Plasmonic- and dielectric-based structural coloring: from fundamentals to practical applications
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Structural coloring is production of color by surfaces that have microstructure fine enough to interfere with visible light; this phenomenon provides a novel paradigm for color printing. Plasmonic color is an emergent property of the interaction between light and metallic surfaces. This phenomenon can surpass the diffraction limit and achieve near unlimited lifetime. We categorize plasmonic color filters according to their designs (hole, rod, metal–insulator–metal, grating), and also describe structures supported by Mie resonance. We discuss the principles, and the merits and demerits of each color filter. We also discuss a new concept of color filters with tunability and reconfigurability, which enable printing of structural color to yield dynamic coloring at will. Approaches for dynamic coloring are classified as liquid crystal, chemical transition and mechanical deformation. At the end of review, we highlight a scale-up of fabrication methods, including nanoimprinting, self-assembly and laser-induced process that may enable real-world application of structural coloring.
KeywordsStructural color printing Color filters Plasmonic color filters Plasmonics Dielectric color filters Metasurfaces Metamaterials Sub-wavelength Nanophotonics Tunable color filters Large scale color filters Up-scale color filters
hydrogenated amorphous silicon
dielectric-based color filters
deep reactive-ion etching
electron beam lithography
extraordinary optical transmission
indium tin oxide
plasmonic color filters
scanning electron microscope
surface plasmon resonances
surface plasmon polaritons
tunable color filters
Color production mechanism are mainly classified by two types: pigmentary or structural coloring. Structural colors, in particular, are caused by microscopic structures that are tiny enough to interfere with visible light. In nature, structural coloring occurs among birds and insects [1, 2, 3, 4, 5, 6]. This method of generating colors has inspired the field of structural color printing. Many artificial and biomimetic colors from nature have been reproduced [7, 8] and applied to photonic crystal research [9, 10, 11]. However, the diffraction limit of light presents a challenge to further development of structural color printing. Recent developments of techniques to fabricate metal-based structures have shown a way to overcome the diffraction and to approach nano-size resolution.
Surface plasmon resonance (SPR) by the electric field along a metallic surface can confine optical excitation to far below the diffraction limit [12, 13]. Furthermore, SPRs can be used to manipulate the polarization, phase and intensity of light [14, 15, 16, 17, 18, 19, 20, 21]. These characteristics offer a capability to generate structural colors from SPRs on plasmonic structures. Plasmonic color filters (PCFs) can have sub-wavelength unit cells and surpass the diffraction limit of light due to SPR from plasmonic structures [22, 23, 24]. Hole-array PCFs achieve high transmittance by exploiting extraordinary optical transmission (EOT) caused by SPRs [15, 25, 26, 27].
The metallic layer in PCFs absorbs visible light; this phenomenon can reduce their efficiency. Use of dielectric-based color filters (DCFs) supported by Mie resonance has been suggested as a method to circumvent this problem [28, 29, 30]. DCFs have relatively low loss, so they can control bandwidth adaptively. Additionally, owing to optically-generated electric and magnetic resonances and low cost, DCFs have substantial potential to complement or even replace pigments and PCFs .
Structural coloring has limitations such as static color, limited-scale fabrication and low throughput. Structural filters produce colors that stay mostly static, so the search for a method to tune them is currently an important research topic. Structural color filters that can be tuned by adjusting external factors can manipulate colors by controlling factors such as polarization angle of incident light, alignment of liquid crystals (LCs) [32, 33, 34, 35], mechanical strain [36, 37, 38, 39] and chemical state [40, 41, 42, 43]. In this review, we summarize research on plasmonic color filtering, with a brief explanation of its working principle. We also introduce recent developments in tunable color filtering and large-scale color filtering, which may lead to real-world application.
2 Plasmonic color filters
Color filters selectively reflect or transmit light of a target wavelength. A unit (pixel) in an array transmits dominantly one color (e.g., red, green, or blue). Each pixel delivers different color information, and a combination of colored pixels can produce a specific image. A structural color filter has a nano-scale structure that interacts with incident light to reflect or transmit light of a specific wavelength. PCFs that exploit plasmonic resonance are promising candidates to replace conventional pigment- or dye-based color filters. We will review research on PCFs sorted by structure.
Transmission of light through a subwavelength aperture in regularly patterned opaque metal film is enhanced at resonant wavelengths. This phenomenon is called EOT, and is one of the most important recent discoveries in optics . The effect is associated with a coupling between excitation of surface plasmon (SP) and incident light in a metallic surface [44, 45]. The interaction can be manipulated by tuning geometric parameters such as periodicity, size and shape of apertures. This observation has triggered a wide variety of related research [46, 47, 48, 49, 50, 51].
Nanohole-shaped PCFs based on silver (Ag) instead of Al or gold (Au) can produce color-enhanced transmissive structural colors . The Ag color diagram has a wide distribution of displayed colors, whereas the Al and Au diagrams focus on certain colors (Fig. 1d). Authors also present three pixels which are resists that were fully, partially and barely exposed to electron beam lithography (EBL) (Fig. 1f, red, yellow and white circles). Although the holes are barely exposed, they show no variation of either color or shape compared to fully-exposed pixels. The fabrication method is a simple two-step process of nanoimprinting and depositing the metal (Fig. 1e). Because this fabrication is simple and compatible with large-scale fabrication, commercialization of these devices as color filters is expected.
Metal–insulator–metal (MIM) nanoresonators with an insulator sandwiched between two metallic materials can also act as color filters. The key principle of MIM is Fabry–Pérot interferometry, in which interference of light within a resonator selectively filters out light of a certain wavelength. The design of the MIM has evolved from films to 2D metasurfaces with unit structures such as gratings or posts [54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64]. To improve efficiencies or color purity, researchers have changed basic materials [65, 66, 67].
Plasmonic stereoscopic printing has been demonstrated . The device can produce a great variety of colors in reflective mode by using polarization-dependent tunable color pixels in pairs of arrays of squares and of ellipses (Fig. 2b). Dual color information can be encoded by two polarization directions in the same pixel (Fig. 2c, d), so this printing technology may have applications as high-density optical data storage, high-resolution 3D display, holograms and anti-counterfeiting measures.
A novel nanorod-shaped plasmonic pixel can produce optical resonances over the entire visible spectrum . The floating plasmonic pixel acts as a plasmonic nearly-perfect absorber with narrow bandwidth, and thereby produces immensely-saturated subtractive colors (Fig. 3ci). The yellow, magenta and cyan of subtractive colors are generated at dipole lengths of 70, 90 and 120 nm, respectively. Experimental results agree well with simulation, except in saturation of the yellow. The discrepancy may result from fabrication imprecision, which broadens a shape of the resonance, but can be overcome by manipulating evaporation parameters. Black is generated by a two-connected floating dipole that acts as a near-perfect absorber, so broad absorption appears in the visible spectrum (Fig. 3c) . This approach can also adjust the color response by tuning geometric parameters such as nanoantenna length and gap between antenna and film.
Development of grating-based 1D PCFs has achieved > 70% average efficiencies of either transmission or reflection [49, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89]. Due to structural periodicity, these devices usually exhibit angle-sensitivity and the momentum-matching constraint of surface plasmon polaritons (SPPs). These problems have been resolved simultaneously using randomly-arrayed nanostructures .
A combined design of MIM and grating can efficiently separate white light into a certain wavelength with distinct color (Fig. 4b) . The MIM-grating color filter consists of Al–ZnSe–Al resonators. Diffraction at the bottom Al grating layer helps couple incident light to plasmon waveguide modes; scattering at the top Al grating layer reconverts the detained plasmon to a propagating wave. The ZnSe layer with 100-nm thickness encourages a coupling of SP in top and bottom, so that the nanoresonators effectively actualize a conversion of photon–plasmon–photon at the resonant wavelengths. This MIM grating design has many advantages of compactness, effective transmission and narrow passband. The dependence on polarization eliminates the need for an individual polarizer layer, so these MIM/grating devices may have applications in liquid crystal displays .
3 All-dielectric structure supported by Mie resonance
Devices that use metallic nanostructures to induce structural colors can manipulate light absorption and scattering beyond the optical diffraction limit. These devices have advantages of compactness, high resolution, robustness, and compatibility with integration in various devices. DCFs have been studied to seek capabilities that complement limitations of metal, such as high loss that leads to peak broadening. Such devices exploit Mie resonance based on Mie scattering that depends on both the geometry and size of particles. In principle, dielectric nanoparticles (NPs) with high refractive index n can affect the results, with both the electric dipole and magnetic dipole having comparable contributions, whereas PCFs mainly control the electric dipole. The resonant magnetic response is caused by coupling of incident light toward the circular displacement current of the electric field as a result of retardation of phase and field penetration. This series of processes arises when the wavelength λ of the light is approximately the diameter 2R of inner particles (2R ≈ λ/n). This mechanism may also provide an opportunity to design DCFs that use Mie resonances to exploit higher-order multipoles .
Optically-generated resonant magnetic responses in dielectric NPs have been observed in particles with different geometries such as ring , spheroid [98, 99], disk and cylinder  and sphere . This diversity of optically-active shapes provides an opportunity to create diverse all-dielectric nanostructures by varying the geometric parameters of NPs to adjust both magnetic and electric resonances. Before choosing design of structure, its material of structure must be selected, because it effects the optical characteristics of the device such as transmission, reflection, optical loss and efficiency.
Many types of dielectric materials are complementarily available; each has advantages and disadvantages. Several materials cannot be used alone because of high optical losses. Hence, these materials are sometimes used in chemical or mechanical mixtures in which each has a property that compensates for the weakness of another. So far, silicon (Si) has been used most commonly, but many researchers have attempted to find optimal materials suitable for devices that require particular optical properties.
3.1 Pure Si
Si have been frequently used in printing technology because of low cost, reliability and compatibility with optoelectronic devices. Si has a high n, and can therefore manipulate light subwavelength scale [102, 103]. Importantly, Si particles with subwavelength size exhibit strong, optically-induced magnetic and electric Mie resonances at visible wavelengths. By exploiting this optical property, Si nanowires can be used as color filters to convert absorbed light to photocurrent . Si can also efficiently tailor the symmetry of light emission and enhance magnetic radiative decay [105, 106, 107, 108, 109]. Many studies have used pure Si in a variety of geometries such as nanopillars  and crosses . The studies mainly accomplished their goals of obtaining high-quality resonances in the entire visible range, which yield a great color gamut of high-purity colors. In devices with cross-shaped Si nanoresonators, the high-quality Mie resonances provide good confinement of energy to the structure .
3.2 Enhanced Si and others
Hydrogenated amorphous Si (a-Si:H) has been evaluated as the material in all-dielectric color filters (Fig. 6b) . Many of existing all-dielectric structural filters consist of crystalline Si (c-Si) [10, 114, 115, 116, 117, 118, 119, 120], but they have low transmission and a challenge to grow high-quality c-Si on foreign substrates. Compared to c-Si, a-Si:H has advantages of low cost, compatibility with complementary metal–oxide–semiconductor process, and efficient growth on foreign substrates at low temperature to achieve high refractive index. The structural filter based on a-Si:H had a higher refractive index than c-Si (Fig. 6bii). Although this design has some losses at short visible wavelengths, a-Si:H has superior optical properties, low cost, and simple fabrication, and therefore may be an alternative to other color filters.
Various materials such as TiO2 and GaP have been evaluated as alternatives to PCFs and other structural color filters [38, 121, 122, 123, 124]. TiO2 is a reasonable candidate; a recent report (Fig. 6c) obtained a suitable n ~ 2.54 at 400 nm, with near-zero extinction coefficient, which means remarkably low loss in the visible range . GaP has also merits in designing all-dielectric metamaterials. The scattering cross section of GaP is ~ 0.5 whereas it is ~ 0.1 in Au disks. The GaP absorption cross section is nearly zero from 500 nm on (Fig. 6d) ; the goal of this study was to discover a material that does not suffer from the visible-spectrum losses of PCFs through the comparison with metal (Au). Results may provide a good alternative to metals that exceeds their far-field and near-field emission efficiency.
4 Tunability and dynamic modulation of colors
Dynamic color printing is essential for practical applications such as dynamic displays, cryptography and camouflage. Research into tunability of structural coloring is increasing, due a to desire for advanced and innovative functionalities of metasurfaces. Most previous designs could only generate one static color with fixed geometry, but color filtering with tunable function would have a diversity of applications. In this section, we introduce various type of tunable color filters (TCFs) that have studied recently. Methods used include applications of LCs [32, 33, 34, 35, 127, 128, 129], of chemical transitions [130, 131, 132, 133] and of mechanical transformations [36, 37, 38, 39].
4.1 Liquid crystals
Tunable color generation can be achieved using an imprinted structure in contacted with LC . This method achieves color tunability by dynamic refractive index tuning by topological reorientation of LC. A shallow imprinted Al layer is surrounded by a high-birefringence LC (Fig. 7c). As unpolarized white light passes through the LC layer, the light couples to plasmonic modes at the imprinted metal surface. The orientation of the LC determines the spectral location of SPR, because SPR Modes depend on the dielectric constant of the surroundings. The LC’s high birefringence causes a large plasmonic shift that leads to high range of color tunability. With no applied electric field, the LC aligns parallel to the Al surface. When an electric field is applied, the LC near the imprinted surface assumes the orientation state that minimizes its internal energy. As the voltage of the electrical field is increased, the LCs keep changing their orientation until they are all normal to the surface. This method achieves higher dpi than a conventional display, and has millisecond-scale response times (Fig. 7d). This research demonstrates the benefits of the LC-plasmonic system, and suggests a method to achieve TCFs.
This tunable device composed of imprinted structure with LC has been shown to be compatible with thin-film-transistor (TFT) technology . The imprinted plasmonic surface was surrounded by a highly birefringent LC and stacked, followed by rubbed amide, ITO, and a supersubstrate layer (Fig. 7e), and the stacked layers were integrated with a TFT array (Fig. 7f). The integrated device is connected to computer so that individual pixels are manipulated via images that the monitor displays. The authors also generated full images and a video of text editing (Fig. 7f, bottom). Although prototype had demerits including image degradation by white reflection from TFT metal lines, and inability to source high voltage, this LC-plasmonic devices shows the possibility of replacing conventional displays.
4.2 Chemical transition
Dynamic displays have been achieved using Fabry–Perot cavity resonators that exploit this metal-to-dielectric transition of Mg . Hydrogen absorption by the Mg layer results in state switch between metal and dielectric; hydrogen desorption causes the reverse process. Upon hydrogenation, a capping layer is changed from Ti/Pd to TiH2/PdH, and as a result light can pass through dielectric spacers (MgH2 + HSQ) and be reflected by an Al mirror. Then Fabry–Perot resonance modes form in the cavity so that reflected light generates vivid colors (Fig. 9c, bottom, d). Under hydrogen exposure, the color changes, and reaches its final state after 78 s. Under oxygen exposure, the image is restored to the original state within 35 s.
4.3 Mechanical deformation
The change in colors can be widened and improved by using a high-contrast metastructure (HCM) composed of metagrating embedded in a transparent and flexible PDMS membrane . HCM pixels were patterned by deep ultraviolet step-lithography, then the Si metagrating was etched to remove the SiO2 layer. The HCM was covered by PDMS and detached from SOI Wafer. The resulting PDMS stamp was further protected by encapsulation in a second PDMS layer. Deformation from ε = 0–10%, tuned the color of a flower image fabricated using the suggested structures (Fig. 10b). The HCM has good repeatability under stretching cycles, so this technology may have applications in camouflage and biolabeling.
Mechanical deformation of all-dielectric metasurfaces can also achieve tunable color at visible frequencies . The design was an array of TiO2 rods embedded in a PDMS layer (Fig. 10c). The array was patterned by EBL, followed by etching. The PDMS was deposited on top of TiO2 array to form embedded TiO2 geometry (Fig. 10d). To test color tunability strain was applied the metasurfaces of TiO2 resonators in PDMS in two directions orthogonal to each other; with only 6% strain, the resonance peaks shifted 5.08% to red under x polarization, and shifted 0.96% to blue under y polarization (Fig. 10e, bottom).
5 Scalable fabrication for further practical applications
The above-mentioned structural color filters are mainly fabricated by conventional patterning methods such as EBL or focused ion beam milling. These methods seem appropriate for manufacturing subwavelength nanostructures due to an ability to fabricate them elaborately. However, these fabrication methods have limitations such as limited area, low throughput, intricate process, and high cost. In addition, the exceptional potentials and advantages of structural color filters remain a challenge to actualize in practice because of a shortage of scalable and high-speed fabrication methods. If color filtering devices can be fabricated over large area at high throughput, the technologies will have many practical applications and will be useful in major industrial fields [53, 65, 135, 136, 137, 138, 139]. Thus, a fabrication method that allows scale-up and fast manufacturing of nanostructures should be developed.
A novel optical nanomaterial based on large-scale network metasurfaces forms vibrant structural colors varied from thickness of an ultra-thin alumina coating (Fig. 12b) . This approach has biomimetic optical properties inspired by a bird, Cotinga maynana, which has blue feathers that are iridescent in a way that cannot be explained by Rayleigh or Mie scattering. A dielectric coating reflects scattered waves to increase scattering, hence generating electromagnetic energy flow and resonant coupling in an Al2O3 layer (Fig. 12c, d). Color responses and resonant reflectance can be adjusted by modulating the coating thickness, and are blue-shifted as the thickness of the dielectric is increased. The device had high mechanical resistance in a scratch test. A last illustration of figures demonstrates that this approach is compatible with large-area production (Fig. 12e).
6 Conclusion and outlook
We have reviewed recent progress in resonance-assisted color generation. Colors achieved using plasmonic resonance and Mie resonance have intriguing features such as exceedingly high resolution, near-permanent lifetime and material simplicity. The resolution may exceed 105 dpi, which surpasses the diffraction limit of light. Resonance-assisted coloring requires only a single or a few nano-size layers, so processing conditions are simple. However, the method has high patterning costs, low throughput and elaborate color tuning mechanisms; these disadvantages must be overcome before commercial applications are possible. Nanoimprinting, self-assembly and laser printing are possible solutions to achieve large-area fabrication and high throughput. LCs, chemical transition and Mechanical deformation may enable accurate and easy color tuning process.
To summarize, resonance-based color printing methods are advancing toward to real-world application. Their features such as ultrahigh resolution, brilliant optical response and compatibility with existing fabrication technologies seem to show a promising future. If fabrication costs can be reduced, and color tuning mechanisms can be improved, these methods will have important potential applications in cryptography, security, imaging, optical data storage and further optical devices.
TL and JJ contributed equally to writing the manuscript. HJ helped writing the manuscript. JR guided manuscript preparation. All authors read and approved the final manuscript.
The authors thank Gwanho Yoon (POSTECH) for the fruitful discussion.
The authors declare that they have no competing interests.
This work is financially supported by the National Research Foundation Grants (NRF-2017R1E1A1A03070501, NRF-2015R1A5A1037668 and CAMM-2014M3A6B3063708) funded by the Ministry of Science, ICT and Future Planning (MSIP) of the Korean government. JJ acknowledges Hyundai Motor Chung Mong-Koo Fellowship.
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