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Abstract
Optically Variable Devices (OVDs) are widely used as security features in anti-counterfeiting efforts. OVDs enable the display of color dynamic effects that are easily interpreted by the user. However, obtaining these elements over large areas poses certain challenges in terms of efficiency. The paper presents a modified approach for manufacturing plasmonic type OVDs through dot-matrix technology, which is a standard origination step of security holograms. By adjusting the spatial filters in the optical scheme, it is possible to double the resolution of the recorded quasi-sinusoidal diffraction gratings. The experiments confirm the creation of diffraction gratings with frequencies from 1600 to 3500 lines per mm, which facilitates the production of plasmonic zero-order spectral filters. The paper shows how the transmission characteristics of the studied elements are affected by the geometric parameters of the diffraction grating, silver layer thickness, angle of incidence, and polarization of light. The results have shown that using the proposed method it is possible to obtain 1D or 2D structural color OVD-image on a large area - several square centimeters and more. High speed recording of such elements is provided: the exposure time was from 120 to 400 ms depending on the grating resolution for a 0.05 mm2 frame, the total printing time for the size of the 25×25 mm2 OVD was about 2.5 hours for a 1D element, and less than 3.5 hours for a 2D element. Thus, the proposed method and the OVD elements produced by it can be useful to designers of optical security elements as a simpler and faster alternative to electron-beam lithographic technologies.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The process of creating color through the interaction of light and nanostructures has been extensively researched over the past two decades. Structural color design is a growing research field that aims to create spectral filters [1], sensors [2], high-resolution printing [3,4], and anti-counterfeiting elements [5,6]. One of the most well-known solutions in this area is plasmonic color printing with sub-diffraction resolution, which uses localized surface plasmon resonances [7,8]. Plasmonic structural colors are made using metal-dielectric diffraction gratings, thin films, nanoaperture multilayered arrays, and other methods. All-dielectric structures that use the resonant interaction of light with metasurfaces are also known to create spectral filters for various applications [9,10]. One of the main ideas in this field was to extract a narrow spectral line at the zero order of diffraction. This enables the replacement of traditional pixelated absorbing color filters in displays and light sensors [11]. Volume nanophotonic devices, such as those obtained using two-photon polymerization, have emerged as worthy competitors to plasmonic devices [12,13]. However, the majority of the proposed solutions, which rely on plasmonic or all-dielectric resonant structures, are smaller than 1 square millimeter in size and require recent advancements in nanoscale lithography and related technologies [14,15].
As documents security get more and more innovative fulfilment, structural colors have found wide application as Optically Variable Devices (OVDs) for product anti-counterfeiting [16,17]. Such application implies dynamically reconfigurable visual effects produced by large scale elements for the comfortable observation. The viewing conditions include rotating or tilting of the sample, and changing the polarization state of the light [18]. Recently, combined solutions using high-resolution surface structures with light-field techniques have gained popularity [19]. The introduction of surface structures into OVD, even for 3D images, allows embossing of the elements on flexible substrates [20,21]. Large scale OVD elements are being developed using both electron lithography [22,23] and interference lithography [24], as well as inkjet printing methods using refractive inks [25,26].
We propose a fabrication approach that enable large-area printing of plasmonic colors based on the resolution enhancement of dot-matrix laser interference patterning, which is conventional for the security holograms origination [27,28]. Experimental results on structural color printing confirm the possibility of creating cost-effective and time-efficient solutions due to the technology used.
2. Methods
In plasmonic OVDs, surface plasmons occurs by the diffraction gratings or subwavelength nanostructures resulting in spectral filtering for reflection or transmission. Back in 1941, investigating anomalies in diffraction gratings, Ugo Fano showed that if the period d and modulation amplitude of the sinusoidal surface topography are small compared to the light wavelength. As a result of interaction, it is possible to find such a value of the coefficient m, at which the k|| = ksp is fulfilled as Fig. 1(a) shows. The excitation of the surface plasmon becomes possible [29,30] and is expressed as follows
where k||0 — wave vector in space with permittivity εMe of the metal in a direction parallel to the surface, θ — angle of incidence, G = 2πm/d, m = 0,±1, ± 2.., c — speed of light in air.
Fig. 1. (a) Illustration of the plasmonic structure under research. Silver is used as metal in the following experiment. (b) Colormap derived from quasi-sinusoidal plasmonic silvered structures obtained in the experiment.
With the help of a periodic relief on a metal-dielectric surface it is possible to excite a surface plasmon by changing, for example, the angle of incidence so as to satisfy the wave number matching condition (Eq. (1)). The relief height as well as the metal thickness influence the performance and efficiency of structure color generation. Equation (1) and 2 describe the geometrical conditions for excitation of a surface plasmon using a diffraction grating. To predict the efficiency, more electromagnetic calculation methods are required. The efficiency depends not only on the geometrical parameters of the structure, but also on the materials used, and the illumination conditions. Referring back to OVD, one can visually observe the color change of the element when the illumination conditions change. The OVDs under study are zero-order elements and should be observed in the transmission light. It is possible to create such an element that changes color when rotated or tilted in linearly polarized light. Since polarizing filters are built into almost every smartphone display, polarized light from the screen can be used as a backlight source. When placing the element on the display and enabling white screen, color effects can be visualized.
OVDs described in this work are based on diffraction gratings with resolution from 1600 lines/mm (period of 555 nm) to 3500 lines/mm (period of 286 nm). Figure 1(b) shows a colormap collected from the experiment results described in this paper. The abscissa axis of the color map shows the frequency of the plasmonic diffraction grating, and the ordinate axis shows the angle of incidence. The illumination of the sample was performed in transmission at TM-polarization, and when observed in unpolarized light, the plasmonic filter selects exactly those colors appearing at TM-polarization. On the colormap, the most evident pastel colors have purple and blue shades, but ochre and green shades are also found. For OVDs we recommend to apply smaller periods to reduce influence of diffraction from different light sources in the room. For higher periodicities, white light diffraction shows less appearance when only illuminated and observed at almost sliding angles to the sample surface.
2.1 Modeling
Rigorous solutions in the framework of the electromagnetic theory of diffraction was used to predict the spectral-angular color behavior of a particular structure [31–33]. Far-field computations for that purpose was carried out with custom RCWA Matlab code. In specifying the shape of the surface relief, the sinusoidal profile was approximated by a stepped profile over 40 layers. Zero order transmission computations were done for varying wavelength (from 380 to 700 nm, in 4 nm steps) and incidence angles (from 0° to 60°, 5° step) and varying grating resolution from (1600 lines/mm to 3500 lines/mm, 50 lines/mm step). Chosen steps correspond with the following experimental spectrum measurements. As a dielectric material of the structure PMMA or resist S1813 is used. We analyzed a wide range of values of surface relief parameters: relief height was from 40 nm to 270 nm, silver layer was from 5 nm to 55 nm thick, diffraction grating frequency was from 1600 to 3500 lines/mm. In this section some results are described to explain the possibilities of color behavior of sinusoidal plasmonic structures. The parameters of silver thickness and profile height were chosen within the parameters achievable in the experiment. The relief height is determined as a full amplitude of the profile from the minimum to the top, as Fig.1a shows. A white LED lamp similar to the common spectrum of the smartphone white screens was used as illumination source in modeling and measurements (Fig. 2(a)).
Fig. 2. (a) Spectrum of the white light source used in simulation and experiment. (b)−(c) Modeling results as spectral-angle diagrams and corresponding colormaps for various diffraction grating frequencies ν, upon which plasmon color is dependent. Illumination in TM polarization is used.
Figure 2(b) − e shows an example of color behavior at TM-polarization for structures with the same height of relief profile a = 185 nm and silver thickness h = 30 nm, but for different grating resolutions from 2200 lines/mm to 3000 lines/mm for 200 lines/mm step. A spectral-angle diagram of the extraordinary transmittance (T0 order) is plotted for each case. A colormap of the color dependence on the incident angle is shown below each spectral-angle diagram. According to the calculation results, two main peaks in the transmittance in the blue and red regions are observed, which appear depending on the surface relief parameters. At higher grating frequencies (smaller periods), the blue shades appear brighter.
Figure 3 presents a comparable instance of color behavior with TE-polarization for plasmonic structures that possess identical surface reliefs as demonstrated in Fig. 2. At TM-polarization, a noticeable color change dependent upon the rotation angle is observed and the transmission spectrum varies based on the period. But at TE-polarization, only shades of ochre or blue colors are visible, and there are no significant changes in spectral characteristics. The spectral-angle transmittance characteristic at TE-polarized light is practically independent of the frequency of the plasmonic diffraction grating.
Fig. 3. Modeling results as spectral-angle diagrams and corresponding colormaps for various diffraction grating frequencies ν. Illumination in TE polarization is used.
Since the proposed recording method allows achieving deep structures with aspect ratio height to period from 0.5, plasmonic color generation is observed for relatively thicker silver layers above 30 nm. The simulated color characteristics for different values of relief profile height from 170 to 210 nm and silver thickness from 20 to 50 nm are shown in Fig. 4. Magenta, purple, violet, blue and green shades of colors are shown for different combinations of relief parameters.
Fig. 4. Example of simulation results illustrating the effect of varying silver thickness and sinusoidal profile height on the color behavior of the plasmonic structures with parameters: grating resolution ν = 2300 lines/mm (period d = 435 nm), silver thickness hAg = [20,30,40,50] nm, dielectric relief height a = [170,190,210] nm. The corresponding color map is shown below each spectral-angle diagram. Illumination in TM polarization is used.
The corresponding analysis of TE-polarization are not provided as the color behavior within simulated values is similar to that described in Fig. 3. Overall, the results of the modeling provide insight into the plasmonic colors that can be generated from deep sinusoidal diffraction gratings. The color always depends on a combination of the topography parameters: period, height, and thickness of the silver layer.
2.2 Fabrication
Figure 5 shows Dot-matrix (or image-matrix) interference recording setup [28,34]. This method for the interference-projection lithography is characterized by the use of a spatial light modulator to dynamically change the period of the gratings during the recording process. After filtering with a micro objective and a pinhole aperture, the light is expanded with the condenser lens. A spatial light modulator with a derived picture of the recorded frame is illuminated in a converging beam and filtered by a ring diaphragm.
Fig. 5. Optical scheme of the recording setup: QW — quarter waveplate, MO — micro objective, SLM — spatial light modulator.
A similar solution by an international group of researchers is based on a reflective 4f correlator scheme [35]. The main difference in the optical scheme is the use of a transmissive SLM, and the difference in the OVD implementation is in the use of silver metallization rather than aluminum. For fabrication, we used the KineMax setup for the Dot-matrix origination of rainbow security holograms. The transmissive SLM used is the Holoeye LC 2012 with a pixel size of 36 µm. The LC 2012 is a very basic SLM based on a translucent liquid crystal microdisplay with a resolution of 1024 × 768 pixel (XGA), 8 Bit (256 Grey Levels). The SLM displays a diffraction pattern generated by the Kinemax software [28,34].
A pattern transferred to the recording plane is output on the LC SLM with maximum contrast. Based on the optical scheme, in the back focal plane of the lens, the Fourier spectrum of this pattern is observed. The back focal plane of the lens is also the front focal plane of the micro objective, where spatial filtration is performed. A spatial filter in the form of a ring-type transmitting aperture passes only the first orders of the Fourier spectrum. In this case, spatial filtering of the axial maximum in the Fourier plane makes it possible to obtain gratings with periods 2 times smaller after the reproduction lens than would be obtained by projection without a spatial filter. Thereby, the gratings with the frequencies of 900 to 1600 mm-1 can be formed in the photoresist plane in the basic setup, and with frequencies of 1800 to 3800 mm-1 in the described modification.
We modified the spatial ring filters to achieve higher resolution of the recorded diffraction gratings without the use prism and immersion fluid. The setup includes diode laser (405 nm). The recording experiments were performed in Shipley microposit S1800 SERIES photoresist. Silver layers were applied with magnetron vacuum deposition unit. Recording of the diffraction grating is carried out within one frame 270 × 210 µm in size, then the sample is shifted to record the next frame. Substrate sizes are up to 127 × 127 mm.
In another interpretation, it can also be said that a holographic diffraction grating (or gratings, depending on the pattern displayed on the LC SLM) is registered in the plane of the photoresist. These gratings are the result of two beams interference if the displayed pattern contains a one-dimensional grating, or the result of interference of several beams if the pattern contains two-dimensional periodicity. The beams are generated by the micro objective from point sources located in its front focal plane. These point sources are the maxima of the Fourier spectrum passed by the spatial filter. The recorded diffraction gratings have a quasi-sinusoidal profile because they are obtained by the optical interference.
2.3 Measurements
The angle-dependent measurements were done with a Jeti Spectraval 1501 spectroradiometer in TM-polarized light corresponding to the diffraction grating. Zero order transmission was directly measured with a linear polarizer between and the light source with the spectrum. The sample was fixed onto a manual rotation stage and measured every 5° for tilt angle. The size of the illumination spot was about Ø5 mm, the measured samples were about 1 × 1 cm for every periodicity. The measured transmission values were transferred into chromatic XY coordinates and RGB values with a custom Matlab script.
An atomic force microscope Solver H47 Pro NT-MDT Spectrum Instruments was used to measure the height and geometry of the relief profile. Optical microscope Zeiss Axio Imager Z2 Vario was used for the period measurements of the diffraction grating, as well as to estimate the quality of exposure uniformity within the frame.
3. Results and discussion
Since a complex diffraction pattern is formed in the Fourier plane of the lens after the light passes through the SLM, it is necessary to use several ring filters with limited capabilities in terms of periods and orientations to ensure sufficient contrast in the recording plane.
The size of the active area of SLM is 36.9 × 27.6 mm (1.8” Diagonal), and the recording frame is respectively 270 × 210 µm. Thus, in the implemented scheme, the frame is transferred with a reduction of the of about 136.5 times. But the diffraction gratings that fill the frame are on a different scale. The maximum grating frequency that the setup allows us to record is 3800 lines per mm, which corresponds to alternating between 1 SLM pixel with minimum phase, and 1 SLM pixel with maximum phase. The pixel size is 36 µm, so the minimum average grating period that the SLM allows to output is 72 µm. The described optical interference-projection scheme allows to transfer such a grating with a reduction of about 273 times. In our case, the resolution of the photoresist used (Shipley microposit S1800 series photoresist) was insufficient for the experiment to record gratings of the highest frequency. Therefore, the resolution limit confirmed in this work is 3500 mm-1 (grating period 286 nm).
The geometry of the generated patterns was tested (Fig. 6) to select optimal spatial filtering conditions. Based on the results, 4 filters were fabricated to record diffraction gratings with the following parameters: 1600–2100 lines/mm (№1), 2100–2600 lines/mm (№2), 2500–3000 lines/mm (№3), 3000–3500 lines/mm (№4). Consequently, when forming a visual image for a single OVD element, it is convenient to use a set of diffraction gratings, which can be obtained without changing the ring filter.
Fig. 6. Patterns in the filtering plane after the SLM (corresponding with Fourier plane of the lens) for recorded diffraction gratings with periodicities: (a) 2000 lines/mm, (b) 2500 lines/mm, (c) 3300 lines/mm. Green shows the area to be selected from the overall pattern in the plane of the ring filter.
For each filter, experiments were conducted to record test palettes of 1D diffraction gratings in 50 lines/mm increments. The size of each diffraction grating was 10 × 10 mm. Profile height varying from 60 nm to 250 nm, and defines with exposure time from 120 to 230 millisecond during the recording. Three lines with square-shaped diffraction gratings with silver metallization of the different thicknesses from 15 to 50 nm, and one control line to be metallized, were implemented on each palette.
3.1 Test palettes of plasmonic colors
Figure 7(a)–(e) shows visual color sets provided by plasmonic quasi-sinusoidal 1D structures with periodicities 1850 to 2000 lines/ mm and step of 50 lines/mm for the different incident illumination angles.
Fig. 7. (a)–(h) Color palette sample №1 presented with different gratings of periodicity from 1850 to 2000 lines/mm, photos are in linearly polarized light for different incident angles θ: (a)–(e) TM-polarization, (f)–(h) TE-polarization. (i) Calculated spectral-angular and color diagrams for the 1850 lines/mm plasmon diffraction grating at TM-polarization, (j) Experimental spectral-angular and color diagrams taken at TM-polarization for the 1850 lines/mm plasmonic diffraction grating in the 2ndrow, magenta circled in (a)–(e). (k) Chromatic diagram with color coordinates shown for the calculation and experiment with an incidence angle step of 5°.
The palettes in Fig. 7(a)–(e) show color changes appearing with inclination of the testing sample in TM-polarized light. The palettes in Fig. 7(f)–(h) show color changes of the same sample appearing in TE-polarized light. The exact color and its modulation depend on the exact profile characteristics: period, profile height and Ag layer thickness. The exposure time was 150 ms for every frame. In the photos, first row corresponds to silver deposition 18 ± 2 nm, second row — 40 ± 5 nm; third row 50 ± 5 nm.
Figure 7(i) and Fig. 7(j) compare the spectral-angular characteristics and those obtained by calculation and experiment for the plasmonic diffraction grating with 1850 lines/mm periodicity (period 541 nm). In Fig. 7(a)–(e), the colors that were subject to angle-dependent spectral measurements are circled in magenta. For these colors the spectral-angle diagram in Fig. 7(j) is plotted. The chromatic diagram in Fig. 7 k illustrates the color coordinates corresponding to the color change of the OVD element for the cases of the calculation and the experiment.
Nanometer accuracy for the profile height and silver thickness cannot be achieved using the dot-matrix experiment in conventional security hologram production. The color mismatch between the experiment and the model was due to the spectral-angle diagram's sensitivity to geometric tolerances of the plasmonic gratings. Although the color coordinates may vary based on the silver thickness and profile height, the color does not disappear completely. There is a correlation between the model colors and the colors observed in real-life (Fig. 7 k).
Despite a noticeable mismatch in the spectral-angular behavior, the maxima of the angular transmittance spectra are consistent. We justify the differences by the following reasons. First, the shape of the topography does not truly correspond to the calculated one, but is quasi-sinusoidal and varies over the area of the sample. Figure 8 shows an example of measurements for a diffraction grating with a frequency of 1950 lines/mm. In the top view (Fig. 8(a),(b)), in addition to the useful modulation of the profile, there is an overlap of the carrier frequency formed due to insufficient order selection on the ring filter. Atomic force microscope measurements over the sample area of eight points indicate that the elevation of the relief profile can vary over the area from 170 nm to 230 nm, as illustrated by the profilograms in Fig. 8(d)–(f). Second, there is a change in the baseline of the surface relief, which is substantiated by the non-uniformity of the residual layer during the the exposure and development of the resist. Consequently, the subsequent silver sputtering also retains a high degree of non-uniformity.
Fig. 8. Measurement example for the plasmonic grating with 1950 lines/mm: (a) frame view from the optical microscope, (b) top view of the grating from the optical microscope, (с) 3D surface relief topography measured by probe-scanning method on an atomic force microscope, (d)–(f) probe-scanning profilogams obtained over the OVD area by atomic force microscopy.
Third, the simulation (Fig. 7(f)) was performed for a uniform relief profile height of 185 nm, and a uniform silver layer of 28 nm. The spectral-angle transmission diagram in the simulation changes when the sinusoidal relief profile height and silver thickness are changed by single nanometer units. Thus, the final color in practice is an additive summation of spectra for the plasmonic structures with different profile height parameters.
The model and experiment inconsistency are caused by manufacturing tolerances and include: error of the mean value of the profile height, variability of the profile height value over the area of the element, error of the mean value of the silver deposition thickness, variability of silver deposition thickness value.
Similar to the sample №1, other spatial ring filters were tested to record diffraction gratings of a higher resolution. Figure 9 shows a color palette №2 provided by plasmonic quasi-sinusoidal 1D structures with periodicities from 2050 to 2500 lines/mm with the step of 50 lines/mm (left to right). The exposure time was 150 ms for every frame. In the photos, the first row corresponds to silver deposition of 18 ± 2 nm, the second row — 40 ± 5 nm; the third row — 50 ± 5 nm. For the outer regions the colors are different from the general series. This is justified by the fact that for the grating with edge periodicities (2050 and 2500 lines/mm), the light energy during the recording is less due to selection on the ring filter.
Fig. 9. Color palette sample №2 presented with different gratings from 2050 to 2500 lines/mm. The photos are in TM-polarized light for the different incident θ and inclination φ angles.
Figure 10(a)–(e) show a color palette №3 provided by plasmonic quasi-sinusoidal 1D structures with periodicities from 2500 to 2950 lines/mm with the step of 50 lines/mm. The exposure time was varied from 180 to 360 ms for every frame. Figure 10(f)–(j) show a color palette №4 provided by plasmonic quasi-sinusoidal 1D structures with periodicities from 3000 to 3500 lines/mm with the step of 50 lines/mm (left to right). The exposure time was 360 ms for every frame. For all the photos at Fig. 10, the first row corresponds to uncovered dielectric resist structure for comparison, the second row — to 18 ± 2 nm silver deposition, the third row — 40 ± 5 nm; the fourth row 50 ± 5 nm.
Fig. 10. (a)–(e) Color palette sample №3 presented with different gratings of periodicity from 2500 to 2950 lines/mm. (f)–(j) Color palette sample №4 presented with different gratings of periodicity from 3000 to 3500 lines/mm. All the photos are taken in linearly polarized light for the different incident angles θ (see Visualization 1).
It should be noted that problems with recording and development can be anticipated due to the following reasons.
- 1) Photoresist photosensitive response is different for different periodicities, i.e. to ensure the exact same profile height for each periodicity requires pretested different exposure.
- 2) There is an instability of the development process. To ensure a deeper profile, a rather concentrated developer and a short developer time (7 seconds) are used. Nevertheless, the development was performed manually, which does not allow us to avoid errors in the development time and some uniformity of the sample immersion.
3.2 OVD with the image «bird»
For the OVD samples used in practice, the observation conditions are important. Since in reality, there will be many light sources around the observed label. A first-order diffraction can complicate a zero-order observation. Hence, for the application of the plasmonic spectral filters with the diffraction gratings, we recommend to use designs with the lowest possible periods (highest periodicities).
Figure 11 shows a sample with an image «1D Bird», which contains eight zones with the gratings of different periodicities from 3000 to 3500 lines/mm and two perpendicular grating orientations (vertical or horizontal) so that some parts of the image are appearing in TM-polarized light, and some in TE-polarized light. The OVD size is 25 × 25 mm.
Fig. 11. «1D Bird» OVD photographed in transmission at different illumination angles for two perpendicular placements in TM-polarization (see Visualization 2 and Visualization 3).
Figure 12 shows a sample with an image «2D Bird», which contains eight zones with the gratings of different two-dimensional periodicities from 3000 to 3500 lines/mm. The OVD size is 25 × 25 mm as for 1D sample (Fig. 11). The gratings are the same so there is much less difference in the appearance for two perpendicular TM- and TE-polarized light condition. Nevertheless, at angles of incidence greater than 20 degrees, the color differences become noticeable. This is explained by the fact that after pinhole filtering the shape of the laser beam has some ellipticity (see Fig. 7). This ellipticity introduces asymmetry in the contrast of interference fringes in perpendicular directions, and after photoresist development the profile has a deeper relief in one of the periodicity directions.
Fig. 12. «2D Bird» OVD photographed in transmission at different illumination angles for two perpendicular polarization states (see Visualization 4 and Visualization 5).
The «1D Bird» and «2D Bird» was fabricated in a single step. If the OVD image necessitates a wide range of frequencies from 1600 to 3200 lines/mm, it is required to record the sample in multiple steps. However, the provided research did not include experiments conducted in multiple steps.
The input frame rate is 60 Hz, so there are two factors influencing the recording speed: exposition time and the sample displacement time from frame to frame. The recording time for samples «Bird» with the area of 25 × 25 mm was:
• 1D sample — 2 hours 28 minutes 45 seconds, with a frame exposure time of 300 ms,
• 2D sample — 3 hours 20 minutes 43 seconds, with a frame exposure time of 200 ms.
The recording speed is restricted by the photoresist sensitivity and movement system. The described experiment involved a stepper displacement system which enabled movement between frames in 300 ms. After each step, the table must remain stationary for approximately 200 ms to eliminate mechanical vibrations prior to exposure. Frame exposure times ranged from 100 ms to 350 ms, depending on the desired surface depth and grating frequency. The displacement system can be improved to expedite the recording process or enhance frame stitching accuracy. Significantly, an extensive duration is invested in exposure periods of time. As a solution to this issue, a high-power laser or a more photosensitive high-resolution photoresist can be employed to accelerate the speed. The SLM frequency plays the least significant role in the rate of recording, whereas an image update takes roughly 17 ms.
The palettes presented have poor coverage in the red region of the spectrum. For example, on the sample with a bird, one can notice the red color not under normal illumination, but only under oblique illumination. In practice, red is easier to realize based on diffraction gratings with large periods and a binary rather than quasi-sinusoidal relief profile. Nevertheless, red filter creation has become widespread, as described by baseline work on plasmonic RGB colors and holography [36–38].
We suppose the visual characteristics of dot-matrix samples described in the paper are compatible to ones obtained with electron-beam lithographic samples or immersion interference lithography [5,24].
4. Conclusions
This work demonstrates how to modificate a dot-matrix holographic printer to record holograms with twice the resolution. An important result is the time efficiency of the patterning method. It takes about 20 seconds per 1 mm2 with realizable large-scale images. We experimentally confirmed the approach feasibility for the production of zero-order spectral filters based on plasmonic diffraction gratings with periods from 285 nm to 625 nm.
The modification refers to increasing the resolution of the recorded diffraction gratings by more than two times, which makes it possible to use the conventional system for rainbow holograms origination to create plasmonic nanograting of the same area. High-speed printing is in general a property of inference lithographs that record a whole frame at once. In the described optical scheme, the frame is formed using SLM.
The research shows reliability of the dot-matrix method for the origination of the plasmonic OVDs. It enables manufacturing OVDs that may be embedded into the documents as window elements that change color when tilted or switch color when rotated for 90° in linear polarized light. OVD may have design or present a solid color label, work in polarized or unpolarized light. Nevertheless, the experiments described revealed that diffraction grating manufacturing errors, which are common in security holography, have a significant impact on producing plasmonic colors based on the same technology.
Proposed method helps to perform large-scale optical security elements with the design based on 1D as well as 2D gratings of arbitrary orientation. The described technological process contains standard steps worked out in security holograms production. Nevertheless, due to the variation of the relief profile height, difficulties may arise during replication by roll-to-roll or planar nanoimprint lithography. Further research will focus specifically on improving the stability of obtaining accurate reproducible relief and the industrial replication.
This approach can be extended to other devices based on the excitation of surface plasmons by diffraction gratings, as well as to applications requiring broadband variable angular-spectral filtering.
Funding
Bauman Moscow State Technical University (Priority 2030).
Acknowledgments
The authors thank Alexander Zherdev for many years of supporting prior research, Ekaterina Drozdova and Ivan Tsyganov for assisting with experiments. In publishing this paper, we would like to acknowledge Sergey Odinokov, who has unfortunately passed away but made significant contributions to this work as its initiator. Additionally, we express gratitude to JSC «RPC «Krypten» for supporting the laboratory's research for many years.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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