Nonlinear optical studies of Bismuth-doped Titanium di-oxide colloids achieved by femtosecond Z-Scan technique

Abstract

In this paper, we explored the nonlinear optical (NLO) properties of Bismuth (Bi) doped titanium dioxide (TiO₂) nanoparticle (NP) colloids in ethanol, with laser pulses of duration ∼ 150 femtoseconds (fs). The Bi-doped TiO₂ NPs were synthesized via the chemical route, sol-gel method. The increased photo response of TiO₂ upon doping it with metals has become a burning issue to extend the applications of metal-doped TiO₂ NPs as integral parts in solar cells, catalysts, phototherapy, etc. The bandgap of anatase TiO₂ NPs was observed to be reduced to 2.89 eV upon doping Bi. The modified band structure of the Bi-doped TiO₂ NPs demonstrates novel chemical, mechanical, and optical properties. NLO studies were conducted on Bismuth-doped TiO2 NPs submerged in ethanol employing femtosecond laser input pulses with incoming wavelengths 700, 750, 800, 850, 900, and 950 nm. It has been detected that open aperture (OA) studies performed at an input peak intensity of 100 MW/cm² on Bi-doped TiO₂ NPs in ethanol were exhibiting complex NLO behavior, i.e., reverse saturable absorption (RSA) in saturable absorption (SA) and SA in RSA with mostly effective two-photon absorption (1 + 1) coefficients. Particularly, at ∼800 nm, the behavior observed was so complex, i.e., RSA in SA in RSA in SA, with an effective 3 PA (2 + 1) co-efficient (γ) ∼6.5 × 10⁻²⁴ m³/W². The closed aperture (CA) studies at ∼37 MW/cm² exhibited self-defocusing effects, i.e., an intensity-dependent refractive index (n₂) with a negative signature.