Metamaterials are a class of artificial materials having structural parameters much smaller than the operative wavelength, usually nanostructured for optical frequencies. Nonetheless, their effective response to ligthwaves shows extraordinary and fascinating properties. Many promising physical behaviours can arise from materials with such unusual electromagnetic response, among which negative refraction, optical cloacking, super resolution imaging, ultra compact optical circuits, plasmonic nanolasers, inversion of the Cherenkov radiation are only few examples. Recently, a new branch of the metamaterial class is receiving an increasing attention in the scientific panorama, which holds an ultra anisotropic behavior not yet found in nature for optical frequencies. These materials are known as hyperbolic metamaterials (HMMs), possessing an extraordinary high anisotropy in the dielectric properties. Due to this extreme anisotropy they represent the way to realize effective bulk meta-structure with particular optical properties in the visible range.
Metal-dielectric multilayers and nanowire arrays with hyperbolic dispersion have been realized across the visible range and various interesting effects have been demonstrated, including subwavelength imaging. By properly mixing a metal with a special dielectric as building blocks, epsilonperp and epsilonpar of the entire system can become either positive or negative, opening the way to an extreme anisotropy. Sputtering and thermal deposition techniques are used in order to obtain multilayer structures with nanometric thickness of the single unit cell. Extraordinary properties have been found.
Metamaterials, being based on metal sub-units, suffer from the high values of ohmic losses, that prevent their use in real applications. The idea to bring gain molecules in close proximity to metallo-dielectric nanostructures is based on coherent effects of excitation energy transfer between resonant bands of the two materials. It is well known that relevant modifications of the fluorescence of dye molecules placed in close proximity to metal NPs are due to mutual interactions with NPs surface plasmons, including resonant energy transfer (RET).
Approaches at meso- and macro- scales have been proposed to fill the gap between the single plasmonic nanostructure and bulk materials. Different systems have been realized at various scale and by following different chemical techniques. At these scales, the plasmon-gain interplay is dominated by the location of the gain medium with respect to the spatial distribution of the local field.
Fabrication of functional nanoparticles and nanostructures
Nano-composite and nano-structured materials feature special physical properties, at the base of innovative technologies. Therefore the fabrication research line is paramount for the different research activities carried out @Nanotec-CS, namely Optical metamaterials, Biosensing, Optofluidics, Plasmonics. The deployed fabrication techniques encompasses both top-down and bottom-up approaches, taking advantage from hybrid strategies.
A new generation of nanostructured plasmonic materials, created for infrared and optical frequencies, can be prepared by the use of nano-chemistry and self-assembly of soft materials as an alternative to standard lithography or multi-beam holography. Nano-scale chemistry offers today a tremendous versatility in terms of constituent materials, morphologies, sizes and surface functions of achievable nano-particles that will constitute the fundamental building blocks of the nano-structured composites. This approach is typically bottom-up, and it can be integrated with top-down techniques, as the traditional UV-mask lithography, or with Multi-photon Direct Laser Writing (DLW) in soft materials. We aim to fabricate arbitrarily complex 3D hybrid structures metallic/dielectric with features in the 100 nm range
In parallel, studies on special resists doped with metallic precursors, to be used for 2-photons DLW are carried out, for creating metallic NPs inside polymeric structures.
PRIN 2012 ‘Gain-Plasmon Coupling in Metal-Dielectric Nanostructures: Loss Compensation towards Laser Action”