M.Sc Thesis

M.Sc StudentYacob Yochai Blau
SubjectShort-wavelength and Negative Index Surface Plasmons
DepartmentDepartment of Electrical Engineering
Supervisor Professor Bartal Guy
Full Thesis textFull thesis text - English Version


Surface Plasmon Polaritons (SPP) are electro-magnetic waves propagating along metal-insulator interfaces with unique properties that enable field concentration into deep sub-wavelength volumes, far beyond the well-known diffraction limit first describe by Abbe. This ability to highly confine optical fields has driven rapid expansion in research and emerging technologies in recent years, utilizing the ability of plasmonic devices to generate, guide, modulate and detect light using nanoscale structures. These remarkable capabilities have the potential for significant advances and future applications such as optical signal processing, nanoscale optical devices, super-resolution optical microscopy, highly-efficient photovoltaics, non-linear optics and optical metamaterials.

   Many of these recently emerging nano-technologies are based on short-wavelength surface plasmons. Theoretically, these waves can exhibit extremely short wavelengths at optical frequencies, yet in practice the wavelength shortening is limited by the inevitable losses in metals which inhibit significant wavelength shortening on single metal-air interfaces. In this thesis we introduce layered platforms which supports truly short wavelengths plasmons, where the wavelength shortening factor can be higher than 2 while maintaining propagation lengths of several microns. These short wavelength modes can enhance the performance of many photonic devices and will be necessary for future applications.

Efficient design of novel devices relies on the ability to characterize the plasmonic wavelength, yet the techniques currently used are either limited to the long-wavelength regime or require highly complex and expensive systems. In this thesis we introduce a novel method for high accuracy far-field measurements of the surface plasmon polariton wavelength, even when it goes well below the optical diffraction limit. This method is capable of measuring a large range of SPP wavelengths including the short-wavelength regime, and moreover, its simplicity and the fact that only conventional far-field optical elements are needed make it feasible in almost any optical lab. We demonstrate this method's capability by experimentally measuring plasmonic wavelengths as short as 231nm for a 532nm excitation wavelength on a metal-insulator-insulator platform.

In addition to their extraordinary wavelengths and field confinement, short wavelength SPP modes in metal-insulator-metal platforms have also been suggested to exhibit negative effective indices. These plasmonic waveguide structures are advantageous since they are much easier to realize than sub-wavelength resonator arrays, which were originally suggested as negative index metamaterials. This striking feature of plasmonic waveguides has been experimentally observed, yet no direct measurement of this phenomenon has been presented.

In this thesis we explore a method for direct measurements of negative refractive index in a plasmonic waveguide using near-field scanning optical microscopy (NSOM). We discuss the necessary modifications to the waveguide structure which enable these measurements, and present calculations of the characteristics of these structures for several materials and geometrical parameters. We further discuss the optimal configuration for the plasmonic waveguide and examine the feasibility of this method. Simulations of these proposed measurements showing negative index behavior in these structures are also presented.