Joris Roels

Tunable optical forces between nanophotonic waveguides

The confinement of light in components with nanoscale cross-sections in nanophotonic circuits significantly enhances the magnitude of the optical forces experienced by these components. Here we demonstrate optical gradient forces between two nanophotonic waveguides, and show that the sign of the force can be tuned from attractive to repulsive by controlling the relative phase of the optical fields injected into the waveguides. The optical gradient force could have applications in optically tunable microphotonic devices and nanomechanical systems.

Biological formation of volatile phosphorus compounds

Phosphine and phosphides are reported to occur at numerous environmental sites such as fresh and marine sediments, landfills, faecal matter, biogas digesters and soils. The concentrations are several log units lower than the time-weighted average exposure standard, i.e. in the order of ng per m3 of gas or ng per kg material. Research about the biological formation of highly reduced gaseous phosphorus compounds dates back more than a hundred years. The early reports had to deal with a lot of scepticism. Thanks to new analytical tools (gas chromatography) it has become clear, during the last decade, that phosphine is a global constituent of the atmosphere. Pure strains of micro-organisms cultivated under highly anaerobic conditions were shown to produce phosphine. Thermodynamic considerations indicate that it is very improbable that the reduction of phosphate to phosphine is endergonic. Therefore the generation of phosphine cannot be compared with sulphidogenesis and methanogenesis. There seems to be a link between the existence of highly reactive gaseous phosphorus compounds and increased levels of metal corrosion. The reactive compounds could be formed by micro-organisms or they are liberated from phosphorus-containing impurities in the iron by the action of bacterial metabolites. The biochemical pathways responsible for the production of gaseous phosphorus compounds have not been characterised yet.

Detection of nanomechanical motion by evanescent light wave coupling

The authors demonstrate a technique allowing sensitive nanomechanical motion detection based on the evanescent light wave coupling between two photonic nanowires. Any relative motion between the nanowires results in a change in light coupling, providing a means of registering motion. The in-plane vibrations of a 220nm×400nm×10μm nanomechanical resonator were recorded using this method. An analysis of the sensitivity reveals the potential of this integrated technique to provide fast and sensitive motion detection.

Silicon-on-Insulator (SOI) Ring Resonator-Based Integrated Optical Hydrogen Sensor

We demonstrate a novel, highly sensitive integrated hydrogen sensor based on a silicon ring resonator on the silicon-on-insulator platform. The hydrogen sensitive element in the sensor structure is a platinum-doped tungsten oxide catalytic coating. The catalytic combustion of hydrogen in air leads to a local temperature rise in the ring resonator. The resulting thermo-optic effect shifts the ring resonator resonance. Resonance wavelength shifts higher than a nanometer are measured for hydrogen concentrations below the 4% lower explosion limit. A potentially tunable sensitivity of around 480 pm shift per %H2 is achieved at an operating temperature of about 40degC.

Ultracompact Phase Modulator Based on a Cascade of NEMS-Operated Slot Waveguides Fabricated in Silicon-on-Insulator

Phase modulation is one of the key functionalities in an integrated photonics circuit. In the silicon photonics platform, several approaches have been undertaken. The thermooptic effect is slow and relatively power hungry, whereas a carrier-based approach is fast ( >; Gb/s) but lossy and weak. Integration of other (e.g., electrooptic) materials typically struggles with fabrication issues that limit the effect. Here, we present a nanoelectromechanical systems-based approach. By applying a voltage over a freestanding slot waveguide, the slot width will change, resulting in an effective index change and thus a phase change. Using a cascaded structure, the effect can be enlarged without reducing the speed. A phase change of 40° was observed for a voltage of 13 V over a cascade of three 5.8-μm-long freestanding slots. The expected speed is in the MHz range.

Determination of phosphine in biogas and sludge at ppt-levels with gas chromatography-thermionic specific detection.

A gas chromatographic (GC) system to measure free phosphine in biogas and matrix bound phosphine in manure and sludge is presented. The system consists of a sample preconcentration trap filled with glass beads, connected with a capillary GC equipped with a thermionic specific detector. With a trap temperature as low as -155 degrees C, a sampling flow of 20 ml/min and a typical total sample volume of 100 ml, free phosphine concentrations in the low ng/m3 range and matrix bound phosphine in the low ng/kg dry matter range, can be accurately and reproducibly determined.

NEMS-based optical phase modulator fabricated on Silicon-On-Insulator

We present a compact low-power optical phase modulator on Silicon-On-Insulator consisting of an under-etched slot waveguide. By applying a voltage of 15V across a 9µm long slot waveguide an optical phase change of 60° was observed.

Noise Analysis and Removal in 3D Electron Microscopy

Recent research in several fields such as Biotechnology and Healthcare has uncovered a vast number of applications where 3D Electron Microscopy (EM) is useful. However, images produced by 3D EM are in most cases severely degraded. These degradations arise due to a multitude of reasons, e.g. the complex electronics in the system, magnetic lens aberration, heating and motion stability, charging, etc. Although the raw, degraded images are currently used for analysis, their usefulness is limited because the degradations make visual distinction and automated analysis of biological features very difficult. In this work, we give an analysis of noise, as one of the most important degradations in 3D EM imaging. Next, we propose a Non-Local Means image restoration algorithm that exploits the derived noise characteristics. The proposed algorithm yields significant improvements compared to other state-of-the-art image restoration algorithms.

Optical gradient force in a slot waveguide on a Silicon-on-Insulator-Chip

We present the fabrication and characterization of a subpicogram micromechanical oscillator in a slot waveguide that is actuated through optical gradient force. The tunable slot width in fact creates an optomechanical phase modulator on a Silicon-on-Insulator-Chip.

Continuous Time-of-Flight Ranging Using a MEMS Diffractive Subwavelength Period Grating (de)Modulator

We present a time-of-flight (TOF) ranging experiment based on optical mixing. The key component in this experiment is the optical (de)modulator. consequently we also report on the design, fabrication, and characterization of a microelectromechanical system diffractive grating (de)modulator with a subwavelength period. The obtained results are an important step toward an optical mixing based TOF 3-D imaging system, which we will further develop in our future work.