M. Halbwax

Recent Progress in High-Speed Silicon-Based Optical Modulators

The evolution of silicon optical modulators is recalled, from the first effect demonstrations to the characterization of high-performance devices integrated in optical waveguides. Among possibilities to achieve optical modulation in silicon-based materials, the carrier depletion effect has demonstrated good capacities. Carrier depletion in Si and SiGe/Si structures has been theoretically and experimentally investigated. Large phase modulation efficiency, low optical loss, and large cutoff frequency are obtained by considering simultaneously optical and electrical structure performances. Integrated Mach-Zehnder interferometers and resonators are compared to convert phase modulation into intensity modulation. Finally, recent results on high-speed and low-loss silicon optical modulator using an asymmetric Mach-Zehnder interferometer are presented. It is based on a p-doped slit embedded in the intrinsic region of a lateral pin diode integrated in a silicon-on-insulator waveguide. This design allows a good overlap between the optical mode and carrier density variations. An insertion loss of 5 dB has been measured with a -3 dB bandwidth of 15 GHz.

Kinetics of Ge growth at low temperature on Si(001) by ultrahigh vacuum chemical vapor deposition

The growth of germanium at low temperature by ultrahigh vacuum chemical vapor deposition on Si(001) is investigated in real time by reflection high-energy electron diffraction. These observations are complementarily checked by atomic force microscopy, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction experiments. It can be seen that the currently observed Stranski–Krastanov-related two-dimensional (2D) to three-dimensional transition is avoided at 330°C and that the major part of the relaxation process occurs during the deposition of the first two monolayers. Then, the measured in-plane lattice parameter evolves slowly and approaches that of bulk Ge after deposition of 50 monolayers. The corresponding relaxation equals 83%. The resulting surface is flat, with a rms roughness of 0.55nm. The relaxation is found to be mainly due to misfits dislocations located at the Ge∕Si interface. Regrowth experiments at 600°C show that the low-temperature films are not stable...

Femtosecond laser for black silicon and photovoltaic cells

We have prepared absorbing structures for photovoltaic cells with different nano-texturization, obtained by means of a femtosecond laser, without the use of corrosive gas (i.e. under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase up to 60 % in the laser texturized zones.

Integration of germanium waveguide photodetectors for optical intra-chip interconnects

This paper presents the potential characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intra-chip optical interconnects. Experimental results on the optical absorption, from 1.1 μm to 1.7 μm of Ge layers epitaxially grown on Si are reported, as well as the measured responsivity of an interdigited MSM Ge photodetector. Light coupling from a rib SOI waveguide to a Ge photodetector is studied for two possible configurations: butt coupling or Ge deposition on top of the waveguide. Comparisons between MSM and PIN Ge detectors are carried out by estimating the dark current, capacitance and time response.

Structure Optimization of Electro-optic Polymer Waveguides For Low Half-wave Voltage Modulators

In this article, we discuss the waveguide dimensions optimization aiming to reduce the Vπ. For that purpose, various cover materials are investigated leading to a minimum effective core area Aeff. The index contrast (core-cladding) at λ= 1550 nm, is varying from 0.07 to 0.21. As a result, the Aeff decreases from 12 μm2 down to 2.3 μm2, the total thickness of the waveguide is thus reduced and consequently the Vπ. Optimal parameters were calculated at λ= 1550 nm for single mode inverted-rib waveguides structure. The PAS1 a new polymer is used as electro-optic material for the core. An analytical model taking account the losses by tunnelling, allowed us to estimate the optimum distance between electrodes to reduce the Vπ which could be about 1.6V ( 0.8 V in a push-pull configuration). Related with the bandwidth of the modulator, permittivity measurements were carried out on core and cladding polymers as well. The process of waveguides fabrication is described in details and several waveguides are performed. Finally, a new experimental technique for precision measurements of the propagation losses in waveguides is presented. The principle is simple, and the propagation losses measured is found to be independent of coupling conditions.