Analysis and synthesis of plasmonic devices and metamaterials at optical and terahertz frequencies

Fabrication technologies have played a major role ever since the ‘60s to the rapid growth of information processing, and to the information revolution we are living today; in the electronic industry, by scaling down the size of devices, it has been possible to improve performance, functionality and reliability, all while reducing cost and increasing the production volume of the devices. Industrial success has given more impetus to further research microfabrication and nanofabrication [1 - 3], which has not only propelled more success, but has also opened to scientists and engineers a new array of physical phenomena to study and apply [4]. Over the last decades, the same fabrication apparatus and approach have allowed significant development in the field of optics, plasmonics, metamaterials, microfluidics and even mechanics [5 - 7]; those are still being extensively researched, sometimes in conjunction, for key applications such as: creation of novel materials, increase in volume of computation and communication, precise medical operations, and sensors for chemistry, biology and telecommunication [8 - 10]. As an example, we can take the plasmonics, where the discovering of a subwavelength confinements [11], coupled to the research of new nano-metamaterials [12, 13], has contributed to different families of the nanodevices with unprecedented functionalities, such as subwavelength waveguides [14, 15], optical nanoantennas [16 – 18], superlenses [19, 20], optical invisibility cloaks [21, 22], hyperlenses [23, 24], planar magnifying hyperlens and light concentrators [25, 26]. In this thesis, I present my research activity as a PhD student which encompasses various projects. My main objective has been the analysis and synthesis of plasmonic filters based on metamaterials; I have also worked on a waveguiding systems and gratings-sensors supporting long range surface plasmon polaritons. The activities were focused mainly on optical frequencies, but some devices worked on THz regime. In the first part of my PhD, I have studied devices working in plasmonic optical frequencies. In order to perform the analysis and design, I have used numerical instruments such as the finite-difference time-domain method (FDTD) included in the software Lumerical FDTD; with the aim to have a numerical comparison for the Lumerical’s results I have also used the Finite Element Method (FEM) present in the Comsol Multyphisics software. The post-processing of the simulation’s results were performed by using Matlab. I could use these tools in the S.B.A.I. (Basic and Applied Sciences for Engineering at Sapienza University of Rome) department and in the LabCEm2 laboratory of the D.I.E.T. (Department of Information Engineering, Electronics and Telecommunications at Sapienza University of Rome). To complete and then expand the analysis performed with numerical instruments in the second part of my PhD I have worked on the fabrication of the devices. With the gradual reduction of the devices’ dimension, various challenges have been encountered: as can be seen in later chapters, while research on nano-optics and metamaterials may be mature from an analytical and numerical point of view, there are still many challenges on the actual fabrication of the devices, because of non-ideality of geometrical shapes, materials and chemical composition; sometimes, even the characterization poses as a limiting factor. Thus, a good part of the activities has been the study of compromises between feasibility and performance of the devices: study on the fabrication tolerances and optimum fabrication dosages. As already told, I have used the same fabrication approach of microelectronics field, which is defined “top down”: externally controlled tools are used to depose, etch, and shape materials (litography) into the desired shape. In contrast, the bottom-up approach the devices are fabricated by using the auto-assembly of molecular components through their chemical bonds; these methods are widely used today to manufacture a large variety of useful chemicals such as pharmaceuticals or commercial polymers. Therefore, I created a nanofabrication procedure following the top-down approach and by using the classics microelectronic processes and techniques: Spin Coating followed by Hot plate for the deposition of polymer materials; Vacuum Evaporator and Sputtering for the deposition of metals and Electron Beam Lithography to imprint the desired geometry to the devices in nano-fabrication; while, for imprint geometry in micro-fabrication field, I used the standard UV-Lithography. I have performed the processes described above in collaboration with the CNR-IMM (Institute for Microelectronics and Microsystem) of Rome and I could use the FESEM Zeiss Auriga system who is kept in the Sapienza’s C.N.I.S. (Centro di Ricerca per le Nanotecnologie Applicate all’Ingegneria) laboratory in order to perform the EBL technique. The characterization of the manufactured samples are finally carried out in collaboration with the CNR-Nanotech institute of nanotechnology and Univesity of Palermo.