Optical Properties of Electrochemically Gated La₁₋<i>_x</i>Sr<i>_x</i>CoO_{3−<i>δ</i>} as a Topotactic Phase‐Change Material

Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion-gel-gated epitaxial films the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) 0.00 ≤ x 0.70 offer a new route to significant, voltage-tuned, nonvolatile modulation metasurfaces, shown here through Kramers–Kronig-consistent dispersion models, structural electronic transport characterization, electromagnetic simulations before after electrochemical reduction. As-grown are high-index insulators < 0.18 but lossy metals > 0.18, due percolation insulator-metal transition. Positive-voltage gating LSCO transistors reveals metal-insulator transition from metallic phase (n 2.5), low-loss insulating phase, accompanied by oxygen-vacancy-ordered brownmillerite transformation at high x. At despite nominally character, undergo remarkable another lower-index, lower-loss state Δn 0.6. In plasmonic these insulator-insulator transitions support varied mid-infrared reflectance modulation, thus framing electrochemically gated as diverse library room-temperature phase-change materials applications including dynamic imaging, camouflage, memories.