1. Introduction

The QR code was invented by Masahiro Hara in 1994, which consisted of a white background and black module pattern. Due to the ability to store large amounts of data, the versatility for anything and everything and the convenience of a quick and error-free scanning process, the QR code has become one of the most-used two-dimensional code in applications in identity verification, currency, electronic payment and information tracking etc [1].

However, QR code security has been a hot debated issue across globe. Conventional printed QR code with poorly visual effect is so cheap to produce and so easy to be duplicated, making it a potential tool for cybercriminals. For example, over 23 per cent of Trojans and viruses are transmitted via QR codes [2]. Hence there are soaring developments of encrypting strategies for the purpose of QR code anticounterfeiting. One can resort to extra re-encrypting the QR code with additional encryption technique, but it requires customized decoding software [2–3]. Several of other studies focus on the adoption of organic and inorganic ink to generate luminescence for security printing. Traditional phosphors, such as Y₂O₃, YVO₄, and CePO₄ doped with red-emitting Eu³⁺ and green-emitting Tb³⁺ are used as down-conversion ink using ultra-violet (UV) light as excitation source [4–5]. Recently, combinations of Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ are doped in both green and blue upconverting inks to print covert QR code readable under infrared excitation. These QR codes are invisible under ambient lighting condition and an authenticated light source is needed to show the security feature [6]. Moreover, the QR code also lacks design elements with vividly appealing appearance [7–8].

Moiré magnifier based on the superposition of a microlens array (MLA) as the revealing layer and a micro pattern array (MPA) as the base layer can provide strikingly stereoscopic and kinetic effect [9]. Because of the engaging visual effect and extreme difficulty in forging, the moiré magnifier has been issued around the world as a premium security feature in several banknotes [10–15]. In order to meet the stringent demand on film thickness, the lattice constant of the MLA usually is less than 50 μm which means high patterning resolution (>8000 dot per inch, dpi) is necessarily required for the MPA. By combining the UV nanoimprinting lithography with the scraping printing technique, only a single color can be generated in the whole format and is not suitable for a variety of micropatterns with multi-scales [12–13]. The nanostructure is introduced to locally behave as bandpass filter in reflection or transmission, thus enabling the generation of the structural color. Chan et al. proposed to employ micro pattern consisting of high aspect-ratio nanopillar array for the purpose of spectral selectivity, but the two-photon polymerization lithography process is time-consuming at high cost [16]. Jiang et al. introduced plasmonic color printing targeting to achieve multiple colors under microlenses for motion effect, however, the low light efficiency results in low image contrast due to the lensing effect [17].

In this paper, we demonstrate the realization of a QR code employing a nearly perfect structural black color with 3D effect based on moiré imaging phenomenon. A design strategy capable of enabling containing more information in one unit and preventing optical signal crosstalk between neighboring elemental images is introduced. The value of the proposed floating QR code device needs more attention due to several facts: (i) Printed QR codes are typically used for authentication and tracking information, but not in security applications. By employing overt feature, we suggest that the QR codes have potential for application in unique and unclonable security printing with visual 3D appealing appearance. (ii) The working perspective is limited to near vertical view which can minimize an invasion of security without any loss in optical efficiency. (iii) The structural black color generated by the tapered dielectric nanostructure coated with chromium metal is able to absorb and trap light polarization-independently in broadband regime, which can offer high-resolution pattern and high contrast to MPA. (iv) The investigated QR code device that presents sharp and distinct image can be easily recognized by both human eyes and commercial smart devices with no need of using any extra special light source.

2. Results and discussion

2.1 Device configuration and illumination of the visual 3D effect

A generic illustration the 3D QR code is shown in Fig. 1. An MLA in hexagonal arrangement is disposed on top surface of the transparent biaxially oriented polypropylene (BOPP) film with a thickness of 50 µm. The lattice constant of the MLA is chosen as 50 µm. The MPA layer composed of the transformed QR code is situated at the focal plane of the microlens, whose focal length is equal to 60 µm and is exactly the same as the thickness of the BOPP film plus the residual layer of the UV resin. The QR code, which is generated through free QR code-generating sites or apps, consists of black squares arranged in a square grid on a transparent background. The dimension size of the QR code is defined as 30 mm × 30 mm with 25 × 25 modules following the QR standard ISO/IEC 18004:2015 [18]. Then, the QR code is encoded to the MPA for generation of moiré image when overlapped by the MLA shown in Fig. 1(b). The computational construction algorithm will be discussed in detail in Sec. 2.3. The tapered nanostructure covered by Cr is adopted for providing the black square grid composing the QR code with high image contrast due to its omnidirectional, broadband and polarization-independent absorption, as shown in Fig. 1(e)–1(f). Finally, the MLA and MPA layers are superposed to form the synthetically magnified moiré image of the QR code floating right above the surface. The floating QR code can be recognized by human eyes or commercially smart devices while the crosstalk between the neighboring elemental unit results in a blurred synthetic image at oblique viewing angle, as shown in Fig. 1(g). Therefore, the proposed floating QR code can reap benefits of concealing authorized information without using other light sources and while also maintaining the visibility of the QR code near vertical view.

 

Fig. 1. Concept of the 3D QR code based on structural black color. (a-f) Process flow of creating the device. a) MLA design. b) Microscopic picture of the fabricated MLA. c) QR code generation. d) Rendering the QR code according to the moiré principle. Scanning electronic microscopy (SEM) pictures of e) the nanostructure and f) MPA. g) After the superposition of the MLA and MPA layer, a synthetically magnified moiré image of the QR code with floating effect is achieved near vertical view.

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2.2 Realization of the patterned structural black color in the 3D QR code

Perfect absorber composed of a split ring resonator and a metal strip was firstly proposed by Landy, which could work in microwave frequency band [19], and since then, a variety of metamaterial perfect absorbers have been investigated for their potential applications in solar energy collection [20–21], biosensing [22], photoelectric detection [23], and absorption filtering [24], etc. The rapid development of the nanofabrication technology leads to the nanostructure-based artificial color that can offer fascinating optical performance. Among various optical absorbers, structural black color, characterized by high absorptive efficiency in the whole visible spectrum in wide angle range, has attracted increasing interest owing to its extensive applicability ranging from high-resolution and high-fidelity optical security devices to advanced cryptographic approaches and from surface decoration, digital display and molecular sensing to durable optical data storage [25–26]. The authors have proposed a tapered nanostructure-based double-sided, omnidirectional, and broadband absorber from the visible to near infrared spectrum [27].

Here, we present a facile fabrication process that allows us to pattern the structural black color in the MPA layer to provide high image contrast for the synthetically magnified moiré image, as shown in Fig. 2. Firstly, an aluminum anode oxide (AAO) template with high order pore array is fabricated by using a well-known two-step anodization process [28]. Then, a tapered nanostructure array is obtained by UV nanoimprinting lithography on transparent substrate, such as poly (ethylene terephthalate) etc. The width and height of the tapered nanostructure is 350 nm and 1.0 μm respectively, and the average distance between neighboring nanostructure is 400 nm. A layer of photoresist with a thickness of 2.0 μm is spin-coated after Cr deposition. The thickness of the Cr layer is 50 nm. Thirdly, the elemental image is patterned through using laser direct-writing (LDW) technique. After development and metal etching, the MPA composed of structural black color is achieved. Finally, superposing the MLA and MPA with high alignment accuracy can realize the 3D QR code device.

 

Fig. 2. Schematic of the steps involved in the fabrication of the MPA. a) The original AAO template. b) Nanostructure is fabricated by UV nanoimprinting lithography and c) a layer of metal Cr is then deposited. d) A layer of photoresist is spin-coated on the surface. e) Elemental images in the MPA are fabricated by using LDW technique. f) After development and Cr etching, the MPA composed of structural black color is achieved. g) A layer of MLA is superposed with the MPA layer to form the floating QR code moiré image.

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SEM pictures of the MPA microstructure after LDW process with gradually increasing magnification factor are shown in Fig. 3(a)–3(d). The patterned micro groove less than 2.0 μm width with steep edge can be conveniently realized by laser direct writing technique meanwhile the profile of the tapered nanostructure contributing to the structural black color is well preserved.

 

Fig. 3. SEM pictures of the patterned micro/nano-structure with gradually increasing microscopic magnification.

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The microlenses, owing to their micro-optic lensing effect, contribute to efficiently sampling the multiple elemental images. The patterned nanostructures, composed of dielectric nanostructure coated with a thin layer of metal, yield omnidirectional and polarization-independent absorption over the entire visible spectrum to generate black color. Full-field electromagnetic wave calculation is performed using Lumerical, a commercially available finite-difference time domain method (FDTD) simulation software package [29]. The shape profile of the tapered nanostructure is fitted with the equation (y/0.175μm)²+(y/1.0 μm)²+(z/0.175μm)²=1, as shown in the inset in Fig. 4(a). The Cr thickness is chosen as 50 nm and its refractive index (RI) is taken from Ref. [30], as shown in the inset in Fig. 4(b). The refractive index of the UV resin is set to 1.5. A converging wave at a wavelength of 550 nm is used as light source. The optical absorption A is calculated by 1-T-R, where T is transmission, R is reflection and both can be obtained directly from the simulation. It can be seen from Fig. 4(a)–4(b) that the absorption can be over 0.92 in the visible spectral regime between 400 nm and 800 nm in a broad-incident-angle range up to 70°, which is much greater than the maximum incident angle of the fabricated microlens with a numerical aperture of 0.45. The superior absorptive performance lies in the fact that the fundamental transverse magnetic (TM) guided-mode between two neighboring metallic sidewalls does not have a cut-off frequency, thus the electromagnetic wave can squeeze into the metal-dielectric-metal structure, instead of scattering off by the nanostructure, demonstrated in the distribution of the electric field, the time-averaged power loss density and Poynting vector shown in Fig. 4(c)–4(d) respectively. It is the non-resonant absorption that leads to the enhanced absorptive behavior [27,31]. Due to the symmetry of the tapered nanostructure and its arrangement, the absorption is independent of polarization.

 

Fig. 4. Calculated reflection/transmission/absorption (a) at normal incidence in the visible spectral region and (b) as a function of incident angle at a fixed wavelength of 550 nm. Electric field $|E |$ field distribution in a cross-section of the tapered nanostructure for (c) TM polarized wave. The amplitude of the time-averaged power loss density Q and the arrows represent the Poynting vector (d). The insets in (a) and (b) are the profile of the fabricated nanostructure and the refractive index of Cr.

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Figure 5 show the measured absorption(A)/reflection(R)/transmission(T) results. Optical transmission/reflection spectra were recorded using a Lambda 750 UV–VIS spectrometer (Perkin Elmer, USA) with an integrating sphere, then absorption is calculated as A=1-R-T. It can be seen that an average absorption of 0.92 in the spectrum range of 400 nm - 800 nm and the angle-dependent absorption spectra are tested. The absorption curves show a slight dispersion as a function of wavelength, but the structural black color is retained at the incident angle in the range of 0^o – 60^o. The difference from the simulation result is due to the imperfect replication and the Fresnel reflection from the covered UV resin. The fabrication process is simple and suitable for scale-up production. The large-format structural black color is shown in the inset in Fig. 5(a). The microscopic photograph of the encoded QR code in the MPA is shown in the inset in Fig. 5(b). The region composing of the nanostructure coated with Cr exhibits structural black color meanwhile other region without Cr is transparent. The patterned black block can be as small as $2.0{\; }\mu m \times 2.0{\; }\mu m$ (more than 12, 500 dpi), whose resolution is much higher than that of conventional printing (∼600 dpi).

 

Fig. 5. (a) Optical performance (measured absorption/reflection/transmission) at normal incidence. The inset shows the picture of the large-format sample. (b) Measured angle-dependent absorption spectra from 400 nm to 800 nm under incident angle of 8^o, 15°, 30^o, 45^o, 60^o. The inset shows the microscopic photograph of the encoded QR code in the MPA.

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2.3 Scheme of the QR code construction by using the moiré principle

The proposed floating QR code is difficult to be realized by classical moiré magnifier. The main reason lies in the limitation set by the MLA lattice imposing on the design of the elemental image. If the floating QR code with a size of 30 mm × 30 mm, the magnification factor M is calculated to be 600 because the length of the MLA lattice is 50 micron here, which puts extremely stringent requirements on both fabrication resolution and alignment accuracy. The magnification factor M has a function with the period ratio r between the MPA and MLA

To address the challenge, a scheme called virtual mask technique, which is capable of not only containing more information in one unit but also eliminating the crosstalk between the neighboring elemental images, is adopted to construct the QR code according to moiré principle, as shown in Fig. 6(a). The virtual mask denoted by the hexagon in orange color is exactly under every microlens, whose side length is 28.3 μm slightly less than that of the MLA. When r is chosen as 1.01, M equals to -100 where the minus sign “-” represents the anti-asymmetric relationship between the magnified QR code image and its corresponding elemental images in the MPA. Now the elemental image is 0.3 mm × 0.3 mm in size, which is much greater than the lattice length of the MLA, but is truncated by the virtual mask. Since the MPA lattice is slightly larger than that of the MLA, the homologous point in elemental images from center to periphery gradually shifts away from the optical axis of the corresponding microlens. After the pickup and reconstruction process, the elemental pattern can be well contained in the encoded MPA with no overlapping. The total thickness of the substrate plus the residual UV resin is about 60 μm. Then, the maximum viewing angle with no crosstalk is 25.3^o, which can be verified by the ray tracing simulation results shown in the spot diagrams in Fig. 6(b)–6(d). The light spot functioning as sampling point in the moiré image gradually moves from center to the edge in the hexagonal virtual mask. If continuing to increase the viewing angle, the light spot shifts out and into neighboring unit, resulting in the crosstalk in the moiré image, as shown in Fig. 6(e). By using the virtual mask technique, the elemental image is allowed to be much complex (such as the QR code) to contain more information without the necessity for ultra-high-resolution fabrication process and sophisticated operation.

 

Fig. 6. Ray tracing simulation result of the micro-optic lensing effect of a single microlens under viewing angle from 0° to 40°.

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The floating height h can be approximately calculated by

There is imperceptible but important difference between the moiré imaging and conventional integral imaging. The latter is based on the physical model of the floating image projecting through the MLA system. However, in moiré imaging phenomenon, the formation of the magnified floating QR code is through sampling the multiple images in the MPA which can offer much flexibility in the design strategy through mathematical operation.

2.4 Optical characterization of the proposed 3D QR code device

The proposed 3D QR code device is actually a refractive/diffractive hybrid imaging system. The lensing effect plays an important role in giving rise to the effect of parallax and contributing to sampling the multiple images. Figure 7(a) shows when there is no MLA superposition, only blurred QR code image with patterned structural black color is observed. After the superposition of the MLA, the captured multiple images are processed to retrieve the image of the target object and the 3D moiré image of the QR code can be visually observed under an incoherent white light source, as shown in Fig. 7(b). Using a QR code scanner, such as a smart phone, a clear QR code image is being on camera and a pop up will appear, as depicted in Visualization 1 and a single-frame except shown in Fig. 7(c). When the device is illuminated by external light source, a shadow of the QR code appears at bottom due to its real image in nature, as shown in Visualization 2 and a single-frame except shown in Fig. 7(d). What is even more interesting is that the position of the shadow follows the azimuth angle of the light source, making it look like a real QR code floating above the surface. The floating QR code can be scanned as quick as that with the printed one, as shown in Visualization 3 and a single-frame except shown in Fig. 7(e). The visual effect facilitates the creation of security feature for anti-counterfeiting and it cannot be duplicated by any kind of printer.

 

Fig. 7. Picture of the QR code (a) without and (b) with MLA. A single-frame except from (c) Visualization 1, (d) Visualization 2, and (e) Visualization 3.

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The QR code based on the moiré principle can offer not only strikingly vivid 3D effect, but also privacy protection without a reduction of the transparency in the normal direction. The schematic of the definition of the viewing angle (θ, φ) is shown in Fig. 8(a), measured as an average angle between the camera and the normal direction of the QR code film. Figure 8(b) shows the diagram of the floating QR code with a height of 6.0 mm in normal direction. The 3D QR code can work until the viewing angle increases to the critical angle (25.3^o), as demonstrated in Sec. 2.3. If the observer moves out of the field of view (FOV), the microlens may relay the elemental image of its adjacent unit, resulting in appearance of an unwanted flipped 3D QR code in the image space, as shown in Fig. 8(c)–8(d).

 

Fig. 8. a) Schematic of the definition of the viewing angle $({\theta ,\varphi } )$. b) Floating 3D QR code. c) and d) Photographs captured at different angles to demonstrate privacy protection illuminated by an incoherent white light.

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Another interesting example of the investigated floating QR code device can comprise of several floating heights, as illustrated in Fig. 9(a). Three positioning detection markers at the corner of the QR code are elaborately designed to have a floating height of 9.0 mm higher than the rest of 6.0 mm. The magnification factor M is proportional to the floating height h according to Eq. (2), thus M for the positioning detection marker and the rest is calculated to be 150 and 100 shown in Fig. 9(b). The microscopic picture of the local microstructure of the positioning detection marker is shown in the dashed box in Fig. 9(c). For comparison, the microscopic picture of the counterpart in the previous case with a magnification factor of 100 is shown in Fig. 9(d). As increasing the viewing angle, the shift distance of the positioning marker is larger than that of the information part, as shown in Fig. 9(e). When the viewing angle is as large as 20^o, the relative shift of the marker is about 1.03 mm which is beyond recognition ability of a mobile phone, thus capable of providing even better privacy protection.

 

Fig. 9. a) The 3D QR code device comprising of several floating heights. b) The magnification factor M is proportional to the floating height h. The inset shows the schematic of the relation between the displacement of the magnified 3D QR code image and the shift distance of the light spot under oblique angle. c) and d) Microscopic pictures of the local microstructure of positioning detection markers with the moiré magnification factor of 150 and 100 respectively. e) Photographs captured at different angle to demonstrate privacy protection illuminated by an incoherent white light.

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3. Conclusions

A novel floating QR code device based on structural black color with the characteristic of privacy protection is proposed and investigated. To our best knowledge, this is the first demonstration of implementing microlenses and nano-structural color for the realization of the 3D QR code coded by moiré principle. The microlenses contribute to efficiently sampling the multiple elemental images and the patterned nanostructures covered by metal show excellent absorption over the entire visible spectrum to generate structural black block. The resolution can be more than 12, 500 dpi which is almost impossible to be realized by traditional printing approach. In addition, by using a scheme of virtual mask technique, the elemental images in the MPA are allowed to be much complex to contain more information than in classical moiré imaging device. To fabricate the vivid floating QR code device, there is no necessity for ultra-high-resolution fabrication process and sophisticated operation. Furthermore, a potential advantage of the proposed floating QR code is its capability of providing privacy protection at oblique angle and immunity to duplication. It can be argued that the combination of physical solutions with digital solutions to the QR code will open up a potential avenue in anticounterfeiting and security-protecting applications in valuable documents, luxury products, currency, drugs, and certificates.