Solvent-dependent fluorescence dynamics and ultrafast optical nonlinearity in Tectona grandis L.f. leaf extract.

The nonlinear optical properties of natural dye extracts derived from Tectona grandis L.f. (teak) leaves were systematically investigated in solvents of varying polarity using the Z-scan technique in the ultrafast domain. UV-visible absorption and FTIR spectroscopy confirm the presence of chlorophylls, carotenoids, anthocyanins, phenolics, flavonoids, tannins, and glycosides in Tectona grandis L.f. (teak) leaf extract, while time-correlated single-photon counting (TCSPC) measurements reveal pronounced solvent-dependent multicomponent excited-state decay dynamics. Femtosecond Z-scan experiments performed using a high-repetition-rate Ti:Sapphire laser (100 fs, 800 nm, 82 MHz) show strong nonlinear absorption accompanied by negative nonlinear refraction for all solvent extracts, indicating reverse saturable absorption (RSA) and self-defocusing behavior. At higher incident powers, the open-aperture Z-scan curves for the toluene and dichloromethane extracts exhibit characteristic W-shaped profiles, signifying saturation of excited-state absorption due to state-filling and population bottleneck effects. Analysis using an intensity-dependent absorption model yields effective nonlinear absorption coefficients (βeff) in the range of 10-6-10-5 m/W and nonlinear refractive index coefficients (n₂) of the order of 10-15 m2/W. The nonlinear response shows strong solvent dependence: carbon tetrachloride exhibits enhanced nonlinear absorption at low intensities due to aggregation-assisted ESA, whereas polar aprotic solvents such as DMSO and DMF display pronounced self-defocusing nonlinearity governed by cumulative thermally induced refractive index changes supported by long-lived excited-state populations. The combined Z-scan and TCSPC analysis establishes teak leaf extract as a solvent-tunable, bio-derived nonlinear optical material with potential applications in optical limiting, optical switching and in various photonics applications.