We consider an ex-situ prepared TiO2 − polystyrene (TiO2 − PS) nanocomposite, with different concentration of TiO2 nanoparticles (NPs), deposited by the spin-coating method on quartz substrates. Nonlinear optical properties of the TiO2 − PS nanocomposite films are studied using the Z-scan technique. Structural properties of the films are investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements. The optical band gap energy of the TiO2 − PS films are determined from the optical absorption spectra. The relation between the optical nonlinearities and the bandgap energy, as a microscopic electronic property of the semiconductor nanocomposite, is established. It is shown that an increase in the content of TiO2 NPs in the nanocomposite films results in bandgap narrowing and concomitant enhancement of optical nonlinearities. It is proposed that bandgap narrowing and enhancement of optical nonlinearities can be ascribed to the introduction of mid-gap states as a result of the formation of surface defects in TiO2 NPs. The results of this work indicate that incorporation of TiO2 NPs in the PS polymeric matrix can be a new way for bandgap narrowing of TiO2 NPs toward the visible light region, which is of great interest for their application in this part of the spectrum. Furthermore, enhancement of the optical nonlinearities of the spin-coated TiO2 − PS films by increasing the TiO2 NP content can open up new avenues for their use as nonlinear optical films in photonics applications.

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