed holograms can no longer be used today to provide primary security for valuable documents.

[0007] Therefore, instead of holograms, preferably security features are used which are based on the use of non-diffracting microstructures, such as micromirrors or microlenses. Such features are significantly more difficult to counterfeit and give the value document equipped with them an unusual and novel appearance and a highly dynamic property that quickly attracts attention. In this context, the use of color-shifting coatings which convey a color impression to the observer that is dependent on the tilt angle is also highly effective and additionally increases the security against counterfeiting.

[0008] The coating with interference color filter (Colourshift) produces color by multiple reflections of light in a (thin) layer structure. The interference condition is only satisfied for one wavelength, and all other wavelengths have destructive interference. Therefore, color is produced. As the number of layers increases, the color becomes "purer" in the spectrum, that is, the interference peak becomes narrower. It is usually a three-layer structure with absorber, dielectric and reflector, such as absorber Cr (about 5 nm), dielectric SiO2 (200-500nm) and reflector Al (about 50nm).

[0009] Micromirrors are facets that reflect incident light essentially in accordance with the laws of radiation optics. They have a size that is significantly (about 10 times) larger than the wavelength of light, in particular a size that is larger than about 5 μm. Therefore, their reflection characteristics are largely achromatic. The size is preferably a maximum of 100 μm, particularly preferably a maximum of 40 μm. The micromirrors can be arranged periodically ("sawtooth grating") or non-periodically. The Fresnel structure can also be composed entirely or partially of a micromirror structure or be approximately formed by a micromirror. The Fresnel structure is formed by a surface of any shape through a cut having a constant height. Therefore, the micromirror can also have, for example, an arc-shaped cross-sectional shape and a three-dimensional shape.

[0010] The micromirrors are usually rectangular, but honeycomb (hexagonal) or arbitrarily shaped micromirrors ("mosaic") are also possible. The surface of the micromirrors is preferably flat with a defined inclination. At least the micromirrors with the largest inclination have a height in the profile that is significantly greater than the wavelength (>1 μm). The steepest micromirror inclination is usually in the range of 20°, and in special designs even in the range of 10°.

[0011] The reflectivity or color of the reflected light can be determined by a coating, which is, for example, a single-layer metallization consistent with the surface, such as aluminum, or a highly reflective coating, such as ZnS, or in the form of a multi-layer color shift structure. Color and / or reflectivity can also be adjusted by superimposed subwavelength structures. If the micromirror is not reflectively coated, but at least translucent, an effect similar to the specular reflection effect in the reflected light can be observed by the prism effect in the transmitted light. For the required light refraction, these structures are not allowed to be embedded or must be on the boundary surface with a material having a significantly different refractive index.

[0012] For many years, optically variable security features have been developed in parallel with microstructures, which are based on the use of nanostructures with typical dimensions in the subwavelength range, i.e. less than 400 nm. One-dimensionally or two-dimensionally structured nanostructures are used in the form of regular subwavelength gratings or in the form of irregular structures, such as bumps or depressions arranged irregularly on a base surface.

[0013] Subwavelength structures are so small that no first-order or higher-order diffraction effects occur in the case of perpendicular incidence of light. The structural dimensions are smaller than the wavelength of light. Different effects are used in different structures: Subwavelength gratings (also referred to as "zero-order gratings" in the literature) are regular one- or two-dimensional gratings with a period in the subwavelength range (≤400nm) and produce color in mirror reflection ("zero order"). Subwavelength gratings with metal coatings use plasmon effects to produce color. In contrast, in gratings with (especially highly refractive) dielectric coatings, color is formed by other effects, namely by resonant excitation of polaritons. The contour shape can be, for example, rectangular or sinusoidal. When illuminated and observed at a gentle angle, first-order diffraction may also occur in addition.

[0014] Moth-eye structures are subwavelength structures that allow light to transition from one medium to another almost without reflection. In dielectric materials, such structures act as "anti-reflection layers". These structures can be arranged regularly (periodically) or irregularly and have a tapering cross section or, for example, also as a binary rectangular structure. If the moth-eye structure is metallized, it appears very dark and especially black.

[0015] In the case of nanostructures arranged regularly in one-dimensional or two-dimensional gratings, resonance effects such as plasmon excitation often lead to the appearance of colors, since the transmission, reflection and absorption of the light falling on the gratings spectrally depend on the parameters that characterize the subwavelength gratings in this respect, such as period, depth, profile shape, coating type and layer thickness of the coating. The period can be used in particular to adjust the formation of a specific color impression. Compared to the color appearance produced by diffraction on holographic gratings, the colors produced in this way and perceived by the observer are usually less brilliant and less optically variable, but rather are more subdued colors that can be observed over a larger angular range without noticeable color changes. When observed without a polarizer, the color impression of subwavelength gratings based on plasmon resonances is often very similar or even almost identical, i.e. independent of the azimuth angle, even when the rotation angle in the plane of the structure is changed. These properties make subwavelength structures particularly interesting as anti-counterfeiting features in combination with microstructures.

[0016] Subwavelength structures are understood here to be structures having dimensions of ≤400 nm. The grating period of the subwavelength grating is preferably between 10 nm and 500 nm, preferably between 50 nm and 400 nm, particularly preferably between 100 nm and 350 nm.

[0017] The anti-counterfeiting security of nanostructures is based in particular on the fact that high-resolution systems and processes, such as those used in electron beam lithography, are required for the production of nanostructures. Compared to embossed holograms, the nanostructures that need to be produced are at least an order of magnitude smaller. In addition, the various forming steps for producing the film, which are primarily formed in the embossing tool production process, must be carried out predictably and shape-true so that an observer can see the structure embossed on the film, for example in the desired color.

[0018] However, regular sub-wavelength gratings can also be produced by conventional photolithography systems and even by direct laser exposure, although the conditions are more stringent. Therefore, the anti-counterfeiting security is not yet optimal.

[0019] In general, subwavelength gratings are more difficult to produce than holographic gratings with a correspondingly larger grating period, since a higher resolution is required for this. This applies here: the smaller the structure, the higher the resolution of the system used must be. For example, laser exposure systems must be built more stable (with respect to mechanical vibrations and temperature), more precise (with respect to positioning accuracy), and use lasers with smaller wavelengths.

[0020] But in most cases, extended regular subwavelength gratings are easier to manufacture than grating arrangements with varying parameters. The smaller the uniformly structured surface area and the greater the number of different parameters, the lower the economic profitability of direct exposure or the greater the difficulty. This also poses increasing difficulties for laser scribing machines: Even if the required grating periods can still be produced with a laser scribing machine, they often deviate from the exact shape originally envisioned. Even electron beam systems that can in principle produce such structures require more complex data processing.

[0021] EP 2 447 743 A1 discloses a security element of the type mentioned at the outset, which provides pixels which are indistinguishable to the naked eye and which have a sub-wavelength grating with a highly refractive dielectric coating. To ensure that the optical impression of the pixel does not change when the security element is rotated perpendicular to its surface, each pixel is composed of sub-pixels whose sub-wavelength gratings have their own azimuthal orientation in the pixel. The patent document refers to the pixels obtained in this way as "isotropic pixels".

[0022] Patent document EP 2 229 287 A2 relates to a security feature, whose surface area filled with sub-wavelength gratings will emit light at certain tilt angles and cause geometric deformation of the perceptible shape. Therefore, the corresponding surface area is pre-distorted in such a way that the surface area appears undistorted in the correct proportions at the specific tilt angle.

[0023] The use of a security feature in the form of a regular nanostructure, i.e. a subwavelength grating, has in some cases a very disruptive optical effect: for light incident at high tilt angles or flat from a certain angle, the observer sees the subwavelength grating glowing brightly, since the first diffraction order becomes visible. This can have a significant adverse effect on color perception, since the original color of the surface area filled with the subwavelength grating is suddenly no longer recognizable. Instead, the first diffraction order is brighter than the color originally assigned to this surface area. Furthermore, annoyingly, due to the sudden brightness of this specific sub-area, the color of the adjacent surface area is no longer clearly visible.

[0024] Such regularly arranged nanostructures, i.e. subwavelength grating structures, can also be found in Euro banknotes, for example. The security films of some denominations contain individual surface areas that glow green when tilted at high tilt angles because they are filled with 300 nm crossed gratings. Summary of the invention

[0025] The technical problem to be solved by the present invention is to provide an anti-counterfeiting feature based on a sub-wavelength grating to generate structural color, and no adverse effects caused by the diffraction effect will be felt when observing the anti-counterfeiting feature.

[0026] The invention is defined in the independent claims. The dependent claims relate to preferred developments.

[0027] The security element for valuable articles or valuable documents has a subwavelength grating structure on a substrate. The subwavelength grating structure can be a one-dimensional or two-dimensional periodic subwavelength grating structure; it displays color in a vertical top view. A plurality of sub-regions that are indistinguishable to the naked eye are formed. Each sub-region has a subwavelength grating structure, and the subwavelength grating structure is uniformly or uniformly designed in the sub-region. The subwavelength grating structure differs between the sub-regions in at least one structural parameter that affects the diffraction of light. A plurality of sub-regions that are indistinguishable to the naked eye cover a surface area of ​​the security element. The sub-regions arranged on the surface area differ in at least one structural parameter, so that when the observer tilts the security element, the observer does not perceive the diffraction effect related to the rotation angle in the surface area occupied by the sub-region. The tilting is achieved around a tilt axis, which is in the substrate plane, thereby changing the elevation angle.

[0028] This concept therefore provides that the color impression of the sub-areas will certainly change with tilting. However, since the sub-areas within the surface area differ from one another in terms of structural parameters, when the security element is rotated about an axis perpendicular to the substrate plane, on average over the entire surface area, i.e. over the sub-areas that occupy the surface area, no significant diffraction-related changes in the color impression will occur within the surface area at large tilt angles. Brightness of the first diffraction order, such as would occur with a (large-scale) uniform grating when tilted at high tilt angles about an axis parallel to the grating lines, does not occur noticeably in the security element according to the invention.

[0029] If the subwavelength grating generates the color on the basis of plasmon resonance, i.e., in particular has a relief profile with a metal coating, which provides the subwavelength grating structure, the color consistency is particularly high. In particular, aluminum, gold, silver, copper, chromium, nickel and / or iron and alloys of these metals can be used as coating materials. The thickness of the metal coating is expediently between 5 nm and 200 nm, preferably between 8 nm and 150 nm, particularly preferably between 15 nm and 80 nm. Advantageously, the metal coating forms a largely opaque coating.

[0030] Furthermore, it is also possible to achieve other effects based on different functional principles, in particular interference effects, by combining with additional thin layers. Thus, HRI layers, multilayer coatings (for example for completing a color shift system with absorber / dielectric / reflector) or layer sequences consisting of HRI layers and LRI layers (HRI=high refractive index; LRI=low refractive index) can be applied optionally to the metal coating. The HRI layer is preferably formed from ZnS or TiO2, the LRI layer, for example, from a polymer or SiO2 or MgF2.

[0031] The structural parameters personalize the sub-regions within the surface area. For this purpose, a plurality of parameters of the sub-wavelength grating structure can be considered, which can be used individually or in combination to achieve the personalization. These parameters include the azimuthal orientation, period, contour (e.g. rotationally symmetric contour shape; non-rotationally symmetric, e.g. rectangular, especially square contour shape, ellipse, parallelogram, etc.) of the protrusions and / or recesses of the sub-wavelength grating structure that are regularly arranged on the base surface.

[0032] In particular, it is possible to combine a plurality of sub-areas into groups, wherein a first parameter individualizes the sub-areas within the group, while the groups are differentiated from each other by a second parameter. It is also possible that within each group, the sub-areas are individualized by different parameters. Thus, for example, individualization can be achieved in one group of sub-areas by azimuthal orientation, while individualization can be achieved in another group by a change in period.

[0033] It is particularly preferred that adjacent different sub-regions of the surface region have an azimuthal orientation variation between 0° and 360°. It is also particularly preferred that the periods of adjacent different sub-regions of the surface region differ by a value of at most 10% (e.g., by 30 nm at 300 nm), preferably by a value of at most 5% (e.g., by 15 nm at 300 nm).

[0034] In an advantageous embodiment, the structural parameters vary substantially isotropically in the area region. In particular, the parameters can vary randomly or pseudo-randomly within the variation range.

[0035] Pseudorandom numbers are sequences of numbers that appear random but are calculated using a deterministic algorithm and are therefore not strictly random. Nevertheless, pseudorandom numbers are widely used because the statistical properties of their distribution, such as the equal probability of individual numbers or the statistical independence of consecutive numbers, are often irregular enough for practical purposes, such as pseudorandom variations in azimuth, and, unlike true random numbers, they are easy to generate using computers.

[0036] The security element can advantageously have a plurality of surface areas. The surface areas here usually form individual areas within the visual object, which have a uniform color in each case, depending on the selected design. If, for example, the visual object is an apple tree, the first surface area can form a brown trunk, the second surface area a green crown and the third surface area a red apple. In a more detailed design, for example, the crown itself can be composed of different surface areas with varying green tones, or the color of the apple can vary between red and yellow, so that each apple represents an independent surface area with a color assigned to it from the spectrum between red and yellow. Each apple can also be composed of different surface areas, whose colors come from the spectrum between red and yellow.

[0037] The configuration of forming a surface region from a plurality of partial regions individualized by at least one structural parameter can advantageously be combined with a micro-mirror arrangement, wherein the surface region is formed on the micro-mirror arrangement.

[0038] Particularly preferably, the surface area is combined with a micromirror arrangement structure so that the surface area is respectively configured as a micromirror of the micromirror arrangement structure. The subwavelength grating structure of each sub-area of ​​the micromirror imparts a specific color to the micromirror, which color is largely independent of the elevation angle. The micromirror arrangement thus produces a colored or tinted visual object. As a result, the tilting security element can produce the desired effect, wherein the color or color intensity of each individual micromirror is not adversely affected by the diffraction effect of the subwavelength structure that produces the color due to the personalized sub-area.

[0039] In these cases, the surface areas are preferably not distinguishable with the naked eye, since the individual micromirrors are preferably below the resolution limit. In other embodiments, larger surface areas can be formed which are distinguishable with the naked eye but still have a uniform color effect caused by the subwavelength structure, which is formed in multiple subareas that are not distinguishable with the naked eye and are difficult for counterfeiters to copy.

[0040] In another particularly preferred embodiment, the surface area corresponds to a micromirror pixel, in which there are multiple micromirrors with uniform orientation. The pixel size of the micromirror pixel is usually in the range of 10 μm to 30 μm, especially 20 μm. The sub-area can be consistent with the micromirror in the micromirror pixel in a congruent manner. But this is not absolutely necessary. In a micromirror pixel, the boundaries of the micromirror and the sub-area can also extend independently of each other. In order to avoid the appearance of boundary surfaces and thus weaken the color effect, the sub-area should not be significantly smaller than the micromirror pixel size. Therefore, the typical size of the sub-area is, for example, each sub-area is at least about 4 to 5 μm, and the size of the micromirror pixel is about 10 μm x10 μm.

[0041] In another particularly preferred embodiment, the surface area corresponds to a plurality of micromirror pixels (i.e., there are regions with uniformly oriented micromirrors). Here, the pixel size of the micromirror pixels is usually also in the range of 10 μm to 30 μm, in particular 20 μm. For example, each micromirror pixel has a size of 20 μm x20 μm, for example. The sub-area is preferably consistent with the micromirror pixels in the surface area in an identical manner. But this is not absolutely necessary. The boundaries of the micromirror pixels and the sub-areas can also extend independently of each other within the surface area. In a typical design, a plurality of micromirror pixels have the same color. It is not absolutely necessary that all micromirror pixels have the same orientation or inclination (or that the orientation / inclination of the micromirror in the micromirror pixel is the same for all micromirror pixels). On the contrary, the orientation / inclination of the micromirror pixels can vary within the surface area.

[0042] The surface area to be filled with structural colors by sub-wavelength gratings is divided into a plurality of sub-areas, the size of which is below the resolution of the human eye. These sub-areas are filled with sub-wavelength gratings, which are clearly defined by a specific parameter set and uniformly fill their corresponding sub-areas. The sub-wavelength gratings in the sub-areas can be one-dimensional and two-dimensional periodic sub-wavelength gratings. The sub-wavelength gratings of adjacent sub-areas in the surface area differ from each other in at least one structural parameter that affects the diffraction characteristics, wherein such a difference has no or almost no visual effect when observed vertically, so that in this state, compared with the structural colors generated by each individual sub-area, the generated structural colors appear to the observer as the same or at least very similar.

[0043] In particular, when illuminated and viewed at a flat angle, no maximum value associated with the first diffraction order occurs, which could significantly change the resulting structural color or be brighter than the resulting structural color. Without auxiliary components, the lack of brightness of the first diffraction order is visible to the naked eye. In addition, any residual diffraction that may occur is uniform in azimuth and has a significantly reduced intensity. The disclosed security feature also provides a more effective protection against counterfeiting attempts.

[0044] The diffraction and / or resonance properties of a sub-wavelength grating are essentially defined by the following structural parameters, which can be used individually or in combination to achieve individualization of the sub-regions:

[0045] - a period between 10 nm and 500 nm, preferably between 50 nm and 400 nm, particularly preferably between 100 nm and 350 nm;

[0046] - a depth between 50nm and 400nm, preferably between 80nm and 300nm;

[0047] - one-dimensional (linear) or two-dimensional gratings; for two-dimensional gratings, for example rectangular, square, hexagonal or parallelogram grating arrangements;

[0048] - azimuthal orientation of the grating in the plane of the security feature relative to a predefined direction;

[0049] - Contour shape of the relief structure: sinusoidal structure, rectangular (binary) structure or other contour shapes with concave and / or convex sections; periodic arrangements of nanoholes or nanodots with various contour shapes are also feasible.

[0050] The grating is preferably uniformly coated with a metal, since this results in a clearly recognizable color that does not change significantly over a large angle range. In another preferred embodiment, the metallization is performed regionally. For example, the top surface of the embossed protrusions ("pillars") is provided with a coating, while the remaining surface areas are not coated. The opposite embodiment, i.e. regularly arranged holes in the metal layer, is also possible.

[0051] The azimuth angle, i.e. the orientation of the regular subwavelength grating in a plane defined by the substrate, is particularly suitable as a structural parameter that can be varied in a subregion, since the azimuth angle has largely no influence on the color generated by the plasmon resonance effect when viewed perpendicularly under unpolarized illumination and substantially only influences the diffraction. It is particularly preferred that the azimuth angle of the subregions filled with the subwavelength gratings is varied randomly or pseudo-randomly within a predefined range of variation, the subregions together representing the entire surface area to be filled with a substantially uniform structural color.

[0052] This means that the azimuth angle of each sub-area can have any value between 0 and 360°, so that due to the division of a substantially uniformly colored surface into a plurality of sub-areas, a sub-wavelength grating with different azimuth angles and approximately the same azimuth angle distribution will appear, i.e., it is substantially isotropic over the entire angular range. This achieves that the observer cannot see the changes caused by the diffraction effect, or at least cannot see the dependence of the diffraction color on the azimuth angle direction in which the security feature is located or the security feature is observed.

[0053] In addition, other structural parameters can also be varied across sub-regions. It is also possible to vary the sub-regions that appear in the surface regions of different colors according to different structural parameters, for example, to vary the period in the surface region filled with the blue structural color and to vary the azimuth angles between the sub-regions in the surface region filled with the yellow structural color.

[0054] Furthermore, it is also possible to vary a plurality of grating parameters simultaneously within a surface region, wherein preferred structural parameters in this case are azimuth and period.

[0055] As already mentioned above, the lateral extension of the subregions is below the resolution of the human eye. The subregions thus have a size below 300 μm, preferably below 100 μm, particularly preferably below 50 μm. This applies to their maximum extension in at least one spatial direction, i.e. the subregions can be very narrow elongated lines, although it is preferred that the subregions have a size below the resolution limit in all directions. The subregions can have irregular contours, but can also appear in the form of regularly arranged pixels.

[0056] In a special embodiment variant, the technical solution of the invention can also be combined with one or more other sub-areas in which the grating parameters do not change at all. For example, it can be provided that on a larger surface there are small sub-areas (e.g. sub-pixels with an edge length of 10 µm), in which the azimuth angle is randomly selected, while in other sub-areas there are gratings with the same azimuth angle (e.g. as values ​​that can be read with the naked eye). In a vertical top view, the small sub-area with the changed azimuth angle and the other sub-areas with the same azimuth angle, which are preferably also not distinguishable with the naked eye, are indistinguishable or at least almost indistinguishable, while in a suitable oblique angle observation (when tilted about an axis parallel to the grating lines), due to the otherwise undesirable first diffraction order of the macroscopic partial area formed by the other sub-areas with the same azimuth angle, a further view becomes visible here.

[0057] Such a combination also makes it easier to unambiguously verify the features according to the invention: the absence of luminous diffraction effects of the first order may not always be unequivocally verifiable, since in practice this may also be due, for example, to poor lighting conditions or to the observer not looking closely enough. However, if a grating with fixed grating parameters is provided in a macroscopic, i.e., visible, sub-region formed by other sub-regions, the observer will receive an unambiguous result if these other sub-regions light up in the first diffraction order, while the other sub-regions do not show such effects.

[0058] According to another example, it can be provided that the outline of a visual object (for example in the form of a symbol that can be read with the naked eye) has a lateral size in one dimension that is below the resolution limit of the eye (for example 60 μm), in which small sub-areas (for example sub-pixels with a side length of 10 μm) are present, in which the azimuth angle is randomly selected, while in other sub-areas there is a (continuous) sub-wavelength grating. As shown in the previous example, in a vertical top-down observation, the small sub-area with the changed azimuth angle is indistinguishable from the other sub-areas. However, when observing at an oblique angle, due to the residual diffraction that occurs to a lesser extent in the small sub-area with the changed azimuth angle, the view of the visual object becomes visible, however, when observing from a direction perpendicular to the grating lines of the other sub-areas (or when tilted about an axis parallel to the grating lines), the first diffraction order is brighter than the view of the visual object, so that in this observation situation, which can be achieved by changing the rotation angle, the visual object formed by the outline disappears again.

[0059] Such a combination also facilitates verification of the features according to the invention. In addition, by providing a partial area comprising small sub-areas with a changed azimuth angle, it is ensured that the entire surface area is perceptible with a uniform color or brightness, the partial area having a lateral size in one dimension that is below the resolution limit of the eye. Therefore, in order to achieve an overall uniform appearance, it is not necessary to subdivide the (macroscopic) other sub-areas into sub-areas that are not perceptible to the naked eye. In this case, a continuous sub-wavelength grating can be provided in the entire other sub-area accordingly.

[0060] For example, subwavelength gratings can be fabricated by:

[0061] - Electron beam lithography;

[0062] - Manufacturing of stamping tools by electroforming steps;

[0063] - Embossing in thermoplastic or radiation-hardening, in particular UV-hardening, embossing lacquers;

[0064] - Metallization (e.g. aluminum, gold, silver, chromium, nickel, copper, iron or their alloys), possibly additionally with an HRI layer (e.g. ZnS); multilayer coatings are also conceivable and can be used advantageously (e.g. color shifting systems with absorber / dielectric / reflector, where subwavelength structures change the color shifting color);

[0065] - Optionally embedding the coated embossed structure (e.g. by means of a protective lacquer).

[0066] In addition to the visually perceptible difference in the arrangement of the sub-wavelength gratings described here compared to the prior art, in which gratings uniformly fill uniformly colored surface areas with a constant set of grating parameters, the security features according to the invention have other advantages: they cannot actually be reproduced by direct exposure with the aid of laser light using interference at a sample position, because the large number of variably filled sub-areas and the small size of these sub-areas would make direct exposure too difficult and too expensive. In addition, due to the small grating period, the possibility of creation using other optical methods is excluded. Production using electron beam systems is feasible, but requires greater expenditure in data processing. This is a double obstacle for counterfeiters. Electron beam systems themselves are very expensive and therefore difficult for counterfeiters to obtain. In addition, the software that needs to be additionally created for data processing is another obstacle.

[0067] These nanostructures are particularly attractive in terms of their anti-counterfeiting features in combination with microstructures. The microstructures primarily ensure the high dynamics and the optical variability associated with them, while the subwavelength grating structures give the features a characteristic color.

[0068] The presently described method for producing a colored view of a security feature based on structural color is therefore particularly effective when superimposed with a microstructure. Such a structure is more complex and also more difficult to manufacture than the subwavelength gratings known in the prior art. The microstructure makes an additional contribution to the suppression of diffraction phenomena, since the diffraction conditions, i.e. when an incident light beam is diffracted towards an observer, also depend on the orientation of the microsurface on which the subwavelength grating is arranged. In addition, the microstructure provides the dynamic properties, while the subwavelength grating provides the corresponding color.

[0069] For example, in a series of experiments, two visual objects in a binary micromirror tilt image with noise in one direction were additionally equipped with nanostructures in order to give them color. The nanostructures on the micromirror consisted of subwavelength crossed gratings with a sinusoidal profile, which were divided into regular subpixels with a size of 20 μm as subregions, wherein the azimuthal orientation of the subwavelength crossed gratings varied randomly between the individual subpixels. Although the remaining (azimuthal uniformly distributed) residual diffraction could still be vaguely seen in the tilt image of the micromirror with the least noise, it gradually decreased with increasing changes in the mirror orientation and could no longer be recognized at the maximum noise.

[0070] In a particularly preferred embodiment of the production method, the azimuth angle varies in the sub-regions, ie the orientation of the regularly periodic subwavelength grating structure in the plane defined by the substrate changes between the individual sub-regions.

[0071] In each individual sub-region, it is particularly preferred to have a two-dimensionally periodically arranged sub-wavelength grating structure, which is preferably applied to the substrate in the form of a rectangular, square, hexagonal or parallelogram grating arrangement. The two-dimensional periodic sub-wavelength grating structure has regularly arranged protrusions and depressions, wherein, in a top view of the security element, the protrusions are in particular not rotationally symmetrical, for example rectangular, particularly preferably square. Of course, the upper side of the protrusions that can be seen in a top view is not limited to a square or rectangular shape. Other non-rotationally symmetrical shapes, such as ellipses and parallelograms, are also conceivable.

[0072] For ease of production, it is preferred that, although the azimuth angles of the sub-regions may change, the orientation of the protrusions in a plan view is always the same in all sub-regions, regardless of the azimuth angles. This is explained in detail below based on an example:

[0073] In one example, the protrusion is rectangular when viewed from above. The edges of all protrusions are parallel to each other, that is, the rectangles that can be identified when viewed from above have the same orientation. In order to change the azimuth, the grating arrangement structure is rotated by a specific angle between the individual sub-areas, wherein the parallelism of the edges of the protrusions remains unchanged in all sub-areas. Each protrusion has a certain registration point, which is the same in all protrusions, and the protrusion is fixed at the intersection of the grating lines of the grating, so that the registration point is exactly located at this intersection. For example, the registration point can be the center of the protrusion that is rectangular when viewed from above, but any other point can also be used as the registration point. The sub-areas differ in the orientation of the grating arrangement structure, but there is no difference in the orientation of the protrusions, and the protrusions are located at the corresponding intersections with their registration points.

[0074] The preferred manufacturing method is also applicable to a one-dimensional periodic sub-wavelength grating structure. In this case, the horizontal or vertical distance between the protrusions is selected to be zero, so a quasi-one-dimensional periodic sub-wavelength grating structure with grating strips and grating gaps is provided.

[0075] By means of the preferred production method, the subwavelength grating structure can be exposed more quickly and is therefore also easier to produce due to the identical orientation of the non-rotationally symmetrical elevations in all subregions. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The present invention is described in more detail according to the embodiments with reference to the accompanying drawings that also disclose the important features of the present invention. These embodiments are for illustration only and should not be understood as limiting. For example, the description of an embodiment with multiple elements or components should not be interpreted as all these elements or components being necessary for implementation. On the contrary, other embodiments can also include alternative elements and components, fewer elements or components or additional elements or components. Unless otherwise stated, the elements or components of different embodiments can be combined with each other. The modifications and variations described for one of the embodiments can also be applied to other embodiments. To avoid repetition, in different drawings, the same or corresponding elements are marked with the same figure numbers and are not repeated. In the drawings:

[0077] Figure 1 A schematic diagram showing a banknote with a security element,

[0078] Figure 2 A schematic diagram of a three-dimensional security element is shown to illustrate the tilting effect,

[0079] Figure 3 Show Figure 1 A top view of the security element,

[0080] Figure 4 Show Figure 3 A cross-sectional view of a portion of the surface of a security element,

[0081] Figure 5 Shown with Figure 4 Other similar embodiments of banknotes with two security elements,

[0082] Figure 6 Another top view of the security element is shown,

[0083] Figures 7 to 9 A top view showing a sub-region of a security element,

[0084] Fig.10 A detail of the protrusion is shown in top view, and

[0085] Fig.11 A plan view of a sub-region of the security element is shown. DETAILED DESCRIPTION

[0086] Figure 1A banknote 2 or other valuable document is schematically shown, which is provided with a security element 4 for protection against copying. The security element 4 displays a visual object to the observer and has a plurality of pixels or surface areas 6 for this purpose. The security element 4 can be formed directly on the banknote paper during the production of the banknote paper. Alternatively, the security element 4 can be provided on a separate substrate applied to the banknote 2, for example by previously configuring the security element 4 as a so-called transfer element. This is known to those skilled in the art.

[0087] Figure 2 A top view of a security element 4 is shown, wherein only one of the pixels 6 is marked for the sake of illustration. For example, the security element 4 and the pixels 6 are viewed largely vertically from the top view direction 8. All pixels 6 of the security element 4 have a sub-wavelength grating which is yet to be explained, wherein the definitions of terms explained above naturally also apply to the sub-wavelength grating. Figure 1-11 Description of an embodiment of the invention. If the security element 4 is tilted about an axis 10 lying in a plane defined by the flat security element 4 and / or the flat banknote 2, the top-down direction 8 will also change accordingly. Nevertheless, the color displayed by the pixel 6 remains essentially unchanged over a large viewing angle range, the boundaries of which are schematically indicated by 12 and 14. Therefore, the sub-wavelength grating also has optical variability, but its optical variability is not as pronounced as in conventional diffraction gratings. In particular, no diffraction effects are perceived within a large tilt angle range and no azimuth-dependent diffraction phenomena occur.

[0088] This characteristic of each pixel 6 is achieved by showing one of the pixels 6 in a top view. Figure 3 As shown, the pixel 6 is formed by a plurality of sub-regions 16, which are in the form of sub-pixels, each of which is formed by a sub-wavelength grating structure. The individual sub-regions 16 differ in one of the parameters that influence the diffraction of light. This is Figure 3 In the top view of FIG. , this is the azimuthal orientation or longitudinal direction 18 along which the one-dimensional periodic sub-wavelength grating structure extends by way of example.

[0089] In an embodiment not shown herein, the grating lines of the one-dimensional periodic sub-wavelength grating structure can be continuously transitioned from one sub-region in the form of a curved line to the grating lines of other sub-regions or other sub-regions with the same period but with a rotated azimuth angle. This can avoid or at least reduce the discontinuity at the boundary of the sub-regions occupied by different sub-wavelength gratings.

[0090] Figure 4 A section through a sub-region 17 along a horizontally extending axis is shown (see Figure 3 ). Figure 4It can be seen from the cross-sectional view that the security element 4 is formed on a substrate 20, on which an embossed lacquer layer 22 is arranged, and the relief of the sub-wavelength grating structure 24 is embossed into the embossed lacquer layer 22. The sub-wavelength grating structure is periodic at least in the cutting plane and extends, for example, in a longitudinal direction perpendicular to the plane of the drawing. The sub-wavelength grating structure consists of a plurality of protrusions 28 and recesses 30, which, in the example shown, are periodically arranged in sequence in one dimension. Of course, a two-dimensional periodic grating structure is also feasible. In this case, there are generally two directions that can be used as the longitudinal direction 18 respectively. The relief structure is provided with a metallization layer 26.

[0091] A sub-region 16 of a pixel 6 can be understood as a sub-pixel. In this case, all sub-regions 16 of a pixel 6 produce the same color, i.e., their sub-wavelength grating 24 is designed so that the sub-regions display the same or at least substantially the same color. However, the sub-regions differ in the longitudinal direction 18 in which the sub-wavelength grating structure 24 extends. Figure 3 In the embodiment shown, the pixel 6 is divided into 36 sub-areas 16, and the longitudinal directions 18 of the sub-areas 16 are 10 degrees different from each other. Therefore, in this embodiment, the longitudinal directions 18 of the sub-wavelength grating structure 24 are uniformly distributed between 0° and 360°. In terms of the change in the longitudinal direction 18, the sub-areas 16 arranged successively do not necessarily have to be arranged as in Figure 3 The longitudinal directions 18 may also be distributed in each sub-region 16 completely randomly or pseudo-randomly.

[0092] In addition, the sub-wavelength grating structure 24 is not limited to a one-dimensional periodic structure, and its change is not limited to the change in the longitudinal direction 18. Other sub-wavelength grating structures can also be used, and other parameters of the sub-wavelength grating structure can be changed, as described in the invention content of the specification. In particular, the sub-wavelength grating structure 24 can be realized by a two-dimensional periodic arrangement of protrusions and / or depressions, and / or as a parameter, the longitudinal direction 18 is not changed, but the period between each individual sub-region 16 is changed.

[0093] Figure 5 It is shown that the division into sub-regions 16 is not directed to the division of pixels 6 , ie into surface regions that are not discernible to the naked eye, but can also be applied to larger regions 32 , 34 , 35 that are divided into sub-regions 16 that are not discernible to the naked eye. Figure 5 It is shown here by way of example that the subregions 16 do not necessarily have to be arranged regularly or have to have the same basic structure.

[0094] Figure 6Another embodiment of a security element 4 is shown, in which the security element 4 is designed as a micromirror arrangement 36 consisting of individually oriented micromirrors 38. Each micromirror 38 comprises a plurality of sub-regions 16, which can differ from one another, for example in the longitudinal direction 18 of the sub-wavelength grating structure 24, for example according to Figure 3 As described in the Pixel 6. Of course, according to Figure 3 and Figure 4 The options already explained and also mentioned in the inventive part of the description regarding how the individual sub-areas 16 can differ from one another also apply to the sub-areas 16 on the micro-mirror surface 38. The advantage of the embodiment of forming the micro-mirror surface 38 from a plurality of sub-areas 16 that differ in at least one parameter is that the individual micro-mirrors 38 of the micro-mirror arrangement 36 display the desired color independently of their orientation, and there is no effect that the final orientation of the micro-mirrors 38 is different from the orientation of the micro-mirrors 38. Figure 2 In a specific orientation corresponding to a tilt angle of 0.1 Å, the desired color is not visible at all and / or is affected or disturbed by higher diffraction orders, such as the first diffraction order.

[0095] Fig.11 A partial top view of another embodiment of the security element is shown, Figure 6 Like the security element shown, this security element is also designed as a micromirror arrangement 36 composed of individually oriented micromirrors 38. Figure 6 The illustrated design differs in that the micromirrors are present in the form of micromirror pixels, each of which has a plurality of micromirrors with a uniform azimuthal orientation (and inclination angle) (indicated by corresponding hatching in the drawings). In this case, the plurality of micromirror pixels have the same color preset by the subwavelength grating structure.

[0096] Here, the larger area 32 is divided into sub-areas 16 which cannot be distinguished by the naked eye, these larger areas can be perceived by the naked eye, one of which is partially indicated by the dashed outline (and the longitudinal direction 18 of the sub-wavelength grating structure) included in the figure for illustration purposes only. The sub-areas 16 of the area 32 are designed with respect to their sub-wavelength gratings so that they appear to have the same color. However, according to Figure 3 As explained in 6, they differ in the parameters that influence the diffraction of light. Fig.11 In the top view of FIG. 3 , this is the azimuthal orientation or longitudinal direction 18 in which the one-dimensional periodic subwavelength grating structure extends by way of example. In the present exemplary embodiment, the micromirror pixels correspond to the subregions 16 of the region 32 in a congruent manner.

[0097] The micromirror pixels of the area 32 all have the same orientation and inclination, i.e. the orientation of the micromirrors within the micromirror pixels is the same for all micromirror pixels of the surface area. However, this is not absolutely necessary. According to a design solution not shown here, the orientation and / or inclination of the micromirror pixels in the surface area can also be varied.

[0098] Figures 7 to 9 shows a top view of the sub-regions 16a to 16c of the security element 4, Fig.10 Details of the protrusions 28 are shown. Figures 7 to 10 In order to illustrate a preferred method for manufacturing the security element 4, this will be described below.

[0099] exist Figure 7 , a top view of the first sub-region 16a of the pixel 6 is shown. The sub-region has a two-dimensional sub-wavelength structure in the form of depressions 30 and protrusions 28, wherein the protrusions are arranged in a square grating arrangement. The grating arrangement forms a first grating 40. For example, the protrusions 28 are present in the first grating 40 so that the center point M of each protrusion 28 is located at the intersection of the horizontal grating lines and the vertical grating lines of the first grating 40. The center point M is used to illustrate the registration of the protrusions 28 in the first grating 40. For simplicity, the center point M is only marked for one protrusion 28. The first grating 40 is likewise only used to illustrate the arrangement of the protrusions 28 in the sub-region 16a; thus, the first grating can be identified in a top view of the security element 4.

[0100] The grating arrangement according to the first grating 40 is exemplarily a square, ie the horizontal grating lines and the vertical grating lines are at an angle of 90° to each other. However, grating arrangements such as hexagonal gratings (not shown) are also possible.

[0101] Figure 8 A top view of the second sub-region 16b of the pixel 6 is shown, wherein the azimuth angle is Figure 7 The ratio is changed according to a predetermined value. This is achieved by arranging the protrusions as a second grating 42, which is rotated by a first angle 44 relative to the first grating 40 but has the same structure in other respects. Figure 8 In the embodiment of FIG. 4 , the angle 44 is 10°. The second grating 42 is rotated relative to the first grating 40 by the angle 44 and is shown in FIG. Figure 8 Regardless of the rotation of the second grating 42 relative to the first grating 40 by the angle 44, Figure 7 and Figure 8 The orientation of the projection 28 in remains unchanged, i.e. Figure 8 In FIG. 4 , the upper edges of the projections 28 are parallel to the edges of the preceding sub-region 16 a ; they do not rotate with the second grating 42 .

[0102] Fig. 9A top view of the third sub-region 16c of the pixel 6 is shown. Figure 7 and Figure 8 In this way, the projections 28 are registered with their center points M at the intersections of the lines of the third grating 46. The basic structure of the third grating corresponds to the gratings 40 and 42, but is rotated by a second angle 48 of 30° relative to the grating 40. Regardless of the rotation of the third grating 46, the orientation of the projections 28 is Fig. 9 The edge of the upper side of the projection 28 also remains unchanged. Fig. 9 The third grating 46 is rotated at a second angle 48 relative to the first grating 40. Fig. 9 The lower right edge of the is shown as a detail.

[0103] Fig.10 In the top view, a single protrusion 28 is shown. Protrusion 28 has a center point M. In addition, a first grating 40, a second grating 42 and a third grating 46 are shown, which are registered with the center point M of protrusion 28, so that the center point M is located at the intersection of the grating lines of gratings 40, 42 and 46. Furthermore, a first angle 44 and a second angle 48 are shown, at which the second grating 42 is rotated around the first grating 40 and the third grating 46 is rotated around the first grating 40.

[0104] Figures 7 to 9 A two-dimensional periodic grating structure with protrusions 28 and depressions 30 is shown, exemplarily arranged in a rectangular grating arrangement in the form of square gratings 40, 42, 46. It is possible to set the horizontal or vertical distance between the protrusions 28 to zero, thereby producing a one-dimensional periodic grating with grating strips and grating columns (not shown in this embodiment).

[0105] exist Figures 7 to 9 16a, 16b, 16c with different azimuth angles are shown in FIG. Figures 7 to 9 In each of the figures (ie in all sub-areas), the edges of the projections or the edges of the top sides of the projections are still oriented in the same way, ie in the embodiment shown, for example, parallel to the grating lines of first grating 40 . Fig.10 In summary, it is shown that in a preferred production method, the orientation of elevations 28 always remains the same, regardless of gratings 40 , 42 , 46 .

[0106] To adjust the azimuth angle, the orientation of the protrusions remains unchanged, but the second grating 42 is rotated relative to the first grating 40 by the azimuth angle, and the third grating 46 is rotated relative to the first grating 40 by the azimuth angle.

[0107] Thus in this manufacturing method, in order to produce different azimuthal angles in different sub-regions 16, the orientation of the regular periodic sub-wavelength grating as a whole is changed, but the orientation of the protrusions 28 remains unchanged.

[0108] List of Reference Numerals

[0109] 2 Banknotes

[0110] 4 Anti-counterfeiting elements

[0111] 6 pixels

[0112] 8 Looking down

[0113] 10 Axis

[0114] 12, 14 Boundary

[0115] Sub-areas 16 and 17

[0116] 18 Vertical direction

[0117] 20 Base

[0118] 22 Embossed lacquer layer

[0119] 24 Subwavelength grating structure

[0120] 26 Metallization

[0121] 28 bulge

[0122] 30 Depression

[0123] Areas 32, 34, 35

[0124] 36 Micro-mirror arrangement structure

[0125] 38 Micro mirror

[0126] 40 First grating

[0127] 42 Second grating

[0128] 44 First Angle

[0129] 46 Third grating

[0130] 48 Second Angle

Claims

1. A security element for a valuable article or valuable document (2), the security element having a sub-wavelength grating structure (24) on a substrate (20), the sub-wavelength grating structure showing a color based on plasma resonance in a vertical top view (8), wherein: A plurality of sub-regions (16, 17) indistinguishable to the naked eye are formed, in which sub-regions a sub-wavelength grating structure (24) is uniformly constructed, wherein the sub-wavelength grating structures (24) differ between the sub-regions (16, 17) in at least one structural parameter that influences light diffraction, and the plurality of sub-regions indistinguishable to the naked eye cover a surface region (6, 32) of a security element (4), characterized in that in the surface region (6, 32), the sub-regions (16, 17) differ in at least one structural parameter, so that when the security element (4) is tilted about at least one tilting axis (10) located in the base plane, an observer does not perceive any diffraction effects that are dependent on the rotation angle in the surface region (6) occupied by the sub-regions (16, 17).

2. The security element according to claim 1, characterized in that The subwavelength grating structure (24) has a relief profile (28, 30) of a metal coating, wherein the metal coating (26) is made in particular of aluminum, gold, silver, copper, chromium, nickel and / or iron or alloys of these metals.

3. The security element according to claim 1 or 2, characterized in that The structural parameters of the sub-wavelength grating structure (24) of each sub-region (16, 17) that are different include at least one of the following parameters of the sub-wavelength grating structure (24): the profile, azimuthal orientation, and period of the protrusions and / or recesses of the sub-wavelength grating structure regularly arranged on the base surface.

4. The security element according to claim 1, characterized in that The sub-wavelength grating structures (24) of the sub-regions (16, 17) differ in their azimuthal orientation, wherein different adjacent sub-regions of the surface region (6, 32) have an azimuthal orientation variation between 0° and 360°.

5. The security element according to claim 1, characterized in that The sub-wavelength grating structure (24) is a two-dimensional periodic sub-wavelength grating structure, wherein the sub-wavelength grating structure (24) has in particular a rectangular, square, hexagonal or parallelogram grating arrangement (40, 42, 44).

6. The security element according to claim 5, characterized in that The sub-wavelength grating structure has regularly arranged protrusions and / or depressions, wherein the sub-wavelength grating structure (24) of each sub-region (16, 17) is separated by an orientation zone of the grating arrangement structure (40, 42, 44).

7. The security element according to claim 1, characterized in that In the surface region ( 6 , 32 ), the structural parameters vary substantially isotropically.

8. The security element according to claim 1, characterized in that The security element has a plurality of surface areas (6).

9. The security element according to claim 1, characterized in that One or more surface regions (6) are formed on the micromirror arrangement (36).

10. The security element according to claim 8, characterized in that The surface areas are respectively constructed as micromirrors (38) of a micromirror arrangement structure (36), as micromirror pixels having a micromirror arrangement structure (36) with a plurality of micromirrors (38) with uniform orientation, or as a plurality of micromirror pixels, wherein the plurality of micromirror pixels respectively have a plurality of micromirrors (38) with uniform orientation of a micromirror arrangement structure (36), wherein a sub-wavelength grating structure (24) of a sub-area (16, 17) of each micromirror (38) or each micromirror pixel or each micromirror pixel in the plurality of micromirror pixels imparts a specific color to the micromirror or micromirror pixel, so that the micromirror arrangement structure (36) produces a colored or tinted visual object.

11. The security element according to any one of claims 1 to 10, characterized in that The surface area(s) (6, 32) are distinguishable with the naked eye.

12. The security element according to claim 1, characterized in that A plurality of sub-regions (16, 17) that differ in at least one structural parameter are combined with one or more other sub-regions that have the same structural parameter.

13. A method for producing a security element for a valuable object or valuable document (2), wherein: A subwavelength grating structure (24) is formed on a substrate (20), the subwavelength grating structure showing a color generated based on plasmon resonance, wherein a plurality of sub-regions (16, 17) that are indistinguishable to the naked eye are formed, in which the sub-wavelength grating structure (24) is uniformly constructed, wherein the sub-regions (16, 17) differ from each other in at least one structural parameter that influences light diffraction, and the plurality of sub-regions (16, 17) that are indistinguishable to the naked eye cover a surface area (6, 32) of a security element (4), characterized in that in the surface area (6, 32), the sub-regions (16, 17) differ in at least one structural parameter, so that when the security element (4) is tilted about at least one tilting axis (10) located in the plane of the substrate, an observer does not perceive a diffraction effect that is dependent on the rotation angle in the surface area (6, 32) occupied by the sub-regions (16, 17).

14. The method according to claim 13, characterized in that A sub-wavelength grating structure (24) having a relief profile (28, 30) with a metal coating is provided.

15. The method according to claim 13 or 14, characterized in that The structural parameters of the sub-wavelength grating structure (24) of each sub-region (16, 17) that are different include at least one of the following parameters of the sub-wavelength grating structure (24): the profile, azimuthal orientation, and period of the protrusions and / or recesses of the sub-wavelength grating structure regularly arranged on the base surface.

16. The method according to one of the preceding method claims, characterized in that The sub-wavelength grating structures (24) of the sub-regions (16, 17) differ in their azimuthal orientation, wherein adjacent different sub-regions (16, 17) of the surface region (6, 32) have an azimuthal orientation variation between 0° and 360°.

17. The method according to one of the preceding method claims, characterized in that In the surface region ( 6 , 32 ), the structural parameters vary substantially isotropically.

18. The method according to one of the preceding method claims, characterized in that A plurality of surface areas (6, 32) are provided, the surface areas being respectively constructed as micromirrors (38) of a micromirror arrangement structure (36), as micromirror pixels having a micromirror arrangement structure (36) with a plurality of micromirrors (38) with uniform orientation, or as a plurality of micromirror pixels, wherein the plurality of micromirror pixels respectively have a plurality of micromirrors (38) with uniform orientation of a micromirror arrangement structure (36), wherein a sub-wavelength grating structure (24) of a sub-area (16, 17) of each micromirror (38) or each micromirror pixel or each micromirror pixel in the plurality of micromirror pixels imparts a specific color to the micromirror or micromirror pixel, so that the micromirror arrangement structure (36) produces a colored or tinted visual object.

19. The method according to any one of claims 13 to 18, characterized in that The surface area / areas (6, 32, 34) are distinguishable with the naked eye.

20. The method according to any one of claims 13 to 19, characterized in that The sub-wavelength grating structure (24) is a two-dimensional periodic sub-wavelength grating structure, wherein the sub-wavelength grating structure (24) is applied to a substrate in particular as a rectangular, square, hexagonal or parallelogram grating arrangement (40, 42, 44).

21. The method according to claim 20, characterized in that The sub-wavelength grating structure has regularly arranged protrusions and / or depressions, wherein the sub-wavelength grating structure (24) of each sub-region (16, 17) is separated by an orientation zone of the grating arrangement structure (40, 42, 44).

22. An article or a valuable object or a valuable document (2) having a security element (4) according to one of claims 1 to 12.

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