Optical Properties of Electrochemically Gated La_1−xSr_xCoO_3−δ 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 of the perovskite cobaltite La_1−xSr_xCoO_3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage-tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig-consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As-grown perovskite films are high-index insulators for x < 0.18 but lossy metals for x > 0.18, due to a percolation insulator-metal transition. Positive-voltage gating of LSCO transistors with x > 0.18 reveals a metal-insulator transition from the metallic perovskite phase to a high-index (n > 2.5), low-loss insulating phase, accompanied by a perovskite to oxygen-vacancy-ordered brownmillerite transformation at high x. At x < 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower-index, lower-loss insulating perovskite state with Δn > 0.6. In simulations of plasmonic metasurfaces, these metal-insulator and insulator-insulator transitions support significant, varied mid-infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room-temperature phase-change materials for applications including dynamic thermal imaging, camouflage, and optical memories.