Publications
2025
Dook van Mechelen, Daan Meulendijks, Milan Koumans
Coffee bean sorting is currently based primarily on visual appearance and near-infrared techniques that probe the bean's skin. However, sorting based on compositional differences has significant potential to optimize the roasting process. We present a novel coffee bean sorting method using terahertz (THz) spectroscopy, which effectively penetrates both green and roasted beans. Our findings show that the optical properties of coffee beans at THz frequencies are primarily governed by internal moisture levels. To demonstrate industrial feasibility, we implement a robot-guided THz sensing system capable of scanning beds of beans for automated sorting. More broadly, our results confirm the potential of THz technology for moisture content analysis across various applications.
Article
26 Mar 2025
MDPI Sensors
M. Koumans, J.L.M. van Mechelen
We report on a novel methodology for extracting material parameters from spectroscopic optical data using a physics-based neural network. The proposed model integrates classical optimization frameworks with a multi-scale object detection framework, specifically exploring the effect of incorporating physics into the neural network. We validate and analyze its performance on simulated transmission spectra at terahertz and infrared frequencies. Compared to traditional model-based approaches, our method is designed to be autonomous, robust, and time-efficient, making it particularly relevant for industrial and societal applications.
Preprint Arxiv:
arXiv:2503.08183
2024
D.B.L. Peeters, G. Geneste, J. Gomez Rivas and J.L.M. van Mechelen
We report on the dielectric properties of yttrium hydride thin films as a function of hydrogen content determined using broad-band optical spectroscopy between 0.25 THz and 650 THz. We discuss the properties of YHx based on the measured optical conductivity and in the light of a spectral weight analysis.
Conference proceeding
1 Sept 2024
49th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz)
Milan Koumans, Daan Meulendijks, Haiko Middeljans, Djero Peeters, Jacob C. Douma and J.L.M. van Mechelen
Signal processing techniques are of vital importance to bring THz spectroscopy to a maturity level to reach practical applications. In this work, we illustrate the use of machine learning techniques for THz time-domain spectroscopy assisted by domain knowledge based on light–matter interactions. We aim at the potential agriculture application to determine the amount of free water on plant leaves, so-called leaf wetness. This quantity is important for understanding and predicting plant diseases that need leaf wetness for disease development. The overall transmission of 12,000 distinct water droplet patterns on a plastized leaf was experimentally acquired using THz time-domain spectroscopy. We report on key insights of applying decision trees and convolutional neural networks to the data using physics-motivated choices. Eventually, we discuss the generalizability of these models to determine leaf wetness after testing them on cases with increasing deviations from the training set.
Article
25 Mar 2024
Scientific reports
2023
Yixi Zhou, Adrien Waelchli, Margherita Boselli, Iris Crassee, Adrien Bercher, Weiwei Luo, Jiahua Duan, J.L.M. van Mechelen, Dirk van der Marel, Jérémie Teyssier, Carl Willem Rischau, Lukas Korosec, Stefano Gariglio, Jean-Marc Triscone & Alexey B. Kuzmenko
Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 – a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.
Article
10 Nov 2023
Nature Communications
Experimental Determination of the Green's function with double probe THz near-field microscopy
J. Gomez Rivas, S.E.T. ter Huurne, N.J.J. van Hoof, D.B.L. Peeters and J.L.M. van Mechelen
We introduce a THz near-field probe microscope capable of measuring the complex electric field in the time domain. With this setup, we retrieve the Green’s function, which allows the direct determination of photonic properties, like the spatial coherence or the local density of optical states.
Conference proceeding
2023
IEEE Photonics Conference (IPC)
Impact of LED Temperature on the Performance of LiFi Optical Wireless Communication Links
Diego R. Vargas Romero, Jean-Paul M. G. Linnartz, Jacobus L. M. van Mechelen
In this paper, we calculate the degradation of an Optical Wireless Communication (OWC) system caused by the junction temperature of the LED. Its throughput depends on the LED frequency response and on the received power versus the noise floor. Thus, the throughput can be calculated from the optimal modulation bandwidth, the efficiency, and the spectral density of the LED, which all are temperature dependent. For an OWC system based on LEDs, the low 3 dB bandwidth limits the achievable throughput of the system. Although, we increase the bandwidth by driving the LED at higher current density, that increases the junction temperature. We explore the effects that temperature has on the dynamic behavior of the LED and subsequently on the SNR and throughput of the OWC link. Our results can be used to optimize the throughput including the effect of self-heating nature of the LED.
Conference proceeding
2023
29th International Workshop on Thermal Investigations of ICs and Systems (THERMINIC)
2022
Sensing moisture patterns using terahertz spectroscopy
M. Koumans, A. Perez-Casanova and J.L.M. van Mechelen
We report on a novel and comprehensive method to determine leaf wetness by using machine learning algorithms applied on transmission THz time-domain data of a vast set of moisture patterns. More generally, our approach opens ways to determine properties of materials with a complex structure for which a model-based signal processing approach is not feasible.
Conference proceeding
2022
47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz)
2021
Jacobus .L.M. van Mechelen, Andreas Frank and Deran J.H.C. Maas
Optical terahertz technology has, despite its exciting properties, such as transparency of visibly opaque materials, 30 years after its technological breakthrough, not found a widespread application with societal relevance. Main causes are its maturity and costs. If, however, the uniqueness of both THz radiation and time-domain spectroscopy is used to close a technological gap in the right market sector, we here show that successful applications are in reach. We have chosen the automotive industry, where the optimization of coatings applied in the paint shop is of longstanding concern for this most expensive unit of the car production line. Here we report on the development of a THz-based sensor system. We study the light-matter interaction of drying polymer coatings and use advanced novel signal processing algorithms to determine the state of matter of drying paints. This very first sensor system for the inspection of wet coatings that accurately predicts the eventual dry thickness without requiring paint-type calibration. Industrial robustness is guaranteed by a bespoke optical beam design. Our sensor demonstrator outperforms state-of-the-art thickness sensors applied on dry coatings by a factor three and is expected to become a game changer for the paint shop, where today’s scrap rates worldwide are as high as one car body per minute.
Article
1 Mar 2021
Optics Express
TU/e
Subgroup of the faculty of Electrical Engineering at the TU/e.
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