Modern Trends in The Creation of New Visual Color Effects in Security Holograms Based on Plasmon Gratings and Three-Dimensional Holograms
Currently, the directions for the development and creation of new optical security elements integrated in the design of the security hologram are actively developing in the security holography to mark the products against counterfeiting and falsification . The most popular is the development of optical security elements (OSE), forming new color effects, namely:
• the elements that form fading color shades, i. e. so-called pastel colors, which are very difficult to counterfeit and which are accepted all over the world as a "canon" in security printing art, e. g., in paper banknotes and currency;
• the elements that form clearly fixed (in a certain angular field of view) colors in the form of a visually perceived volumetric color image of the object.
These developments are being conducted both by leading foreign companies and in Russia. In 2013, SURYS (until 2016 named "Hologram Industries", France – Germany) demonstrated the results of developments to create optical protection elements based on one-dimensional (1D) and two-dimensional (2D) plasmon diffraction gratings (PDGs) integrated in security holograms, www.surys.com. Holographic subdivisions of DNP (Japan, www.dnp.co.jp) and Bayer (Germany, www.baer.com, www.films.covestro.com) are developing three-dimensional color security holograms (3D-CSH) based on three-dimensional holograms forming a single-color or multi-color three-dimensional images of objects.
In this article, the results of some works on the creation of new optical security elements and security holograms with unusual color effects based on plasmon diffraction gratings and three-dimensional holograms in a thick-layer photosensitive media made jointly by Bauman Moscow State Technical University and JSC "SPA Kripten" are given.
CREATION OF OPTICAL PROTECTION ELEMENTS BASED ON PLASMON DIFFRACTION GRATINGS
In recent years, the possibility of integrating plasmon spectral effects into security holograms has been extensively studied. Surface plasmons can be defined as the oscillations of free electrons at the metal-insulator interface. The phenomenon of excitation of surface plasmons at the resonant frequency underlies the creation of a new class of optical spectral filters based on waveguides [2, 3], diffraction gratings [4–7].
Plasmon diffraction gratings (PDGs) are used as wideband optical filters wherein the spectrum bandwidth depends on the radiation incidence angle (spectral-angular dependence). A new class of optical security elements (OSE) was created based on them, and their design is provided by nanostructuring of various image areas.
The researches to create a visually perceived optical security element, different from the effects reproduced by standard rainbow holograms, were carried out in Bauman MSTU, together with JSC "SPA Kripten" (Dubna, Russia). The studies of 2D PDGs are carried out within the search for anomalous peaks in the transmission or reflection spectrum (plasmon resonance). The simulation was carried out for three variants of the relief, which is a set of periodically located elements of a subwavelength scale, with square and triangular packing:
1) perforated metal film made on a substrate;
2) a set of polymer nanocylinders or rectangular steps coated with silver;
3) silver-plated polymer layer with apertures on a polymer substrate.
The table shows the ranges of the parameter values of the investigated PDGs.
As a result of the performed studies, the following regularities are observed. First, when considering structures made in the form of a matrix of apertures or steps coated with a layer of silver, the value of the optimum layer thickness of the sprayed metal is in the range of 20 to 40 nm. Second, when considering structures made in the form of the matrix of apertures perforated in the silver layer, the optimum thickness of the metal layer is in the range of 60 to 120 nm. Moreover, an increase in the thickness of the coating leads to a narrowing of the bandwidth. Third, one can achieve the effect of color conservation in a wide range of variation in the slope of the sample on the structure with hexagonal packing of elements with a smaller periodicity (200–300 nm). The fourth feature is that with the change of polarization of the incident radiation from TM to TE, the bandwidth is narrowed, while remaining in the short-wavelength region, with displacement towards lower angles of radiation incidence.
The developed software allows visualization of color behavior of plasmon structure sample depending on the angle of sample inclination that makes it possible to clearly visualize the colors of structures and facilitates the development of OSE design based on the PDGs, as shown in Fig. 1.
The PDGs, wherein a plasmon effect is observed, consisting in a change of the structure color with a change in the incidence angle, were simulated. Changing the size and shape of elementary nanoscale sections, it is possible to regulate the frequency of plasmon resonance, and, hence, the transmission or reflection spectrum. With period increase, the horizontal band in the entire range of angles is displaced to the red region, and the diagonal band is displaced to the region of smaller angles of incidence. Their joint influence on the sample color is shown in the corresponding color diagrams in Fig. 1. For example, there is a discoloration from turquoise to violet at an incidence angle exceeding 35° on a PDG with a period of 300 nm. Color gradation depending on the period can be traced by choosing one angle for consideration.
Color gradation depending on the period can be traced by choosing one angle for consideration. For example, for a PDG in Fig. 1d, when the illumination angle changes from 60° to 0°, the color changes from purple to green, then to dark orange, and then to dark purple. The influence of the thickness of the silver layer, increasing which, we obtain a shortening of the bandwidth along the angle has been also investigated. According to simulation results, Fig. 2 shows a diagram of color shades changes depending on the angle of inclination (or illumination) for various period PDGs.
Compared to conventional security holograms (SH), the security feature based on the data of plasmon effects will be perceived differently as it is visible not in diffracted radiation, but in reflected and transmitted radiation. Furthermore, if the rainbow effect appears in the full spectral range from blue to red at small rotation of the SH, the effect in the transmitted radiation results in a change of the sample color from blue to red at full rotation at 60–90°, as shown for one of the samples in Fig. 3.
As a result of the theoretical and experimental studies of 2D PDGs, a new security feature design was developed based on 2D PDGs to obtain a new OSE, forming new color effects in visual observation of the images.
In order to obtain fixed or fading color shades in the resulting holographic image, 1D‑2D PDGs with the "metal-insulator" nanostructure with differing period were used: in the logo area – 400 nm, in the background area – 500 nm, in the area of inscription "Kripten" – 600 nm. The relief height should be in the range of 60 ± 20 nm.
An important difference is that it works in both planes: both in horizontal and vertical, since it is based on 2D PDGs. When the OSE is inclined in any plane, a color change will appear, as shown in Fig. 4.
Furthermore, if the sample is inclined in the diagonal plane, the plasmon effect will be manifested differently, with different shades of changing colors. This distinguishes the new OSE both from rainbow holograms and from the OSE based on 1D PDGs. The latter are based on one-dimensional structures, and the two effects, diffraction (rainbow) and plasmon one, appear only when the SH is inclined, and the OSE is in a plane perpendicular to the strokes of the one-dimensional structure.
CREATION OF COLOR HOLOGRAPHIC STEREOGRAMS AS OPTICAL SECURITY ELEMENTS
Color holographic stereograms originally designed for wide-format (up to 1.5 Ч 2 m) visual holograms forming color volumetric images of art objects . This article describes the results of the development of holographic stereograms technology in the form of:
• small-format (up to 40 Ч 40 mm) fully three-dimensional color security holograms (3D-CSH)
• point three-dimensional holograms in the form of individual optical security elements integrated in 3D-CSH.
In the first case, 3D-CSH form a single multicolor volumetric image of objects or two (or more) color volumetric images with horizontal and vertical switching (flip-flop effect), both colored and imaging. In the second case, 3D-CSH form visual effects in the form of color microtext and colored hidden volumetric images.
Such 3D-CSHs are obtained by methods of digital holography and computer synthesis of color volumetric images . A single-stage scheme for creating color hologram stereograms was developed based on hologram modified scheme of Y. N. Denisyuk, wherein one-parallax and full-parallax colored hologram stereograms can be created. In this case, hologram stereograms are the combined digital holograms that, upon creation, consistently record the angles of the object output to the spatial light modulator (SLM). Such SLM is located in the objective branch of the optical scheme and the following 3D-CSHs and optical security elements can be created depending on what images are displayed:
• reflective color hologram stereograms (wherein a set of angles representing stereopairs is displayed on the SLM);
• reflective holograms with colored flip-flop effects (the same image is displayed on the
SLM as the angles, but different angles are recorded in different colors);
• reflective holograms with imaging flip-flop effects (different images are displayed on the SLM as the angles);
• reflective holograms with a hidden color image, restored to reflection;
• reflective holograms with a color microtext, localized both in the hologram plane and outside the hologram plane.
Any display devices, for example, LC-displays, outputting corresponding images (angles) are used as the SLM. Further, these angles are recorded on the photosensitive medium.
By the above-described scheme, full-parallax colored hologram stereograms were created. Photographs of the data matrixes of these holograms ("Earth" and "Cube" design), as well as the images restored using point and extended regenerating sources, are shown in Fig. 5. One-parallax colored hologram stereograms with a flip-flop effect ("Cup‑2018" design) were also created. Fig. 6 presents images of a set of angles used to create color hologram stereograms with a flip-flop effect ("Cup‑2018" design) and Fig. 7 shows a matrix of holograms of "Cup‑2018" design and photographs of vertical and horizontal flip-flop effects in the restored images where the images were restored with point and quasi-extended sources.
It can be further seen that the horizontal flip-flop effect is more pronounced than the vertical one. This is due to the fact that the horizontal flip-flop effect is mainly related to the hardware implementation of the hologram creation scheme. The vertical flip-flop effect is imposed with diffraction effects, determined by the angular and spectral selectivity features of the hologram. These selectivity features, in turn, depend strongly on the size of the reducing source and the spectral composition of its radiation. According to , the approximate mathematical estimates of this effect can be made only at small angular sizes of the restoring source and with an insignificant difference in wavelengths used when restoring images from the wavelengths used to record the hologram.
Fig. 8 shows the process of restoring the color microtext. It can be seen that it is possible to use color microtext in the form of a visual security element. To this end, the individual color components must be displaced relative to each other (due to aberrations of optical elements used in the scheme or deliberate displacement of color components when they are output to the indicator). When restoring this microtext, it is possible to use a color filter (e. g., GBG filter, as shown in Fig. 8).
The results of work on the creation of new optical security elements and security holograms with unusual color effects based on plasmon diffraction gratings and three-dimensional holograms in a thick-layer photosensitive recording medium are given in this article.
The results of OSE computer modeling, forming visually perceivable fading colors or color shades, based on two-dimensional PDGs and the results of experimental studies of such OSE are given. The variants of calculation and fabrication of nanostructures in the form of 2D PDGs for their integration as an optical security element into security holograms produced both by standard roll embossing technology and by the flat UV embossing technology are given. OSE experimental samples were fabricated based on 2D PDGs in transmitted and reflected light.
The experimental results on the creation of three-dimensional color security holograms are given based on color hologram stereograms obtained by the hologram modified scheme of Yu. N. Denisyuk wherein one-parallax and full-parallax colored hologram stereograms can be obtained. Such 3D-CSHs are created by methods of digital holography and computer synthesis of color volumetric images. Furthermore, 3D-CSHs form a single multicolor three-dimensional image of objects or two (or more) colored volumetric images with horizontal and vertical switching (flip-flop effect), both colored and imaging. In addition, 3D-CSHs form visual effects in the form of color microtext and colored hidden volumetric images. It is shown that the color microtext can be used as a security element, and the horizontal flip-flop effect is more pronounced than the vertical one.
The research was carried out and financed both by the instructions of JSC "Scientific and Production Association "Kripten" (Dubna, Moscow region) and within the framework of the agreement on granting the subsidy by the Ministry of Education and Science of the Russian Federation No. 14.577.21.0197.