The project deals with three key aspects in THz technology, which can have a significant scientific impact on the understanding and characterization of properties of wood and cellulose based materials:
Anisotropic optical properties of wood
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. In light of a more efficient and advanced use of wood, a better insight into structure-mechanics relationships and into wood-water interaction are needed.
Publications:
- Zolliker P., Rüggeberg M., Valzania L. & Hack E. "Extracting Wood Properties from Structured THz Spectra: Birefringence and Water Content". IEEE Transactions on Terahertz Science and Technology, 2017, 7(6), 722-731.
- Cao J., Rüggeberg M., & Zolliker P. Towards detection of helical orientated cellulose structures in wood using THz time-domain spectroscopy. In Proceedings of the 44th international conference on infrared, millimeter and terahertz waves IRMMW-THz 2019. Paris, France (2019).
THz imaging of wood
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. Our goal is to build and use a spectroscopic THz imaging setup in a humidity controlled environment. This will allow performing advanced research by in-situ imaging of wood structure and diffusion processes.
Publications:
- Zolliker, P., Shalaby, M., Söllinger, E., Mavrona, E., & Hack, E. "Real-Time High Resolution THz Imaging with a Fiber-Coupled Photo Conductive Antenna and an Uncooled Microbolometer Camera", Sensors 2021, 21, 3757.
Exploring wood and cellulose based materials as THz devices
The strong anisotropic properties of wood mainly caused by the preferred orientation and inhomogeneous distribution of cellulose fibers motivates to synthesize cellulose based materials with tailored optical properties for THz devices (half wave plates, quarter wave plates, polarizers)
Publications:
- Zolliker, P., Mavrona, E., Hack, E., Markus Rüggeberg, M., Zeng, Z., Siqueira, G., & Nyström, G. "Wood: base material for optical elements for terahertz waves?" In 2020 45th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2020.
- Zeng, Z., Mavrona, E., Sacré, D., Kummer, N., Cao, J., Müller L., Hack, E., Zolliker, P. & Nyström, G. "Terahertz Birefringent Biomimetic Aerogels Based on Cellulose Nanofibers and Conductive Nanomaterials", ACS Nano 15 (4), 7451-7462, 2021.
External funding:
SNF project 200021_179061 "Revealing key properties of wood and cellulose with spectroscopic THz imaging" http://p3.snf.ch/project-179061
Contributors and collaborators
Peter Zolliker, Elena Mavrona, Jingming Cao, Daniel Sacré, Laboratory for Transport at Nanoscale Interfaces, Empa
Markus Rüggeberg, Institute for Wood Technology, TU Dresden
Ingo Burgert, Wood Materials Science Laboratory, Institute for Building Materials, ETH Zürich
Gustav Nyström Laboratory for Cellulose & Wood Materials, Empa
Terahertz radiation is electromagnetic radiation between the microwave and infrared band. It penetrates a wide variety of non-conducting materials. Here we describe holographic techniques to increase the resolution beyond the wavelength restrictions of classical imaging.
Holographic principle
Object can be reconstructed if intensity and phase of the diffracted object wave are known in the detector plane.
Phase determination
After subtraction of the thermal background the interferometric signal recorded by the camera is
where subscripts ref and obj refer to the THz reference beam and the object beam, respectively. φ is the respective phase of the waves, while δφ denotes a phase step controlled by mirror translation. In order to reconstruct the object, knowledge of the amplitude and the phase of the object wave is a prerequisite.
While its amplitude is readily available from the diffraction pattern (taking the square root of the intensity of the object wave measured without reference beam), the phase of the object wave is retrieved either using phase stepping algorithms (PSA) or applying the Fourier transform method (FTM) to the carrier frequency frames. PSA retrieve the phase in every image pixel individually by using a set of images each with a varied optical path difference between the object and reference wave. FTM uses a single interference image with a carrier fringe induced by an off-axis reference beam and retrieves the phase from its Fourier transform.
Image reconstruction
The reconstruction method is based on evaluating the Rayleigh-Sommerfeld diffraction integral by use of the fast Fourier transform. The reconstructed complex field at the object plane is obtained by backpropagated to a plane parallel or tilted plane at a given distance and tilt angle.
THz imaging and THz spectroscopy: We perform materials research on selected topics such as studying the interface structure of skin and textiles in mechanical contact, VO2 metal-insulator phase transition, humidity content of solar cell materials, wood structures and materials for new devices (e.g. graphene).
THz laser sources:
- Quantum Cascade Laser (QCL) emitting at 3 THz with 1.2 mW peak power.
- Far infrared gas laser (FIRL100 from Edinburgh instruments). Individual emission lines available in the wavelength range of 60 µm – 600 µm with 10-150 mW continuous power (e.g. 150 mW at 118.8 µm).
THz Detectors
- Uncooled µ-bolometer array with 480x640 pixels on a 25 µm pitch (Devitech).
- Uncooled µ-bolometer array with 480x640 pixels on a 17 µm pitch (Xenics Gobi 640).
- Pyrometer 0.1 - 30 THz NEP: 4x10-10 W/√Hz @ 10,6 μm D*: 5x109 cm √Hz /W
THz Spectrometer
- THz Time Domain Spectrometer „Teraflash“ covering the frequency range from 0.2 to 7 THz. It is based on a fs-laser at 1550 nm and photoconductive switches to generate and detect the THz pulse.
THz optics components
- Parabolic mirrors, wire grid polariser
- Beam splitter plates and gratings
- Lenses (PE-Fresnel, Silicon and Tsurupica)
We offer research collaborations and services in the field of measurement technology, including dimensional and temperature sensors, optical and camera-based methods, monitoring, and validation of measurement procedures. We have experience with electronic, mechanical and civil engineering applications as well as medical technologies.
Types of activities
- Deformation, strain and modal analysis
- Planarity and shape measurement
- Non-destructive Evaluation
- Monitoring and laboratory automation
- Development of prototypes based on optical and thermal methodologies
- Development and validation of measurement methods (including measurement uncertainty)
- Knowledge transfer (teaching at university level)
Processes and equipment
- THz laboratory
- Ultrasonics Lab
- Ellipsometry
- Thermal methods for temperature measurement and NDE
- Speckle Pattern Interferometry (DSPI), laser interferometer, Twyman-Green interferometer
- Fringe projection and moiré system
- Differential, potentiometric and optical displacement transducers
- Force and temperature transducers
- Measuring amplifiers, PC-based systems, wireless data transmission
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