Dielectric chiral metasurface is a new type of planar and efficient chiral optical device that shows strong circular dichroism or optical activity, which has important application potential in optical sensing and display. However, the two types of chiral optical responses in conventional chiral metasurfaces are often interdependent, as their modulation of the amplitudes and phases of orthogonal circularly polarized components is correlated, which limits the further progress of chiral meta-devices. Here we propose a new scheme for independently designing the circular dichroism and optical activity of chiral metasurfaces to further control the polarization and wavefront of transmitted waves. Inspired by mixtures of chiral molecular isomers, we use the dielectric isomer resonators to form “super-units” instead of single meta-atoms for chiral responses in terahertz band, which is called racemic metasurface. By introducing two levels of Pancharatnam-Berry phases between meta-atoms and “super-units”, the polarization rotation angle and wavefront of the beam can be designed without the far-field circular dichroism. We demonstrate the strong control ability on terahertz waves of this scheme through simulation and experiments. In addition, this new type of device with near-field chirality but no far-field circular dichroism may also have important value in optical sensing and other technologies.

Applying the photoexcitation characteristics of vanadium dioxide (VO₂), a dynamic resonant terahertz (THz) functional device with the combination of VO₂ film and dual-resonance metamaterial was suggested to realize the ultrafast external spatial THz wave active manipulation. The designed metamaterial realizes a pass band at 0.28–0.36 THz between the dual-resonant frequencies, and the VO₂ film is applied to control the transmittance of the spatial THz wave. More than an 80% modulation depth has been observed in the statics experiment, and the dynamic experimental results illustrate that this active metamaterial realizes up to a 1 MHz amplitude modulation signal loaded on a 0.34 THz carrier wave without any low noise amplified devices. The electromagnetic properties and photoinduced dynamic characteristics of this structure may have many potential applications in THz functional components, including modulators, intelligent switches, and sensors.

Vanadium dioxide (VO₂), with reversible metal–semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO₂ nanoparticles are particularly superior to VO₂ films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO₂ (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO₂ underwent a two-stage phase evolution from the pure phase comprising VO₂ (A) nanobelts to VO₂ (M) nanorods, instead of the conventional three-stage B–A–M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO₂ (M). The phase-transition temperature of VO₂ (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO₂ (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO₂ (M) nanorods and provides significant insights into their applications in infrared smart windows.

A WO ₃ /RGO film exhibited shorter coloration–bleaching times and larger modulatory range, being attributed to faster Li ⁺ ion diffusion and electron transfer rate.