Johan Stiens

Slab plasmon polaritons and waveguide modes in four-layer resonant semiconductor waveguides

This paper presents a detailed study of the waveguide and plasmon polariton properties of four-layer systems involving highly doped semiconductor material. The dispersion relations of waveguide and plasmon polariton modes are calculated for different geometrical parameters and material properties. Special attention is paid to the transition region between the latter modes, which exhibits a complex behavior. Slab plasmon polaritons at wavelengths slightly larger than the plasma wavelength, yielding a positive real part of the permittivity, have been found. Finally, applications at wavelengths near the transition region and near the plasma wavelength are proposed and discussed.

Integrated Waveguide Structure for Highly Sensitive THz Spectroscopy of Nano-Liter Liquids in Capillary Tubes

Terahertz dielectric spectroscopy permits the study of biomolecular interactions. However, water induces high attenuation of electromagnetic waves in the THz frequency range, obscuring the response of biomolecules. The developed sensor overcomes this problem by concentrating the THz wave propagating in an integrated waveguide on a small liquid volume contained within a capillary tube. Detailed electromagnetic modeling shows efiective interaction between the THz waves and liquids. Transmission measurement results for capillary tubes fllled with water and methanol mixtures demonstrate a substantial increase in sensitivity to changes of liquid permittivity. The current integrated sensor facilitates THz spectroscopy of biological liquids: a case study on bufiered human serum albumin solution demonstrates a great potential to complement biochemical analytical tools.

Study of active millimeter-wave image speckle reduction by Hadamard phase pattern illumination

Active millimeter-wave images typically exhibit characteristic speckle noise, due to the coherence of artificial millimeter-wave sources. We study the Hadamard speckle contrast reduction (SCR) technique, which has been successfully used in laser projection systems, in the context of millimeter-wave imaging. We show the impact of Hadamard pattern order and size and of image and pattern resolution on speckle reduction efficiency. Practical limitations of Hadamard pattern implementations and their effect on speckle reduction efficiency are also discussed.

Multispectral illumination and image processing techniques for active millimeter-wave concealed object detection

Active millimeter-wave imaging systems for concealed object detection offer the possibility of much higher image contrast than passive systems, especially in indoor applications. By studying active millimeter-wave images of different test objects derived in the W band, we show that multispectral illumination is critical to the detectability of targets. We also propose to use image change detection techniques, including image differencing, normalized difference vegetation index, and principle component analysis to process the multispectral millimeter-wave images. The results demonstrate that multispectral illumination can significantly reveal the object features hidden by image artifacts and improve the appearance of the objects.

Comparative study of the intra- and intervalley contributions to the free-carrier induced optical nonlinearity in n-GaAs

We performed a theoretical study about the optical nonlinearity connected with the redistribution of free electrons between the Γ and L valley of highly doped n-GaAs. An extended comparison was made between the intravalley (or nonparabolicity) and the intervalley contribution. In the energy relaxation process, intravalley transitions in the Γ and L valley by means of the emission of optical phonons and equivalent intervalley transitions in the L valley by means of intervalley phonons were considered as the main mechanism of energy transfer to the lattice. It is demonstrated that when the frequency of light matches with the plasma frequency of the n-doped GaAs the nonlinearity is quite big. At a doping concentration of 7×1018 cm−3 we calculated a nonlinear refractive index n2=1.74×10−8 cm2/W at 10.6 μm. The doping and intensity dependent energy relaxation times of electrons are calculated: for Γ-valley electrons τenΓ∼(3–5) ps and for L-valley electrons τenΓ∼(0.3–0.5) ps. It is believed that the response ti...

Precision evaluation of chiral capillary electrophoretic methods in the context of inter-instrumental transfer: Constant current versus constant voltage application

Capillary electrophoresis (CE) is an electrophoretic separation technique that was rapidly increasing in popularity some years ago and that led to high expectations. Because of their different separation mechanisms, CE and HPLC are alternative and complementary separation techniques. Chiral molecules can be directly separated with CE by simply adding a chiral selector to the running buffer solution, leading to flexible and cheap methods. Major drawbacks of capillary electrophoretic separation methods are, however, the lower precision compared to HLPC methods and a more problematic analytical method transfer. Both above stated disadvantages limit the generalized use of CE methods in the pharmaceutical industry. Multiple solutions have been suggested to improve the precision of CE methods. In this work the application of a constant current during the electrophoretic separation instead of the more commonly used application of a constant voltage was studied on two CE instruments with different cooling mechanisms. This was done in the context of optimizing method transfer conditions. A constant current may reduce the generation of heat in the capillary and the consequentially radial and axial temperature fluctuations that both negatively influence the precision of the peak areas, migration times and resolutions of a CE method. The repeatability and time-different intermediate precision of both electrophoretic separation modes were compared on two different CE instruments after a successful analytical method transfer. The chiral separations of three beta-blockers, propranolol, sotalol and betaxolol, were used as test cases. A constant current led to a general improvement of the repeatability and time-different intermediate precision of the relative Area Under the Curve of all three beta-blockers, while that of the migration times remained rather constant. It also led to more similar electropherograms than the application of a constant voltage.

Simulation and Experimental Verification of W-Band Finite Frequency Selective Surfaces on Infinite Background with 3D Full Wave Solver Nspwmlfma

We present the design, processing and testing of a W-band finite by infinite and a finite by finite Grounded Frequency Selective Surfaces (FSSs) on infinite background. The 3D full wave solver Nondirective Stable Plane Wave Multilevel Fast Multipole Algorithm (NSPWMLFMA) is used to simulate the FSSs. As NSPWMLFMA solver improves the complexity matrix-vector product in an iterative solver from O(N(2)) to O(N log N) which enables the solver to simulate finite arrays with faster execution time and manageable memory requirements. The simulation results were verified by comparing them with the experimental results. The comparisons demonstrate the accuracy of the NSPWMLFMA solver. We fabricated the corresponding FSS arrays on quartz substrate with photolithographic etching techniques and characterized the vector S-parameters with a free space Millimeter Wave Vector Network Analyzer (MVNA).

Hot free-electron absorption in nonparabolic III–V semiconductors at mid-infrared wavelengths

A quantum mechanical model based on the second order perturbation theory was constructed for the calculation of the free-electron absorption coefficient in nonparabolic III–V semiconductors. The implemented model allows for the calculation of the absorption changes when the free-electron gas temperature differs from the lattice temperature. Several mechanisms, which assist in the photon absorption process, were taken into account. At the considered lattice and electron temperature range and doping concentrations, the most important scattering mechanisms are impurity scattering, thermal and hot longitudinal optical phonon scattering, and finally acoustic phonon scattering. For all the interaction potentials we included the effect of screening by the conduction electrons. The model was developed in a fully consistent nonparabolic way. The electron dispersion relation as well as the interaction probabilities feature nonparabolic effects. Computations are performed for GaAs, InAs, and InSb at different mid-IR...

New modulator for far‐infrared light: Integrated mirror optical switch

We report on the first prototype IMOS (integrated mirror optical switch), FIR (far‐infrared: 8–12 μm) light modulator. The operation of this type of modulator is based on the reflection properties of a high density, thin electron sheet in semiconductors. The present prototype was built to give experimental backing for earlier theoretical predictions. Low electric power (4 W) and low voltage (12 V) operation have been achieved. A modulation depth of about 30% and a 3 dB frequency of 400 kHz were obtained.

Experimental study of the spatially-modulated light detector

Abstract Usually, integrated detectors in CMOS exhibit long recovery times, limiting the detector bandwidth to only a few MHz. This is due to the long absorption length and the slow diffusion speed of photo-generated carriers. Different approaches have been proposed to solve these problems hereby taxing the compatibility with standard CMOS fabrication processing. We present a novel detector for high-speed light detection in standard CMOS. To solve the problem of slow CMOS-detector recovery, the incident light is spatially modulated and the spatially modulated component of the photo-generated carrier distribution is measured. Though only a single light input signal is required, from the detector on, analog signal processing can be achieved fully differentially. Subsequently, expected good PSRR (Power supply rejection ratio) allows integration with digital circuits. Avoiding hybridization eliminates the conventional problems caused by bonding-pad capacitance, bonding-wire inductance. This reduces the associated signal degradation. In addition, the very low detector capacitance, due to the low effectively used detector area and the low area capacitance of the n-well junction, yields high voltage readout of the detector. This facilitates further amplification and conversion to digital signal levels. The detector will be applicable in arrays due to expected low cross talk. The expected fields of operation involve: serial and parallel optical communication receivers (e.g. for WDM), DVD-reading heads with integrated amplifier, etc. First measurements show 200 Mbit/s operation with a detector-responsivity of 0.05 A/W at λ=860 nm and 0.132 A/W at λ=635 nm. The detector has inherently a low capacitance, in this case only 50 fF (for an effective detector area of 70×70 μm2).