Thierry Camps

Electrical spin injection into p-doped quantum dots through a tunnel barrier

The authors have demonstrated by electroluminescence the injection of spin polarized electrons through Co∕Al2O3∕GaAs tunnel barrier into p-doped InAs∕GaAs quantum dots embedded in a p-i-n GaAs light emitting diode. The spin relaxation processes in the p-doped quantum dots are characterized independently by optical measurements (time and polarization-resolved photoluminescence). The measured electroluminescence circular polarization is about 15% at low temperature in a 2T magnetic field, proving an efficient electrical spin injection yield in the quantum dots. Moreover, this electroluminescence circular polarization is stable up to 70K.

Effect of thermal annealing on the electrical properties of indium tin oxide (ITO) contact on Be-doped GaAs for optoelectronic applications

The effects of thermal annealing on optically transparent electrodes of ITO (indium tin oxide) contact deposited on p-type Be-doped GaAs have been investigated by means of the transfer length method and secondary ion mass spectroscopy measurements. This study shows that the temperature that minimizes the specific contact resistance of ITO/GaAs (500 °C) greatly differs from the temperature that leads to a maximum conductivity of the ITO layer (600 °C) and from the values reported on ITO/GaAs in the literature. The oxygen diffusion in the semiconductor layer and its interaction with the beryllium dopant is pointed out to explain these differences.

A microtip self-written on a vertical-cavity surface-emitting laser by photopolymerization

We present the integration of a self-aligned microtip on a vertical-cavity surface-emitting laser (VCSEL) by near infrared photopolymerization. This one-step fabrication process is triggered by the laser source itself. It is based on the use of photopolymers sensitive at the lasing wavelength and can be applied to VCSEL devices after their process fabrication. We have characterized the fabricated microtips and shown that they focus laser light at few micrometers from the device. The applications of this simple method may concern VCSEL beam shaping as well as the fabrication of microprobes for near-field optical microscopy.

Self-aligned and stray-field-free electrodes for spintronics: An application to a spin field effect transistor

We present a ringlike design for spin field effect transistor electrodes. This configuration solves the local Hall effect drawback as these electrodes do not generate any stray magnetic fields. The shape and size of the electrodes are adjusted in order to match the physical constraints. The gate configuration and channel length are discussed for the [110] growth direction; the GaInAs channel length for the spins to fully switch is calculated to be of the order of 0.15μm for a gate electric field of 150–300kV∕cm.

Preliminary experimental study on the electrical impedance analysis for in-situ monitoring of the curing of carbon/epoxy composite material for aeronautical and aerospace structures

This paper concerns the electrical characterization of T700/M21 unidirectional composite materials using sensors developed specifically for this study. It proposes a reliable and reproducible protocol for the characterization of the material during curing. Prior to the characterization, an analysis was carried out to assess the impact of parasitic access elements (resistance of the electrode/fibre interface or of the feed wire), which was reduced to a minimum by appropriate dimensioning of the electrodes. A study of the electrical conduction in relation to the direction of the fibres made it possible to establish a suitable approach to homogenized measurement of the material. Thermo-electric coupling by self-heating was also evaluated, with a view to obtaining measurements that were not influenced by this phenomenon. Finally, the use of electrical impedance spectral analysis allowed in-situ monitoring of the curing process. The results obtained are compared with those of a rheological analysis of the same material. These results highlight the value of the proposed protocol and demonstrate that, with the aid of these sensors, complete automation of the manufacturing process of composite structures is feasible (optimization of the cure cycle by real-time automatic control).

3D optimization of a polymer MOEMS for active focusing of VCSEL beam

We report on the optimized design of a polymer-based actuator that can be directly integrated on a VCSEL for vertical beam scanning. Its operation principle is based on the vertical displacement of a SU-8 membrane including a polymer microlens. Under an applied thermal gradient, the membrane is shifted vertically due to thermal expansion in the actuation arms induced by Joule effect. This leads to a modification of microlens position and thus to a vertical scan of the laser beam. Membrane vertical displacements as high as 8μm for only 3V applied were recently experimentally obtained. To explain these performances, we developed a comprehensive tri-dimensional thermo-mechanical model that takes into account SU-8 material properties and precise MOEMS geometry. Out-of-plane mechanical coefficients and thermal conductivity were thus integrated in our 3D model (COMSOL Multiphysics). Vertical displacements extracted from these data for different actuation powers were successfully compared to experimental values, validating this modelling tool. Thereby, it was exploited to increase MOEMS electrothermal performance by a factor higher than 5.

A miniaturized VCSEL-based system for optical sensing in a microfluidic channel

We report on design and fabrication of a VCSEL-based Micro-Optical-Electrical-Mechanical-System (MOEMS) suited for portable optical biosensors. It is based on a tunable polymer microlens directly integrated on the surface of the VCSEL laser device. We demonstrate that this method leads to a VCSEL focusing at a working distance of ~300μm suitable for optical analysis in a microfluidic channel. Moreover, as the microlens can be vertically moved up to 8μm with an applied power of only 43 mW (3V), a dynamic scan of the laser spot is possible over 100μm. This integrated approach opens new insights for the use of VCSEL arrays in miniaturized optical sensors.

Optical feedback interferometry for microscale-flow sensing study: numerical simulation and experimental validation

Optical feedback interferometry (OFI) performance for microscale-flow sensing is studied theoretically and experimentally. A new numerical modeling approach for OFI flow meter spectrum reproduction is presented in this work to study the optical effect on the signal due to the micro-scale channel geometry. Two well-defined frequency peaks are found in the OFI spectrum, this phenomenon can be attributed to the reflection of the forward scattered light on the channel rear interface. The flow rate measurement shows good accuracy over a range of fluid velocities from 16.8 mm/s to 168 mm/s, thus providing a promising tool to study and to optimize the OFI microfluidic sensor system.

Study of SU-8 reliability in wet thermal ambient for application to polymer micro-optics on VCSELs

We present experimental data on the reliability of SU-8 polymer when used as a core material for the integration of microlenses on vertical-cavity surface-emitting lasers (VCSELs). The respective effects of a hot and humid environment on structural, mechanical and optical properties of this epoxy resist are investigated. High aspect-ratio SU-8 micropillars are found to keep a good surface morphology and a stable optical transmission, as well as a good adherence on the wafer. Thermal cycling is also studied to check material stability under electro-thermal actuation in SU-8 micro-opto-electro-mechanical system (MOEMS). These results are of great importance for the collective integration of low-cost SU-8-based passive or active microlens arrays onto VCSELs wafers for optical interconnects and optical sensing applications.

Polymer optical MEMS integrated on VCSELs for biosensing

We present our recent advances on design and fabrication of polymer optical MEMS that can be directly integrated on VCSELs arrays for dynamic beam focusing. These studies open new insights for the fabrication of compact optical sensors that require a real-time scan of laser beam position.