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Millimeter and THz waves

General Scientific Contacts: Johan Stiens and Piet Wambacq

General introduction

Millimetre waves and THz waves are electromagnetic waves ranging from 30 to 300 GHz, and from 300 GHz to 3 THz, respectively. They are situated between the microwave and the far infrared region. This frequency range offers high bandwidth and directivity in communication and navigation systems, higher resolution in ranging applications (e.g. radar), and a good compromise between resolution and penetration depth for detection of objects, undetectable in the infrared or visible spectrum. As various materials, including biomaterials, exhibit a pronounced spectral footprint in this spectrum, this offers also unique properties for the development of bio-sensor systems.

The research group tackles problems ranging from the fundamental level towards the system level: interaction of these electromagnetic waves with matter, the design of quasi-optical elements, study of antenna arrays with a wide variety of functionalities, including opto-electronic control, CMOS circuits for telecom and sensor applications, the study and exploitation of meta-materials for enhancing the figures of merit of subsystems, multi-parameter active imaging and sensing for (concealed) object detection,…

The measurement infrastructure comprises a quite unique quasi-optical vector network analyser operating in the 40 to 660 GHz range, with extension potential up to 2 THz.

Dielectric spectroscopy

Scientific Contact: Johan Stiens

The interaction of mm and THz waves with materials is described by the dielectric permittivity and magnetic permeability function. The research work in this area concerns

• Characterization of material properties using quasi-optical vector based measurement techniques (S-parameters)

• Development of powerful and robust hierarchical algorithms (combinations of direct and indirect techniques) to extract material properties of single and multi-layer structures

• Evaluation of dielectric mixing models of compounds and mixtures

• Process control of industrial processes

The materials of our interest include: technology materials for realizing integrated and packaged electronic circuits and systems: semiconductors (Si, GaAs), dielectrics, insulators (BCB), ceramics, polymers, liquids, powders, granular materials, mixtures…The target industries can be very diverse: electronic, chemical, food, pharmacy, biotech, agriculture, construction,…

Meta-materials

Scientific Contact: Johan Stiens

Meta-materials are materials which exhibit in a particular frequency range negative dielectric permittivity and/or negative magnetic permeability. These properties yield new design options for a wide set of applications in the mm-wave and THz frequency range. Our research activities include: modeling and design of single and multi-layer Left Handed Materials, including symmetric and asymmetric split ring resonators, derivation of equivalent circuits and investigations of ultra-sensitive thin film sensors for all kind of materials.

Imaging

Scientific Contact: Johan Stiens

Millimeter waves offer a good combination of resolution and penetration for the development of active mm wave imaging systems for indoor concealed weapon detection. Our research activities focus on physical aperture imaging and includes imaging system architectures, image forming optics, fast scanning techniques, multi-parameter (frequency, angle, polarization, phase) illumination techniques. Through theoretical analyses and experiments, we investigate the cause of glint and interference and speckle artifacts. We study image processing techniques to reveal detailed scene features and enhance the object appearance.

Near-field imaging

Scientific Contact: Johan Stiens

Conventional optical imaging systems in the mm-wave and THz range (0.1 < ? < 10 mm) cannot resolve microscopic details as the resolution in the far field is limited by diffraction (Rayleigh criterion). Super resolution is however feasible by detection of scattered evanescent waves, which are only detectable in the direct neighbourhood of the object by putting a field scattering microprobe.

We investigate the potential of a scanning scattering type near-field mm-wave imaging method based on the interaction between a very sharp probe tip and the object whereby the resolution is not longer determined by the wavelength but by the sharpness of the tip.

Actual challenges in this domain are as follows: (i) study of more advanced tips; which is only possible when the interaction between tip and object is more exactly modelled (in literature reported methods are based on much simpler electrostatic models); (ii) the development of a modulated scattering technique without mechanical movement; (iii) speeding up the scanning process by means of a system of parallel tips.

Antenna arrays

Scientific Contact: Johan Stiens

The goal in this research work is to study quasi-optical functionalities (beam bending, beam splitting, off-axis lens, coherence diffuser, filters,… ) which can be implemented with antenna arrays. We study theoretically and experimentally the beam patterns of antenna elements and (composed) arrays of infinite and finite extent.

Theoretical aspects and analysis methods of frequency selective surfaces are investigated. We study the fundamental geometrical topologies and material parameters which determine their resonance behavior.

One of the most important issues of antenna characterization in the mm-wave frequency (i.e. with limited size coherent illumination) is the finiteness of antenna arrays as numerical design tools almost are no available.

Analog building blocks: telecom and sensors

Scientific Contacts: Johan Stiens and Piet Wambacq

Although silicon based technologies form the core of the semiconductor industry, at mm wave and THz frequencies, historically the device technologies of choice have been those that take advantage of superior properties of III-V semiconductors such as e.g. GaAs, InP and InN. However Si based technologies (CMOS, SiGe) are steadily making inroads into the high frequency market due to ever-increasing operation frequency and their lower cost. In the coming years CMOS, the cheapest technology of all will be able to handle signals well above 100 GHz. E.g. the 20 nm gate length will result in a maximum oscillation frequency of 560 GHz.

The research activities here deal with the design of high performance mm wave telecom transmit-receive front ends and with the study of sensor chip front end architectures.

Opto-electronic control of mm wave components

Scientific Contact: Johan Stiens

The interest in mm wave (passive as well as active) components with a tunable or digitally switching behavior has been increasing over the last years. The approach of direct optical control of mm wave components has unique advantages like galvanic shielding, no DC-biasing networks, extra design freedom in structures and packaging. Theoretical models are developed to describe the interaction between photo-generated carriers in devices and mm waves. We study the properties of opto-electronically controlled devices: isolation, insertion losses, bandwidth, DC and transient effects, cross-talk and heterogeneous integration issues.