1. Introduction

Surface plasmon resonance(SPR) on metallic materials has exhibited a variety of fascinating optical properties [1], such as boosting the transmission of light through subwavelength hole arrays in metal films [2,3] and increasing Raman scattering intensities with localized surface plasmon polaritons [4]. Since the wavelength is sensitive to variations of the medium surrounding a metal surface, SPR is utilized in refractive index sensing [5]. Also, colorfilter based on two dimensional metallic nano-arrays was fabricated [6]. Even further, via the SPR and inherent polarization character, integrated colorfilter and polarizer was achieved in a metal-insulator-metal trilaminar nanowire grating [7], which has great advantages of high optical efficiency and compact structure in display devices. The obstacle hindering the practical application of metallic nanowire gratings is the high cost and complicated techniques for large scale fabrication. In this work, we proposed and fabricated a kind of laterally shifted bilayer metallic nanowire gratings with simultaneous colorfilter and polarizer functions. Although the transmissions are a little lower than previously reported, this structure can be conveniently fabricated by large area laser interference lithography [8]. Based on the unique properties of SPR-induced sharp transmissive peak, tunable colorfilters are achieved, promising a potential in active displays, especially when the refractive index of the ambient materials can be altered on the metal surface by eletrowetting technology [9].

Electrowetting technology has been proposed as an alternative approach to e-paper in both reflective [10] and transmissive modes [11] by manipulating the movement of microfluidic, with the attractive properties of high optical efficiency [12], low power consumption, fast response time and scalable fabrication [13]. However, in conventional electrowetting displays, the images are presented via changing the area of the water droplets immersed in chromophores doped oil film by adjusting the applied voltage [14], the colored oil film absorbs up to 2/3 of the incident light with low optical efficiency. Furthermore, due to the wide transmitted bands of color filters, even the most complicated three-layered full-color electrowetting devices [15] just have no more than 50% in color gamut (NTSC in CIE 1931 UCS). By combining non-absorptive color filter effect [16,17] with refractive index sensitivity character [18,19] of SPR, we present a full-color transmissive electrowetting display with narrow spectrum bands based on bilayer metallic nanowire gratings. It exhibits attractive features of high optical efficiency, wide color gamut, large contrast ratio and compact structure without the need of absorptive chromophores.

2. Basic theory and simulation design

The mechanism of proposed display device depends on the effect of SPR based on the bilayer aluminum (Al) nanowire gratings. When TM-polarized light is incident on the bilayer Al nanowire gratings vertically from the bottom substrate, lateral SPR is excited on the interface between ambient materials and Al by the diffraction of the gratings. With the excitation of SPR, a resonantly transmissive peak through metallic gratings can be obtained. Figure 1(a) shows the diagram of the bilayer Al nanowire gratings and the SPR therein is also presented. The condition to excite a surface plasma wave (SPW) at the interface between a metal and a dielectric can be expressed as Eq. (1):

 

Fig. 1 (a) The schematic of the bilayer aluminum nanowire grating and the SPR therein. (b) The simulated transmissive spectra of gratings with the pitches of 420 nm, 520 nm and 650 nm in the ambient of air. (c) The color gamut of the device according to simulated transmission spectra.

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According to Eq. (1), the peak position is determined by the pitch of gratings and the refractive index of the ambient medium. In order to interpret the relationship between the transmitted light with the parameters of Al nanowire gratings, the rigorous coupled wave analysis-RCWA (RSoft, DiffractMOD^TM) is employed to analyze the spectra for normal incidence of transmission modes. Figure 1(a) shows the model of bilayer Al nanowire gratings for simulation by RCWA and the relevant parameters are also labelled. The configuration can be divided into four layers: the top layer is the ambient dielectric contact with the Al surface, whose refractive index is n_a; the second one is an aluminum film with slits and height h₁ is 50nm; the third layer is photoresist (ARP 3500-6, Allresisit Co.) with a refractive index of 1.5 and height h₂ of 100nm, and the bottom substrate is BK7 glass with a refractive index of 1.5;The width of metal Al and photoresist are both T/2. Figure 1(b) shows the simulated transmission spectra through the Al nanowire gratings in air, where three distinct transmission peaks can be observed at the wavelengths of 460 nm, 546 nm and 690 nm with pitches of 420 nm, 520 nm and 650 nm, respectively. Therefore, the red, green, and blue colors can be obtained with the transmission efficiency of more than 40%. According to the simulated transmission spectra, the color gamut of the device in Fig. 1(c) is 89% NTSC in CIE 1931 UCS, implying a noticeable feature in color rendering over present liquid crystal and e-paper display.

According to Eq. (1), also shown in Fig. 2(a), with the increase of ambient refractive index adjacent to Al gratings surface, the wavelengths of the resonant peaks will extend to invisible light region with the required lowest refractive indexes to be 1.21, 1.42 and 1.65, respectively, meaning the shift from bright to dark state of each color can be achieved. To show the SPR modes clearly, magnetic field profiles Hy of the resonant peak-546 nm in the pitch of 520 nm was calculated by RCWA, shown in Fig. 2(b). The TM light is normally incident from the substrate with incident wavelength of 546 nm, corresponding to the transmitted peak in Fig. 1(b). It is obviously that the light is tunneled through Al films via lateral SPR excited on the interface between aluminum and ambient medium, which is featured with large field on the Al surface. Figure 2(c) shows the simulated transmission spectra of the 520 nm gratings with ambient dielectric changed from air to 60% sucrose solution (whose refractive index is 1.44 at room temperature [21]), it is clear that the resonant peak extend to invisible light region, meaning the tunability of brightness.

 

Fig. 2 (a) The calculated curves of surface plasmon resonance wavelengths with the ambient refractive indexes respectively. (b) Simulated field intensity distribution of Hy component of 546nm TM light via Rigorous Coupled Wave Analysis (RSoft, DiffractMOD^TM). The white lines limn the outline of the gratings with pitch of 520nm. (c) Simulated transmission spectra of the 520 nm gratings with the ambient medium changed from air to 60% sucrose solution.

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3. Fabrication and experimental measurements

Figure 3(a) shows the fabrication process of the bilayer Al gratings. A photoresist layer with a thickness of 100 nm was spin-coated on the BK7 glass substrate. Then, a Lloyd mirrored laser interference system was built to expose the photoresist, as shown in Fig. 3(b). Next, 0.35% NaOH solution in deionized water by mass concentration was used as an etchant with development time of 10 seconds. Lastly, e-beam evaporation was used to deposit a film of 50 nm Al on the photoresist relief. Scanning electron microscopy (SEM) images of the top and side views of the fabricated gratings with three pitches are shown in Fig. 3(c).The grating pitch Λ is derived by laser wavelength (λ) and incidence angle (θ) as Eq. (2):

 

Fig. 3 (a) Fabrication process of the bilayer Al gratings. (b) The setup of laser interference lithography. (c) SEM photos of the top and side views of the Al gratings with the pitch of 650 nm, 520 nm, and 420 nm, respectively.

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Figure 4(a) shows the measured TM light transmission spectra of the three gratings. There are pronounced peaks at the wavelength of 470 nm, 546 nm and 675 nm with the corresponding colors shown by the insets in Fig. 4(b). The transmission efficiency of the peak is no less than 17%. Figure 4(b) shows the color gamut of the device according to transmission spectra measurement (53% NTSC in CIE 1931 UCS). The measured and simulated transmission spectra under the same conditions agree relatively well with each other, except that the measured values are lower than simulations, likely due to the fact that the sidewalls of gratings are covered with deposited Al (especially when the Al film is thick). By optimizing the Al deposition process, transmission efficiency and color gamut can be improved.

 

Fig. 4 (a) The measured transmissive spectra of BMNGs with pitches of 420 nm, 520 nm and 650 nm in air. (b) The color gamut of the full color display based on BMNGs, appended with experimental color pictures, respectively.

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In the following, we choose the gratings with the pitch of 520 nm for further investigation on the active display by altering the ambient refractive index. The brief schematic of the measurement system was shown in Fig. 5(a).A collimated white light beam from a white light emitting diode (LED) was polarized by a Glan–Thompson prism and then was incident on the sample which impinged on one side of the cuvette’s chamber, and a collimating lens mounted on a fiber. The fiber was connected to a spectrometer (QE65-PRO, Ocean Optics) for collecting the transmitted light. Figure 5(b) shows the transmission spectra with ambient liquid changed from air to 60% sucrose solution (whose refractive index is 1.44 at room temperature [21]). It is clear that there is almost no transmitted light in the visible light region because the resonant peak moved to the infrared spectral range, leading to the dark state in vision shown by the inset. Therefore, the state from bright to dark is achieved with a contrast ratio of more than 120:1.The transmission efficiency of bright state is lower than above measurement because of the extra reflection of cuvette’s chamber. All these experimental results fit well with the previous theoretical analyses and demonstrate the feasibility of a full-color transmissive display based on bilayer Al gratings.

 

Fig. 5 (a) The brief schematic of the measurement system. (b) Optical performance of the 520 nm gratings with the ambient medium changing from air to 60% sucrose solution.

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4. Prototype of device structure and operation

The comprehensive characters of the SPR based on the BMNGs from the aforementioned simulation and experiments indicate a new approach to achieve a full-color transmissive display by controlling surface plasmon resonance. Figures 6(a) and (b) illustrate the device configuration of a transmissive electrowetting display (EWD) based on BMNGs, in both on and off states. The EWD devices can be fabricated on an indium tin oxide (ITO) substrate as the transparent electrode. Then a layer of hydrophobic dielectric is formed as an electrical insulation. Three different pitches of bilayer nanowire gratings are obtained on the surface of hydrophobic insulator. The hydrophobic grids are patterned to separate the gratings of each pitch as a subpixel. Finally, three kinds of transparent conductive liquid media with proper refractive index corresponding to subpixels are selected to insert into the EWD cell. For optical switching function, the EWD cell is illuminated by a collimated white light beam with transverse magnetic (TM) polarization to excite SPR.

 

Fig. 6 The cross-sectional schematic of the proposed transmissive EWD cell. The EWD cell in (a) bright state without applying voltage and (b) dark state with applied DC-voltage.

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As shown in Fig. 6(a), the bright transmission is achieved in the voltage-off state. Under the condition of zero voltage biased to the transparent liquid medium, the transparent liquid medium is on the edge of the hydrophobic gratings with a large contact angle because of the robust hydrophobicity on the nano-textured surfaces [22]. The contact angle measured for 60% sucrose solution on the BMNGs is 125°, which can be obviously seen from the inset on the top right corner of Fig. 6. Therefore, most of the incident light propagates directly through gratings with the ambient medium of air, indicating the bright state of the pixel with specific color. Each pixel of the device consists of three sub-pixels which transmit blue, green, and red lights, respectively. Figure 6(b) shows the voltage-on state as an appropriate voltage applied between the bottom ITO electrode and transparent liquid medium. The applied electric field creates a downward electromechanical force that attracts the transparent liquid to wet the hydrophobic dielectric surface. The whole EWD cell displays as a dark state when the transparent liquid media spread over the entire surface uniformly. Therefore, a full-color-capability transmissive display based on surface plasmon resonance can be achieved by combining with electrowetting effect.

5. Conclusion

Tunable spectral filters via surface plasmon resonance on metallic nanowire gratings were demonstrated and an active display model was proposed. Three pitches of bilayer metallic nanowire gratings have been designed and fabricated to achieve the red, green and blue colors. The color gamut of this device can achieve 89% NTSC in CIE 1931 UCS, implying a noticeable improvement of color rendering over the conventional electronic paper display devices. The tunability of brightness was demonstrated on the green color grating with pitch of 520 nm, and the contrast ratios is over 120:1, 12 times of the present electronic paper display. Therefore, the proposed device is featured with low power consumption, compact structure, wide color rendering, and efficient fabrication. These demonstrated characteristics reveal a new approach for bistable full-color electronic paper display by actively controlling plasmonic resonance via electrowetting effect.