1. Introduction

In recent years, with the miniaturization and portable development of COMS image sensor (CIS) technology, reducing the pixel size and improving the spatial resolution of digital imaging have become the main research trends [1–6]. In the conventional color digital imaging, the red, green, and blue (RGB) filter is composed of a dye-doped polymer. Since the absorption coefficient of the organic dye is small, the optical crosstalk between pixels becomes remarkable when the pixel size goes small. In addition, dye-doped polymers are highly sensitive to ultraviolet radiation and high temperature environments. In order to solve the problem of device performance degradation caused by conventional polymer materials, plasmonic metasurfaces consisting of the metal and dielectric layers with the subwavelength periodic structures have been demonstrated on laser and holograms [7–12]. The plasmonic filters are adjustable in the visible spectrum range and well compatible with the CMOS technology [13–17]. However, the efficiency is usually limited due to the high loss of metal, especially at the visible wavelength range. All-dielectric materials are considered as the promising alternatives to metal nanostructures due to the significant advantages such as high-quality resonance effects and low losses of ohmic effects, which is significantly less than the one of metals [18–29]. Furthermore, the magnetic and electrical components of light can be manipulated with the high refractive index contrast of medium.

Despite the rapid development of structural color filters, most nanostructures are static once manufactured with a given size, severely limiting the potential for practical applications. In recent years, flexible devices have been widely investigated due to the high utilization rate, flexible operation and strong adaptability [30–35]. Therefore, the realization of flexible filters for various continuous color generations has attracted widespread attention due to their application in various fields, such as ultra-fast flexible color displays and dynamic holographic imaging [36–45]. Polarization, one of the important parameters of lightwave, can be used to control the response of metasurfaces [46–52]. In this paper, we demonstrate a polarization tunable color filter consisting of asymmetric nanoblocks patterned on a flexible substrate. The nanoblock arrays consist of monocrystalline silicon and exhibit Mie-type electric and magnetic resonances. The color filter can generate a subwavelength resolution in the visible wavelength region and exhibit various bright transmission colors by tuning the physical dimensions of each nanoblock. Since the designed nanoblocks have polarization-dependent properties, the transmission wavelength will shift by tuning the polarization angle. Therefore, we can encode different color information in the same area. Significantly, the flexible color filters are scratchable and can be transferred from the substrate to others, which shows the potential in dynamic color displays.

2. Design and analysis

The schematic diagram of the proposed color filter is shown in Fig. 1. The dielectric nanostructures consist of arrays of silicon nanoblocks upon the SU-8. The upper cladding and the handling substrate are PDMS. Silicon is used as a high-index dielectric material to confine a sufficient amount of light to the nanoblocks. The PDMS upper cladding can protect the silicon nanoblocks from the environment and helps to enhance the color stability. As shown in Fig. 1, the incident light is launched at an incidence angle θ and a polarization angle φ towards the color filter, where θ is the angle from the incident light to the z axis and φ is the angle from the x axis to the orthogonal projection of the incident beam in the x-y plane. The responses for the two orthogonal polarizations are primarily determined by the propagation length in the horizontal or vertical direction. According to Malus’ law [53], the response for a normal incident light of arbitrary polarization can be treated as a combined transmission of the two polarization components. The total transmission T(φ, λ) can be expressed as the following equation [21]:

 

Fig. 1. Schematic diagram of the proposed structural color filter.

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The nanoblock thickness $ t$ and period p are set as 150 nm and 300 nm. The width w_x of each asymmetric nanoblock is 150 nm, and the width w_y is changed from 70 nm to 120 nm in a step of 10 nm. The periodic boundary condition is used for the x/y directions to calculate the infinite arrays, while the boundary condition of perfect matched layer is used for the z-direction. The relative dielectric constant of silicon is determined by Palik silicon in the Lumerical material library. The refractive indices of SU-8 and the material PDMS are 1.579 and 1.40, respectively. We then perform the simulations by varying φ from 0° to 90° with the step of 15°. Figure 2(a) shows calculated transmission spectra for the proposed color filters with different sizes of different polarization states. Strong polarization sensitivity is observed. For example, when w_y is 80 nm and φ is 0°, the lowest Mie resonance wavelength is around 609 nm. When φ increases, the resonant wavelength shifts to the short-wave direction. When the polarization angle φ is 90°, the lowest Mie resonance wavelength is approximately 540 nm. The results show that for a fixed set of geometric parameters, different colors can be obtained for different polarization states.

 

Fig. 2. (a) Calculated polarization-dependent transmission spectra and (b) the corresponding color responses with the width ${w_y}$ changed from 70 nm to 120 nm.

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From Fig. 2(a)(i)-(vi), it can also be found that the sensitivity to polarization state is different for structures with different length/width ratio w_x/w_y. As w_x/w_y increases, the degree of polarization sensitivity also increases, which is due to the total transmission determined by the two polarization components in the x-and y-directions. In order to show the color response of the filter under different polarizations obviously, Fig. 2(b) shows the chromaticity coordinates of the transmission spectra in the standard International Commission for Illumination (CIE) 1931 chromaticity diagram. The figures depict the chromaticity coordinates of the calculated spectra of a filter with ${w_y}$ varying from 70 nm to 120 nm at different polarization angles (φ is changed from 0°to 90° in the step of 15°). The variations in color for the filters in the chromaticity coordinates are marked by the black arrows, which also show that the structure is sensitive to the polarization state.

To further clarify the working principle of the polarization dependent transmission for the proposed color filter, we have calculated the field distribution of two orthogonal states of incident wave polarization at w_y=80 nm, as shown in Fig. 3. The single rectangular nanoblock is treated as a nanopixel. When the electrical vector of the incident wave is aligned with the geometry, the vertical and horizontal segments of nanoblocks respond separately. It can be seen from the electric field distribution map that only the horizontal vertical nanoblocks respond when φ=0° (TM), corresponding to the resonance observed near 609 nm in Fig. 2(a). When φ=90° (TE), the vertical segment of nanoblocks are in the excitation mode, which results in a resonance near 540 nm observed in Fig. 2(a). In both cases, the magnetic field distribution is confined to the center of the structure, thus a strong difference in the electric field distribution is the origin of the polarization sensitivity.

 

Fig. 3. Electric and magnetic field distribution in (x, y) plane: (a) λ=609 nm, TM incident wave and (b) λ=540 nm, TE incident wave.

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We also investigate the influence of the nanostructures period on the function of the color filter, as shown in Fig. 4. Figure 4(a) illustrates the transmission spectra of the nanostructures with different periodic parameters for the TM polarized light incidence. The size of nanoblock is w_x=150 nm, w_y=80 nm, and t=150 nm. The period is arranged in a square lattice with size p changed from 260 to 360 nm in a increment of 20 nm. As the periodic parameters increases, the red shift of resonance is observed, which is due to the subsequent decrease in the effective index. To directly show the color tuning of the filter caused by the periodic parameters, the chromaticity coordinates of the calculated spectra at TM polarization are shown in the standard CIE 1931 chromaticity diagram of Fig. 4(b). The chromaticity coordinate curve expressed by the different periodic parameters (p=260-360 nm) is indicated by a curved black arrow. The color for the filter can be tuned by changing the periodic parameters of the nanoblocks.

 

Fig. 4. (a) Calculated transmission spectra with different pitch and (b) the corresponding color responses for the TM incident polarizations. The nanoblock’s size is w_x=150 nm and w_y=80 nm.

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3. Fabrication and characterization

We then fabricate the flexible color filter by using the nano-fabrication technologies. The fabrication flow is schematically illustrated in Fig. 5(a). The whole device is fabricated on the handler substrate which is a silicon wafer. Firstly, a thin metal aluminum layer is evaporated on the substrate prior to the PDMS coating. The use of an aluminum layer facilitates the removal of the entire structure from the substrate. The PDMS with 10:1 monomer/curing agent mixing ratio (Dow Corning Sylgard 184 elastomer) is spin-coated on the substrate. The thickness for the PDMS layer is controlled to be 3 µm by using a 6000 rpm spin-coating rate for 90 s. The sample is then cured at a temperature of 110 °C for 30 min. After that, the sample is briefly treated in the oxygen plasma to promote adhesion of the coating, following by spin coating of a 0.8 µm-thick SU-8 (Microchem SU-8 2000.5) layer at a 4000 rpm spin-coating rate for 59 s. The sample curing operation is then performed at the temperature of 60 °C for 15 min and 90 °C for another 15 min. The poly-Si film is then deposited by using electron beam evaporation (Denton Vacuum Inc.). A low deposition rate of 3 Å⁄s is used for the evaporation process to avoid the crack of the substrate material. The E-beam lithography (Raith15-II) is used to define the patterns with a negative photoresist maN-2401. The patterns are transferred to the silicon layer by using inductively coupled plasma reactive-ion-etching (ICP-RIE) with a gas mixture of SF₆ and C₄F₈. Finally, the PDMS upper cladding layer with a thickness ∼100 µm is spin-coated at rotation speed 500 rpm for 59 s. Due to the low adhesive force between PDMS and aluminum layers, the whole device can be delaminated from the substrate without using the chemical corrosion. The scanning electron micrograph (SEM) image of the fabricated metasurfaces is shown in Fig. 5(b). Figure 5(c) shows the fabricated flexible color filter after delamination.

 

Fig. 5. (a) Schematic of the fabrication process flow for the stretchable color filters. (b) The SEM image of fabricated metasurface and (c) the photos of the whole flexible device.

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The optical microscope is utilized to measure the transmission spectra. Figure 6(a) shows the measured transmission spectra at different polarization states when φ is gradually changed from 0° to 90°. The corresponding color responses on the standard CIE 1931 chromaticity diagram are given in Fig. 6(b) for visualization. One can find that the measurement results of the device are relatively consistent with the simulations. When the polarization angle of the incident light changes from 0° to 90°, the color can be tuned from purple to red. The widths and height of the fabricated nanoblocks are various not exactly the same as the design values due to the fabrication errors, which will cause the transmission spectra shift.

 

Fig. 6. (a) Measured transmission spectra and (b) the corresponding color responses for different polarization angles. (c) Calculated color responses. The nanoblock’s size is w_x=150 nm, w_y=80 nm and p=300 nm.

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To further investigate the influences of the period and polarization states on the performances of the fabricated device, three nanoblocks designed with different period parameters given in Table 1 are considered in detail. It is clearly observed that different colors will appear under different polarization states of the incident wave. Figure 7 presents the experimental colors observed under the optical microscope in transmission mode, when φ is equal to 0°, 45°, and 90°. For φ=0°, the color response corresponds to the excitation of the horizontal nanoblock segment (x-axis), while φ=90° corresponds to the excitation of the vertical nanoblock segment (y-axis) and the intermediate response between 0°and 90° can be derived from the Eq. (1). For a given filter geometry, a variety of colors can be obtained. Thus, the color of the filter can be easily tuned by changing φ. By comparing the three samples, as shown in Fig. 7, the influences of nanoblocks’ periodic parameters can be illustrated. By appropriately adjusting the period length of each Si nanoblock, two different colors can be formed when φ=0°and φ=90°. Since the loss of the dielectric nano-antenna is very low, various high-quality colors can be formed in the transmission mode. Furthermore, additional degrees of freedom in tunability can be added by changing the structural parameter values of w_x and w_y.

 

Fig. 7. Different colors are visible under optical microscope in transmission mode when φ is equal to 0°, 45°, and 90°.

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Table 1. Different periods of the nanoblocks for three cases.

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4. Conclusion

In summary, we have demonstrated a polarization-dependent color filter based on all-dielectric metasurfaces, which is flexible and can be transferred from the handler substrates to others. The resonance location responsible for color selectivity can be varied in a desired way by properly adjusting sizes and the periods of nanoblocks. The asymmetric nanoblocks are sensitive to the polarization directions of the incident wave. Hence, efficient tuning of colors can be achieved in one structure just by changing polarization angle. Simulation results based on FDTD method show good performances of the proposed color filters and the characterization results are well coincident with the simulation ones. We believe that the suggested devices have potential applications in the areas of functional color display, polarization detection and imaging, and secured optical tag.