Near-field optical study of mesoscopic Au periodic samples: Effect of the polarization and comparison between different imaging modes

This paper presents a complete study, theoretical as well as experimental, of an electromagnetic field scattered by subwavelength metallic pads, alloc ated in a periodic manner on a silica substrate. The simulation of the far field and near field is obtained with the differential method. When the sam ple is illuminated in total internal reflection, the simulations show that the amplification of the electromagnetic field above the Au metallic pads d epends on different parameters (wavelength, polarization of light, angle of incidence, and index of refraction). In this paper, we only consider the e ffect of the probe-to-sample distance and of the polarization of the illumi nating light. As the experimental setup, we used the photon scanning tunnel ing microscope. If we compare these results with the calculations carried o ut with the dielectric pads, we show that the amplification is induced by t he dielectric contrast between the metallic structures and their environmen t. Experimental results are presented in two different imaging modes. In th e "constant intensity" mode, our experimental results are in excellent agre ement with the simulations. Therefore, for a metallic sample analyzed in ou r experimental conditions, it validates the assumption that the signal dete cted is proportional to the square modulus of the electric field in the abs ence of the probe. We particularly show that if the polarization and the pr obe-to-sample distance are suitably chosen, dramatic localizations of the e lectromagnetic field are observed. We then present images obtained in the s o-called "two-wavelength" mode, where two light sources illuminate the samp le. The first beam is used for feedback regulation; it consequently allows a control of the tip motion, and permits us to determine the reference surf ace. By using such feedback regulation for this first beam, we are able to compare quantitatively the effect of the polarization on the field distribu tion of the second beam. The results are confirmed by the corresponding sim ulations.