ARTICLE https://doi.org/10.1038/s41467-019-12689-2 OPEN Ultrahigh resolution and color gamut with scattering-reducing transmissive pixels June Sang Lee1, Ji Yeon Park1, Yong Hwan Kim2, Seokwoo Jeon3, Olivier Ouellette 4, Edward H. Sargent 4*, Dong Ha Kim 1,5,6* & Jerome K. Hyun1* While plasmonic designs have dominated recent trends in structural color, schemes using localized surface plasmon resonances and surface plasmon polaritons that simultaneously 1234567890():,; achieve high color vibrancy at ultrahigh resolution have been elusive because of tradeoffs between size and performance. Herein we demonstrate vibrant and size-invariant trans- missive type multicolor pixels composed of hybrid TiOx-Ag core-shell nanowires based on reduced scattering at their electric dipolar Mie resonances. This principle permits the hybrid nanoresonator to achieve the widest color gamut (~74% sRGB area coverage), linear color mixing, and the highest reported single color dots-per-inch (58,000~141,000) in transmis- sion mode. Exploiting such features, we further show that an assembly of distinct nanor- esonators can constitute a multicolor pixel for use in multispectral imaging, with a size that is ~10-folds below the Nyquist limit using a typical high NA objective lens. 1 Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 03760, Republic of Korea. 2 R&D Center, KOS, Inc., Hanam, Gyeonggi-do 12930, Republic of Korea. 3 Department of Materials Science and Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. 4 Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON M5S 3G4, Canada. 5 Division of Chemical Engineering and Materials Science, College of Engineering, Ewha Womans University, Seoul 03760, Republic of Korea. 6 State Key Laboratory of Molecular Engineering of Polymers, Fudan University, 200433 Shanghai, China. *email: [email protected]; [email protected]; [email protected] NATURE COMMUNICATIONS | (2019) 10:4782 | https://doi.org/10.1038/s41467-019-12689-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-12689-2 fi he ability to lter vibrant colors within submicron spatial aligned 1D Ag-TiOx-Ag nanoresonators that use resonant scat- scales is key to achieving advanced display1,2, optical tering reduction at the fundamental (electric dipole) resonance to T 3,4 2,5,6 7,8 fi storage , security encryption , multiplexing , bio- lter light polarized along the long axis of the optical element (i.e., imaging/sensors9, and multispectral/hyperspectral imaging10–12 s-polarized light). Color selectivity is determined not by the exhibiting enhanced spatial and spectral resolution. The filter period of an array nor is it determined by the coupled fields needs to support the display of spectrally pure individual colors, subtending the interfaces of two metallic substructures. Instead, access to a wide-gamut color space that includes highly vibrant or we form an enhanced photonic resonance within the oxide core saturated colors, size-invariant response, maintenance of similar assisted by the metallic shell, corresponding to a spectrally pure intensities among different colors, use of intuitive color mixing fundamental mode. Exploiting the oxide-metal core-shell nano- algorithms, and ability to independently control saturation and wire geometry, we show, in this work, one of the highest sRGB hue without disturbing the other. Filtering colors from micron space coverages of ~74% corresponding to a wide gamut that and submicron pixels has already been demonstrated in includes high color vibrancy, and highest lateral resolution of transmissive3,13–16 and reflective17–22 designs. Many of these 141,000 DPI reported for transmissive color pixels at a fixed filters have relied on the use of surface plasmon polaritons (SPPs), polarization. The realization of ultrahigh resolution and color which can be excited diffractively in metallic gratings, or the use saturation further allows the nanoresonators to achieve linear of localized surface plasmons (LSPs) generated with metallic multicolor mixing at confined spatial scales. These features are substructures. These designs provide tunable colors via structural well suited for snapshot multispectral imaging filters, which are control; however, a compromise must be made between color subject to stringent size requirements several times higher than vibrancy and spatial resolution23 which challenges the filters from that of the underlying pixel array. simultaneously satisfying the conditions described above. Plas- monic gratings involving the coupling of SPPs offer highly saturated colors, but lack spatial resolution because a large Results number of grating elements is needed to facilitate momentum Vibrant and transmissive color pixels based on reduced scat- matching between light and SPPs13. Moreover, since the period tering. A schematic showing blue, green and red transmissive fi dictates color selectivity, mixing colors within a con ned space is pixels (Fig. 1a) depicts an array of TiOx nanowires sandwiched difficult and unintuitive. LSP-based metal substructures including between two layers of Ag film, supported by a glass substrate nanodisks above a metal film provide sub-diffraction-limited (detailed fabrication process is described in the Methods section resolution but lack color vibrancy because plasmon damping at and Supplementary Fig. 1). The choice of the core dielectric arises the metal interface attenuates the quality factor of the mode17. from the need to minimize the nanowire size and optical loss for Although such structures can achieve ultrahigh resolutions improved spatial resolution and color saturation, respectively. fl exceeding 100,000 dots-per-inch (DPI), they operate in re ection TiOx exhibits a large real part and negligible imaginary part of the mode20,24, with applications aimed primarily toward displays or permittivity, satisfying both conditions. S-polarized light is nor- prints. One can question the need for such ultrahigh resolution in mally incident from the top. In this example, the period between these cases since it exceeds the resolving power of the human eye the nanowires was made constant for all pixels. The bottom and by several orders of magnitude25, while the color gamut is poorer top Ag film thicknesses were 20 and 26 nm, respectively, which than that of standard commercial pixels. provided the optimum pixel contrast by balancing the quality of Other designs that achieve ultrahigh resolution in structural the photonic resonance of the nanoresonator with light leakage colors have been demonstrated with dielectric Mie resonators in through the bare Ag background. The detailed role of each film is the form of Si nanowires26, nanoblocks27,28, and Cr/Si hybrid further described in Supplementary Figs. 2 and 3. nanoblocks29. The large index contrast between resonator and air As shown schematically in Fig. 1a, the base width and height of fi helps to prevent mode overlap between adjacent elements to the TiOx nanowire determines the wavelength of the ltered light, achieve high DPIs but at the expense of introducing multipolar whereby an increased width shifts the filtered color to a longer resonances that compromise the spectral purity. Furthermore, Si wavelength. SEM scans of the fabricated blue, green and red color absorbs in the visible range, resulting in nontrivial optical losses. pixels are shown in Fig. 1b from left to right respectively, imaged Photonic (Mie) resonances have previously been investigated at a 63° tilt angle. Well-defined and regular nanoresonators are in horizontally-resting bare nanowires using photons30 and fast inside each pixel of 10 µm × 10 µm size. To evaluate the shape of electrons31. When a metallic shell of an appropriate thickness is the nanoresonators, we prepared cross-sections by FIB milling introduced, these resonances are strongly enhanced, enabling each pixel. SEM images of the cross-sections (bottom right inset enhanced nonlinear signals in ZnO nanowires32. Scattering is also of Fig. 1b) reveal trapezoidal shapes for wide nanowires (width reduced, leading to the creation of a single Ag-ZnO-Ag hybrid above 310 nm) and triangular shapes for thin nanowires (width nanowire color filter33 and an invisible photodetector based on a below 80 nm). We discuss this effect more in detail later, as it Si nanowire34. In the quasi-static approximation, complete scat- provides an important pathway for achieving full sRGB colors. tering cancellation occurs when the electric field of the incoming The filtering function of the fabricated pixels is depicted in light encounters a two-component core-shell nanoparticle or Fig. 1c. As shown in the transmission spectrum of a representa- nanowire whose inner and outer components exhibit out-of- tive, orange pixel in Fig. 1c, the color is dominated by a single phase overall electric dipole moments of identical strengths35.At peak in the visible range. The calculated and measured an appropriate geometric ratio of inner and outer components, transmission spectra agree qualitatively, except near the deep- the two opposing overall electric dipole moments effectively blue wavelength region because an increased uncertainty in cancel and the incident wave passes through the hybrid structure spatial parameters of the calculated structure, modeled from undisturbed. Such a phenomenon can also be found from an cross-sectional SEM images, is introduced