Nonlinear Optical Properties of 0D Chiral Hybrid Bismuth Iodides

Abstract

Chiral hybrid metal halides show great promise for nonlinear optical (NLO) applications like circularly polarized second harmonic generation (SHG). The inherent toxicity of lead is a concern for the widespread adoption of frequently explored lead-based chiral hybrid halides. Here, we report the second and third-order NLO properties of lead-free 0D chiral compounds, (R-/S-MBA)4Bi2I10, and their achiral counterpart, (Rac-MBA)4Bi2I10 (MBA: methylbenzylammonium) under excitation wavelength ranging 1360–1590 nm. Chiral (R-/S-MBA)4Bi2I10 exhibits strong SHG along with stronger third harmonic generation (THG). The chiral crystals showed high sensitivity to the handedness of circularly polarized pump light (gSHG − CD ≈ 9% at 1510 nm). The THG response shows resonance enhancement matching the excitonic absorption. Achiral (Rac-MBA)4Bi2I10 exhibits the maximum THG response (χ(3) = 1.05 × 10⁻¹⁸ m2 V−2). Z-scan measurements with non-resonant femtosecond pulse excitation at 800 nm yield high nonlinear absorption coefficients (β) and nonlinear refractive index (n2) for all three samples, with (Rac-MBA)4Bi2I10 exhibiting the highest values. These hybrid chiral metal halides, with efficient second and third-order nonlinearity, and high optical stability, are potential candidates for NLO applications such as Kerr-based optical switching, circularly polarized up-conversion, and communication.