Doping a semiconductor with foreign atoms enables the control of its electrical and optical properties. We transplant the concept of doping to macroscopic photonics, demonstrating that two-dimensional dielectric particles immersed in a two-dimensional epsilon-near-zero medium act as dopants that modify the medium's effective permeability while keeping its effective permittivity near zero, independently of their positions within the host. The response of a large body can be tuned with a single impurity, including cases such as engineering perfect magnetic conductor and epsilon-and-mu-near-zero media with nonmagnetic constituents. This effect is experimentally demonstrated at microwave frequencies via the observation of geometry-independent tunneling. This methodology might provide a new pathway for engineering electromagnetic metamaterials and reconfigurable optical systems. Doping-the judicious inclusion of impurities into a material, with the aim of controlling the material's macroscopic parameters-has been essential in the development of the semiconductor industry (1). Arguably, the success of semiconductor devices lies largely in the possibility of engineering their electrical, optical, and/or magnetic properties with a relatively small number of randomly located impurities. For instance, tailoring the electrical conductivity via doping facilitated the development of diodes and transistors, which are the basis of microelectronics (2). Different doping mechanisms are also central to a number of optical and optoelectronic devices, including lasers, light-emitting diodes, and solar cells (3).In principle, the concepts behind doping could be transplanted to other structures, length scales, and, in general, fields of physics. This was successfully done, for instance, when applying doping principles from bulky semiconductors to nanocrystals or quantum dots (4). In our work, the concept of doping is applied to the fields of macroscopic photonics and electromagnetic metamaterials (5, 6) and, in particular, media with near-zero parameters (5, 7-9). However, the extrapolation is nontrivial. In fact, when a homogeneous host body, characterized by macroscopic material parameters such as relative permittivity e h and relative permeability m h , is filled with a macroscopic dopant (characterized by e d and m d ), this foreign body scatters the internal electromagnetic fields, and the response of the system in general differs from that of a homogeneous body with effective macroscopic parameters e eff and m eff .Traditionally, it is only in the long-wavelength regime that a body filled with impurities can be substituted with a homogenized body by using suitable effective-medium theories (EMTs) (10). Conventional EMTs apply usually under three conditions: The original body must contain a sufficiently large number of impurities, and both their size and separation must be much smaller than the wavelength of operation (10). Although these conditions are actually relaxed in advanced homogenization techniques (11, 12), these proce...

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer "metamolecules" as the number of nanocrystals is systematically varied. Precise, hexagonally ordered Au nanocrystal oligomers with 1-31 members are assembled via capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual oligomers is measured by spatially resolved, polarized darkfield scattering spectroscopy. The response is highly sensitive to in-plane versus out-of-plane incident polarization, and we observe an exponentially saturating red shift in plasmon resonance wavelength as the number of nanocrystals per oligomer increases, in agreement with theoretical predictions. Simulations further elucidate the modes supported by the oligomers, including electric dipole and magnetic dipole resonances and their Fano interference. The single-oligomer sensitivity of our measurements also reveals the role of positional disorder in determining the wavelength and character of the plasmonic response. The progression of oligomer metamolecule structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic clusters and extended two-dimensional arrays.

Next-generation 'smart' nanoparticle systems should be precisely engineered in size, shape and composition to introduce multiple functionalities, unattainable from a single material. Bottom-up chemical methods are prized for the synthesis of crystalline nanoparticles, that is, nanocrystals, with size- and shape-dependent physical properties, but they are less successful in achieving multifunctionality. Top-down lithographic methods can produce multifunctional nanoparticles with precise size and shape control, yet this becomes increasingly difficult at sizes of ∼10 nm. Here, we report the fabrication of multifunctional, smart nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic ZnFeO and plasmonic Au nanocrystals. The superparamagnetism of ZnFeO prevents these nanorods from spontaneous magnetic-dipole-induced aggregation, while their magnetic anisotropy makes them responsive to an external field. Ligand exchange drives Au nanocrystal fusion and forms a porous network, imparting the nanorods with high mechanical strength and polarization-dependent infrared surface plasmon resonances. The combined superparamagnetic and plasmonic functions enable switching of the infrared transmission of a hybrid nanorod suspension using an external magnetic field.

Materials with designed electromagnetic response have a wide range of exotic applications

Spontaneous emission, stimulated emission and absorption are the three fundamental radiative processes describing light-matter interactions. Here, we theoretically study the behaviour of these fundamental processes inside an unbounded medium exhibiting a vanishingly small refractive index, i.e., a near-zero-index (NZI) host medium.We present a generalized framework to study these processes and find that the spatial dimension of the NZI medium has profound effects on the nature of the fundamental