Nonlinear behavior of silver nano-films in cw regime

Transmission Z-scan measurements of silver nano-films in the cw regime were carried out to investigate the local response in the films’ nano particles. The films were evaporated onto glass substrates at room temperature. The... [ view full abstract ]

Transmission Z-scan measurements of silver nano-films in the cw regime were carried out to investigate the local response in the films’ nano particles. The films were evaporated onto glass substrates at room temperature. The dependence of the far-field intensity on the sample position due to intensity-dependent optical nonlinearities was examined based on the Gaussian decomposition analysis of Shiek-Bahae model (SBM) and the thermal-lens model (TLM). In prefocal region, the experimental data were fitted with the SBM, whereas in the postfocal region the data were better fitted with the TLM. The thermal-lens model is nonlocal in space and time, whereas the Gaussian decomposition model is based strictly on local response. I discuss this behavior based on the context of the two models.

Introduction

A major difference between the thermally induced nonlinearities and those induced by the local interaction is in the material’s response time.  Recently, there have been reports about ultrafast thermal nonlinearities in nanocomposite materials due to plasmonic effects. From the context of these reports, it was interesting to ask if the local model used to interpret z-scan measurements could be utilized to interpret the ultrafast thermal nonlinearities in these nanomaterials based on some sort of local behavior that arises from confining effects of the radiation fields in nano-volumes and shed some light on the cause of this behavior.  

Results

The thermal load from the laser source that produced the nonlinear behavior was manipulated by an optical chopper with 11% duty cycle. Fig. 1 shows the fitting of the measured data points with the SBF and TLM curves without using the optical chopper. The local model was excellent in the pre-focal region whereas the data were better fitted with the thermal model in the post-focal region. Fig. 2 shows the results of the SBF fitting when the optical chopper was used. Here, good agreement between the measured data and the SBF calculated curves suggest that the nonlinear response in the fabricated films is local which means it is in the ultrafast scale.