Tunable work function and optical nonlinearity of nanocomposites

We fabricated Au nanoparticles on the surface of TiO₂ by photo-reduction method. The structural property of film is investigated using transmission electron microscopy technique. The surface plasmon resonance absorption peaks of TiO₂ and Au-TiO₂ were recorded using Shimadzu (UV-2450) UV-visible spectrophotometer. The work function of films was measured by scanning Kelvin probe microscopy (SKP5050) from KP technology, United Kingdom. The nonlinear optical refractive index and nonlinear optical absorption coefficient of Au-TiO₂ nanocomposites were simultaneously measured using z-scan technique.

The nonlinear optical response of Au-TiO₂ nanocomposites is due to pure electronic transfer effects in Au nanoparticles. The observed nonlinearity is due to the dielectric constant of Au, which is due to the surface plasmon resonance and surface polarization between Au nanoparticles and TiO₂. The dielectric constant of Au shows maximum value at 532 nm wavelength (2.33 eV). Surface plasmon resonance effect of Au nanoparticles is directly related to the metal dielectric constant, therefore it is increased. The increase in surface plasmon rsonance of Au-TiO₂ nanocomposites can be observed from UV-visible absorption spectra. The optical nonlinearity depends strongly on the dielectric constant of Au. Therefore, the optical nonlinearity increases with the increase in surface plasmon resonance peak. The absorption peak of surface plasmon resonance at 544 nm is inversely proportional to the work function. Therefore, we can say that the Au dopant decreases the work function that effectively increases the surface plasmon resonance absorption peak at 544 nm, so that the nonlinear response is enhanced. The systematic change in the work function with Au concentration plays a major role in optical nonlinearity. The estimated optical nonlinearity was found to increase from 3.80×10^-6 to 9.69×10^-6 esu with increase in Au concentrations from 0 to 1.0x10^-2 mole. This observed increment in nonlinearity is due to the enhancement of local electric field created by excitation of surface plasmon resonance that affects the work function. Therefore, the surface plasmon resonance and work function help in tuning the optical nonlinearity. The tunable nonlinear optical response of the Au-TiO₂ nanocomposites may find applications in nonlinear optics at wavelength 532 nm and the development of materials for generating the higher order harmonics.