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, allocated 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 sample is illuminated in total internal reflection, the simulations show that the amplification of the electromagnetic field above the Au metallic pads depends on different parameters (wavelength, polarization of light, angle of incidence, and index of refraction). In this paper, we only consider the effect of the probe-to-sample distance and of the polarization of the illuminating light. As the experimental setup, we used the photon scanning tunneling microscope. If we compare these results with the calculations carried out with the dielectric pads, we show that the amplification is induced by the dielectric contrast between the metallic structures and their environment. Experimental results are presented in two different imaging modes. In the "constant intensity" mode, our experimental results are in excellent agreement with the simulations. Therefore, for a metallic sample analyzed in our experimental conditions, it validates the assumption that the signal detected is proportional to the square modulus of the electric field in the absence of the probe. We particularly show that if the polarization and the probe-to-sample distance are suitably chosen, dramatic localizations of the electromagnetic field are observed. We then present images obtained in the so-called "two-wavelength" mode, where two light sources illuminate the sample. The first beam is used for feedback regulation; it consequently allows a control of the tip motion, and permits us to determine the reference surface. By using such feedback regulation for this first beam, we are able to compare quantitatively the effect of the polarization on the field distribution of the second beam. The results are confirmed by the corresponding simulations.