T. Angelova

A systematic optimization of design parameters in strained silicon waveguides to further enhance the linear electro-optic effect

The electro-optic Pockels effect may be generated in silicon photonics structures by breaking the crystal symmetry by means of a highly stressing cladding layer (typically silicon nitride, SiN) deposited on top of the silicon waveguide. In this work, the influence of the waveguide parameters on the strain distribution and its overlap with the optical mode to enhance the Pockels effect has been analyzed. The optimum waveguide structure have been designed based on the definition and quantification of a figure of merit. The fabrication of highly stressing SiN layers by PECVD has also been optimized to characterize the designed structures. The residual stress has been controlled during the growth process by analyzing the influence of the main deposition parameters. Therefore, two identical samples with low and high stress conditions were fabricated and electro-optically characterized to test the induced Pockels effect and the influence of carrier effects. Electro-optical modulation was only measured in the sample with the high stressing SiN layer that could be attributed to the Pockels effect. Nevertheless, the influence of carriers were also observed thus making necessary additional experiments to decouple both effects.

On the influence of interface charging dynamics and stressing conditions in strained silicon devices

The performance of strained silicon devices based on the deposition of a top silicon nitride layer with high stress have been thoroughly analyzed by means of simulations and experimental results. Results clearly indicate that the electro-optic static response is basically governed by carrier effects. A first evidence is the appearance of a variable optical absorption with the applied voltage that should not occur in case of having a purely electro-optic Pockels effect. However, hysteresis and saturation effects are also observed. We demonstrate that such effects are mainly due to the carrier trapping dynamics at the interface between the silicon and the silicon nitride and their influence on the silicon nitride charge. This theory is further confirmed by analyzing identical devices but with the silicon nitride cladding layer optimized to have intrinsic stresses of opposite sign and magnitude. The latter is achieved by a post annealing process which produces a defect healing and consequently a reduction of the silicon nitride charge. Raman measurements are also carried out to confirm the obtained results.

Recent advances in strained silicon devices for enabling electro-optical functionalities

More than ten years ago it was demonstrated that the Pockels effect could be feasible by breaking the crystal symmetry of silicon. Since then, strained silicon devices have been developed by means of a highly stressing cladding layer (typically silicon nitride, SiN) deposited on top of the silicon waveguide. However, it has been recently shown that carrier effects could play a significant role and the induced Pockels effect could have been overestimated. In this work, we will review the recent advances in the field and we will present our last results showing that there is also a strong influence of the interface traps on the electro-optical response.

Electrical switching in hybrid VO 2 /Si photonic structures

The integration of active materials on silicon is emerging as a promising field in silicon photonics to improve the performance metrics of key photonic components, in particular active components. Active materials allow tuning their optical properties as function of external stimuli. Amongst them, vanadium dioxide (VO2) has been largely investigated for different applications due to its controllable change between an insulating and a metallic phase. For photonic applications, VO2 shows a promising performance due to the abrupt change in the refractive index between the two phases across the semiconductor to metal transition (SMT). In this work, we will present our recent results for enabling disruptive electrical switching performance in hybrid VO2/Si photonic structures. Results have been obtained in the framework of the FP7-ICT-2013-11-61456 SITOGA project.

Hybrid silicon-ferroelectric oxide platform for tunable nanophotonics on silicon

In this communication, we present strategies to implement novel or enhanced optoelectronic functionalities on silicon via the monolithic integration of functional oxides as active layers for nanophotonic devices. We focus on the use of ferroelectric oxides with naturally strong electro-optical coefficients such as BaTiO3 (BTO) on SOI for the realization of electro-optical modulators around 1.55 μm. Specifically, we present methods to monolithically integrate monocrystalline BTO on silicon and to design devices that leverage their strong Pockels coefficient for efficient and fast electro-optical modulation. We show preliminary results on the fabrication, process and characterization of a slotted BTO waveguide platform. We further discuss on first electro-optical modulation results and how to improve the current optoelectronic platform. In particular, we will explain the potential origins of optical losses in such systems and how to avoid them.

Optimization of stress to enhance electro-optical modulation performance based on strained silicon waveguides

The influence of the stress on the electro-optical performance in strained silicon waveguides is analyzed by means of simulation and experimental results. There is a strong dependency between them in the static regime although carrier effects are also observed when the applied voltage increases.

Integration of functional oxides on SOI for agile silicon photonics

Photonic devices enabling light modulation and switching are of major importance for modern telecommunication. Silicon-based electro-optic modulators are intensively investigated because of their direct compatibility to CMOS fabrication processes. However, the performances of Si based modulators are intrinsically limited, in terms of data transmission rate and energy consumption. To overcome this limitation, ferroelectric oxides with naturally strong electro-optical coefficients combined to small footprint could be ideal candidates for high-speed modulators. We have designed an integration scheme to implement these materials on SOI photonic platform, for 1.55 μm Datacom systems. Monolithic integration of ferroelectric oxides on SOI via SrTiO3 templates is presented. The devices are based on a horizontal (crystalline Si / ferroelectric oxide / amorphous Si) slot waveguide configuration.