Nonlinear Optical Properties of 0D Chiral Hybrid Bismuth Iodides

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 (g_{SHG - 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.