Low temperature silicon nitride waveguides for multilayer platforms

Several 3D multilayer silicon photonics platforms have been proposed to provide densely integrated structures for complex integrated circuits. Amongst these platforms, great interest has been given to the inclusion of silicon nitride layers to achieve low propagation losses due to their capacity of providing tight optical confinement with low scattering losses in a wide spectral range. However, none of the proposed platforms have demonstrated the integration of active devices. The problem is that typically low loss silicon nitride layers have been fabricated with LPCVD which involves high processing temperatures (<1000 ºC) that affect metallisation and doping processes that are sensitive to temperatures above 400ºC. As a result, we have investigated ammonia-free PECVD and HWCVD processes to obtain high quality silicon nitride films with reduced hydrogen content at low temperatures. Several deposition recipes were defined through a design of experiments methodology in which different combinations of deposition parameters were tested to optimise the quality and the losses of the deposited layers. The physical, chemical and optical properties of the deposited materials were characterised using different techniques including ellipsometry, SEM, FTIR, AFM and the waveguide loss cut-back method. Silicon nitride layers with hydrogen content between 10-20%, losses below 10dB/cm and high material quality were obtained with the ammonia-free recipe. Similarly, it was demonstrated that HWCVD has the potential to fabricate waveguides with low losses due to its capacity of yielding hydrogen contents <10% and roughness <1.5nm

Scattering of a plasmonic nanoantenna embedded in a silicon waveguide

Plasmonic antennas integrated on silicon devices have large and yet unexplored potential for controlling and routing light signals. Here, we present theoretical calculations of a hybrid silicon-metallic system in which a single gold nanoantenna embedded in a single-mode silicon waveguide acts as a resonance-driven filter. As a consequence of scattering and interference, when the resonance condition of the antenna is met, the transmission drops by 85% in the resonant frequency band. Firstly, we study analytically the interaction between the propagating mode and the antenna by including radiative corrections to the scattering process and the polarization of the waveguide walls. Secondly, we find the configuration of maximum interaction and numerically simulate a realistic nanoantenna in a silicon waveguide. The numerical calculations show a large suppression of transmission and three times more scattering than absorption, consequent with the analytical model. The system we propose can be easily fabricated by standard silicon and plasmonic lithographic methods, making it promising as real component in future optoelectronic circuits.Comment: 10 pages, 5 figure

Tensile strain engineering of germanium micro-disks on free-standing SiO2 beams

Tensile strain is required to enhance light-emitting direct-gap recombinations in germanium (Ge), which is a promising group IV material for realizing a monolithic light source on Si. Ge micro-disks on free-standing SiO2 beams were fabricated using Ge-on-Insulator wafers for applying tensile strain to Ge in a structure compatible with an optical confinement. We have studied the nature of the strain by Raman spectroscopy in comparison with finite-element computer simulations. We show the impacts of the beam design on the corresponding strain value, orientation, and uniformity, which can be exploited for Ge light emission applications. It was found that the tensile strain values are larger if the length of the beam is smaller. We confirmed that both uniaxial and biaxial strain can be applied to Ge disks, and maximum strain values of 1.1 and 0.6% have been achieved, as confirmed by Raman spectroscopy. From the photoluminescence spectra of Ge micro-disks, we have also found a larger energy splitting between the light-hole and the heavy-hole bands in shorter beams, indicating the impact of tensile strain

Photonic crystal waveguides on silicon rich nitride platform

We demonstrate design, fabrication, and characterization of two-dimensional photonic crystal (PhC) waveguides on a suspended silicon rich nitride (SRN) platform for applications at telecom wavelengths. Simulation results suggest that a 210 nm photonic band gap can be achieved in such PhC structures. We also developed a fabrication process to realize suspended PhC waveguides with a transmission bandwidth of 20 nm for a W1 PhC waveguide and over 70 nm for a W0.7 PhC waveguide. Using the Fabry–Pérot oscillations of the transmission spectrum we estimated a group index of over 110 for W1 PhC waveguides. For a W1 waveguide we estimated a propagation loss of 53 dB/cm for a group index of 37 and for a W0.7 waveguide the lowest propagation was 4.6 dB/cm

Tensile strain of germanium micro-disks on freestanding SiO2 beams

Tensile strain is crucial to expect the direct recombination in germanium (Ge), towards monolithic light sources on silicon (Si). Freestanding beams of Ge are known to produce strong tensile strain, however, it is not trivial to construct a cavity in a freestanding structure. Here, we fabricated Ge micro-disks on freestanding oxide beams, and observed Whispering-Gallery-Modes (WGM) by photoluminescence. The tensile strain was larger in shorter beams, which is consistent with simulations

Hot-wire chemical vapour deposition for silicon nitride waveguides

In this work, we demonstrate the use of HWCVD as an alternative technique to grow SiN layers for photonic waveguides at temperatures <400ºC. In particular, the effect of the ammonia flow and the filament temperature on the material structure, optical properties and propagation losses of the deposited films was investigated. SiN layers with good thickness uniformity, roughness as low as 0.61nm and H concentration as low as 10.4×1021 atoms/cm3 were obtained. Waveguides fabricated on the studied materials exhibited losses as low as 7.1 and 12.3 dB/cm at 1310 and 1550nm respectively

Si-rich silicon nitride for nonlinear signal processing applications

Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5dB/cm) and enhanced Kerr nonlinear response (Re{γ} = 16 Wm-1). Thanks to these properties, our nonlinear waveguides are able to produce a pi nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications

Recent breakthroughs in carrier depletion based silicon optical modulators

The majority of the most successful optical modulators in silicon demonstrated in recent years operate via the plasma dispersion effect and are more specifically based upon free carrier depletion in a silicon rib waveguide. In this work we overview the different types of free carrier depletion type optical modulators in silicon. A summary of some recent example devices for each configuration is then presented together with the performance that they have achieved. Finally an insight into some current research trends involving silicon based optical modulators is provided including integration, operation in the mid-infrared wavelength range and application in short and long haul data transmission link