A. D’Orazio

Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings

A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to enhance light absorption in a monolayer graphene exploiting guided mode resonances. Numerical findings reveal that the optimized configuration is able to absorb up to 60% of the impinging light at normal incidence for both TE and TM polarizations resulting in a theoretical enhancement factor of about 26 with respect to the monolayer graphene absorption (≈2.3%). Experimental results confirm this behavior showing CVD graphene absorbance peaks up to about 40% over narrow bands of a few nanometers. The simple and flexible design points to a way to realize innovative, scalable and easy-to-fabricate graphene-based optical absorbers.

Nonlinear control of absorption in one-dimensional photonic crystal with graphene-based defect

Perfect, narrow-band absorption is achieved in an asymmetric 1D photonic crystal with a monolayer graphene defect. Thanks to the large third-order nonlinearity of graphene and field localization in the defect layer we demonstrate the possibility to achieve controllable, saturable absorption for the pump frequency.

Photonic band gap filter for wavelength division multiplexer

An optical multiplexer-demultiplexer based on an index-confined photonic band gap waveguide is proposed. The dropping of electromagnetic waves having a given frequency or a certain frequency band is obtained via a phase-shifted grating obtained by breaking the uniform period sequence to include a defect layer. The selective filtering properties of the proposed structure are simulated by means of a computer code relying on a bi-directional beam propagation method based on the method of lines.

Graphene-based perfect optical absorbers harnessing guided mode resonances

We investigate graphene-based optical absorbers that exploit guided mode resonances (GMRs) attaining theoretically perfect absorption over a bandwidth of few nanometers (over the visible and near-infrared ranges) with a 40-fold increase of the monolayer graphene absorption. We analyze the influence of the geometrical parameters on the absorption rate and the angular response for oblique incidence. Finally, we experimentally verify the theoretical predictions in a one-dimensional, dielectric grating by placing it near either a metallic or a dielectric mirror, thus achieving very good agreement between numerical predictions and experimental results.

Experimental demonstration of a novel bio‑sensing platform via plasmonic band gap formation in gold nano‑patch arrays

In this paper we discuss the possibility of implementing a novel bio-sensing platform based on the observation of the shift of the leaky surface plasmon mode that occurs at the edge of the plasmonic band gap of metal gratings, when an analyte is deposited on top of the metallic structure. We report numerical calculations, fabrication and experimental measurements to prove the sensing capability of a two-dimensional array of gold nano-patches in the detection of a small quantity of Isopropyl Alcohol (IPA) deposited on top of sensor surface. The calculated sensitivity of our device approaches a value of 1000 nm/RIU with a corresponding Figure of Merit (FOM) of 222 RIU(-1). The presence of IPA can also be visually estimated by observing a color variation in the diffracted field. We show that color brightness and intensity variations can be ascribed to a change in the aperture size, keeping the periodicity constant, and to different types of analyte deposited on the sample, respectively. Moreover, we demonstrate that unavoidable fabrication imperfections revealed by the presence of rounded corners and surface roughness do not significantly affect device performance.

Semiconductor-based superlens for subwavelength resolution below the diffraction limit at extreme ultraviolet frequencies

We theoretically demonstrate negative refraction and subwavelength resolution below the diffraction limit in the UV and extreme UV ranges using semiconductors. The metal-like response of typical semiconductors such as GaAs or GaP makes it possible to achieve negative refraction and superguiding in resonant semiconductor/dielectric multilayer stacks, similar to what has been demonstrated in metallodielectric photonic band gap structures. The exploitation of this basic property in semiconductors raises the possibility of yet-untapped applications in the UV and soft x-ray ranges.

Color control through plasmonic metal gratings

We experimentally demonstrate the color tuning abilities of two-dimensional periodic arrays of gold nano-patches on silicon substrate. We observe that changes in the geometrical parameters of the array can shift significantly the plasmonic resonance that occurs at the edge of the plasmonic band gap. Experimental proof of this shift is provided by the observation of an important change in the color of the diffracted field. Calculations of the diffracted spectra match the observed color changes very well and provide an efficient means for the design of sensing platforms based on color observation.

Plasmonic bandgap formation in two-dimensional periodic arrangements of gold patches with subwavelength gaps

We report on the formation of plasmonic bandgaps in two-dimensional periodic arrangements of gold patches. Orthogonal arrays of subwavelength slits with different periodicities have been studied by means of a three-dimensional finite-difference time-domain (FDTD) code, changing incident polarization and geometrical parameters. Spectral response of gold patches having different a form factor and surrounded by different media have been also investigated and compared in order to give a full description of bandgap shifts paving the way for the design of polarization-sensitive devices.

Refinement of Er3+-doped hole-assisted optical fiber amplifier.

This paper deals with design and refinement criteria of erbium doped hole-assisted optical fiber amplifiers for applications in the third band of fiber optical communication. The amplifier performance is simulated via a model which takes into account the ion population rate equations and the optical power propagation. The electromagnetic field profile of the propagating modes is carried out by a finite element method solver. The effects of the number of cladding air holes on the amplifier performance are investigated. To this aim, four different erbium doped hole-assisted lightguide fiber amplifiers having a different number of cladding air holes are designed and compared. The simulated optimal gain, optimal length, and optimal noise fig. are discussed. The numerical results highlight that, by increasing the number of air holes, the gain can be improved, thus obtaining a shorter amplifier length. For the erbium concentration NEr=1.8x1024 ions/m3, the optimal gain G(Lopt) increases up to ~2dB by increasing the number of the air holes from M=4 to M=10.

Design of Er3+ doped SiO2–TiO2 planar waveguide amplifier

Er-doped silica–titania channel waveguide amplifiers are investigated by means of a home-made computer code to test their feasibility. The optical, spectroscopic and geometrical parameters measured on the fabricated slab waveguides, are used in the simulation model, which takes into account the pump and signal propagation, the amplified spontaneous emission, and the secondary transitions pertaining to the ion–ion interactions. By simulation, in the small signal operation, a gain close to 4.6 dB is demonstrated for a channel waveguide 5 cm long, by using a pump power of 200 mW; the signal wavelength being ks ¼ 1533 nm, pump and signal co-propagating.