LaZrGaO: Band-Inversion Strategy in Topology-Protected Octahedron for Large Nonlinear Response and Wide Bandgap.

The contradictory relationship between band gaps and the second-harmonic generation (SHG) response constitutes a formidable challenge in the rational design of infrared nonlinear optical (IR NLO) crystals. In oxide-based crystals, the incorporation of strongly distorted octahedra containing d° cations as central elements are a common approach to enhance SHG responses, while inadvertently leading to a significant decrease in band gaps due to the unfavorable energy level splitting. In this study, an innovative "4d/5s electron band-inversion" strategy is introduced to enhance SHG response while preserving a wide band gap within the octahedron-symmetry-protected langasite structure. A novel high-performance IR NLO crystal, LaZrGaO (LGZr) is successfully synthesized, where the unoccupied 4d orbitals of the Zr cation underwent a transition from the valence band to the bottom of the conduction band, and the ZrO octahedra exhibited minimal distortion. Consequently, LGZr exhibited the largest SHG response observed to date (reaching up to 2.4 × LaNbGaO) and the broadest band gap (5.16 eV) within the langasite family. Furthermore, LGZr is revealed with a remarkable laser damage threshold (1.66 GW/∼cm) and broad IR transmission capabilities (∼7.8 µm) and supported the growth of centimeter-sized single crystal. The "band-inversion strategy" offers significant advantages to realize high-performance IR NLO crystals.