Theoretical Description and Design of Optical Nanomaterials

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Patrick Grahn Name of the doctoral dissertation Theoretical Description and Design of Optical Nanomaterials Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 22/2014 Field of research Nano-optics Manuscript submitted 17 December 2013 Date of the defence 11 April 2014 Permission to publish granted (date) 29 January 2014 Language English Monograph Article dissertation (summary + original articles) Abstract Recent developments in nanotechnology have made it possible to create a large variety of nanoparticles with predefined shapes. Nanoparticles that are much smaller than the optical wavelengths can appear as artificial atoms to light and, when assembled into a periodic threedimensional lattice, they form a crystalline optical nanomaterial. The main difference from natural materials is that the optical response of the material can be purposefully tailored by designing the constituting nanoparticles. In fact, optical nanomaterials can exhibit extraordinary optical properties that cannot be found in natural materials. However, in order to successfully design such artificial media, advanced theoretical methods are required. The main goal of the research described in this thesis was to develop a theoretical basis for a comprehensive treatment of optical nanomaterials. In the developed formalism, the microscopic optical response in the unit cell of a nanomaterial is characterized in terms of elementary electric current multipoles. These multipoles are fundamentally connected to the nanoparticles' geometry, which provides an efficient way to adjust and tune the optical response. The influence of higher-order multipole excitations is demonstrated by designing a nanoscatterer in which light cannot excite any electric dipole moment. In the thesis, it is shown that the macroscopic optical properties of nanomaterials can straightforwardly be described in terms of the interaction of optical plane waves with the planar arrays of nanoscatterers that compose the medium. Effective material parameters, such as the refractive index and wave impedance, naturally appear in this description in the form of simple analytical expressions. In contrast to existing theories, the introduced approach correctly handles also spatially dispersive materials, including those composed of noncentrosymmetric nanoscatterers. In such materials, two counter-propagating waves can experience the medium differently. The developed theory reveals the fundamental role of higher-order multipoles and spatial dispersion in realizing extraordinary optical properties with designed nanomaterials. In particular, materials composed of asymmetric nanoparticles may find novel light-guiding and lightharvesting applications. Furthermore, nanomaterials can be designed to suppress optical reflection at certain interfaces only, which can be exploited, e.g., in new interferometric optical devices. Most of the introduced theory can be straightforwardly applied to other artificial media, such as radio-frequency metamaterials and, in some cases, photonic and phononic crystals.Recent developments in nanotechnology have made it possible to create a large variety of nanoparticles with predefined shapes. Nanoparticles that are much smaller than the optical wavelengths can appear as artificial atoms to light and, when assembled into a periodic threedimensional lattice, they form a crystalline optical nanomaterial. The main difference from natural materials is that the optical response of the material can be purposefully tailored by designing the constituting nanoparticles. In fact, optical nanomaterials can exhibit extraordinary optical properties that cannot be found in natural materials. However, in order to successfully design such artificial media, advanced theoretical methods are required. The main goal of the research described in this thesis was to develop a theoretical basis for a comprehensive treatment of optical nanomaterials. In the developed formalism, the microscopic optical response in the unit cell of a nanomaterial is characterized in terms of elementary electric current multipoles. These multipoles are fundamentally connected to the nanoparticles' geometry, which provides an efficient way to adjust and tune the optical response. The influence of higher-order multipole excitations is demonstrated by designing a nanoscatterer in which light cannot excite any electric dipole moment. In the thesis, it is shown that the macroscopic optical properties of nanomaterials can straightforwardly be described in terms of the interaction of optical plane waves with the planar arrays of nanoscatterers that compose the medium. Effective material parameters, such as the refractive index and wave impedance, naturally appear in this description in the form of simple analytical expressions. In contrast to existing theories, the introduced approach correctly handles also spatially dispersive materials, including those composed of noncentrosymmetric nanoscatterers. In such materials, two counter-propagating waves can experience the medium differently. The developed theory reveals the fundamental role of higher-order multipoles and spatial dispersion in realizing extraordinary optical properties with designed nanomaterials. In particular, materials composed of asymmetric nanoparticles may find novel light-guiding and lightharvesting applications. Furthermore, nanomaterials can be designed to suppress optical reflection at certain interfaces only, which can be exploited, e.g., in new interferometric optical devices. Most of the introduced theory can be straightforwardly applied to other artificial media, such as radio-frequency metamaterials and, in some cases, photonic and phononic crystals.