TiO ₂ metasurfaces: From visible planar photonics to photochemistry

Abstract

TiO2 metasurfaces have been intensively studied in the past few years. To date, the TiO2 metadevices only used their high reflective index (n). The controllable light extinction coefficient (k) of TiO2 has not been exploited yet. Here, we converted TiO2 metasurfaces to black TiO2 metasurfaces and explored their new opportunities in photochemistry. A complementary metal oxide semiconductor (CMOS)–compatible technique has been developed to reversibly and precisely control the absorption of TiO2 metasurfaces without spoiling their internal nanostructures. Consequently, two types of black TiO2 metasurfaces were realized for photochemical experiments. The metasurface with an ultrawide absorption band can substantially enhance the white light absorption and accelerate the solar-based photochemistry process by a factor of 18.7. The other metasurface with an absorption band of <20 nm only responded to the resonant wavelengths, making the photochemistry process capable of being monitored in real time. In addition, the reversible switch between normal and black states makes TiO2 metasurfaces suitable for dynamic metadevices as well.

All-dielectric metasurface for high-performance structural color

“…More than the wavelength shift, the FWHMs of main peaks are around 34-40 nm in Fig. 3c, close to the FWHM of TiO 2 metasurfaces 30,43 and 40% narrower than the ones in air (see Fig. 3b).…”

Embedded metal oxide plasmonics using local plasma oxidation of AZO for planar metasurfaces

“…Vertical depth profile of the carrier concentration modulation of the films is uniform in our devices but could potentially be controlled by grey scale lithography to create a whole new category of materials. [54] Other materials amenable to local carrier concentration include TiO 2 [55] or 2D transition metal dichalcogenides (TDMC) such as MoS 2 [56,57] which could be the topic of future investigations. We believe that plasma patterning techniques have broad potentials for electronic and photonic applications to achieve novel structures once unimaginable or beyond the existing manufacture capability.…”

Dynamic plasmonic color generation enabled by functional materials

“…Wu et al (30) introduced a CMOS (complementary metal-oxide semiconductor)-compatible technique to alter the absorption properties of functional dielectric TiO 2 metasurfaces for dynamic color generation. In contrast to the WO 3 -based switching strategies, oxidation and reduction were realized by implanting H + ions and O − ions in an inductively coupled plasma etcher, respectively.…”

“…The optical properties of magnesium (Mg) and magnesium hydride (MgH 2 ), respectively, for example, can be reversibly controlled by hydrogenation and dehydrogenation, suggesting Mg as a plasmonically active ( 26 ), functional material for dynamic color control ( 14 , 27 – 29 ). In analogy to plasmonic coloration, structural colors produced by dielectric metasurfaces can be actively controlled by the intrinsic optical properties, e.g., the absorption of the dielectric material ( 30 ).…”

“…Copyright 2020, Macmillan Publishers Limited. (C) Adapted from ( 30 ). © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.…”