|Ph.D Student||Simkhovich Boris|
|Subject||Linear and Nonlinear Plasmonic Effects in Gold|
|Department||Department of Nanoscience and Nanotechnology||Supervisor||ASSOCIATE PROF. Guy Bartal|
|Full Thesis text|
Gold is one of the main Plasmonic materials. Its linear and non-linear properties were heavily studied in various forms - bulk, film, multilayer, nanoparticles and nanoantennas. Nevertheless, here we report a number of new results, shinning new light on the basic properties of gold.
By studying a linear reflection form gold plated prisms, we determined the existence of two plasmonic modes instead of commonly assumed one. The determined relative positions of the modes and their collective response are fully explained theoretically, providing the new description of slab plasmonic modes. While this work demonstrate this finding in gold, the implications of the results are relevant to all metals and entire field of Plasmonics.
Yet, the main focus of the Thesis is the nonlinear properties of gold. Due to gold crystal symmetry its lowest bulk nonlinear response is of the third order where four electromagnetic waves mix together. Our theoretical investigation of Four Wave Mixing in gold revealed its capability to convert near field information to the far field by the nonlinear interaction. Moreover, the folding of spatial information is found sustainable for wide band of angular frequencies and suitable for imaging purposes.
Another third order nonlinear process existing in gold is the Kerr-type effect, in which all the four mixing waves are of the same frequency. In order to measure the nonlinear parameter for that process, a number of techniques were examined. It was found, that standard Z-scan technique is inaccurate in the case of gold and other non-transparent materials. In addition, our investigation revealed some basic confusion with respect to the intensity definition in opaque or lossy media. The derived rigorous intensity expression is shown to be inappropriate for Kerr-type effects modeling in such materials. Using this knowledge, we developed a new model for technique known as reflection Z-scan. The technique was further used for determination of gold's third order susceptibility. The experimental results suggest that gold becomes a better metal by its Kerr-type response, with less absorption and real permittivity higher in the absolute value. Furthermore, the estimated permittivity change can get up to 30% , which is unprecedentedly high for metals.
We validated our results in a separate experiment, utilizing prism-coupling to gold plasmonic modes and their characteristic reflection pattern in order to infer the permittivity nonlinear change. The results confirm the nonlinear change trend in the real part of gold's permittivity with effective ∼ 13% change. Surprisingly, the nonlinear coupling to the two plasmonic modes, found previously in the linear study, is a function of the incident phase front shape. The nonlinear interplay of the modes together with the strong nonlinear response provide a road-map for nonlinear devices, e.g., optical ultrafast plasmon modulation.