G. Magno

Controlled reflectivities in self-collimating mesoscopic photonic crystal

We propose a simple, fast, and accurate method to design complex layered photonic crystal structures that exhibit mesoscopic self-collimation. We apply this method to the control of the overall reflectivity of such structures, and we numerically demonstrate high-transmissivity (>99%) self-collimating waveguides and high-reflectivity (>99%) self-collimating Bragg mirrors.

Label-Free

Immunosensors are devices that exploit immobilized antibodies to promote the binding of specific analytes related to diseases of medical importance, such as cancer or cardiac dysfunctions. Label-free immunosensors have an important role, due to their simplicity and fast read-out. Here, the proof of concept for an immunosensor based on a 2-D photonic crystal silicon nitride membrane is presented. The device has been fabricated by means of a well-tuned nanofabrication protocol, achieving a high-quality photonic pattern on a large-area membrane (1 mm × 1 mm), and it has been tested for the detection of interleukin-6, getting protein detection at pg/mL concentrations.

\hbox{Si}_{3}\hbox{N}_{4}

In this paper we describe the design, fabrication and characterization of gold nano-patches, deposited on gallium nitride substrate, acting as optical nanoantennas able to efficiently localize the electric field at the metal-dielectric interface. We analyse the performance of the proposed device, evaluating the transmission and the electric field localization by means of a three-dimensional finite difference time domain (FDTD) method. We detail the fabrication protocol and show the morphological characterization. We also investigate the near-field optical transmission by means of scanning near-field optical microscope measurements, which reveal the excitation of a localized surface plasmon resonance at a wavelength of 633 nm, as expected by the FDTD calculations. Such results highlight how the final device can pave the way for the realization of a single optical platform where the active material and the metal nanostructures are integrated together on the same chip.

Photonic Crystal Based Immunosensors for Diagnostic Applications

Abstract. Ultra-short vertical plasmonic couplers were devised for the efficient excitation of long-range surface-polariton-plasmon mode, in the visible regime, between a polymeric waveguide and a plasmonic waveguide in two different configurations. Numerical simulations suggest the realization of coupling efficiencies as high as 90% and insertion losses as low as −5.5  dB, with a coupling length of few micrometers. Thus the proposed design clearly proves that is possible to optimize contemporaneously the coupling efficiency and the coupling length. Therefore the compactness and the lower fabrication requirements make the proposed device very promising in a variety of applications.

Localized surface plasmon resonances in gold nano-patches on a gallium nitride substrate.

Mesoscopic self-collimation (MSC) in mesoscopic photonic crystals with high reflectivity is exploited to realize a novel high Q-factor cavity by means of mesoscopic PhC planar mirrors. These mirrors efficiently confine a mode inside a planar Fabry-Perot-like cavity, that results from a beam focusing effect that stabilizes the cavity even for small beam sizes, resembling the focusing behavior of curved mirrors. Moreover, they show an improved reflectivity with respect to their standard distributed Bragg reflector counterparts that allows higher compactness. A Q-factor higher than 10⁴ has been achieved for an optimized 5-period-long mirror cavity. The optimization of the Q-factor and the performances in terms of energy storage, field enhancement, and confinement are detailed.

High-efficient ultra-short vertical long-range plasmonic couplers

Mesoscopic self-collimation offered by mesoscopic photonic crystals can be efficiently used to control light lateral dispersion in conjunction with controlled reflectivity. An accurate control of this phenomenon is crucial for the exploitation of peculiar effects such as the outwitting of instabilities in planar cavities and the collimation in arbitrary directions inside the mesoscopic photonic crystal.

Stable planar mesoscopic photonic crystal cavities.

Graphene, a 2D carbon sheet with a honeycomb lattice, is a two-dimensional material with outstanding thermal, mechanical, electronic and optical properties. In particular, graphene is a gapless material with high mobility that exhibits remarkably high absorption values (~2.3%) for the visible and near-infrared wavelengths. In this paper, we will illustrate some applications of graphene photonics and plasmonics, reported in literature, in different research fields and we will investigate theoretically and experimentally the linear and nonlinear properties of graphene-based nanostructures such as one-dimensional (1D) photonic crystals (PhCs) and 1D gratings. In particular, we will show how to exploit the large nonlinear response and the saturation effects of graphene monolayers, sandwiched in the defect layer of an asymmetric 1D photonic crystal, to dynamically change the structure from a perfect absorber (100%) to a mirror. We will also show how it is possible to tune the working wavelength by tilting the angle of incidence of the impinging electromagnetic field for both TE and TM polarizations. Finally, we will report on a 1D dielectric grating, incorporating a graphene monolayer, that resembles the 1D PhC optical response exploiting guided mode resonances. Therefore, the proposed nanostructures could efficiently exploit the linear and nonlinear properties of the graphene monolayer for the realization of tunable absorbers or saturable mirrors improving and boosting the performance of optical devices such as photo-detectors and short-pulse lasers.

Self-collimation in mesoscopic photonic crystals: From reflectivity management to stable planar cavities

It was first thought that this mesoscopic self-collimation only occurred when the phase index averaged to 0 over a mesoscopic period: <;n>=(nPhC.lPhC+nb.lb)/(lb+lPhC)=0, requiring a negative index nPhC in the photonic crystal (PhC) slab to compensate for the positive phase index nb in the bulk material. This stringent condition leaves no room for other optimization or functionality and imposes air as the bulk material (nb=1). However, we have recently demonstrated that mesoscopic self-collimation condition does not actually rely on phase index in the PhC slab but on a new concept: the curvature index, nc. This new condition of zero averaged curvature <;C>=nc/lPhC+nb/lb=0 leaves room for many other optimizations. In particular, we have numerically demonstrated mesoscopic self-collimation in all positive index material with extremely low filling factor in air of 3%. This opens the way to applications of mesoscopic self-collimation to active structures like laser cavities or amplifier. Moreover, as this condition does not impose a particular mesoscopic period L=lPhC+lb and loosely constraints the phase index, it is possible to make mesoscopic self-collimating reflectors with arbitrary reflectivity value using appropriately designed slabs. It is thus possible to tune reflectivity anywhere between high reflectivity. This opens the way towards multifunctional mesoscopic structures. We have also shown that mesoscopic self-collimation can be combined with slow light in structures essentially made of high index and potentially active or non-linear materials.

Graphene-based photonic nanostructures for linear and nonlinear devices

Graphene is a perfect two-dimensional (2D) carbon sheet in honeycomb lattice with unique, multi-faceted properties. Very recently plasmons have been identified in graphene thus opening a new research field called “graphene plasmonics” which provides a suitable alternative to noble-metal plasmonics, since the associated surface plasmons (SPs) exhibit much larger light confinement abilities and relatively long propagation distances, with the advantage of being highly tunable via electrostatic gating. Moreover graphene can be efficiently integrated in photonic nanostructures in order to increase the optical absorption in the visible and infrared regions (graphene photonics). In this paper we will illustrate some applications of graphene plasmonics and graphene photonics, reported in literature, in different research fields and we will review our recent numerical and experimental results concerning the linear and nonlinear optical response of graphene plasmonic and photonic nanostructures emphasizing their interaction in terms of absorption and optical resonances.

Multifunctionnal self-collimating mesoscopic photonic crystals

In this Letter, the study of a periodic structure composed of gold strips arranged in double-period unit cells, in a symmetric and asymmetric environment, is reported. The spectral maps show that the formation of the plasmonic bandgap and the extraordinary optical transmission are subjected to the proportion between the strip widths. Moreover, when the asymmetric environment is considered, high-transmittance and high-absorbance states arise. Hence, by controlling the geometrical parameters of the binary-periodic structure, it is possible to tailor the spectral response of the grating enhancing the desired features and exploiting them for different applications.