Florian Merget

Pockels effect based fully integrated, strained silicon electro-optic modulator

We demonstrate for the first time a fully integrated electro-optic modulator based on locally strained silicon rib-waveguides. By depositing a Si3N4 strain layer directly on top of the silicon waveguide the silicon crystal is asymmetrically distorted. Thus its inversion symmetry is broken and a linear electro-optic effect is induced. Electro-optic characterization yields a record high value χ(2)(yyz) = 122 pm/V for the second-order susceptibility of the strained silicon waveguide and a strict linear dependence between the applied modulation voltage V(mod) and the resulting effective index change Δn(eff). Spatially resolved micro-Raman and terahertz (THz) difference frequency generation (DFG) experiments provide in-depth insight into the origin of the electro-optic effect by correlating the local strain distribution with the observed second-order optical activity.

Three-dimensional simulation model of switching dynamics in phase change random access memory cells

Switching dynamics associated with reset and set operations of vertical phase change random access memory (PCRAM) cells are studied using a three-dimensional simulation model. Based on a finite difference method, the numerical algorithm simulates the electrical, thermal, and phase change dynamics in the PCRAM device during switching operations taking into account electrical and thermal percolation characteristics of the phase change material. Toward a better understanding of switching operations and the optimization of cell designs, the obtained simulation results provide unprecedented insight into temporally and spatially resolved kinetics of device temperature, current densities, and phase transitions. Threshold conditions for reset and set operations are identified in close agreement to existing experimental data, and the scaling ability of the investigated vertical PCRAM cell design to a minimum feature size of at least 40 nm is demonstrated.

Silicon nitride CMOS-compatible platform for integrated photonics applications at visible wavelengths

Silicon nitride is demonstrated as a high performance and cost-effective solution for dense integrated photonic circuits in the visible spectrum. Experimental results for nanophotonic waveguides fabricated in a standard CMOS pilot line with losses below 0.71dB/cm in an aqueous environment and 0.51dB/cm with silicon dioxide cladding are reported. Design and characterization of waveguide bends, grating couplers and multimode interference couplers (MMI) at a wavelength of 660 nm are presented. The index contrast of this technology enables high integration densities with insertion losses below 0.05 dB per 90° bend for radii as small as 35 µm. By a proper design of the buried oxide layer thickness, grating couplers with efficiencies above 38% for the TE polarization have been obtained.

Electrical percolation characteristics of Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films during the amorphous to crystalline phase transition

Percolation effects are studied in phase change materials used for optical and electrical nonvolatile memory applications. Simultaneous measurements of the electrical resistivity and optical reflectivity allow experimental assessment of the percolation behavior associated with the mixed amorphous and crystalline phases in this class of chalcogenide materials. Dynamic analysis of the isothermal amorphous to crystalline phase transition in as-deposited amorphous Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films are performed. A significantly earlier onset and saturation for the change of electrical resistivity in comparison to the optically measured phase transition is observed. This behavior reflects a significant influence of percolation on the electrical properties of this class of materials, leading to a nonlinear relationship between crystallization degree and electrical resistivity. This nonlinearity is associated with the formation of highly conducting crystalline paths in a lower conducting amorphous matr...

On the measurement of the Pockels effect in strained silicon

We measure the voltage-dependent phase shift in silicon waveguides strained by a silicon nitride layer and show that, in our measurements, the phase shift is due to free carrier accumulation inside the waveguides. Nonetheless, inverting the applied voltage also inverts the applied phase shift-an effect due to a quasi-static surface charge in the silicon nitride. Since the measured effect is on the same order as recently published second-order nonlinearities attributed to the Pockels effect, inclusion of these carrier-based effects in the analysis of experimental data is of paramount importance.

High-frequency electro-optic measurement of strained silicon racetrack resonators

The observation of the electro-optic effect in strained silicon waveguides has been considered a direct manifestation of an induced χ(2) nonlinearity in the material. In this work, we perform high-frequency measurements on strained silicon racetrack resonators. Strain is controlled by a mechanical deformation of the waveguide. It is shown that any optical modulation vanishes, independent of the applied strain, when the applied voltage varies much faster than the carrier effective lifetime and that the DC modulation is also largely independent of the applied strain. This demonstrates that plasma carrier dispersion is responsible for the observed electro-optic effect. After normalizing out free-carrier effects, our results set an upper limit of (8±3) pm/V to the induced high-speed effective χeff,zzz(2) tensor element at an applied stress of -0.5 GPa. This upper limit is about 1 order of magnitude lower than previously reported values for static electro-optic measurements.

Optical Peaking Enhancement in High-Speed Ring Modulators

Ring resonator modulators (RRM) combine extreme compactness, low power consumption and wavelength division multiplexing functionality, making them a frontrunner for addressing the scalability requirements of short distance optical links. To extend data rates beyond the classically assumed bandwidth capability, we derive and experimentally verify closed form equations of the electro-optic response and asymmetric side band generation resulting from inherent transient time dynamics and leverage these to significantly improve device performance. An equivalent circuit description with a commonly used peaking amplifier model allows straightforward assessment of the effect on existing communication system architectures. A small signal analytical expression of peaking in the electro-optic response of RRMs is derived and used to extend the electro-optic bandwidth of the device above 40 GHz as well as to open eye diagrams penalized by intersymbol interference at 32, 40 and 44 Gbps. Predicted peaking and asymmetric side band generation are in excellent agreement with experiments.

Edge Couplers With Relaxed Alignment Tolerance for Pick-and-Place Hybrid Integration of III–V Lasers With SOI Waveguides

We report on two edge-coupling and power splitting devices for hybrid integration of III-V lasers with sub-micrometric silicon-on-insulator waveguides. The proposed devices relax the horizontal alignment tolerances required to achieve high coupling efficiencies and are suitable for passively aligned assembly with pick-and-place tools. Light is coupled to two on-chip single mode SOI waveguides with almost identical power coupling efficiency, but with a varying relative phase accommodating the lateral misalignment between the laser diode and the coupling devices, and is suitable for the implementation of parallel optics transmitters. Experimental characterization with both a lensed fiber and a Fabry-Pérot semiconductor laser diode has been performed. Excess insertion losses (in addition to the 3 dB splitting) taken as the worst case over both waveguides of respectively 2 ± 0.3 dB and 3.1 ± 0.3 dB, as well as excellent 1 dB loss placement tolerance range of respectively 2.8 μm and 3.8 μm (worst case over both in-plane axes) have been measured for the two devices. Back-reflections to the laser are below -20 dB for both devices within the 1 dB misalignment range. Devices were fabricated with 193 nm DUV optical lithography and are compatible with mass-manufacturing with mainstream CMOS technology.

Silicon photonics plasma-modulators with advanced transmission line design.

We have investigated two novel concepts for the design of transmission lines in travelling wave Mach-Zehnder interferometer based Silicon Photonics depletion modulators overcoming the analog bandwidth limitations arising from cross-talk between signal lines in push-pull modulators and reducing the linear losses of the transmission lines. We experimentally validate the concepts and demonstrate an E/O -3 dBe bandwidth of 16 GHz with a 4V drive voltage (in dual drive configuration) and 8.8 dB on-chip insertion losses. Significant bandwidth improvements result from suppression of cross-talk. An additional bandwidth enhancement of ~11% results from a reduction of resistive transmission line losses. Frequency dependent loss models for loaded transmission lines and E/O bandwidth modeling are fully verified.

Influence of Si and N additions on structure and phase stability of Ge2Sb2Te5 thin films

The influence of Si and N in Ge(2)Sb(2)Te(5) (space group [Formula: see text]) on structure and phase stability thereof was studied experimentally by thin film growth and characterization as well as theoretically by ab initio calculations. It was found that Si and N most probably accumulate in the amorphous matrix embedding Ge(2)Sb(2)Te(5) grains. The incorporation of Si and N in these samples causes an increase of the crystallization temperature and the formation of finer grains. N is more efficient in increasing the crystallization temperature and in reducing the grain size than Si which can be understood based on the bonding analysis. The incorporation of both Si and N in Ge(2)Sb(2)Te(5) is energetically unfavourable, leading to finer grains and larger crystallization temperatures. While in the case of Si additions no significant changes in bonding are observed, N additions appear to enable the formation of strong Te-N bonds in the amorphous matrix, which are shown to be almost twice as strong as the strongest bonds in unalloyed Ge(2)Sb(2)Te(5).