Research

My research activity is centered around linear and nonlinear electromagnetic interactions in photonic and plasmonic nanostructures, metamaterials and 2D materials.

Plasmonics

Second harmonic generation from dipole nanoantennas

We have studied SH scattering from symmetric and asymmetric metallic dipole nanoantennas with small gaps. We have assessed the roles of field enhancement and antenna modes when both the fundamental and harmonic frequency fields are resonant with the antenna. We find that field enhancement in the gap has minimal impact on the amount of radiated SH light. The only significant effect of gap reduction in symmetric configurations is a strong red-shift of the maximum conversion efficiency: for very small gap sizes (size s < 5 nm) SH emission is maximized at the low-frequency resonance associated with the first even mode, whereas for larger gaps (size s > 5 nm) the SH emission peaks on the high-frequency resonance associated with the second-order even mode. SH light shows a strong sensitivity to asymmetries introduced by gap displacements. The asymmetry enables coupling of SH light to quasi-odd antenna modes, which introduce additional channels to enhance the scattering efficiency. The multipolar analysis reveals that the asymmetry may be engineered to achieve strong dipolar and octupolar radiation components in the SH radiation pattern.

Plasmonic beam steering

We have demonstrated controllable light deflection in thick metal gratings with periodic subwavelength slits filled with an active material (e.g., liquid crystal). Under specific illumination conditions, the grating becomes nearly transparent and acts as a uniform optical phased-array antenna where the phase of the radiating elements is controlled by modifying the index of refraction of the material that fills each slit. The beam-steering operational regime occurs in a wide wavelength band, and it is relatively insensitive to the input angle.

Transmission resonances in metallic gratings

We have analyzed the fundamental properties of enhanced transmission in metal gratings, showing that it is possible to achieve full control of the transmission efficiency. The combined study of a single slit system and its periodic counterpart reveals virtually identical spectral features, except at wavelengths that nearly match the grating pitch. In particular the effect of the interaction of Fabry-Perot modes and surface modes is the opening of a plasmonic band gap. The simultaneous coupling and back-reflection at the surface plasmon band gap prevent light from coupling into the cavity mode inside the slit. For incident wave-vectors and grating pitches expected to exactly match and excite a surface plasmon, absorption is comparable to the bulk metal case of ∼2% and transmission is suppressed by several orders of magnitude.

Super-resolution in transparent metal-dielectric multilayers

We have investigated the extraordinary guiding properties of realistic, transparent, resonant MD-PBG structures, outlining their capabilities in terms of super-guiding and super-resolution across the entire visible range under hyper-diffractive conditions. The phenomena behind the strong lateral confinement provided by these structures are strictly related to the localization of the electromagnetic fields and the energy transport of both propagating and evanescent modes excited by sub-wavelength, realistic sources. The inhibition of diffraction is achieved over a wide spectral range thanks to the field localization properties peculiar to transparent metal stacks. Vortex-like patterns of the energy flux develop due to the presence of evanescent modes and their interference with propagating modes. We have proposed two practical designs based on Ag/GaP and Ag/TiO2 multilayer stacks, demonstrating that super-resolution is preserved even in the case of lower-refractive-index dielectric materials.

Metamaterials

Enhanced harmonic generation from epsilon-near-zero films

We have experimentally demonstrated efficient third harmonic generation from an indium tin oxide (ITO) nanofilm (λ/42 thick) on a glass substrate for a pump wavelength of 1.4 µm. A conversion efficiency of 3.3x10-6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero (ENZ) mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

2D Materials

Control of Fano resonances in graphene-based gratings

We have shown that the introduction of graphene in dielectric gratings operating at infrared telecom wavelengths may pave the way for the development of tunable resonators and absorbers. Guided-mode resonances of graphene-based gratings have been analyzed through the quasi-normal mode theory. The ability of graphene to modulate the complex eigenfrequency of such modes has been investigated by means of a first-order perturbative approach. The agreement of this approach with full-wave simulations has been verified in a wide range of graphene doping levels. The modal analysis has also been instrumental in describing resonant effects using a generalized Fano formula. By increasing the graphene chemical potential one is able not only to reduce the resonance linewidth and shift the resonance wavelength, but also to significantly alter the resonance shape. In particular, we identify two distinct operational regimes. Below a certain gating threshold, the grating displays a symmetric absorption resonance whose characteristic wavelength and linewidth are virtually insensitive to the graphene chemical potential. Above this threshold, the real part of the graphene conductivity decreases significantly so that the resonance becomes narrower and asymmetric. In this regime, one can exploit changes of the imaginary part of the graphene conductivity in order to modulate the resonance wavelength.