Metamaterials (MMs) constitute the frontier in material science technology. Unlike classic natural materials, MMs optical properties can be easily tailored as desired by suitably arranging the geometry, shape or size of their fundamental constituents. MMs are usually made of deeply subwavelength metal/dielectric building blocks arranged in periodic patterns showing one, two or three dimensional symmetry, possibly embedding reconfigurable and/or gain material in order to provide them smart responsive properties. Our research is focused upon two main branches: Hyperbolic Metamaterials and Chiral Metamaterials. Thanks to their extreme optical parameters, they both offer unprecedented design flexibility and open to a plethora of fascinating new properties in photonics, plasmonics, nanotechnology and bio-nano science fields.
Hyperbolic Metamaterials are artificial nanostructures featuring extreme optical anisotropy. With respect to the in plane (εx= εy = ε||) and out of plane (εy = εperp) dielectric permittivities, four anisotropies can be identified, two of which imply the sign of ε|| and εperp to be opposite. In these cases the HMM can behave as an effective dielectric for an in-plane polarized lightwave and, at the same time, as a metal for an out-of-plane one (Type I anisotropy, ε|| > 0 and εperp < 0), or completely inverse (Type II anisotropy, ε|| < 0 and εperp > 0). Two main geometries are considered to behave in this way: (1) deeply subwavelength array of metallic nanowires, embedded in a dielectric matrix and (2) 1D metal/dielectric periodic multilayers. In the latter case (the easiest to fabricate), the design of the optical parameters of the overall HMM can be easily carried out in the framework of the Effective Medium Theory, simply by acting on the thickness of the fundamental materials and on their fill fraction. This way, the refractive index of such materials can be tailored as desired in order to unlock many fascinating exotic behaviors, such as ultra-subwavelength light collimation, perfect lensing and so on. Our research consists both in exploring new frontiers in the exciting field of HMMs (mainly investigating about new conception fundamental materials and/or periodic arrangements) and designing and developing HMM based devices such as perfect lenses, biosensors, HMM based photovoltaic modules and HMM based nanolasers.
Tunable Epsilon Near Zero and Pole (ENZP)HMM (Perfect Lens and Supercollimator)
- A. Ramakrishna et. al. demonstrated the possibility of using extremely anisotropic media, such as hyperbolic metamaterials (HMMs), to reach the so called “canalization regime”, a condition we renamed εNZP under which the medium shows εx = εy = 0 (in the plane) and εz = ∞ simultaneously.
The iso-frequency surface for the extraordinary (TM-polarized) waves propagating in such an HMM is given by
Here εm and εd are the permittivities of the two materials chosen as building blocks (m for metal and d for dielectric). The simultaneous condition of ε∥ ~ 0 and ε┴ ~ ∞ can be fulfilled if Ld = Lm and, contemporarily, εm = – εd.
We demonstrated the experimental realization and characterization of a particular HMM showing the aforementioned condition (εNZP), in the visible range, opening the way to extreme applications as supercollimation effect and perfect lens behavior for high resolution bio-imaging.
HMM based applications (Improved transmission, emission rate enhancement and nanolasers)
A two-dimensional (2D) silver diffraction grating coupled with an Ag/Al2O3 HMM shows 18-fold spontaneous emission decay rate enhancement of dye molecules with respect to the same HMM without grating. The experimental results are compared with analytical models and numerical simulations, which confirm that the observed enhancement of grating-coupled HMM (GCHMM) is due to the outcoupling of non-radiative plasmonic modes as well as strong plasmon-exciton coupling in HMM via diffracting grating.
Chiral metamaterials are a new class of metamaterials where a specific chiral geometry induces an optical response to the incident light field resulting from a mixture of electric and magnetic dipoles. This leads to many intriguing phenomena and applications, such as strong circular dichroism or optical rotatory dispersion, and provides interesting potential applications as broadband circular polarizers or to enhance the optical response of chiral molecules by superchiral light, offering also a simpler route to negative refraction. In particular, three-dimensional nanoscale geometries provide a wider set of functionalities, as broadband chirality to manipulate circular polarization at optical frequencies, but their fabrication becomes challenging as their dimensions get smaller. Our objective is to study the optical response of these nanostructures towards the development of miniaturized optical components such as circular polarizers, beam splitters, optical insulators, or for the dichroic spectroscopy of biological molecules.
3D Chiral Metamaterials in the VIS range
Plasmonic nanohelices have been realized by a bottom-up fabrication approach based on focused ion and electron beam induced deposition, providing a nanometer scale control on geometrical features. The fabricated arrays show chiro-optical properties at the optical frequencies and extremely high operation bandwidth tailoring dependent on the dimensional features.
Highly Pure Broadband Circular Polarizers based on Intertwined Fully 3D Nanostructures
Three dimensional triple-helical nanowires have been engineered by the innovative tomographic rotatory growth, on the basis of focused ion beam-induced deposition. These three dimensional nanostructures show large circular dichroism between 500 and 1000 nm, with a high signal-to-noise ratio. Optical activity of up to 8 only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices that can be integrated in optical nanocircuits to modulate the visible light polarization.