Sylvain Maine

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.

Electrical characterization of devices based on carbon nanotube films

Statistical study of electrical conduction on a large array of devices based on carbon nanotube films shows a weakly dispersive film conductivity, and a specific contact resistance of 1.1 10−6 Ω cm2, which is four orders of magnitude lower than previously reported values. This allows identifying the conductivity of the carbon nanotube films as driven by a fluctuation induced tunneling mechanism. Such results pave the way to the realization of optoelectronic devices, such as highly sensitive light or gas sensor arrays.

Complex optical index of single wall carbon nanotube films from the near-infrared to the terahertz spectral range

We retrieve the complex optical index of single-walled carbon nanotube (CNT) films in the 0.6-800 μm spectral range. Results are obtained from a complete set of optical measurements, reflection and transmission, of free-standing CNT films using time domain spectroscopy in the terahertz (THz) and Fourier transform infrared (IR) spectroscopy in the visible-IR. Based on a Drude-Lorentz model, our results reveal a global metallic behavior of the films in the IR, and confirm their high optical index in the THz range.

Design Optimization of a SiGe/Si Quantum-Well Optical Modulator

In this paper, design optimization of a SiGe/Si quantum-well optical modulator integrated on silicon-on-insulator (SOI) substrate to achieve high-frequency operation is reported. The structure, based on free-carrier depletion in a PIN diode, is integrated in a rib waveguide. Influence of geometrical parameters, layer doping, and metal contacts is determined through numerical simulations and optimized structures are defined. The obtained figure of merit VpiLpi is 1.8 V-cm.

Potential of carbon nanotubes films for infrared bolometers

We investigate in this paper the potential of carbon nanotubes for infrared bolometers. A method to obtain CNT film layer and technological processes to obtain matrix of devices are presented. The electrical characterization of samples establishes the quality of our technology i.e. low contact resistance, and weak dispersion between devices. The potential of carbon nanotubes films as bolometric material is investigated by measuring the thermal dependence of their resistance and by comparison with amorphous silicon (one of the leading material for bolometric applications). Optical measurements of CNT films in the infrared and THz ranges show a relatively high absorption for a few hundreds nanometers thick material. Eventually the infrared (8-12 μm) photo-response of a first demonstrator is presented and discussed.

Design optimization of SiGe/Si Modulation-doped multiple quantum well modulator for high speed operation

A structure based on the free-carrier-induced electrorefractive effect in Si/SiGe modulation-doped quantum wells, placed in the intrinsic region of a PIN diode has been proposed. Effective index variation produced by carrier depletion under a reverse bias leads to a phase modulation of a guided wave. The measured variation of the effective index is typically 2.10-4 for a 0V to 6V variation of the reverse bias voltage. This study is focused on the integration of modulation doped SiGe/Si quantum-well optical modulator in SOI submicron rib waveguides with optical losses lower than 0.4dB/cm. The influence of the geometrical parameters, of layer doping and of the metallic contacts has been determined through numerical simulations and optimized modulation structures are defined. The obtained factor of merit LχVχ is then of 1.26 V.cm which can be favorably compared with the best published results obtained with other optimized modulators.

Comparison between multiple quantum wells and carbon nanotubes to generate high power in mode-locked fiber oscillator

Ultrashort pulse laser sources are deployed in a wide variety of applications ranging from basic scientific research and metrology to eye surgery and material processing. The majority of fiber lasers produce ultrashort pulses thanks to nonlinear optical elements called saturable absorbers (SA). Currently, multiple quantum wells (MQW-SA) structures are regularly used as SA. Recently, at 1.5 µm, we report the highest pulse energy extracted from a fiber oscillator passively mode-locked by MQW-SA [1]. The discovery of the nonlinear optical behaviour of carbon nanotubes (CNT-SA) makes them attractive materials for replacing MQW-SA [2].

Silicon-on-insulator microphotonic devices

SOI microwaveguides and associated devices (splitters, turns,...) are used for light distribution. Rib SOI geometries obtained by shallow etching of the silicon film offer definite advantages for the integration of active devices while fulfilling efficiency and compactness. Propagation losses of such waveguides are one order of magnitude smaller than for single mode strip waveguides. Rib-based compact and low loss optical signal distribution from one input to up to 1024 output points has been demonstrated. Light injection in submicron SOI waveguides is discussed. The indirect bandgap of silicon is not in favor of light emission and modulation. Realization of silicon sources and efficient high speed silicon-based modulators is a real challenge. For light detection, germanium can be grown on silicon and Ge photodetectors with -3dB bandwidths up to 30 GHz have been demonstrated.