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Abstract
Plasmonic metallic nanostructures with anisotropic design have unusual polarization-selective characteristic which can be utilized to build nanopolarizers at the nanoscale. Herein, we propose a dual-color image display platform by reconfiguring two types of silver nanoblocks in a single-celled metasurface. Governed by Malus’s law, the two types of silver nanoblocks both acting as nanopolarizers with different orientations can continuously modulate the intensity of incident linearly polarized red and green light pixel-by-pixel, respectively. As a result, an ultra-compact, high-resolution, and continuous-greyscale dual-color image can be recorded right at the surface of the meta-device. We demonstrate the dual-color Malus metasurface by successfully encoding and decoding a red-green continuously-grayscale image into a metasurface sample. The experimentally captured meta-image with high-fidelity and resolution as high as 63500 dots per inch (dpi) has verified our proposal. With the advantages such as continuous grayscale modulation, ultrathin, high stability and high density, the proposed dual-color encoded metasurfaces can be readily used in ultra-compact image displays, high-end anti-counterfeiting, high-density optical information storage and information encryption, etc.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurfaces are artificial two-dimensional materials composed of a series of sub-wavelength unit structures. By changing the material, shape, size, and other parameters of the unit-cell structure, metasurfaces have shown many extraordinary physical characteristics which can be used to achieve phase modulation [1–11], amplitude modulation [12–16], polarization control [17–20], and wavelength selection [21–23]. In recent years, metasurfaces have been widely employed to display holographic images [24–30] and nano-printing images [31–39] at the nanometer scale. Among them, the polarization-controlled metasurfaces based on Malus’s law can continuously modulate the intensity of incident linearly polarized light pixel-by-pixel, which has been applied to display meta-images with ultra-compactness and ultra-high resolution [40–42]. In 2018, a dielectric metasurface for image display was designed, which successfully achieved simultaneous control of light intensity and polarization, and realized color image display [43]. However, the nanoblocks with ultrahigh aspect ratio (∼ 3 for red light and ∼ 4 for green light) would burden the metasurfaces fabrication process for practical purposes. In the same year, a metasurface based on a metal-dielectric-metal (MIM) structure was proposed for image display [44]. As a result, grayscale image information is encoded into the spatial distribution of polarization states and decoded into a light intensity distribution after inserting a bulk analyser after the metasurface. Although the MIM-typed metasurfaces can be utilized to realize high-performance image display, the broadband response of their nanostructures would limit their display application to be only monochromatic display rather than color display.
In this paper, we use single-layered metallic structure to replace the MIM structure, propose a new metasurface composed of silver nanoblock arrays for color image display. The two types of silver nanoblocks, with narrowband responses of incident red and green light respectively, both act as high-performance nanopolarizers rather than half-wave plates like MIM structure. As a result, a color image can be encoded into the orientations of the two types of nanoblocks corresponding to red or green light respectively, both with continuous grayscale modulation. With both single-cell and single-layer design, the proposed metasurface for image display hold the advantages of both high-resolution and ultra-simple structures, and it can find its markets in ultra-compact image display, high-end anti-counterfeiting, high-density optical information storage, information encryption, and many related fields.
2. Design of two types of silver nanoblocks for narrowband responses
The plasmonic metallic nanostructure with anisotropic design can show different response characteristics to the linearly polarized (LP) incident light in two cross-polarized directions, which gives it the potential to be designed as a nanopolarizer. In this paper, an array of silver nanoblocks with length of long axis L, width of short axis W, height H, and orientation angle α is reconfigured on a silica substrate to design a nanopolarizer array. The schematic diagram of unit-cell structure of nanoblock arrays is shown in Fig. 1(a), and the period of unit-cell structure is CS. Numerical simulations of the nanoblocks arrays with α=0° by using electromagnetic simulation software CST Studio Suite are conducted, and the direction of the incident light is set as normal incident. By adjusting the geometric parameters of the nanoblocks, the reflectivity Rx and Ry of incident light whose electric field is oriented along the long or short axis of the nanoblock respectively can be different.
Fig. 1. Illustration and simulated results of the plasmonic metallic metasurface based on silver nanostructures. (a) Schematic diagram of the unit structure based on silver nanostructures for polarization separation. The orientation angle $\alpha $ is the acute angle between the long axis of the nanoblock and the x-axis. (b) Reflectivity of incident LP light along the long axis (Rx) and along the short axis (Ry) under different periods CS. (c) Reflectivity Rx of incident LP light along the long axis under different lengths L of long axis. (d) Reflectivity Ry and transmittivity Ty of incident LP light along the short axis under different lengths L of long axis. The orientation angle $\alpha $ of the nanoblocks in (b-d) is 0°, and the direction of the incident light is set as normal incident.
As shown in the red curve of Fig. 1(b), the geometric parameters of the nanoblock are L = 100 nm, W = 70 nm, H = 70 nm, CS = 300 nm, and the orientation angle α is 0°. We can find that the reflectivity Rx has a maximum value of 78% when the wavelength of the incident LP light is 542 nm, while the reflectivity Ry of the LP light along the short axis is only 4.1%, which mean the most of x-axis polarized light is reflected while only a small part of y-axis polarized light is reflected. In other words, the silver nanoblock can act as a nanopolarizer. And in the visible light range, it is shown that the full width at half maximum (FWHM) of reflectivity Rx curve is relatively large, which signifies that silver nanopolarizers have broadband response to the LP incident light. However, the broadband response is not conductive to design dual-color meta-image display metasurface. By changing the period CS of unit-cell structure, as shown in the red, blue, and green curves of Fig. 1(b), we can find that with the value of CS is increasing, the response wavelength with the highest reflectivity Rx has a red shift. More importantly, the FWHM of the reflectivity curve gradually decreases. Moreover, the reflectivity Ry remains still small when the incident wavelength is greater than 525 nm. In addition, as shown in the blue and green curves, their maximum reflectivity is 76.6% and 74.6%, respectively. Although the reflectivity has been reduced, these changes are relatively small. However, as shown in the purple curve of Fig. 1(b), when the period CS is 420 nm, the maximum reflectivity along the long axis of the nanoblock is only 36.3%. This should be due to the reason that a part of the incident light is reflected by other diffraction orders.
In order to achieve dual-color display metasurface, it is necessary to design nanopolarizers with narrowband response. Here, we choose 400 nm as the design period while other parameters keep unchanged, and only change the length of the long axis of the nanoblock. As shown in the green, blue, and red curves in Fig. 1(c), we can find that with value of L is increasing, the response wavelength with the highest reflectivity has a red shift, unfortunately the FWHM of the reflectivity curve is increasing. And when the value of L is less than 100 nm, the spacing between the nanoblock becomes too larger, which leads to the result that the efficiency of the excitation of the localized Surface Plasmon Resonance (LSPR) is no longer efficient given the dimensions of the nanoblock and the material employed. Therefore, the reflectivity is decreased. In addition, as shown in Fig. 1(d), when value of L is changed, the reflectivity Ry and transmittivity Ty of LP light along the short axis of nanoblock can hardly change. And when the wavelength is less than 585 nm, the metal has a strong absorption of transmitted light.
Here, we choose two types of nanoblocks which have relatively narrowband responses to design the dual-color display metasurface. As shown in the green and red curves of Fig. 1(c), we can find that at a wavelength of 585 nm, the green curve reaches its maximum value of 74.6%, while the red curve is only 17.2%. At a wavelength of 650 nm, the red curve has a maximum of 86.9%, while the green curve has only 3.5%. The reflectivity Rx of the two types of nanopolarizers to LP incident light reaches the highest and relatively low values at the two designed wavelengths of 585 nm and 650 nm, respectively. This means that the two types of nanoblocks just have relatively few interfere with each other. Two types of nanopolarizers have the same height H = 70 nm, short axis width W = 70 nm, and period CS = 400 nm, but different long axis length of L1 = 100 nm and L2 = 135 nm, respectively. They are used as the basic unit-cells to design an ultracompact metasurface for dual-color meta-image display.
3. Design of meta-image display
The above two types of nanoblocks can act as nanopolarizers with the polarization direction along the long axis at the design wavelengths of 585 nm and 650 nm, respectively. According to the Malus’s law, when a LP light beam polarized along the x-axis is incident on the nanopolarizer, the Jones matrix of the reflected light beam can be expressed as:
In fact, since the reflectivity Rx of the nanopolarizer is not 100%, the actual reflected light intensity should be expressed as:
When the orientation angle α of the nanopolarizer is 0, the intensity of reflected light reaches the maximum value Imax=RxIin. According to Eq. (3), we can find that the intensity of the reflected light can be changed arbitrarily and continuously only by changing the orientation angle α of the nanopolarizer.
As shown in Fig. 2(a), a red and green dual-color image with 360 × 360 pixels and 256 continuous gray levels is selected as the original design. The gray value of each pixel in this image can be used as the relative light intensity, according to Eq. (3), pixels of red and green parts of this image can be decoded into the corresponding orientation angles of two types of nanoblocks, as shown in Fig. 2(b). When light with the polarization direction along the x-axis is incident, the designed meta-image can be observed at the surface of the metasurface. An area with 90 × 90 pixels as shown in Fig. 2(c) was selected from Fig. 1(a), we can decode it into the corresponding orientation angles of the nanoblocks, and then the normalized electric field distribution of the nanobricks are simulated in the electromagnetic simulation software FDTD Solutions. When the wavelength of the incident LP light is 585 nm and 650 nm, the simulation results are shown in Figs. 2(d) and 2(e). We can find that when the incident wavelength is 585 nm, only the green leaf part will respond, as shown in Fig. 2(d). Similarly, when the incident light wavelength is 650 nm, only the red flower part will respond, which is consistent with our design.
Fig. 2. (a) Original design with 360 × 360 pixels and 256 continuous gray levels. (b) Schematic diagram of the dual-color meta-image display. (c) A zoom-in-view area with 90 × 90 pixels shown in (a). (d,e) Normalized electric field diagram at the wavelength of 585 nm and 650 nm, respectively.
As shown in Figs. 3(a)–3(f), when the wavelength λ increases, the reflect ability of the green corresponding nanoblock to incident light decreases, and the light intensity of the corresponding area also decreases, while the red nanoblock gradually increases their reflect ability to the incident light. The change of electric field distribution in the x-y plane above the metasurface is consistent with the green and red reflectivity curve shown in Fig. 1(c). when white light with polarization direction along the x-axis illuminates the metasurface, the actual light field will be the superposition of the fields at different wavelengths.
Fig. 3. (a-f) Normalized electric field diagrams at a wavelength of (a) 585 nm, (b) 605 nm, (c) 615 nm, (d) 625 nm, (e) 635 nm, and (f) 650 nm.
4. Experiments
Here, the silver nanoblock array composed of two types of nanopolarizers is reconfigured on a silica substrate to further verify the feasibility of the designed dual-color display metasurface. The geometric parameters of two types silver nanoblocks are designed with L1 = 100 nm, W = 70 nm, H = 70 nm and L2 = 135 nm, W = 70 nm, H = 70 nm, respectively. And the period of unit-cell structure in the x-axis and y-axis direction is 400 nm. Each pixel of the target image corresponds to a unit-cell structure, and the gray value of the pixel can be decoded into the orientation angle of the corresponding nanopolarizer by Eq. (3). We used standard electron beam lithography (EBL) to fabricate the nanoblock array based metasurface samples, and the size of the sample is 144 × 144 μm2. The partial scanning electron microscopy (SEM) image is shown in Fig. 4(a). As shown in Fig. 4(b), when a white light beam passes through the polarizer whose polarization direction is along the x-axis, and then normally illuminates the designed dual-color display metasurface, due to the light intensity modulation of each unit-cell structure to the LP light, an experimentally captured meta-image as shown in Fig. 4(c) will be displayed on the reflection direction. In our experiment, an optical bright-field microscope (Motic BA310Met) was used for observing the meta-image, and a CMOS camera was employed to capture the experimental meta-image. By adjusting the intensity of the illumination source and the exposure settings of the CMOS camera software, the white point in the field of view was calibrated, and a clear color meta-image was finally captured. Additionally, the objective with high magnification brings high resolution and faded colors simultaneously, therefore, we chose a 50× objective to obtain a vivid color image with relatively high magnification. It can be found that the details of the experimentally captured meta-image are clearly reproduced. The detailed outlines of the red flowers and green leaves shown in the photo are very clear, indicating that the light intensity modulation of the incident LP light by the two narrowband nano-polarizations is not confused, which is very consistent with the simulation results. The experimental results fully confirmed the feasibility of the dual-color image display metasurface design method based on silver nanoblocks, which is helpful to increase the metasurface information capacity. In fact, we redistribute the orientation angle of the nanopolarizers under the condition of monochromatic light, and assume that the light of each wavelength has the same light field intensity. However, in the experimental environment, the light of different wavelengths in the spectral distribution of the incident light field has different hue, saturation, and brightness, which led to a certain difference between the meta-image we observed and the original design. This difference can be reduced by considering the spectral data related to the color of the incident light in the design of metasurface.
Fig. 4. (a) The SEM image in partial view of the metasurface sample. (b) Schematic diagram of the meta-image display based on the Malus’s law. (c)The high-resolution and high-contrast meta-image captured with an optical microscope.
5. Conclusions
In summary, we propose a dual-color meta-image storage and display method based on the silver nanoblock metasurface. Due to the resonance phenomenon inside the silver nanoblock, each nanoblock can act as a nanopolarizer which reflects most of the LP light along its long axis. That is, we can achieve light intensity control only by adjusting the orientation angle of the nanoblocks. When the metasurface is illuminated with a LP incident light beam, the target image is clearly reproduced. Our proposed image display unit can display two colors at the same time, and has the advantages of continuous gray scale modulation, high stability, and high density. This method possesses a great potential in high-density optical information storage, high-end anti-counterfeiting, information encryption and so on.
Funding
National Key Research and Development Program of China (2019YFB2203904); National Natural Science Foundation of China (62075047, 61965006, 61975038, 61964005, 62065006); Natural Science Foundation of Guangxi Province (2020GXNSFDA297019, 2020GXNSFAA238040, 2021GXNSFAA075012, 2019GXNSFAA245024, 2020GXNSFBA159059, 2018GXNSFAA281272); Science and Technology Major Project of Guangxi (AD19245064); Guangxi Key Laboratory of Automatic Detection Technology and Instrument Foundation (YQ20107, YQ19108); the Innovation Project of GUET Graduate Education (2020YCXS089).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available within this article.
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