Multilayer Hyperbolic Metamaterials for Tunable Nanophotonics Applications

Materials with reconfigurable optical properties and possessing the ability to actively manipulate light at the nanoscale are among the greatest requirements of advanced nanophotonics and quantum optics. In particular, the enhancement and the control of optical nonlinearities and quantum emitters photoluminescence represent a fundamental challenge for the development of many optoelectronic devices for all-optical signal processing in integrated photonic circuits. In recent years, the scientific and technological research has strongly pursued the development of novel nanocomposites with peculiar optical properties that are absent in natural materials, and derive from the tailoring of their geometrical and compositional features by subwavelength nanofabrication. These are called metamaterials, taking their name from the ancient greek Mετα ("meta"), which means "beyond". Indeed, their properties go beyond the conventional ones in nature. In this framework, hyperbolic metamaterials (HM) stand out among the most promising cutting-edge nanomaterials owing to their epsilon-near-zero (ENZ) properties and their unique hyperbolic dispersion. These originate from the combination of constituent materials with opposite optical features (i.e. a metal and a dielectric) in a structural design bringing forth to an extreme uniaxial anisotropy of the effective permittivity (whose real part goes to zero at the ENZ wavelength), and a potentially unlimited photonic density of states. Strong optical nonlinearities and enhancement of the radiative decay rate of quantum light emitters can be obtained owing to these properties, which can be finely engineered across the optical spectrum by geometry and composition, or tuned by external parameters, thus achieving an active control of the phenomena deriving from them. Therefore, HM have great potential to be employed for applications in several fields, ranging from nonlinear nanophotonics to biosensing. The aim of the present work is the investigation of the tunability of the linear and nonlinear optical properties in multilayer HM by structure design and by external parameters, and the ability of these materials to control the photoluminescence of quantum emitters. To this purpose, hyperbolic multilayers with different metal filling fraction, layers thickness and composition are fabricated. The effective permittivity and the linear optical properties of the produced samples are shown to be tailorable by changing the metal filling fraction via the layers thickness, strictly dependent on plasmonic effects arising from the coupling of surface plasmon polaritons (SPP) sustained at the metal-dielectric interfaces, and controllable by light incidence angle and polarization. The tuning of the optical Kerr effect via the metal filling fraction (i.e. spectrally shifting the ENZ wavelength) is evidenced. A continuous modulation of the nonlinear optical response as a function of the incidence angle with TE- and TM-polarized light is demonstrated around the ENZ wavelength, and a phenomenological model is implemented to simulate the angle and polarization-dependent nonlinear parameters accounting for the optical anisotropy of the HM and the local intensity enhancement inside them. The intensity dependence of the nonlinear optical response is studied. A figure of merit is defined to optimize the multilayer structure and composition, and take full advantage of the local intensity enhancement favoured by the SPP coupling in the ENZ regime for boosting the nonlinear optical response. Finally, a selective control of specific transitions of Eu3+ emitters coupled to hyperbolic multilayers is pointed out in their hyperbolic regime, and by changing the distance from them. The obtained results highlight multilayer HM as potential platforms for all-optical switches, optical limiters, single-photon sources and nanolasers.