THz Spectroscopy of wood
Background: Wood is increasingly used in more demanding, load bearing applications such as high-rise buildings, and a number of new engineered wood products have been developed fairly recently such as cross-laminated timber. In addition, hardwood species such as beech or ash are becoming more important for such products. In light of these new applications and with the goal of a more efficient and advanced use of wood, a better insight into structure-mechanics relationships and into wood-water interaction are needed.
Key parameters are structure and orientation of the cellulose microfibrils in the wood cell walls and the distribution and diffusion of water in wood. Terahertz (THz) technology provides the unique possibility to analyze and image these key parameters and properties of wood with the big advantage of being a non-hazardous technique allowing for immediate sample manipulation during the experiments. This makes the THz technology in particular suitable for in situ diffusion or mechanical experiments, applying various climatic conditions and imaging over longer times. Furthermore, due to the small size of sources and detectors, it is well-suited for the development of mobile instruments for on-site investigations and has the potential of becoming a frequently used key technology in wood science.
Project description: We have identified three key aspects in THz technology, which can have a significant scientific impact on the understanding and characterization of wood properties:
(i) THz radiation is very sensitive to humidity and water. Due to the different spectral properties of water compared to those of the cell wall polymers it allows for a spatially and temporally resolved measurement of water content and diffusion in wood.
(ii) As the anisotropic optical properties of wood and cellulose correlate strongly with the cellulose microfibril orientation, THz radiation is an excellent medium for probing the spatial structure of wood relevant to mechanical properties.
(iii) Internal stress in a material is also a cause for anisotropic optical properties. We presume that we can correlate changes in optical anisotropy at THz wavelengths with stress changes in wood.
Previous studies, including own work, dealt with these topics; however, they are mainly based on single spot measurements. Our goal is to build and use a spectroscopic THz imaging setup in a humidity controlled environment with the possibility to integrate additional devices such as simultaneous weighting or in-situ tensile tests. This will allow performing advanced research by in-situ imaging of wood structure and diffusion processes.
SNF project in collaboration with Daniel Sacré (PhD student), Laboratory for Transport at Nanoscale Interfaces, Empa, Markus Rüggeberg, Laboratory for Applied Wood Materials, Empa and Prof. Ingo Burgert Wood Materials Science Laboratory, ETH Zürich (since 2016)
THz Spectroscopy on phase transition of VO2
The optical properties of the VO2 films were measured using a THz time-domain spectrometer in the transmission configuration to monitor the semiconductor-to-metal phase transition as a function of the sample treatment with extensive γ-irradiations.
in collaboration with Itani Madiba, University of South Africa (UNISA), Pretoria, South Africa and Artur Braun, High Performance Ceramics Lab, Empa (since 2016)
Mechanical contact of skin and textiles: THz imaging and modelling of the interface
In diesem Projekt soll mit bildgebenden Verfahren die Kontaktstelle zwischen Haut und Materialoberfläche untersucht werden. Um eine genügende Auflösung zu erreichen werden Experimente aufgebaut, die von der Laserinterferometrie inspiriert sind. Zusätzlich werden Auswerteverfahren entwickelt, um den Feuchtegehalt, die Oberflächenstruktur und die Kontaktfläche sowie deren druckabhängige Veränderung quantitativ zu ermitteln. Möglich wird dies durch die Verwendung von unschädlicher Wärmestrahlung, sogenannten Terahertz-Wellen, die eine Wellenlänge im Bereich von 0.1-0.5 mm haben. Diese Strahlung kann Textilien durchdringen und wird von der Haut je nach Feuchtegehalt reflektiert oder absorbiert.
SNF Project in collaboration with Erwin Hack and Lorenzo Valzania, Laboratory for Transport at Nanoscale Interfaces, Empa, and Prof. Th. Feurer, University Bern (since 2015)
Selection of past projects
Exploration of digital holographic for Terahertz imaging transmission and reflection. A Quantum Cascade Laser as well as a high-power continuous wave (CW) laser are used as THz sources together with a high resolution (640×480 pixel) uncooled micro-bolometer array as imaging device. Wave propagation to parallel and non-parallel planes is used to reconstruct the objects. Synthetic aperture methods are applied for resolution enhancement. We aim for a lateral resolution of about 100 μm and a relative phase sensitivity of about 0.5 rad corresponding to a depth resolution of 5-10 μm.
In collaboration with Erwin Hack, Laboratory for Transport at Nanoscale Interfaces, Empa (2013-2015)
Realistic visualization of material surface appearance on a handheld device
(as project leader)
CTI project in collaboration with X-Rite Regensdorf (2012-2013)
(as principal investigator)
PhD project for Matthias Scheller Lichtenauer under supervision of Prof. J. Giesen
- Measurement and Modeling of Multi-Attribute Psychometric Scales
- Psychometric Scaling of Dynamic Multi-attribute Stimuli
SNF funded project in collaboration with University of Jena (2010-2013)
Bestimmung papierabhängiger Parameter zur Anpassung digitaler Druckdaten
CTI project as a collaboration of The Media Technology Lab with Ugra, St. Gallen 2007 - 2009
Gamut Mapping as Optimization Problem
(as principal investigator)
PhD Project for Zofia Baranczuk under supervision of Prof. J. Giesen
funded by Hasler foundation in collaboration with Universität Jena (2007 - 2010)
Einbau einer Kalibrationsmethode zur Linearisierung des Photopapiers
(as project leader)
funded by Swiss Imaging Technologies, Regensdorf (2004) in a collaboration with Media Techonolgy Lab at Empa
Neural networks in photofinishing
(as industry collaborator)
KWF Project (now Innosuisse) with Gretag Imaging and ETH Zürich (Prof. G. Tröster) for PhD work of Michael Kocheisen with (1994-1997)
High Tc Superconductors
Postdoc projects "Analyse structurale sur poudre par la radiation synchrotron aux rayons X et par la diffraction neutronique" and "Détermination des structures en utilisant l'effet de la diffraction anomale"
at Brookhaven National Lab in collaboration with University of Geneva funded by SNF (1987-1989)
(as PhD student)
PhD Project "Synthèse, structure et propriétés des hydrures métalliques ternaires Mg2FeH6, Mg2CoH5 et Mg2NiH4"
at University of Geneva under supervision of Prof. K. Yvon, partially financed by SNF/NEFF, (1983 - 1987).