J. Stiens

Vrije Universiteit Brussel | VUB · Electronics and Informatics (ETRO)

Textile Reinforced Cementitious (TRC) sandwich composites provide a load-bearing, noncorrosive, lightweight, and durable alternative for steel-reinforced sandwich elements, and/or traditional steel-reinforced concrete. However, the composite nature of the material and slender nature of the facings render the fracture behavior complex. Insufficient...

Background: General pathophysiological mechanisms regarding associations between fluid administration and intra-abdominal hypertension (IAH) are evident, but specific effects of type, amount, and timing of fluids are less clear. Objectives: This review aims to summarize current knowledge on associations between fluid administration and intra-abd...

Non-destructive testing (NDT) techniques are used to study the mechanical properties of the materials without destroying, nor compromising, such properties. This paper focuses on two types of NDT methods in order to follow the curing process of textile-reinforced cementitious (TRC) composites and their cementitious matrix for the first 24 h after h...

Introduction: Increased intra-abdominal pressure (IAP) has an important impact on morbidity and mortality in critically ill patients. The SERENNO Sentinel system (Serenno Medical, Yokne’am Illit, Israel) is a novel device that allows automatic and continuous IAP measurements. Aims: Pre-clinical validation in a bench model study comparing the new de...

Rubber is one of the most used materials in the world; however, raw rubber shows a relatively very low mechanical strength. Therefore, it needs to be cured before its ultimate applicatios. Curing process specifications, such as the curing time and temperature, influence the material properties of the final cured product. The transient radar method...

In this manuscript a multi-modal non-destructive testing (NDT) approach is presented for hydration monitoring of fresh mortar based on ultrasonic pulse velocity (UPV) and electromagnetic (EM) transmission measurements. The EM millimeter wave velocity and permittivity are measured for the first time throughout cement hydration and accurately demonst...

Finite-difference time-domain (FDTD) method is an efficient full-wave numerical method for simulating graphene-based optoelectronic devices. The previous investigation on modeling graphene in leapfrog FDTD methods shows that dispersive errors increase with tuning up the chemical potential of graphene. In this article, a new implementation of graphe...

The grown interest in healthcare applications has made biomedical engineering one of the fastest growing disciplines in recent years. Photoplethysmography (PPG) has gained popularity in recent years due to its versatility for noninvasive monitoring of vital signs such as heart rate, respiratory rate, blood oxygen saturation and blood pressure. In t...

Transient radar method (TRM) is introduced as a novel non-destructive testing (NDT) technique that is capable of analyzing fully blindly and simultaneously the electromagnetic (EM) properties as well as geometric parameters of deep sub-wavelength thin single layer slabs. This study focuses on the minimum detectable layer thickness by means of TRM,...

Intra-abdominal hypertension, defined as an intra-abdominal pressure (IAP) equal to or above 12 mmHg is one of the major risk-factors for increased morbidity (organ failure) and mortality in critically ill patients. Therefore, IAP monitoring is highly recommended in intensive care unit (ICU) patients to predict development of abdominal compartment...

Textile Reinforced Cementitious (TRC) composites have gained recent attention due to several advantages compared to traditional steel-reinforced concrete, such as concrete usage reductions and design freedom. The implementation of this technology on a sandwich design offers light-weight and slender structural elements with superior mechanical perfo...

SiCN thin film is one of the most attractive silicon-based materials due to its excellent electrical, mechanical and optical properties. In this study, SiCN thin films have been prepared by the radio frequency reactive magnetron sputtering method under different sputtering power, pressure, and substrate temperature. The microstructure, morphology,...

This review presents an overview of previously reported non-invasive intra-abdominal pressure (IAP) measurement techniques. Each section covers the basic physical principles and methodology of the various measurement techniques, the experimental results, and the advantages and disadvantages of each method. The most promising non-invasive methods fo...

In the study of the blue-violet emission properties of SiCN thin films, the emission intensity depends on the crystal structure of thin films. The annealing process is widely used in the preparation process of thin films as one of the effective means to remove defects and improve crystal quality. In this study, SiCN thin films were prepared by RF m...

In this conference paper, we investigated the main concept behind sub-terahertz and microwave reflectometry in intra-abdominal pressure (IAP) monitoring. At first, the direct impact of IAP on the geometric and consequently, electromagnetic properties of the abdominal compartment was studied. Subsequently, each influencing geometric parameter impact...

A Vector Network Analyzer (VNA) is an indispensable piece of equipment for the electromagnetic characterization of materials. With continuous development, the testing frequency band of the VNA has entered the terahertz domain. Previous studies have shown that carbon nanofibers (CNFs) prepared by the vapor method have excellent absorbing characteris...

The mechanical properties of Textile Reinforced Cementitious (TRC) composites have been extensively investigated in recent years. This material is constituted by a fine-grained cementitious matrix and a continuous textile reinforcement, a combination that has proven to be a lightweight, durable, and sustainable alternative for reinforced concrete....

Transient Radar Method (TRM) was recently proposed as a novel contact-free method for the characterization of multilayer dielectric structures including the geometric details. In this paper, we discuss and quantify the intrinsic and systematic errors of TRM. Also, solutions for mitigating these problems are elaborated extensively. The proposed solu...

In this paper, we report an ultra-sensitive millimeter wave (mm-wave) reflection liquid sensor, which is electronically tunable, non-invasive, and label-free. The reflection liquid sensor is based on the impedance match technique. It is implemented in a rectangular waveguide structure, which makes the interaction between mm-wave and liquid under te...

In this paper, ZnO films were deposited on the surface of few-layer graphene films by RF magnetron sputtering and its photoluminescence performance was studied. Large-area few-layer graphene films were prepared on a Cu substrate by CVD. To improve the crystalline quality and c-axis preferred growth orientation of ZnO films, we systematically optimi...

Controlling the number of graphene layers is crucial in graphene synthesis for the development of graphene-based electronics. Numerous investigations have found that a reasonably designed binary alloy can effectively overcome the defects of pure metal and activate the self-limited growth of a monolayer of graphene. Graphene films were prepared on t...

Zinc oxide (ZnO), a wide direct band gap semiconductor, has attracted extensive attention for its potential optoelectronic applications, such as coherent ultraviolet light emitters. Pure one-dimensional ZnO, structures such as nanorods and nanowires, etc. inevitably have some crystal defects, which makes them to emit not only in the ultraviolet reg...

We propose a novel strategy for applying Compressed Sensing to Stepped Frequency Continuous Wave Synthetic Aperture Radars. The measurements are performed by adhering to a sequential measurement strategy. The sensor autonomously adapts, on the fly, the number of samples needed to reconstruct the reflectivity function and guarantees an imposed recon...

Background: Synthetic compounds with pharmaceutical applications in brain disorders are daily designed and synthesized, with well first effects but also seldom severe side effects. This imposes the search for alternative therapies based on the pharmaceutical potentials of natural compounds. The natural compounds isolated from various plants and ar...

This paper reports on the performance of a sub-THz liquid sensor tool coupled to a microfluidic platform. Non-invasive, non-destructive and on-line measurements are demonstrated for the determination of ethanol concentration and for controlling via micromixer device. We have developed a label-free chemical sensing methodology coupling a sub-teraher...

Graphene-based terahertz (THz) devices, such as modulators, detectors, and absorbers, play important roles in terahertz applications due to graphene’s effectively tunable terahertz absorption. The features of these terahertz devices are greatly related to the THz absorption of graphene. However, the monolayer graphene shows limited absorption to te...

Millimeter (mm) waves are of interests to monitor the properties of interest (POIs) of food products which are opaque for optical and infrared frequencies as they not suffer from all the atomic and intermolecular absorption processes situated in the latter bands of the Electromagnetic (EM) spectrum. They also offer a higher penetration depth into w...

This work explores an innovative strategy for increasing the efficiency of compressed sensing applied on mm-wave SAR sensing using multiple weighted side information. The approach is tested on synthetic and on real non-destructive testing measurements performed on a 3D-printed object with defects while taking advantage of multiple previous SAR imag...

Ground penetrating radars for mine detection can profit from the many advantages that compressed sensing can offer through random subsampling in terms of hardware simplification, reduced data volume and measurement time, or imagery simplification. An intrinsic antenna-ground model is used, canceling the undesired reverberation effects and the very...

Weighted Laguerre finite-difference time-domain (Laguerre-FDTD) method is an efficient and unconditionally stable time-domain numerical method. The complex frequency-shifted perfectly matched layer (CFS-PML) is the main absorbing boundary condition in the Laguerre-FDTD method. The implementations of CFS-PML in the Laguerre-FDTD method, which have b...

This paper reports on the performance of a subterahertz (THz) liquid sensor tool coupled to a microfluidic platform. Contactless and on-line measurements are demonstrated for the determination of ethanol concentration and controlling by a micromixer device. The authors have developed a label-free chemical sensing methodology coupling a subterahertz...

Non-destructive technique determines surface or inner cracks, moisture content during curing, failure etc. In this paper, we combine two types of NDT methods: elastic waves in the passive (Acoustic Emission, AE) and active form (Ultrasonic Testing, UT) and Millimeter wave (MMW) spectroscopy to investigate physical and chemical parameters of Ferro-c...

Non-destructive technique determines surface or inner cracks, moisture content during curing, failure etc. In this paper, we combine two types of NDT methods: elastic waves in the passive (Acoustic Emission, AE) and active form (Ultrasonic Testing, UT) and Millimeter wave (MMW) spectroscopy to investigate physical and chemical parameters of Ferro-c...

An in-house developed Label-free Immobilization-free Terahertz (THz) sensor has shown its potential in various applications [1]. In this paper, we present the experimental results in the field of High Performance Liquid Chromatography (HPLC) and protein crystallization. Ultra-Violet (UV) detectors are very commonly used as a detection technique in...

Here, we demonstrate a novel Terahertz (THz) wave characterization technique and modulation devices developed at ETRO-IR (VUB) that facilitate the detection of Chemical Biological Radiation and Nuclear (CBRN) hazards. First, a novel transient radar technique is presented for a Non-Destructive Testing (NDT) of a multilayered dielectric object. A tra...

Characterizing and understanding carbon nanotube (CNT)-based polymer composites plays a crucial role for the ability to customize their properties. The (an)isotropy of CNT networks inside a composite can have significant effects on the final material's properties. However, characterizing the (an)isotropy of seemingly random CNT networks can be diff...

The development of millimeter-wave (mm-wave) technology has enabled the study of bio-molecular interactions by means of electromagnetic waves with frequencies between 30 and 300 GHz. In this study, an attempt has been made to exploit the possibility of mm-wave technology as alternative detection technique for liquid chromatographic (LC) systems. Th...

Despite the numerous benefits of microwave applications in our daily life, microwaves were associated with diverse neurological complaints such as headaches and impaired sleep patterns, and changes in the electroencephalogram (EEG). To which extent microwaves influence the brain function remains unclear. This exploratory study assessed the behavior...

A novel technique enabling ultrafast nondestructive characterization of multilayer dielectric structures is proposed. Actual estimations indicate that the data acquisition performance of electronic measurement systems of today allow deep submillimeter depth resolution, almost independently of the frequency. For a 10 GHz signal, e.g., this correspon...

The propagation of TE modes in a rectangular metal waveguide with an integrated structure of finite length containing a graphene monolayer was studied by a modal decomposition method. The modal decomposition method generates a number of linear algebraic equations related to waveguide modes taken into account. The system of linear algebraic equation...

A sensor system for sensing a level of frozenness of a predetermined product is described. The sensor system comprises at least one emission antenna for directing electromagnetic radiation with at least one frequency within a frequency range of 0.1 GHz to 10 THz to a product and at least one receiver for receiving said electromagnetic radiation aft...

In ultra-wideband non-destructive testing of large multilayered polymers, data collection and reduction can be achieved by applying compressed sensing techniques. In this work, using effective modelling of possible defects, such as air gaps between layers, we construct defect dictionaries and use them as support data for a signal similarity-based c...

GSolver software is used to optimize the parameters of a GaAs-based layers structure for modulating the reflectivity of light. This structure can be used in an Integrated Mirror Optical Switch (IMOS) for the Q-switching technology. A system of low doped GaAs and highly doped AlGaAs structure is built on a binary diffraction grating composed of germ...

the excitation of coupled metal-graphene surface plasmons, in the Otto configuration at the WR 3.4 band was demonstrated experimentally. The influence of the air gap on the excitation efficiency was investigated. The results enable us to characterize graphene samples, and can be used for potential applications, such as in design and fabrication of...

Sub-terahertz (THz) frequency is attracting extensive interests for a wide range of applications. Electronically tunable phase shifters as one of the key building blocks in sub-THz systems, especially waveguide based phase shifters, encounter difficulties in both design and fabrication. In this paper, an electronically tunable rectangular metallic...

The dispersion equation for waveguide modes of rectangular metal waveguides containing a dielectric buffer/graphene/air gap layered structure has been derived in the approximation of perfectly conducting metal waveguide walls. Dispersion dependences of TE0m modes for sub-terahertz frequencies were analysed against buffer layer thickness and Fermi e...

Graphene draws massive interests due to its outstanding physical properties. During the recent years, extensive studies have shown its promising features in THz and infrared frequency ranges. However, the electromagnetic properties of graphene in microwave and millimeter wave spectrums are still not well understood. In this paper, we employed finit...

Dispersion equation for waveguide modes of rectangular metal waveguide containing a dielectric buffer/graphene/air gap in the approximation of perfectly conducting waveguide metal walls, and the dispersion equation for waveguide modes of planar metal waveguide containing a dielectric metal/buffer/graphene/air were presented here. Dispersion depende...

A new approach to generate high-quality broadband frequency swept millimeter-wave (mm-wave) waveforms based on photonics-assisted frequency up conversion of a low-frequency chirped drive signal has been proposed. This is made possible by using an unbalanced temporal pulse shaping system involving two cascaded Mach-Zehnder modulators both biased at...

We proposed a multilayer structure, aimed to unravel the complex solution space: coupled graphene-metal plasmons and waveguide modes which are supported by this structure, we described these plasmon modes behaviors at the sub-THz range, to our knowledge; this could be the first study of its kind. The proposed structure is shown in Fig. 1 Figure 1....

The increased interest in the separation of peptides, proteins, immunoglobulins and polynucleotides, led to an increased demand for appropriate analytical methodologies and instrumentation. CE is because of its unique separation mechanism and high efficiency, frequently used in the analysis of those molecules. In this study a CE method, for the sep...

4th NSE PhD

In this paper we focus on the dispersion characteristics of TM and TE waveguide modes and the reflectivity of plane waves, incident on the four-layer structure consisting of air/graphene monolayer/dielectric buffer layer/metal substrate. The TM waveguide modes split up into two branches for small frequencies, one of the branches (cutoff waveguide b...

TM surface plasmon (SP) characteristics of a four-layer structure, consisting of air as the superstrate, a monolayer of graphene, a dielectric buffer layer and metal as the substrate are analyzed at sub-THz frequencies. TM SPs in such case are represented by metal-like and graphene-like branches. For small frequencies the metal-like plasmon splits...

In spite of the fact that graphene plasmons were investigated in a number of structures containing a few graphene monolayers or graphene bilayers, the case of coupled graphene plasmons with other types of structure modes (surface metal-plasmon or waveguide modes) has not been thoroughly examined. The coupling of graphene plasmons with surface metal...

Objectives: Caffeine (CAF) and maltodextrin (MALT) mouth rinses (MR) improve exercise performance. The current experiment aims at determining the effect of CAF and MALT MR on cognitive performance and brain activity. Methods: Ten healthy male subjects (age: 27±3yrs) completed 3 experimental trials. Each trial included 4 Stroop tasks: 2 familiarizat...

A Polymethylmethacrylate (PMMA) rectangular tapered probe with metal coating on the sides is analyzed as a near-field imaging probe at 100 GHz in Ansoft High Frequency Structure Simulator (HFSS). Normally, highly resistive silicon and sapphire, which are costly, are used as a near-field probe due to their low loss and high permittivity. PMMA near-f...

Nitride-based materials still offer opportunities for the development of new electronic and optoelectronic applications. From the seventies of the 20th century, GaAs was considered as mature family material for many electronic applications. In the beginning of the 21th century, GaAs is step by step replaced by GaN in many applications due to its hi...

In this paper we investigate how one can take additional advantage of coupled metal-graphene plasmons. The dispersion relation for coupled metal-graphene plasmons is presented here for a proposed multilayer structure. Graphene and couple metal graphene modes supported by this structure are shown below, these modes can depend very strongly on Fermi...

Capillary electrophoresis (CE) is an electrodriven separation technique that is often used for the separation of chiral molecules. Advantages of CE are its flexibility, low cost and efficiency. On the other hand, the precision and transfer of CE methods are well-known problems of the technique. Reasons for the more complicated method transfer are t...

In this paper we investigate how one can take add itional advantage of coupled metal-graphene plasmons. Hereto we derived the dispersion relation for coupled metal-graphene plasmons in multilayered structures. It is shown that an optimized multilayer structure, the solution of this dispersion relation of this structure is presented below. The coupl...