Plasmonic- and dielectric-based structural coloring: from fundamentals to practical applications

2.1 Nanohole

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 [15]. 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 [46–51].

In PCFs with asymmetric cross-shaped apertures, one nanoaperture can generate two colors (Fig. 1a). PCFs in aluminium (Al) film can have polarization-adjustable properties that result in selectivity between two transmitted colors [52]. By manipulating geometric parameters such as long arm (LA), short arm (SA) and periodicity of nanocavity apertures, various colors are obtained. Exchanging the polarization directions of the LA and SA result in dual color variations. No color changes in SA-variable palettes appear at a same period when light is transmitted through the LA and polarized along the SA. In contrast, the color changes gradually when light is transmitted through the SA and polarized along the LA (Fig. 1b). Therefore, by controlling polarization direction of incident light an operator can selectively obtain one of two colors from the plasmonic device.

Nanohole-shaped PCFs based on silver (Ag) instead of Al or gold (Au) can produce color-enhanced transmissive structural colors [53]. 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.

2.2 Metal–insulator–metal

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 [54–64]. To improve efficiencies or color purity, researchers have changed basic materials [65–67].

A research based on Ag-alumina (Al₂O₃)–Ag exploiting tandem nanodisks demonstrates a wide range of color generation by adjusting geometric parameters such as periodicity and radius of structures (Fig. 2a) [68]. Previous works on plasmonic color printing have mostly concentrated on hue instead of color brightness and saturation. These methods suffer from relatively broad full width at half-maximum, shallow peaks, and dips in spectra. A device based on Ag–Al₂O₃–Ag consists of an array of tandem nanodisks to solve these limitations, and achieved bright and saturated colors with red, green and blue in reflective mode, and cyan, magenta and yellow in transmissive mode (Fig. 2ai, ii). These complementary near-full colors are attributed to a combination of Wood’s anomaly and an in-phase electric dipole. This device is relatively easy to fabricate because it uses comparatively large nanodisks.

Plasmonic stereoscopic printing has been demonstrated [69]. 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.

2.3 Nanorod

Color imaging devices based on nanorod arrays can be made to demonstrate a color spectrum by manipulating their resonances [70–76]. The generated colors are mainly attributed to an interaction between plasmon resonance that is related to peaks in spectra, and Fano resonance that is related to dips in them. Although the influence of the Fano resonance is not critical, it helps to narrow the spectral peaks to sharpen and enrich the colors. Near-full-color printing has been achieved using Ag and Au nanodisks on a backreflector that encodes the color information (Fig. 3a) [77]. Each color pixel consists of four nanodisks; the results are achieved by varying the diameter of disks and the gap between them, but are not affected by their periodicity. Use of a metallic capping layer can also intensify the color (Fig. 3b). This approach is compatible with nanoimprint lithography, and may therefore be amenable to scale-up and high-throughput fabrication.

A novel nanorod-shaped plasmonic pixel can produce optical resonances over the entire visible spectrum [78]. 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) [30]. This approach can also adjust the color response by tuning geometric parameters such as nanoantenna length and gap between antenna and film.

2.4 Grating

Development of grating-based 1D PCFs has achieved > 70% average efficiencies of either transmission or reflection [49, 79–89]. 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 [90].

Polarization-dependence can also appear in grating-based color-printing devices. A silver-based grating color filter that uses a plasmonic phase retarder causes dependence on incident polarization by simple rotation of a polarizer [91]. By changing the different polarizer settings, the same input image can be given very different color properties (Fig. 4a). The 45° and 135° polarization angles generate strongly-contrasting colors of purple, dark blue, orange and yellow. Photomask used to make the images is divided into five areas according to the grating thickness of the Ag layer. The analyzing polarizer has two inverse regions to improve optical appearance. Polarization-sensitive color printing technology has potential applications in security [92, 93], sensing [94], polarizing detectors [76] and magneto-optics [95].

A combined design of MIM and grating can efficiently separate white light into a certain wavelength with distinct color (Fig. 4b) [62]. 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 [96].

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 [104]. Si can also efficiently tailor the symmetry of light emission and enhance magnetic radiative decay [105–109]. Many studies have used pure Si in a variety of geometries such as nanopillars [110] and crosses [111]. 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 [111].

Research using pure Si nanodisks obtained colors and spectra that are distinct from those obtained using metal (Al, Ag) nanodisks [112] (Fig. 5a). Si provided monotonous hue variations, whereas metals exhibit rapid color changes. These differences occur because peaks are clearer in Si spectra than in Al and Ag. Colors and their intensity produced by the DCFs can be adjusted by tuning the diameter and gap of nanodisks (Fig. 5b). This color exhibits little angle dependence (Fig. 5c). A reproduction of a painting replicated the original well (Fig. 5d).

3.2 Enhanced Si and others

To enhance the optical properties of Si, many researchers have attempted to find improved materials and designs. Germanium (Ge) has been evaluated as a substitute for Si (Fig. 6a) [113]. Ge does not have high-quality resonance due to high absorption in the visible range, and therefore cannot be used alone as a color filter. A material composed of Si (0.8) and Ge (0.2) achieved higher refractive index and lower absorption coefficient than pure Si in most of the visible region. The ratio of Ge must be minimized to avoid high absorption.

Hydrogenated amorphous Si (a-Si:H) has been evaluated as the material in all-dielectric color filters (Fig. 6b) [28]. Many of existing all-dielectric structural filters consist of crystalline Si (c-Si) [10, 114–120], 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, 121–124]. 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 [125]. 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) [126]; 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.

4.1 Liquid crystals

One method of achieving tunability is to exploit the anisotropy and rearrangement of LCs under applied voltage; this approach achieved rotating polarization of incident light at will. An electrically-tunable color filter with a LC polarization rotator has been achieved [127] by exploiting guided mode resonance with waveguide layers, and diffracted wave by an asymmetric Al pattern to govern resonance in proposed resonators. The design used Al nanoresonators that have an asymmetric gap distance between the resonators along x and y directions, and therefore respond differently to x-polarized and y-polarized light (Fig. 7a). With no voltage applied to a unit cell, polarization of incident light is rotated to 90°, but when applied voltage exceeds a threshold V_a, polarization is maintained at 0°. At voltage < V_a, x and y polarizations coexist. Transmission spectra by polarization control with voltage manipulation is expressed as T _θ  = T₀(λ) cos²θ + T₉₀(λ) sin²θ where T₀ and T₉₀ are the transmissions of x and y polarization, and θ is polarization angle. Consequently, the proposed visible dichroic resonator can be used to tune expressed color in a narrow range of CIE 1964 color coordinates (Fig. 7b).

Tunable color generation can be achieved using an imprinted structure in contacted with LC [128]. 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 [129]. 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 plasmonic-nanostructures formed by electrodepositing and electrostripping were evaluated as biomimetic camouflage [130]. The device exploited electrodeposition-induced color transformability of Au/Ag core–shell nanostructures. The plasmonic arrays were composed of a bottom indium tin oxide (ITO) layer, and a perforated SiO₂ layer with Au/Ag core shells embedded in the holes (Fig. 8a). A plasmonic unit cell is produced by packaging the arrays into a device and filled with gel electrolyte that contain Ag⁺ ions. The coloring process exploits two phenomena: the physical configuration of the plasmonic core shells is tuned by applying a voltage, and the redox reaction of Ag is hysteretic (Fig. 8b). As Ag deposition times increase, the nanodomes inside holes grow, and as a result the cells change color. The plasmonic cells achieved dynamic camouflage (Fig. 8c, d). The plasmonic cells were attached to the body of a mechanical chameleon that was equipped with miniature color sensors. The information from color sensors was analyzed and sent to each cell, and the colors of the chameleon were altered to match the acquired signals. The plasmonic cell device was also fabricated on a flexible substrate; this success shows the possibility of wearable dynamic camouflage.

Catalytic magnesium (Mg) metasurfaces can be used as dynamic plasmonic color displays [131]. Plasmonic colors were printed, tuned, erased and restored by exploiting time-dependent hydrogenation and dehydrogenation of Mg NPs. When placed between a titanium (Ti) adhesion layer and a Ti/palladium (Pd) complex and exposed to hydrogen, Mg NPs absorb ~ 7.6 wt% hydrogen and is changed from a metal to a dielectric (MgH₂) (Fig. 9a). The Mg NPs lose their plasmonic features, and reflectance spectra lose all of the features that they showed before the transition. Therefore, hydrogenation can be regarded as an erasing process; dehydrogenation can be regarded as a restoring process. These erasing and restoring processes have possible applications as optical data encryption and decryption (Fig. 9b). This new concept of plasmonic coloring has potential application such as anti-counterfeiting, optical information encryption, and animation printing.

Dynamic displays have been achieved using Fabry–Perot cavity resonators that exploit this metal-to-dielectric transition of Mg [132]. 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

Tunability of metasurfaces [39] has applications and possibilities for commercialization. Mechanical deformation is one approach to achieve this tunability. Structural color can be generated by mechanical deformation [36]; specifically by stretching a photonic-crystal slab patterned on a flexible and elastic substrate. This method has achieved resonance-induced reflectance that has sharp resonance peaks; this process is related to Fano resonance [134]. Optical Fano resonance can be regarded as an interference effect between directly reflected light and modes that leak into the environment. Because the reflected light and radiated light have same phase information, constructive interference sharpens the resonance. The reflectance peak can be controlled at will by changing the periodicity a in an array of nanodots (Fig. 10a). An array of Si nanorods was patterned on polydimethylsiloxane (PDMS) substrate, then attached to a black balloon to stretch the sample isotropically. Stretching to 10% yielded 32-nm spectral shift; repeated stretching did not cause degradation.

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 [37]. 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 [38]. 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).