CN110936750A

CN110936750A - Optical anti-counterfeiting element and anti-counterfeiting product

Info

Publication number: CN110936750A
Application number: CN201811109920.1A
Authority: CN
Inventors: 孙凯; 朱军
Original assignee: China Banknote Printing and Minting Corp; Zhongchao Special Security Technology Co Ltd
Current assignee: China Banknote Printing and Minting Corp; Zhongchao Special Security Technology Co Ltd
Priority date: 2018-09-21
Filing date: 2018-09-21
Publication date: 2020-03-31

Abstract

The invention relates to the technical field of anti-counterfeiting, and discloses an optical anti-counterfeiting element and an anti-counterfeiting product. The optical security element comprises: a substrate; an optically reflective facet formed on the substrate having a tilt angle within a predetermined range; a sub-wavelength surface microstructure formed at least partially on the optically reflective facet; and a metal film formed on the sub-wavelength surface microstructure, wherein the optically reflective facet is configured to increase a visible range of color characteristics of the sub-wavelength surface microstructure formed on the optically reflective facet. The invention can realize the anti-counterfeiting characteristic with color and color change under the general lighting environment, and can also increase the visible range of the color and color change characteristic.

Description

Optical anti-counterfeiting element and anti-counterfeiting product

Technical Field

The invention relates to the technical field of anti-counterfeiting, in particular to an optical anti-counterfeiting element and an anti-counterfeiting product.

Background

Nowadays, optically variable (optically variable) technology is widely used for public anti-counterfeiting of high-anti-counterfeiting securities such as banknotes, and the technology has the characteristics of dynamic images, color changes and the like which can be observed by naked eyes, and cannot be imitated or copied by electronic equipment such as cameras, scanners, printers and the like. Holography is a widely used optically variable anti-counterfeiting technology. Holograms, while capable of creating color variations and motion-like features, are highly demanding on illumination light and are difficult to accurately describe and distinguish.

Chinese patent CN102712207B uses a reflective facet design with a large size (10 um) to form rich dynamic features, but the reflective facet mainly generates geometric reflection for visible light and cannot generate color features. Chinese patent CN103534622A discloses that when incident light encounters metal with periodic structure, it is diffracted, and the light has different momentum under the condition of keeping the energy unchanged, and plasma and corresponding light absorption can be generated at the interface, thereby generating color characteristics. However, the visible range of the color generated by the absorption is very small, and the color has no characteristics such as dynamic feeling and the like which are more beneficial to the human eye to recognize.

Disclosure of Invention

The invention aims to provide an optical anti-counterfeiting element and an anti-counterfeiting product, which can generate anti-counterfeiting characteristics with colors and color changes under a general lighting environment and can increase the visible range of the colors and the color change characteristics.

The present invention provides an optical security element comprising: a substrate; an optically reflective facet formed on the substrate having a tilt angle within a predetermined range; a sub-wavelength surface microstructure formed at least partially on the optically reflective facet; and a metal film formed on the sub-wavelength surface microstructure, wherein the optically reflective facet is configured to increase a visible range of color characteristics of the sub-wavelength surface microstructure formed on the optically reflective facet.

Optionally, the metal film is one of a metal single layer film or a metal multilayer film.

Optionally, the thickness of the metal monolayer film is in a range of 5nm to 100 nm.

Optionally, the thickness of the metal monolayer film is 10 nm.

Optionally, the metal film is a film made of any one of gold, silver, copper, aluminum, platinum, or palladium or an alloy formed of one or more of gold, silver, copper, aluminum, platinum, and palladium.

Optionally, the optical security element further includes: a reflection enhancing coating formed on the metal film.

Optionally, the preset range is 0 to 20 degrees.

Optionally, the preset range is 0 to 15 degrees.

Optionally, the sub-wavelength surface microstructure is one of a one-dimensional grating or a two-dimensional grating.

Optionally, the shape of the grooves of the sub-wavelength surface microstructure is one of sinusoidal, rectangular, or saw-tooth.

Optionally, the depth of the grooves of the sub-wavelength surface microstructure is in the range of 20nm to 200 nm.

Optionally, the depth of the grooves of the sub-wavelength surface microstructure is in the range of 50nm to 100 nm.

Optionally, the period of the sub-wavelength surface microstructure along at least one of two directions in which the two-dimensional plane extends is in a range of 200nm to 500 nm.

Optionally, the period of the sub-wavelength surface microstructure along at least one of two directions in which the two-dimensional plane extends is in a range of 300nm to 400 nm.

Optionally, the average orientation of the optically reflective facets is determined by the tilt angle and/or azimuth angle of the optically reflective facets.

Optionally, a period of the optically reflective facet along at least one of two directions in a two-dimensional plane in which the optically reflective facet lies is in a range of 5 μm to 50 μm.

Optionally, a period of the optically reflective facet along at least one of two directions in a two-dimensional plane in which the optically reflective facet lies is in a range of 10 μm to 30 μm.

Correspondingly, the invention also provides an anti-counterfeiting product which comprises the optical anti-counterfeiting element.

Optionally, the anti-counterfeit product comprises a banknote, an identification card, a bank card or a money order.

The invention creatively forms the sub-wavelength surface microstructure covered with the metal film on the optical reflecting facet with the inclination angle within the preset range, thereby being capable of generating the anti-counterfeiting characteristic with color and color change under the general illumination environment and increasing the visible range of the color and color change characteristic.

Additional features and advantages of the invention will be set forth in the detailed description which follows.

Drawings

FIG. 1 is a block diagram of an optical security element provided in one embodiment of the present invention;

FIG. 2 is a schematic diagram of the tilt angle of a sub-wavelength surface microstructure provided in one embodiment of the present invention;

FIG. 3 is a block diagram of an optical security element provided in one embodiment of the present invention;

FIG. 4 is a cross-sectional view of a sub-wavelength surface microstructure provided in accordance with one embodiment of the present invention;

FIG. 5 is a cross-sectional view of a sub-wavelength surface microstructure provided in accordance with one embodiment of the present invention;

FIG. 6 is a reflection spectrum of a sub-wavelength surface microstructure provided by an embodiment of the present invention at different duty cycles; and

fig. 7 is a diagram of an anti-counterfeit product according to an embodiment of the present invention.

Description of the reference numerals

1 substrate 2 optical reflective facets

3 sub-wavelength surface microstructure 4 metal film

70 banknote 71 windowing safety line

72 paste mark 73 width

730 window

Detailed Description

The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

For optically reflective facets, which are randomly tilted and azimuthally distributed in nature, a variety of features can be formed depending on the orientation and arrangement of the optical facets. These features include: (1) substantially random optical scattering features that create a matte effect to eliminate some discomfort to the viewer from specular reflection; (2) the method comprises the steps of scrolling, zooming, shape illusion and dynamic characteristics with stereoscopic impression generated according to binocular parallax; and (3) features that make the viewer feel as protruding forward and/or backward relative to the surface on which they are actually located. The specific optical characteristics and the principle of generating the optical characteristics can be defined by the Chinese patents CN102712207B, CN102905909B, CN103282212B and CN103229078B, the contents of which are all incorporated in the present invention.

For metallic-plated sub-wavelength surface microstructures that selectively absorb visible light to create reflected and/or transmitted color, the process can be described by the principle of surface plasmon absorption. In general, when photons and electrons have the same energy, as in a flat metal surface, energy transfer cannot occur due to their different momentums, and surface plasmon at the interface cannot be excited. However, when the incident light encounters the metal of the periodic structure, the light is diffracted, has different momentum while maintaining its energy, and if the light with certain momentum matches the periodic structure of the metal, plasma and corresponding light absorption can be generated at the interface between the metal and the medium, thereby generating a color feature, but the color feature can be observed only when the observer faces the sub-wavelength surface structure. The absorption characteristics and color characteristics are closely related to the structural parameters (such as period, depth and cross-sectional shape) of the periodic structure, the optical properties of the metal and the medium, the thicknesses of the metal layer and the medium layer, the angle of incident light and the like.

Based on the optical reflection facet and the sub-wavelength surface microstructure in the prior art, the invention provides an optical anti-counterfeiting element, and the corresponding structure is shown in fig. 1. As shown in fig. 1, the optical security element may include: a substrate 1; an optically reflective facet 2 formed on the substrate with a tilt angle within a predetermined range; sub-wavelength surface microstructures 3 formed at least partially on the optically reflective facet 2; and a metal film 4 formed on the sub-wavelength surface microstructures 3, wherein the optical reflecting facet 2 is used for increasing the visible range of the color characteristics of the sub-wavelength surface microstructures 3 formed on the optical reflecting facet 2; the substrate 1 may be one of paper, a thin film (e.g., a PET film or an aluminum-plated film, etc.). The optical anti-counterfeiting element creatively forms a sub-wavelength surface microstructure covered with a metal film on an optical reflection facet with an inclination angle within a preset range, so that the anti-counterfeiting characteristic with color and color change can be generated under a general illumination environment, and the visible range of the color and color change characteristic can be increased.

In order that the optically reflective facets do not diffract visible light, the present invention provides that the period of the optically reflective facets in at least one of the two directions in the two-dimensional plane in which they lie (i.e., the plane in which the substrate lies) is in the range of 5 μm to 50 μm. Preferably, the period of the optically reflective facet along at least one of its two directions in the two-dimensional plane in which it lies (i.e., the plane in which the substrate lies) is in the range of 10 μm to 30 μm.

The average orientation of the optically reflective facets may be determined by the tilt angle and/or azimuth angle of the optically reflective facets. As shown in fig. 2, the inclination angle of the optical reflective facet is an included angle θ between the optical reflective facet and the two-dimensional plane (i.e. the plane of the substrate) where the optical reflective facet is located, and the azimuth angle of the optical reflective facet is an included angle between the projection of the normal vector of the reflective optical facet in the two-dimensional plane where the normal vector is located and the reference direction x

As shown in fig. 2. The predetermined range of tilt angles for the optically reflective facets provided in the present invention can be 0 to 20 degrees, with azimuth angles selected from 0 to 360 degrees. Preferably, the preset range may be 0 to 15 degrees.

The sub-wavelength surface microstructure may be one of a one-dimensional grating or a two-dimensional grating. The shape of the grooves of the sub-wavelength surface microstructure can be one of various shapes such as a sine shape, a sine-like shape, a rectangle shape or a sawtooth shape. The grid distribution of the two-dimensional grating can be an orthogonal structure, a honeycomb structure, a two-dimensional bravais lattice structure, a random structure and the like. It should be understood that the sub-wavelength surface microstructures provided by the present invention are not limited to the above-described structures, and a splice or combination of these sub-wavelength surface microstructures can also be used in the optical security element. By designing the sub-wavelength surface microstructure, patterns such as characters and marks required by anti-counterfeiting can be realized.

Computational simulation of the plasma absorption characteristics generated by a variety of simple or complex sub-wavelength surface microstructures can be performed using the time domain finite difference method (FDTD) method and associated commercial software (e.g., FDTD Solutions software). The simulation method can accurately predict the reflection spectrum or the transmission spectrum formed by the absorption of the plasma, and particularly can perform careful scanning optimization on the structural parameters of the periodic structure, so as to guide the design and the selection of the sub-wavelength surface microstructure and the metal film.

The period in at least one of the two directions along which the sub-wavelength surface microstructure extends (i.e., the plane of the corresponding optically reflective facet below the sub-wavelength surface microstructure) is in the range of 200nm to 500 nm. Preferably, the period of the sub-wavelength surface microstructure in at least one of two directions along which the two-dimensional plane in which it extends is in the range of 300nm to 400 nm. The grooves of the sub-wavelength surface microstructure have a depth in the range of 20nm to 200 nm. Preferably, the depth of the grooves of the sub-wavelength surface microstructure is in the range of 50nm to 100 nm.

The metal film may be a film made of any one of gold, silver, copper, aluminum, iron, tin, zinc, nickel, and chromium or may be a film made of an alloy formed of one or more of gold, silver, copper, aluminum, iron, tin, zinc, nickel, and chromium. The metal film may be one of a metal monolayer film or a metal multilayer film. Wherein the thickness of the metal single layer film may be in a range of 5nm to 100 nm.

The invention can also design the thickness of the metal film to make the incident light semi-reflective and semi-transmissive, namely, the thickness of the metal single layer film can be designed to be 10 nm. As shown in fig. 3, when incident light is incident on the subwavelength surface microstructure covered with a 10 nm-thick metal single layer film in the region a and the subwavelength surface microstructure covered with a 10 nm-thick metal single layer film formed on the optical reflection facet in the region C, not only the color of the generated light can be observed in the reflection direction but also the color of the generated light can be observed in the transmission direction, and the colors observed in the reflection direction and the transmission direction can be designed to be significantly different colors. For example, red for reflection and blue-green for transmission. But the color characteristics within the two regions A, C have a distinct difference: the color generated by the area A is mainly determined by the sub-wavelength surface microstructure and the metal single-layer film, and the selective absorption can be generated on the visible light wave band through the physical mechanism of plasma absorption under specific conditions, so that the color is generated. When the viewing angle is changed, the wavelength of the absorbed light is changed, and thus the color is also changed. The absorption of the plasma is very sensitive to the viewing angle, which results in a too fast color change or disappearance, which is not conducive to practical viewing. That is, the range of visibility that the observer can observe the color feature is small and is almost limited to the right area a. The color generated by the region C is mainly determined by the sub-wavelength surface microstructure and the optical reflection facet which are covered with the metal single-layer film, the random inclination angle and/or azimuth angle of the optical reflection facet can change the angle of incident light of the sub-wavelength surface microstructure, so that different plasma absorption can be generated on the metal film interface of the sub-wavelength surface microstructure, and further, an observer can observe the generated color in a larger visual field range and can observe different colors in different visual field ranges, namely, the color displayed by the sub-wavelength surface microstructure is modulated by the optical reflection facet. That is, the viewer can see the dynamic feature and the change of color simultaneously by changing the viewing angle in a wide range of visual field through the structure in the region C. Furthermore, when incident light is incident on the optically reflective facets in the B region covered with a 10nm thick metal monolayer film, scattering, motion and/or optical features protruding from the surface (e.g., relief) can be produced. The metal single-layer film can be used as a reflection enhancement coating, so that the reflectivity of the reflective facet is greatly increased, and the dynamic characteristic is more remarkable.

Note that the preset range of the tilt angle of the optically reflective facet in fig. 3 is 0 degree to 20 degrees. Only when the inclination angle is within the preset range, an observer can simultaneously see the dynamic characteristics and the color change through changing the observation visual angle in a larger visual field range through the structure in the area C. Preferably, the preset range is 0-15 degrees, and the observer can simultaneously see more obvious dynamic characteristics and color changes in a larger visual field range through the structure in the area C by changing the observation visual angle. If the inclination angle is in a range other than the preset range (i.e., 0 to 20 degrees), the observer can observe only very weak dynamic features and colors through the structures in the region C, and it is difficult to see changes in the dynamic features and colors by changing the observation angle.

The sub-wavelength surface microstructures and the optically reflective facets can be prepared by: firstly, a master plate is manufactured by adopting a holographic interference method, a laser direct writing technology or an electron beam etching technology and the like; then, according to the master plate, a working plate is manufactured through an electroforming process; finally, the working master is transferred to the surface of the substrate 101 by a production process such as embossing, UV replication, or the like. Due to the large difference in magnitude of the dimensional parameters of the sub-wavelength surface microstructure and the optically reflective facet, there are different requirements on the photoresist material of the master or the process of making the master. In fact, combining two structures on the same mother board or working board is quite complicated and difficult to be completed by a single process, so in actual operation, the two structures can be prepared by a two-step method, namely, a sub-wavelength surface microstructure is prepared by a holographic interference method or an electron beam direct etching method, and then an optical reflection facet is prepared by an overlay process by laser direct writing.

The metal film can be realized by vacuum coating processes such as thermal evaporation, electron beam evaporation, high-frequency sputtering, magnetron sputtering, ion sputtering, reactive sputtering or ion plating, and can also be realized by processes such as chemical plating, electroplating or coating.

FIG. 4 is a cross-sectional view of an asymmetric sub-wavelength surface microstructure covered with a single metal layer film. The subwavelength surface microstructure is asymmetric, i.e. its unit duty cycle r ═ b/a ≠ 0.5, where b is the width of the unit at half the depth h of the groove and a is the maximum width of the unit. Incident light I1 is incident from the upper surface of the subwavelength surface microstructure, which corresponds to the subwavelength surface microstructure with R being 0.7, and the reflected light is R1 and the transmitted light is T1; incident light I2 enters from the lower surface of the subwavelength surface microstructure, which corresponds to the subwavelength surface microstructure with R being 0.3, and the reflected light is R2 and the transmitted light is T2. Due to the asymmetry of the subwavelength surface microstructure, the reflected light R1 and R2 are different, and the reflected light R1 and R2 have significantly different reflection spectra, as shown in fig. 6. Thus, a viewer may see different reflected colors from the top and bottom surfaces by selecting an asymmetric sub-wavelength surface microstructure.

FIG. 5 is a cross-sectional view of a symmetric sub-wavelength surface microstructure covered with a single layer of metal film, according to one embodiment of the present invention. Due to the symmetry of the subwavelength surface microstructure (R ═ 0.5), the reflected light R3 and R4 produced were identical, and the corresponding reflectance spectra were identical, regardless of whether incident from the top or bottom surface of the subwavelength surface microstructure, as shown in fig. 6. If a symmetrical subwavelength surface microstructure is used, that is, r is 0.5, since the surface plasmon absorption effect is significantly affected by the surrounding environment (that is, the refractive index of the medium), it is possible to realize that the upper and lower surfaces of the metal single layer film of the subwavelength surface microstructure generate different reflection colors by covering the upper and lower surfaces with coatings having different refractive indexes. The thickness of the coating needs to be much greater than the depth of the grooves of the sub-wavelength surface microstructure, e.g. 1-10um, and its surface remote from the sub-wavelength surface microstructure is flat (i.e. without the undulations of the sub-wavelength surface microstructure). For example, using a two-dimensional sinusoidal sub-wavelength surface microstructure with a period of 350nm and a depth of 100nm, a refractive index of 1.4 for the upper surface coating and a refractive index of 1.7 for the lower surface coating, the peak position of the plasma absorption will differ by about 140nm, as evidenced by the color: the upper surface appeared blue and the lower surface appeared orange.

Specifically, the subwavelength surface microstructure in fig. 3 is designed to be a sine-like groove type with a duty ratio r of 0.8, a period of 400nm, a depth of 65nm, and an orthogonal two-dimensional grid distribution. And the sub-wavelength surface microstructure is covered with a 15nm aluminum film, the upper surface and the lower surface of the aluminum film are covered with reflection enhancement coatings with the refraction rate of 1.45, and the thickness of the coatings is 2 um. When viewed from a reflective angle, the upper surface of area a appears red to present a specular appearance (i.e., to produce only positive area a) and the lower surface appears yellow to present a specular appearance (i.e., to produce only positive area a); the region B generates scattering, dynamic and/or protruding characteristics (such as embossment) on the surface, and the characteristics are silvery white of the aluminum film when observed from any side; the region C simultaneously generates scattering, motion and/or features protruding from the surface (e.g., relief), and the upper and lower surfaces respectively appear red and yellow, and the chroma and shade of the color change with the change of the viewing angle (i.e., modulated by the optical reflective facets), and the viewing range becomes larger. And viewed from a transmission angle, the areas A and C are blue, the area B is silver, the three areas have different visual characteristics and form strong visual contrast with each other, so that the optical anti-counterfeiting element is ensured to have strong anti-counterfeiting capability.

Of course, the optically reflective facets in the present invention are not limited to the periodic structures described above, and other non-periodic structures are possible.

In summary, the present invention creatively forms the sub-wavelength surface microstructure covered with the metal film on the optical reflection facet having the inclination angle within the preset range, thereby generating the anti-counterfeiting feature with color and color variation under the general illumination environment and increasing the visible range of the color and color variation feature.

The invention also provides a security product which can contain the optical security element. The optical anti-counterfeiting element can be placed in the anti-counterfeiting product in a window opening safety line mode, a window opening sticking strip mode or a labeling mode and the like. The anti-counterfeit products may include products having high added values such as bills, identification cards, bank cards, money orders, or securities.

Fig. 7 shows the optical security element in a different manner in a banknote 70. The optical anti-counterfeiting element can form a windowing safety line 71 through an anti-counterfeiting paper manufacturing process in the prior art, the windowing safety line 71 is embedded into the bank note 70 in a segmented mode, and certain parts are located on the surface of the bank note 70; the optical security element may also be attached to the surface of the banknote 70 by means of a label 72; alternatively, the optical security element may be adhered to the surface of the banknote 70 by means of a wide strip 73, and a window 730 (referred to as a window) is present in the area of the wide strip 73, and the shape and size of the window 730 are not limited, and various regular or irregular shapes such as a circle, rectangle or square, and sizes larger than, smaller than or equal to the width of the wide strip are feasible. The window 730 is obtained by removing a portion of the banknote 70 and the transmissive security feature of the wide strip 73 is visible through the window 730. The optical security element may be applied to the banknote 70 using any one of a windowed security thread 71, a label 72, a wide strip 73, or any combination thereof.

The preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications can be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention.

It should be noted that the various features described in the above embodiments may be combined in any suitable manner without departing from the scope of the invention. The invention is not described in detail in order to avoid unnecessary repetition.

In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.

Claims

1. An optical security element, comprising:

a substrate;

an optically reflective facet formed on the substrate having a tilt angle within a predetermined range;

a sub-wavelength surface microstructure formed at least partially on the optically reflective facet; and

a metal film formed on the sub-wavelength surface microstructure,

wherein the optically reflective facet is configured to increase the visible range of color features of the sub-wavelength surface microstructures formed on the optically reflective facet.

2. An optical security element according to claim 1, wherein the metal film is one of a metal monolayer film or a metal multilayer film.

3. An optical security element according to claim 2, wherein the thickness of the metal monolayer film is in the range of 5nm to 100 nm.

4. An optical security element according to claim 2, wherein the thickness of the metal monolayer film is 10 nm.

5. An optical security element according to claim 1, wherein the metal film is a film made of any one of gold, silver, copper, aluminum, platinum or palladium or a film made of an alloy formed of one or more of gold, silver, copper, aluminum, platinum and palladium.

6. An optical security element according to claim 1, further comprising: a reflection enhancing coating formed on the metal film.

7. An optical security element according to claim 1, wherein the predetermined range is 0 to 20 degrees.

8. An optical security element according to claim 1, wherein the predetermined range is 0 to 15 degrees.

9. The optical security element of claim 1 wherein the sub-wavelength surface microstructure is one of a one-dimensional grating or a two-dimensional grating.

10. An optical security element according to claim 1 wherein the grooves of the sub-wavelength surface microstructures are one of sinusoidal, rectangular or saw-tooth in shape.

11. An optical security element according to claim 1, wherein the grooves of the sub-wavelength surface microstructure have a depth in the range of 20nm to 200 nm.

12. An optical security element according to claim 1, wherein the grooves of the sub-wavelength surface microstructure have a depth in the range of 50nm to 100 nm.

13. An optical security element according to claim 1, wherein the period of the sub-wavelength surface microstructures in at least one of the two directions along which the two-dimensional plane extends is in the range of 200nm to 500 nm.

14. An optical security element according to claim 1, wherein the period of the sub-wavelength surface microstructures in at least one of the two directions along which the two-dimensional plane extends is in the range 300nm to 400 nm.

15. An optical security element according to claim 1, wherein the average orientation of the optically reflective facets is determined by the tilt angle and/or azimuth angle of the optically reflective facets.

16. An optical security element according to claim 1, wherein the period of the optically reflective facet along at least one of the two directions in the two-dimensional plane in which it lies is in the range 5 μm to 50 μm.

17. An optical security element according to claim 1, wherein the period of the optically reflective facet along at least one of the two directions in the two-dimensional plane in which it lies is in the range 10 μm to 30 μm.

18. A security product comprising an optical security element according to any one of claims 1 to 17.

19. A security product as claimed in claim 18 which comprises a banknote, an identification card, a bank card or a money order.