The Thermo-Optic Effect in Norbixin: Characterization by Z‑Scan and Self-Phase Modulation under CW Laser

This work investigates the thermal nonlinear optical properties of norbixin, a natural dye extracted from Bixa orellana L. seeds, dissolved in acetone. The molecular structure and optical response were characterized using UV-vis, FT-IR, and Raman spectroscopies. Nonlinear optical properties were investigated using the Z-scan technique and the analysis of far-field diffraction ring patterns under continuous-wave (CW) laser illumination. The optical band gap was estimated to be 2.40 eV using the Tauc plot method, indicating that the excitation at 532 nm (hν ≈ 2.33 eV) occurs in a near-resonant regime. Open-aperture Z-scan measurements revealed saturable absorption (SA) behavior, characterized by a negative nonlinear absorption coefficient of (β = -1.27 × 10^-4 cm²/W), whereas closed-aperture Z-scan showed a clear peak-valley trace indicative of a strong self-defocusing effect, from which a negative nonlinear refractive index (n _2,th) on the order of -5.28 × 10^-9 cm²/W was determined. A discrepancy of approximately 3 orders of magnitude was observed between the nonlinear parameters obtained from Z-scan and spatial self-phase modulation (SSPM). This difference is discussed in terms of concentration-dependent thermal loading and the presence of large on-axis phase shifts. The transition from a radially symmetric to a vertically compressed diffraction pattern confirms the influence of a convection-dominated regime under high-intensity CW. These results establish norbixin as a promising sustainable nonlinear optical material for photonic applications, including optical limiters and all-optical switches, while providing fundamental insights into thermo-optic effects and convection in nonlinear media.