Nonlinear optical re-orientation effect induced by external electric field in colloidal TiO2 nanorods

Abstract

In this study, optical re-orientation of titanium dioxide (TiO₂) nanorods (NRs) was studied using Z-scan method. TiO₂ NRs have been synthesized using solvothermal synthesis method and were characterized by UV–Vis spectroscopy, X-ray diffraction and transmission electron microscopy methods which indicate formation of TiO₂ NRs in anatase phase with dimensions below 50 nm. Z-scan setup containing a He–Ne laser and a laser power meter was used for open and close aperture Z-scan measurements. A uniform electric field was applied perpendicular to the laser beam to orient the NRs and third order nonlinear absorption and refraction indexes were measured in different applied voltages. It was found that the sign of the nonlinear refraction index switched from negative to positive, indicating the dominance of the re-orientation effect over the thermal-lensing effect as voltage increased. Also, the effect of the needle shape of TiO₂ NRs on the threshold value of the switch voltage was investigated. In addition to experiments on NRs, nonlinear optical phase change equations resulting from the recirculation effect were developed and used to confirm the experimental results. The results of this investigation can be very important in adjusting the threshold voltage in changing the sign of nonlinear refractive index in laser switches and modulators.