La3ZrGa5O14: Band-inversion Strategy in Topology-Protected Octahedron for Large Nonlinear Response and Wide Bandgap

Abstract

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 d0 cations as central elements is 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, we introduced an innovative "4d/5s electron band-inversion" strategy to enhance SHG response while preserving a wide band gap within the octahedron-symmetry-protected langasite structure. We successfully synthesized a novel high-performance IR NLO crystal, La3ZrGa5O14 (LGZr), where the unoccupied 4d orbitals of the Zr4+ cation underwent a transition from the valence band to the bottom of the conduction band, and the ZrO6 octahedra exhibited minimal distortion. Consequently, LGZr exhibited the largest SHG response observed to date (reaching up to 2.4× La3Nb0.5Ga5.5O14) and the broadest band gap (5.16 eV) within the langasite family. Furthermore, LGZr was revealed with a remarkable laser damage threshold (1.66 GW/cm2) and broad IR transmission capabilities (~7.8 μm), and supported the growth of centimeter-sized crystals. The "band-inversion strategy" offers significant advantages to realize high-performance IR NLO crystals.