Metamaterial, nanophotonic and plasmonic components for applications in integrated optics

We live in an era of great change, inside societies heavily reliant on technological innovation. We are now in the middle of the information revolution: the increasing amount of data created, processed, stored and moved, as well as the related technological platforms, are profoundly changing economy, markets, and industry structures. Scientific achievements from multiple disciplines, in particular in life sciences and physics, grow expectations on further revolutions. In this framework, optical technologies are a major driving force in research, necessary not only to respond to the increasing demands from telecommunications and computation, but also for new areas of big interest and expectations such as biology, chemistry, medicine and metrology. A special role in this is expected from integrated optics, which aims to build optical devices by combining components such as waveguides, optical filters, modulators, amplifiers, lasers, photodetectors, etc [1]. Classical technologies to construct such optical elements are Silicon Photonics, InP or InGaAsP, and the “exotic” Photonics crystals [1]; although those achieved many interesting scientific results, and allowed to build some high-performance devices for telecommunications [2,3], the long-term goal of reaching the complexity of micro/nano electronics is yet to be fulfilled, because of limitations due to optical losses, incapacity to reduce dimensions under the wavelength, and impossibility to miniaturize some optical elements [1,4]. In the last two decades, three sub-disciplines of Integrated Optics that defy the aforementioned limitations, namely optical metamaterials, nanophotonics and plasmonics [5,6], have grown greatly thanks to the use of the fabrication tools and methods developed initially for electronic scaling, as well as the introduction of powerful computers on a mass scale; despite being researched ever since the ‘50s and ‘60s, and being considered as minor and exotic options until 15 years ago, the scientific feats they demonstrated in the last years make the scientists envision also 7 industrial success in various directions, in particular on intra/inter-chipcommunication, computation and chemical/biological sensing. Some devices are already established at an industrial level [7], but the core potential of those fields is still under research; in general, the components obtained from those technologies focus on one, or a combination, of the following effects: the particular physics of nanometric sizes [8,9], enhanced sensitivity of resonances [10], the possibility of localizing the electromagnetic field in small regions [11], the possibility to tailor specific non-conventional optical effects [12], and the possibility to integrate with other types of systems [13] in order to produce inexpensive and reusable measurement systems. The most popular and relevant topics of research are: subwavelength waveguides [14, 15], optical nanoantennas [16 – 18], elements for computation [9, 19], slow light [20], active elements, optical tweezers [21], superlenses [22, 23], optical invisibility cloaks [24, 25], hyperlenses [26, 27], planar magnifying hyperlens and light concentrators [28, 29]. While those are still at research level, most of them are moving away fast from the proof-of-principle stage towards that of mature applications [5]. Many potential devices have been presented in literature, and there are usually multiple technological solutions in the same topic (i.e. there are many types of waveguiding systems [30], or various possible geometries for circular dichroism sensors). Defining the best option for specific applications (choice between different configurations, parameters, physics) is also important, nowadays. Also, while research in those fields may be mature from a theoretical perspective (including analytical and computational tools), the synthesis into fabricated elements becomes sometimes challenging due not only to limitations of the technologies themselves, but also to the non-idealities of the fabrication processes; in this case, the boost of the sensibility of metamarerials, plasmonics and nanophotonics backfires, as the behavior of the devices may be significantly altered due to fickle working conditions and/or compromised repeatability. In order to design actual devices, it is important to take into account of this, which means an increase of computational complexity. In this thesis I present my research activity as a PhD student, which is composed of a variety of projects concerning Metamaterials, Plasmonics and Nanophotonics for various types of application. My work was focused in numerical/analytical study of designed devices or experimental data, and in micro/nano-fabrication; I also handled measurements on some of the investigated structures, while others were operated 8 by collaborators. My main projects involve improved coupling method for intra-chip communication, and highly dichroic metasurfaces for potential applications as biosensors. My research includes also study on efficient edge coupling between fiber and plasmonic waveguides and strongly directional optical antenna. Most of my design efforts were focused on structures using surface plasmons, in particular LRSPPs, since the related phenomena and structures offer great flexibility, and because many technological concepts may be imported from microwave devices, scaled down to micro/nanometers. As for fabrication, I have adopted a top-down approach for the construction of my devices. Overall, the multidisciplinary aspect of these works was handled by managing feedbacks from each step, which quantitatively increased the complexity of the design process. Logistics was another difficulty that needed to be handled, since the many resources needed for fabrication and measurements required the collaboration with multiple institutions:  CNIS, Sapienza, for access to Electron Beam Lithography machine;  University of Ottawa-SITE, with the group of Prof. Berini, for the access to clean room with instrumentations focused on surface plasmons, as well as related expertise; in Canada I have been supported also by the expertise of prof. Sun from Jilin University, China.  CNR-ARTOV for access to clean room;  CNR-IFN for access to another EBL and evaporators;  DIET, Sapienza (Department of Information Engineering, Electronics and Telecommunications) for logistic support;  CNR-Nanotech, operated FIBID fabrication and optical measurements.  Department SBAI for optical and photoacoustic measurements; This thesis is organized as follows. Chapter 2 contains the fundamentals of plasmonics (with particular regard for long range surface plasmon polaritons), metamaterials, circular dichroism, fabrication tools. In chapter 3 I have presented a scheme for intra-chip communication by using multilayer optical routing through vertical directional couplers; it contains an analysis of vertical directional couplers (material from the paper [31] “Analysis on vertical directional couplers with long range surface plasmons for multilayer optical routing”), fabrication processes flow operated on a plasmonic signal bridging system, as well as schemes for measurements. Chapter 4 contains a numerical analysis on a strongly directional 9 optical antenna based on leaky wave antenna theory. In chapter 5 I have reported the results of the analysis on the circular dichroism of a metamaterial composed of an array of nano-helices, published as [32] “Precise detection of circular dichroism in a cluster of nano-helices by photoacoustic measurements”. In Chapter 6 a new planar geometry for dichroic filters (named “nano-beans”) is presented, with design, fabrication, and first measurements. Finally, in chapter 7 I will draw the conclusions of this work. In Appendix I have presented some content of interest of the projects that have been moved away from the chapters for the sake of readability, or some additional minor works: A1 contains the study of plasmonic chirped gratings, in A2 I have shown the analytical development of Green’s Tensor used in chapter 4, in A3 I have presented the fabrication processes required to build a nanoscopic sieve for the mechanical separation of cells within lab-on-chips, and in A4 I have presented the supplementary information of the numerical and analytical analysis done for the chapter 5