Conference Agenda

Output power is a key characteristic of any radiation source. Therefore, reliable measurements of radiant power are of critical importance for academia and industry. However, the results of such measurements are only reliable if the employed power detectors are properly calibrated. Providing calibrations for measurement devices—including power detectors—with high accuracy is one of the main tasks of National Metrology Institutes (NMIs) such as the Physikalisch-Technische Bundesanstalt (PTB), the NMI of Germany.

The need for calibrated terahertz (THz) power detectors became urgent with the advent of novel terahertz (THz) sources such as quantum-cascade lasers or photoconductive switches. However, detector calibrations for the relevant spectral ranges were not available at this time. An important step for the solution of this problem was the development of a detector standard by PTB for frequencies between 1 THz and 5 THz [1], which allowed PTB to provide detector calibrations as a service to customers worldwide.

The available THz detectors at this time fell short of the often-required high power sensitivity or high speed paired with a large sensitive area and an approximately frequency-independent spectral power responsivity. PTB together with the German company SLT Sensor- und Lasertechnik GmbH [2] developed a novel type of pyrolectric detector that indeed fulfills these requirements. A unique feature of this detector type is its approximately frequency-independent spectral power responsivity from 100 GHz [3] to at least 3 THz. For many years now, this detector type is commercially offered by the company SLT and is always delivered with a calibration certificate issued by PTB.

These developments enabled trustworthy power measurements worldwide. Very recently, PTB together with SLT Sensor- und Lasertechnik developed a prototype for a waveguide-coupled power detector for the so-called WR-3.4 band (220 GHz to 330 GHz). This detector is crucial to support the research and development of mm-wave components required for the communication technology of the 6^th generation (6G).

Building on both its earlier and more recent developments, PTB continues to serve the THz community with state-of-the-art detectors and calibrations, contributing to sustained progress toward ever more powerful THz technologies.

A larger proportion of technical textiles are usually coated to achieve the desired properties, such as water impermeability and chemical resistance. The coating is often applied in liquid form with a coating knife blade acting as squeegee and obtains its strength and functionality through curing by drying or chemical bonding. The final coating thickness and absence of air bubbles are decisive for the function. All DIN standards for determining the quality of coated textiles are based on destructive testing on samples cut out of the finished product.
The aim of a completed cooperation project between all three institutions was to demonstrate, for the first time, a non-destructive and non-contact measurement method using THz radiation inline on a coating line of the DITF [1]. The textile to be coated was a commercially available warp knitted fabric made of polyester. The coating medium was an emulsion based on acrylate as a binder, a crosslinker and water as solvent. After drying and fixation the thickness THz measurements of the coating layer were carried out in reflection geometry. A fast-measuring THz time-domain spectrometer (THz-TDS) from TOPTICA (TeraFlash smart) with a measurement rate of 1600 THz pulse traces was used with a reflection measuring head developed by PTB with 2” off-axis parabolic mirrors in a polarisation-maintaining setup.
Samples of the textile and a specially produced flat layer of the coating had previously been measured individually at PTB using THz-TDS (TeraFlash pro, TOPTICA Photonics AG) in transmission. The data evaluation revealed the effective refractive index and loss coefficient of the textile and the coating. These values were used to analyse the inline measurements on a deflection roller at the end of the coating system. In reflection geometry, layer thicknesses are determined from measured time differences of the received THz pulses when reflected at the coating surface, at the interface and at the back of the textile, which is pressed firmly on the deflection roller acting as metallic mirror.
At present, the coating thickness is typically adjusted during the production run by the distance between the blade of the coating knife and the fabric. In this measurement campaign, the distance was varied from 30 μm to 80 μm. To our surprise, however, the online measured coating thicknesses did not correlate at all with the distance setting. We verified our results by offline THz measurements on samples, which were taken from the finished textile for the different knife settings [2]. The reason for this behaviour is the open-pored knitted fabric through which the liquid coating material is partially pressed by the coating knife. The uncorrelated variation of the mean values of the coatings seems to be an inherent problem of the coating process, which was revealed by our THz measurements. Yet, this finding underlines the need for an online-capable thickness measurement tool.
To summarise, it can be stated that a successful first step towards inline control of the coating process was demonstrated in the cooperation project. Despite the penetration of the liquid coating into the textile, a clear reflection signal could be detected at the textile/coating interface. Dominated by the inhomogeneity of the coating, its thickness could be determined with an average standard deviation of 10 μm. With significantly finer textiles, the accuracy could be reduced to a few μm in the future. In addition, further measurements in an industrial environment are required to overcome the challenges and confirm the broad applicability of the THz measurement method.

References:
[1] A. Steiger et al. Melliand International - Textile Technology 31, Issue 1, 42-46 (2025). [2] N. Vieweg et al. Journal of Industrial Textiles 53, 1-21 (2023) DOI 10.1177/15280837231207396

As the demand for higher data rates is growing exponentially and the EM spectrum is nearly saturated below 300 GHz, THz frequencies beyond this are shifting into focus of applied research for the sixth generation (6G) of wireless communication. As the dimensions of blockage objects or parts thereof approach the wavelengths present in the THz bands of interest (0.3 – 2.5 THz), it seems likely that diffraction effects change drastically. To investigate these effects in practice, we constructed an optical setup for THz-TDS with a large beam diameter that allows for measurements with household objects and people in the THz beam. Based on the TDS measurements we discuss the limitation of modelling the blockage by everyday objects as well as a person in the THz beam through a simple geometrical optics approach from the objects shadow. Furthermore, we apply Fresnel’s diffraction formulae as a more advanced model for frequency dependent blockage.

Additive manufacturing (AM) of terahertz (THz) components is a cost-effective and efficient method of rapid prototyping. This work discusses the THz characterization of AM-compatible polymers to evaluate their potential use in the fabrication of THz components. The materials under investigation are primarily used in the inkjet 3D-printing (IP) process, and are compared with other AM-compatible materials. The authors analyzed heat resistant AR-H1 material, transparent AR-M2 material and flexible silicon elastomer material. After the THz time domain spectrocopy of these materials, it can be deduced that the materials AR-M2 and AR-H1 can be used in manufacturing of THz components, with the benefit of a superior printing resolution of 50 µm.

Terahertz (THz) generation from Lithium Niobate (LN) platform using nonlinear optical processes has garnered significant interest due to its potential in numerous applications. In particular, THz generation in periodically poled LN (PPLN) via chirped and delayed laser pulses (CDLP) manifests a great potential in applications such as spectroscopy and particle accelerator. Recent studies have demonstrated such approach using large-scale PPLN pumped by Ti:Saphire-based laser sources with pulse energies of several Joules. However, these systems face substantial challenges for miniaturization, limiting the development of compact, portable THz sources essential for integration. In this work, we demonstrate the generation of narrowband THz waves in a PPLN waveguide using low energy pulses of several μJ at a wavelength of 1 μm. In addition, by precisely adjusting the delay between two chirped pulses, we show the potential of this technique for modal phase-matching, selecting specific THz modes within a waveguide. This approach underscores the potential of the CDLP technique in achieving precise mode confinement and dynamic mode switching, paving the way for compact and versatile THz sources.

Spintronic THz emission, based on optically induced spin-to-charge conversion (SCC) such as Inverse Spin-Hall Effect (ISHE) and Inverse Rashba-Edelstein Effect (IREE), recently emerged as a novel method for generation of electromagnetic waves within a large THz bandwidth up to 30 THz owing to the absence of phonon absorption [1]. In this talk, after having presented our THz-emission spectroscopy set-up and optical bench, we will present our main results dealing with THz emission from Rashba states of spintronics materials such as i) sputtered topological insulator BiSb [2], ii) GeTe ferroelectric material and iii) from 2d material stacking based PtSe₂/MoSe₂ interfaces grown on the ferroelectric Lithium Niobate (LiNbO₃ or LNO) substrate [3]. More particular, the latter system provides a clear control of the magnitude of the THz emission vs. the direction of the ferroelectric polarization in z-cut LNO substrate with polarization normal to the 2D layers which makes it very appealing for THz applications and devices.

References:

[1] T. Seifert et al., ‘Efficient metallic spintronic emitters of ultrabroadband terahertz radiation’, Nature Photonics 10, 483–488 (2016)

[2] E. Rongione et al., ‘Spin-momentum locking and ultrafast spin-charge conversion in ultrathin Bi_1-xSb_x topological insulator’, Adv. Sci. 2023, 10, 2301124

[3] S. Massabeau, O. Paull, A. Pezo, F. Miljevic et al., « Inverse Rashba Edelstein THz emission modulation induced by ferroelectricity in CoFeB/PtSe2/MoSe2//LiNbO3 systems” APL Materials in press (2025)