Unidimensional hyperbolic metamaterials: synthesis and characterization

The emerging field of optical metamaterials offers a new paradigm to engineer the optical response of a system, in ways that are not found in conventional materials. A metamaterial, indeed, derives its properties not only from its constituent media but also from its geometry, which typically consists of a dense lattice of subwavelength blocks. Hyperbolic metamaterials (HMMs), in particular, are non-magnetic media characterized by a tensorial dielectric permittivity with principal components of different signs. This peculiar property allows the system to sustain light modes with arbitrarily high wavevector (at a given frequency), causing the photonic density of states to diverge. Recent theoretical and experimental results obtained with HMMs composed of a multi-layered system with alternating gold and alumina thin films are discussed in this work. Scattering matrix simulations and ellipsometric measurements have been used to validate the effective medium formulas for the metamaterial permittivity. The nonlinear optical constants of the metamaterial have been measured by means of the z-scan technique, observing a transition for the nonlinear refractive index. Far-field excitation of the high-k modes of the HMMs has been obtained via a simulated hexagonal grating of nanoholes in a gold layer on top of the multilayer, which provides the additional momentum to the impinging light. Moreover, the influence of the HMMs on the spontaneous emission of nearby fluorescent emitters has been investigated with time-resolved photoluminescence measurements. Preliminary results have shown a strong decrease of the emitter lifetime, which can be related to the near field coupling between the emitter and the HMMs.