EP3898248B1 - Security element active in the thz range and method for production thereof - Google Patents

Description

The invention relates to a security element that acts in the THz range for the production of ID cards, cards, passports or documents of value, such as banknotes, checks or the like, which has a lattice structure that is not visible to the naked eye and is formed, for example, by a metal layer. The invention further relates to a method for producing such a security element. Finally, the invention further relates to a document of value with such a security element.

Security elements are used to protect documents of value, such as banknotes, checks or the like, against counterfeiting. There are basically two types of security elements to be distinguished here, covert security elements which are not readily recognizable to a user of the value document and are generally subjected to a machine authenticity check, and open security elements which are recognizable to a user. An example of overt security elements are holograms. However, it is also known to design a security feature in such a way that it is both an overt security feature, i.e. a security feature recognizable by a user, and a covert security feature that can usually only be checked with the naked eye with a machine and possibly a user not noticed at all. In addition to holograms in accordance with the prior art DE 102015009584 A1 , which discloses a security element according to the preamble of claim 1, for example overt security features based on metalized sawtooth structures or color-shift inscriptions or embossed structures that are coated with a thin metal layer, combined with grid structures that act in the THz range. These lattice structures have a period of approx. 20 µm - 100 µm and consist of metallized Strips spaced by narrow slits. Such structures can also be overlaid with metallic relief structures. The disadvantage is that the narrow slits in the metal film have to be realized by an additional etching step or by laser demetallization. Since these slits preferably have a width of about 1 µm, this operation is challenging in industrial production.

In general, security elements for documents of value must meet a number of requirements. On the one hand, they should be difficult or impossible to reproduce with simple means, i. H. the effort for a one-off production should be as high as possible. On the other hand, production in series production should involve as little effort as possible.

The invention is therefore based on the object of creating a security element that can be detected with THz radiation and is therefore covered, which has even more favorable properties in the THz range and requires little additional effort in series production. It should also be able to be combined particularly advantageously with overt security features.

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

A security element is provided for the production of documents of value, such as banknotes, checks or the like, which has a grid structure that is not visible to the naked eye and is formed in a first layer, preferably a metal layer, which is arranged in a dielectric and is opaque to THz radiation is, wherein the first layer is in a first plane and has, for example, a layer thickness between 6 nm and 1 micron. In the first layer there are longitudinal slits that are transparent to THz radiation and are next to one another educated. The first layer is embedded in a dielectric that is transparent to THz radiation. The longitudinal slits are arranged next to one another periodically or quasi-periodically with a period, for example between 8 μm and 200 μm. The width of the longitudinal slits is not more than 1/5, preferably 1/10, of the period. Preferably the slots are at least 5 times as long as the period. A second layer is also arranged in the dielectric in a second plane, which is parallel to the first plane. This is also formed from a layer material that is opaque to THz radiation and has a second line grating structure. This is inverted to the first line grating structure and offset by half a period. As a result, the second line lattice structure forms longitudinal webs which, when viewed from above, exactly fill the gaps left by the first longitudinal slots in the first layer.

It is thus provided in particular that the second line lattice structure is formed from longitudinal webs running in parallel, with one of the longitudinal webs of the second layer lying under each of the longitudinal slots of the first layer. The superimposed pairs of longitudinal slots and longitudinal webs each have essentially (i.e. within the scope of manufacturing tolerances, which can be e.g. 5-10%) the same width, so that the mentioned gap filling is realized.

In a particularly preferred embodiment, the distance between the first and second levels is between 50 nm and 100 μm, particularly preferably between 500 nm and 5 μm. The period is more preferably in the same interval.

Quasi-periodic means that the grating period fluctuates around a mean value. This fluctuation can preferably be up to half a period, particularly preferably up to 1/10 period. In particular, periodic structures are covered by a quasi-periodic arrangement, in which the period varies due to production. Such quasi-periodic arrangements of slot structures also have an increased TM transmission in the THz range.

In a preferred configuration, the first and/or the second layer comprises a color-shift coating. This improves visual recognition - in addition to machine evaluation, which takes place in the THZ area.

Next are those from the DE 102015009584 A1 known combinations with security features that can be seen with the naked eye. Here, in particular, hologram structures, sub-wavelength gratings with periods between 200 nm and 500 nm, sawtooth structures, etc. come into question DE 102015009584 A1 is fully integrated here in this regard.

The method for producing a security element produces the security element described. For this purpose, a first layer, which is formed from a layer material that is opaque to THz radiation, is arranged in a first plane in a dielectric that is transparent to THz radiation. It has a periodic or quasi-periodic first line lattice structure, which cannot be seen with the naked eye, and consists of parallel longitudinal slits, which produce longitudinal slits in the first layer. A width of the longitudinal slits is not larger than 1/5 of the period, preferably not larger than 1/10 of the period. A second layer, which is also formed from a layer material that is opaque to THz radiation, is arranged in the dielectric in a second plane, which is parallel to the first plane. In the second shift a second line grating structure is formed. It is inverted to the first line grating structure and offset by half a period. This in turn ensures that the longitudinal webs, which are formed in the second layer, exactly fill the longitudinal slots in a plan view.

The configurations of the security element already mentioned can, of course, also be implemented in further developments of the method.

Finally, a document of value is also provided which is provided with a security element having the properties mentioned.

The security element according to the invention can be checked in the THz range, since the slot structure is transparent for THz radiation with TM polarization, but opaque for TE polarization, or vice versa. Due to the slit structure, the security feature acts as a polarizer that lets through THz radiation with one polarization. If the incident THz radiation is correspondingly polarized, a large proportion of this polarization component passes through the security element. The security element can thus be subjected to a machine authenticity check very easily. A THz radiation source and a THz detector are used for this purpose. Ideally, the THz radiation is polarized, with the security element also being able to be detected with unpolarized THz radiation. In this case, a polarizer, which acts as an analyzer, must be arranged in front of the detector. The authenticity check can be carried out with a measurement in transmission as well as in reflection. In the exemplary case of transmission, the security element acts as a polarizer that lets the THz radiation with TM polarization through. If the THz radiation is correspondingly linearly polarized, this component passes through the security element for the most part and becomes polarized with the same polarization completely detected by the analyzer. If the polarization directions of the radiation source and detector are perpendicular to one another, the trivial case of a hole in the security element can be ruled out. This would also be visually verifiable. With a rotated arrangement of the security feature, a vertically polarized THz radiation is rotated when passing through the security element and the analyzer with horizontal polarization can receive the THz signal. The contrast can be increased by recording with two or more different polarization directions. The machine authenticity check can thus be carried out both with parallel orientation of the polarization of beam source and detector and with crossed orientation. The rotation of the security feature is a rotation of the security feature in the plane that is defined by the periodically or quasi-periodically adjacent longitudinal slots.

The slit structure cannot be seen with the naked eye, at least for an inexperienced observer and/or in a mere top view, since the width of the longitudinal slits is not greater than 1/5 of the period. If this upper limit is lowered, the slit structure becomes very difficult or impossible to recognize even for a practiced observer who is looking for the slit structure and/or when using special viewing techniques (e.g. certain tilting and rotating of the security element). It is particularly preferred that the width of the longitudinal slits is not greater than 1/10 of the period, since the slit structure then produces a security feature that is particularly well concealed.

Due to the double structure, which provides longitudinal webs under the slits that are formed in the first layer, which exactly match the slit width, the transmission and reflection properties in the THz range are improved.

In a development, the security element is designed in such a way that the layer comprises a plurality of fields, between which a longitudinal direction of the longitudinal slits differs. So the fields have an individual angular orientation of their directions of the longitudinal slots. Such individual fields then appear with different brightness depending on the rotational position and/or orientation to the THz detector.

The configuration of the lattice structure in the layer is virtually imperceptible to the naked eye. A potential counterfeiter thus receives no indication of the presence of such a security feature. In terms of reflection and transmission, the layer hardly differs from a layer that is formed without the longitudinal slits. In particular, it is possible to combine the security element with a visually visible structure, e.g. by additionally providing the areas of the layer between the longitudinal slits with a further security feature that can be seen with the naked eye. In this way, the security element has covert properties that can be seen with THz radiation and additional ones that can be seen with the naked eye, i. H. open security properties. The additional security feature that can be seen with the naked eye can be, in particular, a metalized hologram, a sub-wavelength grating with periods of 200 nm to 500 nm, a sawtooth structure and/or a color-shift coating.

The production method according to the invention can be designed in such a way that the preferred configurations and embodiments of the security element described are produced.

Fig. 1

eine Schemadarstellung einer Banknote mit einem Sicherheitselement,

Fig. 2A bis 2D

Schnittdarstellungen durch das Sicherheitselement der Fig. 1 in verschiedenen Ausführungsformen,

Fig. 3A bis 7B

Kurven zur Veranschaulichung der Wirkung des Sicherheitselementes der Fig. 2A bis 2D auf Strahlung im THz-Bereich,

Fig. 8

eine Draufsicht auf eine Ausführungsform des Sicherheitselementes mit Feldern unterschiedlicher Längsausrichtung einer THz-Strahlung beeinflussenden Struktur und

Fig. 9

verschiedene Ausführungsformen eines Feldes zur Codierung verdeckter Informationen mit einem der Sicherheitselemente der Figuren 2A bis 2D.

The invention is explained in more detail below by way of example with reference to the accompanying drawings, which also disclose features that are essential to the invention. Show it:

1

a schematic representation of a banknote with a security element,

Figures 2A to 2D

Sectional views through the security element 1 in different embodiments,

Figures 3A to 7B

Curves to illustrate the effect of the security element Figures 2A to 2D for radiation in the THz range,

8

a plan view of an embodiment of the security element with fields of different longitudinal alignment of a structure influencing THz radiation and

9

various embodiments of a field for coding covert information with one of the security elements of Figures 2A to 2D .

1 shows a banknote 2 in plan view, which is provided with a security element 4 . In all of the embodiments, the security element 4 is designed in such a way that it filters radiation in the THz range in a specific way and at the same time is designed in such a way that it cannot be seen with the naked eye. The security element 4 thus provides a concealed security feature that can be read with a corresponding detector. The THz structure is im visible wavelength range almost opaque or at least not recognizable. This does not preclude the structure from being combined or overlaid with a visually transparent structure, as in the embodiments of FIGS Figures 2B to 2D the case is. The security element 4 can be open on both sides (eg when used as an element spanning a window) or on one side. In addition, it can be completely embedded in the substrate, which is transparent to THz radiation. Examples of the substrate or the dielectric are paper or plastic.

The Figures 2A to 2D FIG. 1 shows a sectional representation of the security element 4, which has a dielectric 6 arranged on a carrier, which is not described in more detail. A first line grating structure 8 is embedded in the dielectric 6 , which is transparent to THz radiation, and is made up of a coating that absorbs THz radiation and is located in a first plane 10 . In a second plane 12 parallel thereto, which is a distance h lower, there is a second line grating structure 14 in the dielectric 6, which is also made of a THz radiation-absorbing material, preferably made of the same material as the first line grating structure 8.

The first and the second line grating structure 8.14 form a bi-layer grating 16. The first and the second line grating structure 8.14 are inverted to each other. The first line grating structure 8 consists of metallic strips 18 which are spaced apart from one another by longitudinal slots 20 . This structure is arranged according to a period p. The second line grating structure 14 is inverted for this purpose. It has longitudinal slits 24 at those points where the first line grating structure 8 has the strips 18 and strips 22 at the points of the longitudinal slits 20. The layer that forms the strips 18 has a thickness t1, for example, the layer that forms the strips . 22 forms a thickness t2. The strips 18, 22, and thus the line grating structures 8,14, have a refractive index n and are completely surrounded by the dielectric 6, which preferably and optionally has the same refractive index n above and below the line grating structures 8,14. The refractive indices in the dielectric 6 can also vary.

As can be seen, the inverted shape that the second line grating structure 14 has in relation to the first line grating structure 8 means that the width d of the strips 18 of the first line grating structure 8 corresponds exactly to the width of the longitudinal slots 24 of the second line grating structure 14. The same applies to the width s of the longitudinal slots 20 of the first line grating structure 8 and the strips 22 of the second line grating structure 14. The inverted line gratings are also shifted relatively by half a period in the plane 10 or 12 so that in a top view of the security element 4 (according to the viewing direction in Figure 2A from top to bottom) a gapless layer is formed by the strips 18 and 22.

In Figure 2A it is shown that a plane wave is incident on the security element 4 at an azimuth angle ph and an elevation angle th. This incident radiation S is partially transmitted as transmitted radiation T and partially reflected as reflected radiation R. The properties of this effect on radiation in the THz range are described below on the basis of Figures 3 to 7 explained in more detail.

The embodiments of Figures 2B to 2D differ from those of Figure 2A by the design of the strips 18. They are used to additionally create a visually perceptible effect. Despite this visually perceptible effect, the structure effective in the THz range cannot be recognized. The covert security property is therefore retained. In this context, reference is made expressly to the DE 102015009584 A1 referenced, the disclosure content of which with regard to the combination of a structure effective in the THz range and a visually perceptible structure is incorporated here in its entirety.

The structure described above for authenticity detection in the THz range is preferably produced on film substrates and can then be applied to banknotes 2, for example. However, a metallically reflective surface is not very attractive to an observer. It is possible to overprint this area. However, it is advantageous to overlay this structure with other security features based on foil elements. This is because known metallized security features, such as holograms, micromirror arrays or metallic sub-wavelength gratings, are primarily human features that are difficult to check mechanically for authenticity. By overlaying these structures with the THz feature described above, a machine check of these structures in the THz range is possible.

A superimposition with various metalized security features known per se is therefore advantageous. Such an overlay can be done with embossed holograms, as in Figure 2B is shown schematically. In Figure 2B the strips of the first line grating structure 8 consist of a hologram structure 26. It is opaque to THz radiation, so that the hidden security property of the security element 4 is retained. Such holograms consist of grating arrangements with periods of about 500 nm to 1500 nm. In known embossed holograms, the grating profile has a sinusoidal or rectangular shape with pitches of about 100 nm to 300 nm. The structure is metallized over the entire surface. Aluminum, silver or copper with layer thicknesses of approx. 30 nm to 80 nm are preferably used as metals. In this case, the superimposition with the THz structure described above means that narrow periodic regions of the embossed hologram or mirror strips lie on the lower level 12 . Since the period of the hologram grating is significantly smaller than the grating period of the THz structure, there is no additional interaction in the THz range with this structure. This is because the period of the hologram is orders of magnitude smaller than the wavelength of the THz radiation. There is therefore a comparable transmission in the THz range as in the case of the bi-layer grating 16 described above.

In Figure 2C the strips include a color-shift coating 28, which can optionally also be applied to the strips of the second line grating structure 14. The color shift coatings 28, 30 produce a visually perceptible effect. Since they include a metal layer or another coating that is opaque to THz radiation, the effect of the security element 4 as a covered security feature is retained here as well. The color-shift structure consists, for example, of a semi-transparent chromium layer, a dielectric spacer layer, preferably made of silicon dioxide, and a metallic mirror layer underneath, e.g. e.g. aluminium. This layer structure is finally formed as a bi-layer structure 16 . The surface area of the underlying structure is small compared to the total area. Therefore, the visual impression of these security features is hardly affected by the overlay with the THz structure.

In 2D the strips of the first line grating structure 8 are in the form of a sawtooth structure 32 . So there is a superimposition with a sawtooth structure such. B. Fresnel structures. Known sawtooth arrangements have a lateral extent of between 1 μm and 10 μm and a height of between approximately 0.3 μm and 4 μm. Such arrangements are used to create motion and spatial effects in reflection. They are either covered with a simple metal layer or they are vaporized with a so-called color-shift structure in order to create an additional color effect. The metallized structure is interrupted by the periodic arrangement of the longitudinal slots 20, under which the strips 22 lie on the lower level. In the THz range, only this combination affects the transmission, since the interaction with the sawtooth structure 32 itself is low.

The overlay can also be performed with (optical) sub-wavelength structures. These are 1-dimensional or 2-dimensional periodic gratings with periods between 100 nm and 500 nm, which are metallized. It remains to be mentioned that so-called metallic moth-eye structures, which can serve as an absorbent background, can also be overlaid with the structure explained above. Also, the metalized ridges will be raised instead of recessed as in the drawings above. The transmission in the THz range is identical with this vertically mirrored arrangement.

Furthermore, the above-mentioned coatings that absorb THz radiation are not limited to a simple metal layer or color-shift structures. Other multilayer coatings can also be used as long as they are opaque to THz radiation - either in combination or due to an absorbing component or layer.

Below is the effect of the security element 4 on radiation in the THz range, ie on radiation between, for example, 1 and 12 THz, using the example of the security element Figure 2A explained. Since the visually perceptible structures of the strips of the first line grating structure 8 in the embodiments according to Figures 2B to 2D have no effect on the effect in the THz range, the embodiments also apply here analogously.

The following calculations refer to an aluminum grille with a rectangular cross-section. The refractive index of the surrounding dielectric is n=1.4. Figures 3A-C show the spectral transmission for TM- ( Figure 3A ) and TE polarizations ( Figure 3B ) and the degree of polarization for a bi-layer grating 16 with webs of different widths s=2; 4; 6 µm with constant period d=50 µm. For a frequency of 1 THz, the transmission with TM polarization is 32%, 43% or 49% for the ridge width s=2 µm, 4 µm or 6 µm. For TE polarization, the transmission for these ridge widths is approximately zero. The thickness t1=t2 of the coating was 50 nm, the distance between the planes h=2 μm. The contrast of transmission between TM and TE polarization is in Figure 3C ) shown. Here the calculated degree of polarization (T _{TM -T TE} ) / (T _TM + T _TE ) is plotted as a function of frequency. The more these values differ from zero, the stronger the polarization effect of the bi-layer grating 16. It can be seen that the grating as a whole frequency range shown has pronounced polarization properties.

The influence of the height distance h on the transmission behavior in the THz range is now explained. Figures 4A-C show the transmission at normal incidence for an aluminum grating with a period d=50 µm, s=2 µm, t=50 nm for height spacings of 0.5 µm, 1 µm, 1.5 µm and 2 µm. The other parameters are identical to those of Figures 3A-C . Again, the figure labeled A shows the transmission for TM polarization, the figure labeled B shows the transmission for TE polarization, and the figure labeled C shows the contrast. Here it can be seen that a variation of the height difference h does not have a significant effect on the transmission behavior in the THz range. The polarization properties are hardly affected. This means that the process window in series production is not critical with regard to this parameter.

Now the influence of the grating period on the transmission in the THz range is examined. In Figures 5A-C shows the transmission for gratings with periods d=25 µm, 50 µm, 75 µm and 100 µm. The ratio of the gap width to the period is constant and amounts to s=0.04 * d. Out of Figures 5A and 5B it can be seen that the spectral characteristic of the transmission is shifted towards lower frequencies for increasing periods. Figure 5C it can be seen that the polarizing effect of the grating is very good for these periods in the entire spectral range shown. This shows that the transmission characteristics can be adjusted for the desired frequency band by appropriately selecting the grating period. It should also be mentioned that the transmission and polarization properties shown here are hardly affected if the grating profile deviates from the ideal rectangular shape. Therefore are in the making Such grids do not have to meet high requirements in order to achieve the desired transmission or polarization effect in the THz range. The figures are based on a height of h=1.8 μm and a thickness of the metal layer (here Al) of t=50 nm.

6 shows the calculated spectral reflectance of a bi-layer grating 16, a simple wire grating and a smooth 60 nm thick aluminum film - in the visible spectral range. All structures are embedded in a dielectric with a refractive index of 1.52. The perpendicularly incident light is unpolarized. The reflection in the zeroth order for both grating types averages about 73% compared to 82% for a smooth 60 nm thick aluminum layer, which is also embedded in a dielectric with n=1.52. For larger periods, the difference between the reflection at the bi-layer grating 16 and a smooth surface decreases increasingly. This means that a bi-layer grating 16 with periods d>50 μm can hardly be distinguished from a smooth metallic surface by an observer. This is all the less the case when such bi-layer gratings 16 are overlaid with other structures such as hologram gratings.

Finally, a metallized embossed hologram overlaid with a bi-layer grating 16 was analyzed experimentally. The structure corresponds schematically to the drawing of Figure 2B . The structure consists of a 60 nm thick aluminum film embedded in UV varnish between two PET foils. The embossed hologram consists of gratings of different azimuthal orientation with periods between 500 nm and 2 µm. The profile shape is sinusoidal. The parameters of the bi-layer grating are: d=50 µm, h=1.8 µm, s=2 µm and t=60 nm. Figure 7A shows the spectral transmission in the range from 0.1 to 3 THz for TM and TE polarization. From this, the degree of polarization was calculated and Figure 7B shown. The structure is here in embedded in a UV-curable embossing varnish with a refractive index of 1.4. The measurement confirms the previously presented properties of the bi-layer grating 16. The unexpected high transmission is also present for superimposed grating structures. The blocking effect for TE polarization is somewhat lower for this sample. However, this is due to small defects in the sample through which the THz radiation penetrates unhindered. Despite this, the degree of polarization of the sample is high. The bi-layer grating 16 superimposed in the hologram can be reliably detected.

As mentioned earlier, the overlays coming from the DE 102015009584 A1 (there for another covert security feature) also in question here. The superimposition of a motif with the THz structure described above is illustrated using the example of 8 explained. The motif "Butterfly" with the number "25" is formed in front of a background using a metallic embossed hologram. Here the entire surface is overlaid with a bi-layer grid 16 . The longitudinal slots 20, 24 are oriented horizontally in the background 34 and vertically in the areas 36-42. In the THz range, this security element 4 shows a different transmission in the areas 34 on the one hand and 36-42 on the other hand. With a spatially resolving detector, the subject can be detected in the THz range.

Furthermore, this approach can be used to encode information that can be evaluated automatically in the THz range. An example is the coding of the denomination or the value of banknotes, e.g. 5, 10, 20, 50 and 100. These numerical values (or other values) can be encoded by differently oriented areas of bi-layer gratings, preferably with areas rotated by 90°. An example with four fields 46, which in front of a background 48 with a horizontal longitudinal slit direction have several coding fields 50-56 with a vertical longitudinal slit direction 9 .

1. a) Prägen der Struktur in UV-Lack auf Folien,
2. b) vollflächige gerichtete Metall-Bedampfung, wobei die Flanken der Stege nicht mit Metall bedeckt werden,
3. c) Kaschierung mit Deckfolie.

The security element 4 can be produced on an industrial scale using known methods. The main steps in production are:

1. a) embossing the structure in UV varnish on foils,
2. b) directed metal vapor deposition over the entire surface, whereby the flanks of the webs are not covered with metal,
3. c) Lamination with cover film.

The basic principle corresponds to that from the above

DE 102015009584 A1 , the advantage now being achieved that no metal has to be removed in the deepened embossed longitudinal slots 20 . The longitudinal slots 20 are created as depressions by the corresponding embossing structure, and the metal is also deposited in the depressions in order to form the strips 22 there.

As already explained in the general part of the description, the security element can simply be subjected to an authenticity check by examining its polarization properties for radiation in the THz spectral range. Possible devices are shown in FIGS. 13 and 14 of FIG DE 102015009584 A1 . The security feature 4 acts as a polarizer that lets through THz radiation with TM polarization. If the THz radiation is correspondingly linearly polarized, this component mostly passes through. In order to direct linearly polarized radiation with TM polarization onto the security element 4, the THz source can be followed by a polarizer; this can be omitted if the THz source already emits the corresponding polarized radiation. After passage through the security element 4 z. B. provided an analyzer that filters the direction of polarization according to the THz detector. By recording a signal from With two or more different polarization directions, the contrast can be increased, ie the device is first set in the configuration with the same alignment of source radiation and detector and then with mutually orthogonal alignment. If the security element has 4 areas with differently oriented slit structures, a spatially resolving detector measures different intensities for the individual areas. This increases the reliability of the authentication of this feature.

To check the authenticity of the security element 4 described above, a THz radiation source and a THz detector are used, which are arranged opposite one another. The security element 4 is located in between and is preferably irradiated approximately perpendicularly. The radiation from the THz radiation source is preferably linearly polarized and the detector is also polarization-sensitive. The security element 4 is arranged in such a way that TM polarization is present for at least one area of the bi-layer grating 16 and the THz radiation reaches the detector there almost unhindered. In contrast, the transmission for the grating areas in TE polarization is blocked. In an alternative arrangement, the polarization directions of the radiation source and detector are perpendicular to one another. In this way, the trivial case of a hole in the security element, through which the THz radiation would also pass unhindered, can be ruled out. With a twisted arrangement of the THz grating, a vertically polarized THz radiation is rotated as it passes through and the analyzer with horizontal polarization can receive the THz signal. The contrast can be further enhanced by recording a signal with two or more different polarization directions.

THz analysis can be performed at a single frequency or in a single frequency band, as well as for multiple frequencies or separate frequency bands. The latter two embodiments refine the authenticity check.

Reference List

2

bank note

4

security element

6

dielectric

8th

first line grid structure

10

first floor

12

second level

14

second line grid structure

16

Bi-layer grid

18, 22

stripes

20.24

longitudinal slots

26

hologram

28, 30

Color Shift Layer

32

sawtooth structure

34, 56

background

36-44, 46, 50-56

Field

S, T, R

radiation

H

Distance

p

period

t1, t2

thickness

n, nM

refractive index

th

elevation angle

Ph

azimuth angle

s, d

Broad

Claims (11)

1. Security element for producing valuable documents, such as banknotes, cheques or the like, wherein, in a dielectric (6) that is transparent to THz radiation in a first plane (10), a first layer is arranged which is formed from a layer material that is opaque to THz radiation and forms a periodic or quasi-periodic first line grating structure (8), which is not perceivable with the naked eye, made of parallel longitudinal slits (20) that produce gaps in the first layer, wherein a width of the longitudinal slits (20) is no greater than 1/5 of the period (p), preferably no greater than 1/10 of the period (p),
   characterized in that,
   in the dielectric (6) in a second plane (12) that is parallel to the first plane (10), a second layer is arranged which is likewise formed from a layer material that is opaque to THz radiation and forms a second line grating structure (14) that is inverse to the first line grating structure (8).
2. Security element according to Claim 1, characterized in that the second line grating structure (14) is formed from parallel longitudinal webs (22), wherein under each of the longitudinal slits (20) of the first line grating structure (8) lies one of the longitudinal webs (22) of the second line grating structure (14) and the longitudinal slits (20, 24) lying thereabove and longitudinal webs (18, 22) each have substantially the same width (d, s), with the result that the longitudinal webs (24) of the second line grating structure (14) cover in plan view of the first plane (10) the longitudinal slits (20) in the first line grating structure (8).
3. Security element according to Claim 1 or 2, characterized in that a distance (h) between the first and second planes (10, 12) is between 50 nm and 100 µm.
4. Security element according to any of Claims 1 to 3, characterized in that the period (p) is between 10 µm and 100 µm.
5. Security element according to any of Claims 1 to 4, characterized in that the first and/or second line grating structure (8, 14) has/have a multilayer coating which is opaque to THz radiation, in particular a colour shift coating (28, 30).
6. Method for producing a security element for producing valuable documents, such as banknotes, cheques or the like, wherein, in a dielectric (6) that is transparent to THz radiation in a first plane (10), a first layer is arranged which is formed from a layer material that is opaque to THz radiation and is provided with a periodic or quasi-periodic first line grating structure (8), which is not perceivable with the naked eye, made of parallel longitudinal slits (20) that produce gaps in the first layer, wherein a width of the longitudinal slits (20) is no greater than 1/5 of the period (p), preferably no greater than 1/10 of the period (p),
   characterized in that,
   in the dielectric (6) in a second plane (12) that is parallel to the first plane (10), a second layer is arranged which is likewise formed from a layer material that is opaque to THz radiation and is provided with a second line grating structure (14) that is inverse to the first line grating structure (8).
7. Method according to Claim 6, characterized in that the second line grating structure (14) is formed from parallel longitudinal webs (22), wherein under each of the longitudinal slits (20) of the first line grating structure (8) is arranged one of the longitudinal webs (22) of the second line grating structure (14) and the longitudinal slits (20, 24) lying thereabove and longitudinal webs (18, 22) each have substantially the same width (d, s), with the result that the longitudinal webs (24) of the second line grating structure (14) cover in plan view of the first plane (10) the longitudinal slits (20) in the first line grating structure (8).
8. Method according to Claim 6 or 7, characterized in that a distance (h) between the first and second planes (10, 12) is between 50 nm and 100 µm.
9. Method according to any of Claims 6 to 8, characterized in that the period (p) is between 10 µm and 100 µm.
10. Method according to any of Claims 6 to 9, characterized in that the first and/or second line grating structure (8, 14) has/have a colour shift coating (28, 30).
11. Valuable document having a security element according to any of Claims 1 to 5.

Images