Daniele Modotto

Second-harmonic generation in silicon waveguides strained by silicon nitride

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40 pm V(-1) at 2,300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near- to mid-infrared spectrum from 1.2 to 10 μm.

Dynamics of the modulational instability in microresonator frequency combs

The generation of optical Kerr frequency combs by microresonators has attracted much interest in the last few years [1]. The equidistant and highly resolved spectral lines of these combs are expected to help facilitate numerous applications such as optical clocks, sensing and spectroscopy. The theoretical descriptions of microresonator frequency combs has to date mostly been carried out using a modal expansion approach, which describes the slow evolution of the comb spectrum using time-domain rate equations [2]. However, this approach has several disadvantages when it comes to modelling of temporal structures and is also computationally expensive to use when applied to broadband combs, which in the case of octave spanning combs can comprise thousands of resonant modes. An alternative description of microresonator frequency combs has recently been proposed [3] that allows the comb to be described in the time-domain under quite general conditions, by means of a mean-field Lugiato-Lefever equation, which is a driven and damped nonlinear Schrödinger equation that has previously been applied to great success in the description of fiber-ring lasers [4].

Novel Design of UWB Antenna with Band - Notch Capability

An ultra-wideband (UWB) antenna with a narrow frequency notch is presented. The antenna has been fabricated on a Duroid 5870 substrate and occupies an area of only 30times35 mm2. Starting from a trapezoidal planar patch exhibiting a VSWR smaller than 2.5 in the 3.5-10 GHz band, a frequency notch at 5.65 GHz is introduced by two slots near the coplanar waveguide feeding the patch. The measured return loss shows a good agreement with the simulation results and proves that this kind of antenna is suitable for reducing the detrimental interference effects of WLAN, operating around 5.5 GHz, on UWB radio links.

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.

On the numerical simulation of Kerr frequency combs using coupled mode equations

Abstract It is demonstrated that Kerr frequency comb generation described by coupled mode equations can be numerically simulated using Fast Fourier Transform methods. This allows broadband frequency combs spanning a full octave to be efficiently simulated using standard algorithms, resulting in orders of magnitude improvements in the computation time.

Diffraction engineering in arrays of photonic crystal waveguides.

Light propagation in uniform arrays of photonic crystal waveguides is studied. We demonstrate that, in stark contrast to the case of conventional waveguide arrays, diffraction can be tailored both in magnitude and sign by varying only the spacing between adjacent waveguides. Diffraction management in ultracompact arrays of straight photonic crystal waveguides is demonstrated by solving Maxwells equations through the time-domain finite-element method.

Optical filter based on two coupled PhC GaAs-membranes

We demonstrate an ultracompact optical filter based on two coupled high-index contrast GaAs photonic crystal (PhC) membranes. The PhC membranes consist of a square lattice of air holes and behave as a Fabry-Perot cavity whose reflectivity and transmissivity depend on the air gap between the two membranes. The normal-incidence reflectance measurements and the numerical simulation of reflection spectra show a high sensitivity to the geometrical parameters, such as the distance between the slabs, whose control would make the device suitable for a new class of tunable optical filters.

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.

Second harmonic generation in AlGaAs photonic wires using low power continuous wave light

We report modal phase matched (MPM) second harmonic generation (SHG) in high-index contrast AlGaAs sub-micron ridge waveguides, by way of sub-mW continuous wave powers at telecommunication wavelengths. We achieve an experimental normalized conversion efficiency of ~14%/W/cm2, obtained through a careful sub-wavelength design supporting both the phase matching requirement and a significant overlap efficiency. Furthermore, the weak anomalous dispersion, robust fabrication technology and possible geometrical and thermal tuning of the device functionality enable a fully integrated multi-functional chip for several critical areas in telecommunications, including wavelength (time) division multiplexing and quantum entanglement.

Nonlinear bidirectional beam propagation method based on scattering operators for periodic microstructured waveguides

Beginning with a recently proposed bidirectional beam propagation method based on scattering operators, we develop an accurate and efficient method for the analysis of periodic microstructured waveguides in a nonlinear regime. This novel numerical tool allows us to describe the role played by losses that are due to diffraction and the effects of the finite size of the beams in nonlinear optical devices with strong lateral confinement of the optical intensity. We demonstrate the effectiveness and the efficiency of our method with numerical examples.