Six-order amplification of third-order optical susceptibility (χ(3)) and bandgap engineering in ion-doped chitosan biopolymers

Chitosan (CS) is a biocompatible and environmentally friendly biopolymer, which is promising for optoelectronic applications. However, its large optical bandgap and low electrical conductivity restrict its functioning in state-of-the-art optical and electronic devices. In this study, sodium 4-nitrophenolate (SNP) was embedded in a chitosan matrix using a simple and inexpensive solution-casting method to modify the structural, optical, electrical, and nonlinear optical characteristics. The gradual incorporation of SNP into the chitosan structure led to a transition to a more amorphous state with an increase in structural disorder and localized electronic states. Consequently, the direct optical bandgap was remarkably lowered from 5.13 eV for the pure chitosan to 2.41 eV for the 300 mg SNP-doped film, while the Urbach energy rose from 0.398 to 0.760 eV, implying greater electronic disorder. The static refractive index was enhanced from 1.119 to 2.793 with a considerable amplification in the optical dielectric response, which is indicative of higher polarization and charge redistribution within the polymer matrix. The optical resistivity was reduced by more than an order of magnitude to 0.0012 Ω.m with a sheet resistance of 1.4 × 10⁻⁶ Ω/sq. Furthermore, the third-order nonlinear optical susceptibility and nonlinear refractive index were enhanced by around six orders of magnitude to 1.463 × 10⁻¹¹ esu and 3.168 × 10⁻¹⁰, respectively. These results demonstrate that SNP incorporation effectively modulates the electronic and optical properties of chitosan, with the enhanced nonlinear optical response suggesting potential for future applications in nonlinear optical and optoelectronic technologies, including optical switching, data storage, and optical limiting.