Multifunctional Terahertz Transparency of Thermally Oxidized Vanadium Metasurface over Insulator Metal Transition

Abstract
Vanadium dioxide (VO2) is one of the most promising materials for active metasurfaces due to the insulator-metal transition, urging the development of an etching-free patterning method and realization of multifunctionality in various spectral bands. Here, without etching, photolithography of vanadium metal followed by thermal oxidation achieve all-VO2 slit array metasurfaces that can be exploited as a multifunctional terahertz (THz) transparent electrode. The metasurfaces retain approximately constant THz transparency over the phase transition while the electrical conductivity of the VO2 lines changes about a thousand times, and near-infrared (NIR) diffraction is switched selectively. Numerical simulation shows that, during the phase transition, a decrease in THz transmission through the VO2 lines is compensated for by funneling through the slits, which is especially efficient with a deep subwavelength period. On the contrary, at the NIR range, the optical path difference between the slits and the VO2 lines is controlled according to the VO2 phase, enabling switching between constructive and destructive interferences for a specific diffraction order. It is expected that the demonstrated patterning method and multifunctional THz transparency will promote VO2-based metasurfaces, finding multispectral applications such as THz/NIR hybrid communication.

A research team, led by the Nano Optics Group within the Department of Physics at UNIST has reported that they succeeded in achieving all-vanadium dioxide (VO2) multifunctional metasurfaces which perform as a transparent window in the broadband THz regime with variable DC conductivity of dynamic range over three decades and selective switchability of near-infrared (NIR) diffraction.

Figure 1. Fabrication and characterization of VO2 metasurfaces. Schematics of fabrication steps: A vanadium slit array is patterned on a c-plane sapphire substrate by photolithography. After that, the vanadium slit array is annealed under optimized oxygen pressure and temperature. After the annealing process, the thickness of the film doubled.

Metasurfaces are made of artificially two-dimensional (2D) subwavelength-scaled nanostructures whose properties could not be found in natural materials. The newly-proposed metasurfaces are expected to be used in various applications, such as the next-generation wireless communication systems, such as 6G.

In this study, the research team successfully fabricated slit arrays with micrometer scale dimensions by conducting photolithography, followed by thermal oxidation of pre-defined vanadium metal structures without etching damage on the substrate.

Figure 2. Selective switching of the zeroth-order diffraction at 790 nm. a) Modulation of zeroth-order transmission intensity by heating and cooling of the VO2 metasurface (p = 22 µm, w = 4 µm). b) Camera images of diffraction patterns for the insulating (30 °C) and metallic (78 °C) states and simulated diffraction intensities. c) Repetitive modulation of zeroth-order intensity under 10 Hz optical heating.

The research team further examined the transmission, reflection, and absorption in the THz regime across the phase change temperature experimentally and theoretically and revealed that the metasurface is constantly transparent while the VO2 electrodes change from insulating to metallic due to light funneling through the subwavelength slits.

Furthermore, by thermally controlling the NIR optical path difference between the slits and the VO2 electrodes, the research team could achieve successful switching between destructive and constructive interferences of the zeroth-order diffraction.

“The etching-free fabrication method and multifunctional THz transparency demonstrated here will be fruitful for VO2 metasurface applications, such as multispectral smart windows and hybrid communications,” noted the research team.

The findings of this study have been published in the November 2022 issue of Laser & Photonics Reviews, a renowned scientific journal journal covering research on all aspects of optical science. This study has been supported by the National Research Foundation of Korea (NRF).

Journal Reference
Hyosim Yang,Dai-Sik Kim,Hyeong Seok Yun, et al., “Multifunctional Terahertz Transparency of a Thermally Oxidized Vanadium Metasurface over Insulator Metal Transition,” Laser Photonics Rev., (2022)