Tunable Metasurfaces Based on Charge Density Waves in 1T-TaS₂

Tunable metasurfaces, especially with tunable color, are promising for many applications in adaptive optics and sensing when compactness and integration are necessary. Many techniques enable tunability – field-effect, structural phase change, mechanical stretching, electrochemistry, photochromic effects, and so on. However, the tunability with these approaches suffers from either being too slow or too small. For example, the state-of-the-art spatial light modulators (SLMs) using liquid crystals operate at less than 1 kHz. MEMS deformable mirrors are also limited to about 1 kHz. This limitation arises from the fact that the fundamental light-matter interaction in these approaches relies upon individual electrons interacting with photons. This limitation could be overcome by adopting a fundamentally new regime of light-matter interaction. In quantum materials or strongly correlated materials, a photon interacts with many ‘connected electrons.’ Thus, a stimulus such as light interaction can perturb not just a single electron, but the sea of ‘connected electrons’ to result in far greater tunability while still being fast. Here we propose 1T-TaS₂, a 2D quantum material with giant electrical and optical tunability at room temperature, for low-power, MHz-fast tunable meta-devices.<br/>1T-TaS₂ is a quantum 2D material exhibiting a strong charge order even at room temperature. Previously, the electrical properties of 1T-TaS₂ have been shown to greatly change extensively change with various stimuli such as strain, chemical environment, and electrical bias. However, the optical properties remained little known. Recently, we studied the optical properties of 1T-TaS₂ under electrical bias and different illumination conditions. We reported fast tunable optical properties of this material arising from the reorganization of charge density wave (CDW) stacking. Our experimental investigation suggested that CDW domains in 1T-TaS₂ respond to light or electrical bias by switching their stacking configuration in a sub-microsecond timescale. The change in stacking configuration results in a unity-order change in refractive index at room temperature. Such huge tunability at such low-level stimulus (100 mW/cm² ~ 1 Sun) makes 1T-TaS₂ promising for tunable nanophotonic applications, especially on mobile platforms.<br/>The giant index change observed in 1T-TaS₂ results in a huge change in bulk properties when combined with plasmonic/dielectric structures. Plasmon resonances induce strong vertical fields that interact with the change in c-axis permittivity of 1T-TaS₂ and thereby enhance the effects of tunability of bare 1T-TaS₂ films. We experimentally demonstrate a tunable meta-grating fabricated on 1T-TaS₂. The blazed grating shows 100% modulation depth at just 5 mW/cm² intensity of 514 nm wavelength laser. The grating has a bandwidth of 1 MHz and operates reliably in ambient air for at least two continuous days. Further, we designed a Fano-resonant metasurface on 1T-TaS₂ that exhibits a large change in its reflectance spectrum with different illumination intensities. The reflectance change corresponds to the blue and red colors for the 5000 K illumination source. Such MHz-fast, color-changing metasurfaces are promising for many applications including LiFi and 3D displays.