Stefano Boscolo

Coupling and decoupling of electromagnetic waves in parallel 2D photonic crystal waveguides

The behavior of two nearby straight photonic crystal waveguides is analyzed. It is shown that the two guides, considered as a single system, may realize a very efficient wavelength selective directional coupler, that may be used as a channel interleaver in a WDM communications system. In addition, we also show that, by properly designing the geometry of the dielectric region between the guide cores, waveguide decoupling can be obtained. Necessary conditions for this feature to be obtained are analytically derived, and an electromagnetic explanation of the decoupling process is given.

Numerical analysis of propagation and impedance matching in 2D photonic crystal waveguides with finite length

A novel way of approaching wave propagation in two-dimensional (2-D) photonic crystal guides with finite length is presented. It is shown that the main propagation features can be captured by borrowing simple concepts of propagation in transmission lines and combining them with other concepts taken from the theory of periodic structures.

Graphene-assisted critically-coupled optical ring modulator.

Graphenes conductivity at optical frequencies can be varied upon injection of carriers. In the present paper, this effect is used to modulate losses of an optical wave traveling inside a ring cavity. This way an optical modulator based on the critical-coupling concept first introduced by Yariv can be realized. Through numerical simulations, we show that a modulator featuring a bandwidth as large as 100 GHz can be designed with switching energy in the order of few fJ per bit. Also, we show that operations with driving voltages below 1.2 volt could be obtained, thus making the proposed modulator compatible with requirements of low-voltage CMOS technology.

A Compact MIMO Array of Planar End-Fire Antennas for WLAN Applications

An approach to the design of multiple-input multiple-output (MIMO) arrays exploiting planar directive antennas is presented. It is well known that pattern orthogonality is a key aspect to reach low correlation, and thus to improve channel capacity in rich multipath environments. However, attention is often focused on reducing mutual coupling rather than optimizing the active element patterns. In this communication a planar MIMO array of printed Yagi-Uda antennas with integrated balun is presented. The end-fire radiation mechanism of the Yagi-Uda is exploited to obtain a triangular array of three sectoral antennas. This allows to achieve nearly orthogonal patterns, while keeping a low mutual coupling among radiating elements. A properly shaped ground at the feeding points allows to increase the isolation between the antennas, even in such a compact layout. A laboratory model has been characterized experimentally, and the effectiveness of the proposed design in terms of theoretical achievable capacity is demonstrated through numerical simulations considering IEEE 802.11n multipath fading channel models.

Modeling of enhanced field confinement and scattering by optical wire antennas

We describe the application of full-wave and semi-analytical numerical tools for the modeling of optical wire antennas, with the aim of providing novel guidelines for analysis and design. The concept of antenna impedance at optical frequencies is reviewed by means of finite-element simulations, whereas a surface-impedance integral equation is derived in order to perform an accurate and efficient calculation of the current distribution, and thereby to determine the equivalent-circuit parameters. These are introduced into simple circuits models, directly borrowed from radio frequency, which are applied in order to model the phenomena of enhanced field confinement at the feed gap and light scattering by optical antennas illuminated by plane waves.

Three-dimensional multiple-scattering technique for the analysis of photonic-Crystal slabs

A numerical technique allowing theoretical analysis of propagation in three-dimensional (3-D) photonic-crystal (PC) slabs is presented. The method is an extension to the 3-D case of the two-dimensional multiple-scattering technique that was originally developed by Tayeb and Maystre. As an application, the method is used to numerically estimate the out-of-plane scattering losses in a straight PC waveguide of finite height.

Highly directional planar ultra wide band antenna for radar applications

We describe a novel planar highly directive ultra wide band (UWB) antenna based on a disc monopole fed by a 50-Ohm microstrip line. The key feature of the proposed antenna is a careful engineering of the ground plane that permits to increase directionality for radar applications. We demonstrate through numerical simulations and measurements in anechoic chamber that the designed antenna exhibits low return loss, high directivity and good time-domain properties in the band of interest between 6 and 8 GHz.

Graphene sustained nonlinear modes in dielectric waveguides.

We discuss the existence of nonlinear modes sustained by graphene layers in dielectric waveguides. Taking advantage of the almost two dimensional nature of graphene, we introduce the nonlinear effect as a parameter in the continuity equations. We then apply our modeling to a simple slab waveguide to enlighten how graphene can be used to induce huge nonlinear phase shifts at easily accessible power levels.

Planar, Compact Dual-Band Antenna for Wireless LAN Applications

A compact, printed dual-band antenna for WLAN applications is proposed. The radiating elements consist of a combination of a printed dipole for the lower resonant frequency and a bow-tie antenna for the upper resonant frequency. The operation on two separate bands is guaranteed by the use of two surface-mounted-device inductors. The antenna has been manufactured in antipodal configuration. Measurement results show good omnidirectionality and low cross-polarization levels.

Graphene-assisted control of coupling between optical waveguides.

The unique properties of optical waveguides electrically controlled by means of graphene layers are investigated. We demonstrate that, thanks to tunable losses induced by graphene layers, a careful design of silicon on silica ridge waveguides can be used to explore passive PT-symmetry breaking in directional couplers. We prove that the exceptional point of the system can be probed by varying the applied voltage and we thus propose very compact photonic structures which can be exploited to control coupling between waveguides and to tailor discrete diffraction in arrays.