Transmissive subtractive color filters are proposed and demonstrated that take advantage of an all-dielectric metasurface based on a lattice of TiO2 nanopillars (NPs), rendering a high transmission efficiency that exceeds 90%. TiO2 NP elements have been created that exhibit a high aspect ratio. Specifically, a series of lithographic processes are conducted to form a narrow and deep hole in the photoresist, which is accompanied by atomic layer deposition of TiO2. A broad palette of vivid colors encompassing the visible band has been obtained by adjusting the NP diameter for a constant duty ratio of 0.35. For the NP resonator, the electric and magnetic field profiles in conjunction with the scattering cross-sections have been meticulously investigated to theoretically validate that the resonant transmission dips are primarily governed by the simultaneous excitation of an electric dipole and a magnetic dipole via Mie scattering.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Color filters have been extensively exploited for a variety of applications, encompassing color imaging and display, organic light-emitting diode devices, solar-cells, color printing, plastic consumer products, anti-counterfeiting, and biosensors [1–10]. Inspired by nature, as in the case of wings of the butterfly, high-performance nano-structural colors can be produced in artificially devised structures from light scattering, diffraction, absorption, or interference [1–6]. Structural color filters are perceived as a prominent alternative to pigment/dye-based materials, which are susceptible to high loss, environmental hazards, difficulty in scalability, performance degradation caused by ultraviolet irradiation and high temperature and incompatibility with the complementary metal-oxide-semiconductor (CMOS) fabrication process [1–11].
Recently, nanostructured metasurfaces, which refer to an array of ultrathin planar subwavelength elements, have been intensively explored to serve as an alternative platform to conventional diffractive optical elements, rendering the tailoring of amplitude, phase, and polarization of light [12–15]. Diverse nanoscale devices such as color holograms, lenses, and color filters were suggested in metal and/or dielectrics [15–18]. It is observed that plasmonic devices are subject to intrinsic losses in the visible band, which adversely affect their transmission in terms of the spectral shape, bandwidth, and efficiency, resulting in low color gamut and purity. Their scalability is also critically limited [6–10]. Although Fabry-Perot based color filters have been suggested for improving the color gamut and purity [4,5], they suffer lower output efficiency and require the multiple material to form different layer which make the fabrication complicated and costly. High-index nano-resonators that are made up of dielectric materials like silicon [17–22], which are based on electric dipole (ED) and magnetic dipole (MD) resonances mediated by Mie scattering, have been proposed to suppress material absorption and improve efficiency. However, the window of transparency was confined to the infrared region or longer wavelength region of the visible band. Mie scatters in silicon are supposed to face a daunting challenge in generating a distinctive color impression with higher efficiency, especially for blue and yellow color filters . Although SiO2 and polymers may provide a far wider transparency windows that span the entire visible bands, they are not suitable for producing high quality structural colors because of their low refractive indices. It is of paramount importance that materials and nanofabrication techniques be newly developed, thereby substantially extending the spectral domain of operation of such dielectric metasurfaces beyond the visible band. Amorphous titanium dioxide (TiO2) can be a prime candidate as base material, in view of its broad transparency window that stretches beyond the visible band. The high-index TiO2 material, whose interband transition lies just outside of the visible regime, can afford to readily mediate light−matter interactions . In this context, an array of TiO2 nanocylinders was built through the conventional lift-off method , where a coin-shape structure with a low aspect ratio of ~0.5 was introduced. For the filter, the spectral tuning was barely mechanically performed . Although a reflective color filter was embodied tapping into a complicated shape of trapezoidal TiO2  by virtue of the lift-off technique, the device fabrication entails relatively long-time and multi-step processes. In an attempt to avoid the aforementioned obstacles, we propose atomic layer deposition (ALD) of amorphous TiO2  to enable the construction of high-aspect-ratio NPs, so that negligible surface roughness and low optical loss could be obtained in the visible band. Our approach is differentiated from previous techniques [26,27] in which the patterns are mostly transferred by a template (e.g., anodic alumina or inverse opal). Highly sophisticated nanostructures can be reliably reproduced thanks to the conformal coating of ALD upon the resist patterns.
In this work, a highly efficient transmissive color filter has been proposed and realized that capitalizes on an all-dielectric metasurface made of TiO2 NPs of high aspect ratio. Primary subtractive colors of cyan, magenta, and yellow were successfully demonstrated providing large color gamut and purity. The color tunability of the TiO2 metasurface exhibiting transmission dips has been thoroughly scrutinized depending on the NP diameter, so as to realize a broad range of color images. By inspecting near-field electric and magnetic profiles in relation to the NP and its scattering cross-sections, it has been confirmed that the mechanism bringing about the transmission dips is closely related to the coexistence of ED and MD resonances. The angular tolerance for the device has been finally explored. The proposed device is anticipated to play a vital role in the implementation of image sensors and optical nanoantennas.
2. Proposed subtractive color filters capitalizing on an all-dielectric metasurface
Figure 1(a) illustrates the configuration of the proposed color filters that incorporate an all-dielectric metasurface, which is composed of an array of TiO2 NPs of 400-nm thickness (Hg), formed on a glass substrate. For the NP metasurface, the duty cycle is defined as the ratio of the diameter (D) to the period (Λ), which is set at 0.35, and the aspect ratio (Hg/D) is appropriately chosen to ensure enhanced transmission. Figure 1(a) shows that the polarization direction of the incident light is marked by the electric (E) field, which is parallel to the x-direction is transverse electric (TE) polarization, while the magnetic (H) field is parallel to the y-direction. The transverse magnetic (TM) polarization refers to the E-field aligned parallel to y-axis. The proposed devices are presumed to give rise to transmissive subtractive colors of yellow, magenta, and cyan, which can deliver double light throughput compared with their additive counterpart, generating a strong color signal . The proposed filters were designed with the assistance of a simulation tool based on the finite-difference time-domain (FDTD) method (FDTD Solutions, Lumerical Inc., Canada). The dielectric NPs with a high aspect ratio were prepared by utilizing ALD of TiO2 at a low temperature of 130 °C, thereby avoiding the deformation of resist patterns. The refractive index of the material was measured by ellipsometry (JA Woollam M2000D). Figure 1(b) shows the fabrication procedure adopted for the device, which is addressed in the methods section.
Figure 2(a) shows the scanning electron microscope (SEM) images of the manufactured devices corresponding to primary subtractive colors. Each color filter, with a footprint of 40 µm × 40 µm, produces distinct and vivid colors of yellow, magenta, and cyan, as illustrated in the inset of micrographs for diameters of D = (140, 175, and 210) nm, respectively. A couple of dark spots are witnessed in the images, which stem from fabrication defects such as fallen NPs, and in the case of the yellow filter, they are pronounced. Figure 2(b) shows that the NPs give the highest aspect ratio of about 3. The tilt view (52°) SEM images correspond to the three primary color filters, while the inset for the case of the magenta filter shows the cross-sectional SEM image after focused gallium ion beam (FIB) milling. On the whole, the created devices give birth to decent correlations with the design specifications, while minimal surface roughness for the vertical profile is noticed for the TiO2 NPs. Figure 3(a) shows the simulated and measured transmission spectra for the primary color filters. It was observed that the transmission spectra exhibit double suppressions at the resonance wavelengths for the subtractive colors of yellow, magenta, and cyan. The filter devices delivered high off-resonance transmissions that surpassed 90% in the spectral regime longer than the resonance wavelengths. Figure 3(b) depicts the standard International Commission on Illumination (CIE) 1931 chromaticity diagram, which is commonly used for color evaluation. In the cases of yellow, magenta, and cyan, the simulation and measurement results were in reasonable agreement.
Figure 4(a) shows the evolution of the resonance wavelengths relating to the dips in the transmission spectra for the TiO2 metasurface for the (i) simulated and (ii) measured cases when the NP diameter varies from (126 to 224) nm in steps of 7 nm. The pattern signifies the influence of the NP diameter on the color tuning. The black dashed (for the ED) and the blue dashed (for the MD) lines, traces the resonant dip positions, apparently overlap for the simulated and measured spectra. The inset in Fig. 4(a) (ii) presents the color images for the fabricated filters with different diameter and Fig. 4(b) displays the corresponding CIE 1931 chromaticity diagram for (i) the simulated and (ii) the measured cases. The transmission spectra with broader bandwidth in the subtractive mode, as depicted in Fig. 4(a), is desired to produce the vivid colors response as plotted in CIE 1931 chromaticity diagram shown in the Fig. 4(b). The disparities between the simulated and measured spectra are principally ascribed to the fabrication inaccuracies, including deviations from the perfect cylindrical shape of TiO2 NPs, in conjunction with surface and sidewall roughness. The sidewall roughness of TiO2 pillars is determined by that of ZEP520 resist holes, which is known to be 1-2 nm RMS roughness. Also, the pillar top roughness, resulting from the TiO2 deposition and shallow blanket etching (~20 nm etch depth), has a similar range of roughness. Therefore, the effect of roughness on the optical performance of the filters might be marginal. We measured the dispersion of TiO2, film deposited based on ALD technique, as shown in appendix A. The extinction coefficient was below 0.001 in the visible spectrum, indicating a negligible absorption loss from 400-nm-thick TiO2 film. The deviation from the perfect cylindrical shape to other shapes like ellipsoidal pillars might result in the different spectral shape and hence the color saturation. It is stated that the saturation of the subtractive colors is dependent on the bandwidth, near-zero resonant dip in conjunction with a high off-resonance transmission efficiency . It has been reported that the bandwidth is controlled to generate different colors and improve saturation of the colors using an ellipsoidal pillar .
3. Mechanism underlying the transmission dips for the metasurface based on TiO2 NPs
For a metasurface comprised of an array of TiO2 NPs, each NP element is perceived to act as a resonator, allowing for the excitation of ED and MD modes by virtue of the Mie scattering [17–20]. Each NP is equivalent to a dielectric waveguide that is truncated on both ends, serving as a low-quality factor Fabry-Perot etalon. The effective refractive indices of the NP resonator are keenly dependent on its diameter . In an effort to discover the key mechanism underpinning the spectral transmission dips, the NP has been rigorously checked in terms of the E- and H-field profiles at resonance in combination with the scattering cross-section. Figure 5(a) (i) depicts the spectral transmission that exhibit dips centered at the wavelengths of (530 and 560) nm, for a typical filter with D = 175 nm. Figure 5(a) (ii) shows the calculated scattering cross-section for the NP, where the cross-section peaks are concurrent with the resonance dips, supporting the thesis that the scattering mostly dictates the spectral transmission. Figures 5(b) and 5(c) present the field profiles in relation to the resonant ED and MD modes initiated by the scattering, respectively. In particular, Figs. 5(b) (i) and 5(c) (i) show the respective E- and H-field profiles in the xy plane at the height of 300 nm above the TiO2-glass interface. The ED mode, monitored at the wavelength of 530 nm, is signified by the reinforced E-field as shown in Fig. 5(b) (ii) in combination with the current loop for the H-field shown in the vector plot in Fig. 5(b) (iii), on the yz cross-section that cuts through the center of the TiO2 NP. While the MD mode at 560 nm wavelength is proclaimed by the circular displacement current associated with the E-field, which is shown in the vector plot in Fig. 5(c) (ii), in combination with the reinforced H-field shown in Fig. 5(c) (iii) on the xz cross-section that cuts through the center of the TiO2 NP. The black dashed line stands for the interface between two different media. The presence of the neighboring ED and MD modes is responsible for the spectral resonance broadening [19,31]. The ED and MD excitations can be better distinguished under different aspect ratios and duty ratios [16,17]. A strong MD is deemed to be induced when the wavelength within the nanostructure becomes comparable to its dimension, under the condition of 2R ≈λ/n, where n is the refractive index, R the radius, and λ the wavelength. A subwavelength nanoparticle (R < λ) requires a large index of refraction and thus facilitates such resonance . Simultaneous occurrence of the ED and MD has been attributed to the Kerker’s-type scattering [29,30,33].
The proposed color filter drawing upon an all-dielectric metasurface is notable for its capability to attain affordable angular tolerance, as shown in Figs. 6(a) and 6(b), for TE and TM polarizations, respectively. The transmission dips and the maximum off-resonance transmission greater than 90% are observed to be maintained up to an incidence angle of 10°, for both polarizations. The observed relaxed angular tolerance makes the proposed filters suitable for their potential implementation in the color displays, imaging, and color printing. The transmission spectra were also scrutinized with respect to the TiO2 height, for both polarizations, as shown in Fig. 6(c). The height of the TiO2 NP was fixed at 400 nm, which guarantees an appropriate spectral response and decent coloration throughout the visible band, with the height ranging from (100 to 500) nm.
4.1 Device fabrication
The cell constituting the proposed color filter was designed and created to exhibit dimensions of 40 µm × 40 µm. For the fabrication of the TiO2 metasurface, a 400-nm-thick electron-beam resist (ZEP 520A from Zeon, Japan) was spin-coated on a glass substrate; then, electron-beam lithography (Raith150) was applied to make a pattern of circular holes. The distortion of pattern dimension and shape due to the proximity effect was minimized by applying high electron beam energy (30kV) and the small duty ratio of the holes. ALD was used to conformally deposit TiO2 on the developed holes, where the deposition rate was around 60 nm per hour. We applied around two hours’ deposition for complete filling of the largest nanopillars (D = 210 nm). Subsequently, a TiO2 blanket etch was performed using CHF3 plasma to reach up to the resist surface. Finally, the electron-beam resist was removed by oxygen plasma.
4.2 Optical characterization
The fabricated TiO2 pattern was visually inspected by dual beam (SEM/FIB) high-resolution scanning electron microscopy (FIB II, Quanta 3D FEG, FEI). The transmission spectra were checked by launching a collimated beam from a halogen lamp (HL-2000-FHSA, Ocean Optics), which was polarized through a calcite crystal polarizer (GTH 10M-A, Thorlabs), to the prepared color filter mounted on a motorized rotation stage. The optical output was captured by spectrometry (Avaspec-3648, Avantes) via a multimode fiber. Images for each color were taken via digital microscopy (Leica DM4000 M).
4.3 Numerical simulations
The transmission/reflection spectra, scattering cross-sections, and field profiles for the proposed devices were investigated by means of a simulation tool based on the finite difference time domain (FDTD) method (FDTD Solutions, Lumerical Inc., Canada) . A normally incident plane wave was illuminated to a unit cell that satisfies a periodic boundary condition, thereby mimicking an array of TiO2 NPs. The dispersion characteristics of TiO2 in the visible spectral range, which were observed through an ellipsometer (JA Woollam M2000D), were reflected in the simulations, and the dispersion characteristics of SiO2, which are used for the simulations were derived from the multi-coefficient model supplied by the simulation tool .
We realized highly efficient transmissive-type color filters that utilize an all-dielectric metasurface that incorporates high aspect ratio of TiO2 NPs on a glass substrate. Primary subtractive colors of cyan, magenta, and yellow were demonstrated that exhibited large color gamut and purity. A suite of filter devices with different NP diameter was manufactured to demonstrate the broad range of color tuning in the visible band. The prepared color filters provide a transmission efficiency beyond 90%, enabling high resolution and excellent color fidelity. The transmission dips are supported by the simultaneous excitation of the ED and MD modes, which are well depicted with the help of the near-field profiles of E-field and H-field. The dips in the transmission spectra and the position of the peaks in the calculated scattering cross-sections correlate with each other. The proposed color filter will make it an outstanding candidate for implementing various applications that include high-performance optical nanoantennas, beam-steering, beam-shaping devices, and so forth.
Appendix A Dispersion characteristics of TiO2
Figure 7 shows the refractive index vs wavelength of TiO2 in the visible band used for the simulation. The dispersion characteristics of the TiO2 were measured by ellipsometry (JA Woollam M2000D) in the visible spectral range and were reflected in the simulations. The ALD was used for the deposition of TiO2 at the relatively low temperature of ~130 °C.
National Research Foundation of Korea (NRF), funded by the Korean government (MSIP) (No. 2016R1A2B2010170); Kwangwoon University; Australian Research Council Future Fellowship (FT110100853, Dr. Duk-Yong Choi).
The authors are grateful to Mr. Chul-Soon Park and Mr. Song Gao for their valuable help. This work was performed in part at the ACT node of the Australian National Fabrication Facility.
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