Full-Color-Tunable Nanophotonic Device Using Electrochromic Tungsten Trioxide Thin Film

Abstract

Color generation based on strategically designed plasmonic nanostructures is a promising approach for display applications with unprecedented high-resolution. However, it is disadvantageous in that the optical response is fixed once the structure is determined. Therefore, obtaining high modulation depth with reversible optical properties while maintaining its fixed nanostructure is a great challenge in nanophotonics. In this work, dynamic color tuning and switching using tungsten trioxide (WO 3 ), a representative electrochromic material, are demonstrated with reflection-type and transmission-type optical devices. Thin WO 3 films incorporated in simple stacked configurations undergo dynamic color change by the adjustment of their dielectric constant through the electrochromic principle. A large resonance wavelength shift up to 107 nm under an electrochemical bias of 3.2 V could be achieved by the reflection-type device. For the transmission-type device, on/off switchable color pixels with improved purity are demonstrated of which transmittance is modulated by up to 4.04:1.

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

“…The change in extinction coefficient is bounded in the vis–UV spectrum, where the Pr 4+ energy levels are located, while no change is obtained below 2 eV. This is in contrast to other optically tunable materials, e.g., in vanadium dioxide metal–insulator phase transitions, [ 47 ] phase change materials [ 9 ] or cation intercalated oxides [ 10,15 ] where the change in k is broad, extending into the NIR spectrum, thus increasing unwanted optical losses. The maximum Δ k is shifted from the maximum Δ n r , consistent with the Kramers–Kronig relations, yielding a more than unity figure‐of‐merit for a low loss tunable optical material [ 9 ] at the real refractive index change maximum (at 2.23 eV), Δ n r /Δ k ≈ 3.4.…”

“…The magnitude of transmission modulation obtained here is comparable to previous reports on tunable optical devices, e.g., based on the formation of silver inclusions in oxide thin films, [ 6 ] or on cation intercalation into oxide thin films. [ 15 ]…”

“…[ 34,35 ] These earlier studies serve as the basis for the here reported chemical control of PCO's complex refractive index within the visible–UV spectrum. The results reported here point to the future potential for thermally enhanced tuning of the optical properties of MIEC oxides and thereby the implementation of active thin film optical devices in the vis–UV spectrum based on them, e.g., dynamic color displays, [ 4,5,15 ] smart windows, [ 16–18 ] tunable plasmonics, [ 6 ] and reprogrammable metasurfaces. [ 10 ]…”

“…One of the possible optical tuning approaches is based on chemically or electrochemically driven exchanges of atomic species between matrix host materials and external solid, liquid, or gaseous sources. This approach has been successfully applied in a number of materials systems, for example, implementation of dynamic color displays [ 5 ] and metasurface holograms [ 7 ] based on plasmonic particles with differing optical responses upon reaction with hydrogen versus oxygen; voltage‐controlled plasmonic device, where humid atmospheres serve as a proton source or sink upon bias application [ 4 ] ; self‐holding optical actuator, [ 14 ] tunable nanosized color pixels, [ 15 ] and reconfigurable metasurfaces [ 10 ] based on the electrochemical intercalation of lithium ions; full‐color switching in plasmonic nanoslit arrays using electrochromic polymers [ 13 ] ; and gasochromic and electrochromic transition metal oxide smart windows. [ 16–18 ] Tuning of optical properties by electrical bias was previously realized in, e.g., liquid‐crystal‐based tunable ring resonators [ 19 ] ; plasmonic color filters [ 20 ] ; Fresnel plate lenses with variable focus length [ 12 ] and dielectric metasurfaces [ 11,21 ] ; phase‐change‐material‐based optical switches [ 9 ] and dynamic optical beam steering; [ 22 ] ionic [ 6 ] and field effect [ 23 ] devices.…”

“…Mixed ionic–electronic conductor (MIEC) materials, supporting both mobile ionic defect and electronic charge carriers, have been utilized for some time in electrochromic applications [ 15 ] with the suggestion that other MIECs could as well exhibit tunable optical properties. [ 10 ] MIEC oxides that contain transition and lanthanide metal ions are particularly amenable to changes in their optical properties, e.g., their apparent color in the visible spectrum, given the ability to modify the oxidation states, i.e., δ, of the multivalent ions, e.g., Pr 4+/3+ in PCO [ 26 ] or Fe 3+/4+ in SrTi 1– x Fe x O 3– δ .…”