MEMS-actuated terahertz metamaterials driven by phase-transition materials

Abstract

The non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO₂). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO₂, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems.

Keywords: MEMS; Metamaterials; Phase-transition material; THz; VO2.

Fig. 1

a Schematic of counterclockwise spiral cantilevers fully flat (top) and curving up (bottom) (Note: clockwise spiral cantilevers are also made). b Simulated azimuth polarization rotation angle θ for flat spiral cantilevers. c Simulated azimuth polarization rotation angle θ for curved spiral cantilevers. d Simulated ellipticity angle η for flat spiral cantilevers. e Simulated ellipticity angle η for curved spiral cantilevers

Fig. 2

a Cantilever curvature vs. overall thickness for different Au/Cr/VO₂ ratios. (I) and (M) indicate the VO₂ in the insulating or metallic phases. Inset: Schematic of the tri-layer cantilever and the thin film materials of the cantilevers. b Cantilever curvature change during actuation for different Au/Cr thicknesses and 120 nm VO₂. A deeper color indicates a larger curvature change against the VO₂ phase transition. The blue line indicates 0 curvature when VO₂ is in the insulating phase at 30 °C. The yellow line indicates 0 curvature when VO₂ is in the metallic phase at 90 °C

Fig. 3

a SEM image of the VO₂ thin film, the scale bar size is 500 nm. Inset: Cross-sectional view of ~ 120 nm VO₂ on a 500 nm SiO₂/Sapphire substrate, the dash line indicates the interface between VO₂ and SiO₂, the scale bar is 500 nm. b Resistance–temperature change of the VO₂ thin film. c Optical microscope image of the spiral cantilevers curving up at 30 °C, the scale bar is 200 μm. Inset: SEM image of the spiral cantilevers curving up. The scale bar is 50 μm. d Optical microscope image of the spiral cantilevers fully flat at 90 °C, the scale bar is 200 μm

Fig. 4

a Schematic of the THz-TDS system. b Measured azimuth polarization rotation angle changes between the curved state, θ (30 °C), and the flat state θ₀ for clockwise and counterclockwise spirals. c Measured ellipticity angle changes between the curved state η (30 °C), and the flat state η₀, for clockwise and counterclockwise spirals. d Measured azimuth polarization rotation angle change θ–θ₀ and ellipticity angle change η–η₀ vs. temperature at 0.72 THz