Conference Agenda

Overview and details of the sessions of this conference. Please select a date or location to show only sessions at that day or location. Please select a single session for detailed view (with abstracts and downloads if available).

 
 
Session Overview
Session
SES-02: THz Imaging
Time:
Wednesday, 25/June/2025:
11:00am - 12:30pm

Session Chair: Pascale Roy
Session Chair: Peter Haring-Bolívar
Location: Dorint Parkhotel Siegen

Patmosweg 60, 57078 Siegen

Show help for 'Increase or decrease the abstract text size'
Presentations

Towards real-time all-electronic holographic THz imaging

Hartmut G. Roskos1, Hui Yuan1, Mingjun Xiang1,2, Aparajita Bandyopadhyay1, Kai Zhou2,3

1Physikalisches Institut, Goethe University Frankfurt am Main, Germany; 2Frankfurt Institute for Advanced Studies (FIAS), Frankfurt am Main, Germany; 3School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, P.R. of China

With THz radiation, it is fairly straightforward to generate three-dimensional (3D) images of scenes, exploi­ting coherent and/or time-of-flight measurement capabilities, or tomo­graphic techniques, all readily avai­lable in THz photonics. All of these can, however, until now not be performed under (close to) real-time conditi­ons, except perhaps with abundant THz power from large-scale radiation sources. As a consequence, various high-volume practical applications of THz imaging remain closed to THz photonics, including robotic sensing and motion control, gesture detection, security scanning or rapid industrial quality control. With the long-term goal to overcome this limitation in a cost-effective manner, we have set out to explore Fourier-domain imaging with all-electronic radiation sources and detectors. The image data are recorded in the focal plane of an optical system, where the scene-induced complex-valued radiation field pattern is as compact as nowhere else in the beam, which favors the use of detector arrays. As the field pattern is proportional to the spatial Fourier transform of the scene, one obtains a low-resolution overview of the scene by recording a limited amount of low-k-vector values, and can decide later for resolution improvement via mapping out the full lens-transferrred Fourier spectrum.

This presentation gives an overview of the status of our quest for rapid Fourier imaging. Initially, we started out with sub-1-THz raster-scan heterodyne detection of the field pattern at the funda­men­tal frequency [1] and the 2nd subharmonic [2], and corroborated diffraction-limited Fourier imaging. Hetero­dyne detection is, however, not a good choice because coherent detector arrays will not be available in the near future, but the use of arrays is a conditio sina qua non for real-time imaging. We hence asked us the question whether 3D Fourier imaging is feasible with power-detection alone instead of heterodyne detection, exploiting the phase information in the interference patterns in the focal plane. As direct phase retrieval is a mathematical ill-posed problem, we decided to explore the use of deep learning for image reconstruction. As a sufficiently large measured image database is not available for the training of our convolutional neural networks, we prepared a synthetic database. In order to avoid complications introduced by the imaging optics such as astigmatism, we first explored phase retrieval and image reconstruction from power-only data for the case of inline holography. This turned out to be successful. We demonstrated image reconstruction for simple 2D objects [3], identified object distances using a multi-head neural network [4], and reconstructed partially obscured objects with the help of a self-learning network [5]. At present, we extend this work to lens-based Fourier imaging and to simple 3D objects in both transmission and reflection geometry, and, in the future to the use of high-power RTD-emitter arrays and CMOS detector arrays.

References (all work funded by DFG):
[1] H. Yuan et al., APL Photonics 4, 106108 (2019); for industrial-type imaging: IRMMW-THz 2025.
[2] H. Yuan et al., Opt. Express 31, 40856-40870 (2023).
[3] M. J. Xiang et al., IEEE Trans. Terah. Sci. Techn. 14, 208-215 (2024).
[4] M. J. Xiang et al., Comput. Phys. Commun. 312, 109586 (2025).
[5] M. J. Xiang et al., Opt. Express, accepted (2025).



A TERAHERTZ CAVITY FOR THE SPECTROSCOPIC STUDY OF NON-THERMAL PLASMAS

Fabien Simon1, Francis Hindle1, Olivier Pirali2, Marie-Aline Martin-Drumel2, Georges Humbert3, Jean-François Lampin4, Gaël Mouret1

1Laboratoire de Physico-Chimie de l’Atmosphère, UR 4493, LPCA, Université du Littoral Côte d’Opale, 189A Av. Maurice Schumann, F-59140 Dunkerque, France; 2Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France; 3Institut XLIM, CNRS UMR 7252, Université de Limoges, 123 Avenue Albert Thomas, Limoges 87060, France; 4Institut d'Électronique, Microélectronique et Nanotechnologie (IEMN), CNRS UMR 8520, Université de Lille, Avenue Poincaré, F-59652 Villeneuve d'Ascq Cedex, France

Terahertz (THz) spectroscopy can distinguish polar molecules in gas mixtures thanks to the narrow and intense transitions specific to this band. An ultrasensitive cavity spectrometer using a metallic corrugated waveguide has already been demonstrated [1] for the detection and quantification of gas traces [2] in the THz range.

Plasma-based treatment of pollutant emissions, combined with catalysts, can offer high efficiency and low-cost implementation [3]. However, the physicochemical processes that occur in this non-thermodynamically balanced environment still require a better knowledge and an analysis with a dedicated instrument. We are developing a THz cavity to probe stable and unstable species forming in non-thermal plasmas. The objectives are to optimize the plasma for species production and pollution treatment via the ultrasensitive detection of radicals in the plasma.

We present here the specific features of the instrument, including the use of an innovative hollow core dielectric waveguide [4] whose losses are characterized, and achieved sensitivity compared to a direct absorption spectrometer. We deliver the first results of the spectroscopic analysis of ammonia and methanol plasmas created by radiofrequency discharge. Conversion and production rates are evaluated as a function of the radiofrequency power. Our work will continue with tests to detect short lifetime radicals in plasmas containing carbon.

Reference:

[1] F. Hindle et al. Optica 6.12, p. 1449 (2019).

[2] C. Elmaleh et al. Talanta Volume 253, 124097 (2023). [3] M. QU et al. Process Safety and Environmental Protection 153, p. 139-158, (2021).

[4] G. Humbert. Peng, GD. (eds) Handbook of Optical Fibers, p. 1-49, (2019).

This thesis work received co-funding from the Hauts-de-France Region and the National Research Agency, as part of the WASPE project which brings together four partners: the LPCA, the Orsay Institute of Molecular Sciences (ISMO), the XLIM Institute and the Institute of Electronics, Microelectronics and Nanotechnologies (IEMN).



Hz-level resolution photonic spectrum analyzer using an electro-optical THz comb

Benedikt Krause, Jesko Fröhlich, Sascha Preu

Technical University of Darmstadt, Germany

The expansion of the carrier frequencies for data communication into the terahertz (THz) region, i.e. for 6G [1], requires also technological advancements for measurement devices like spectrum analyzers (SAs). Despite excellent performance, electronic SAs require multiple frequency extender modules if harmonics are to be investigated besides the fundamental tone. Photonic variants using DFB laser diodes and photoconductive mixers showed a promising frequency coverage of more than 1 THz [2]. Unreferenced DFB laser diodes achieve resolutions in the low MHz range [2], and if locked to a precise electronic source in the 100s of Hz range [3]. We show that an electro-optical (EO) THz comb based on an EO phase modulator (EOM) can achieve single digit Hz resolution. The EO THz comb generates multiple side-modes from a single laser by driving an EOM with a sinusoidal radio frequency (RF) signal [4]. As all of the modes stem from a single laser, they inherit the optical properties of the laser, and contain mostly common noise. This common noise subtracts out during the difference frequency generation. The RF generator limits the linewidth and the linewidth scaling [5]. For our measurements, we select one of the side-modes with an optical filter and use the laser directly for the difference frequency generation (Fig. 1a). We determine the linewidth capabilities of the EO THz comb as a photonic SA by investigating the signal generated by an electronic source (Keysight PNA) with an extender module (Anritsu 3740A-EW) at fixed frequencies between 100 GHz and 110 GHz. Higher frequencies are straight forward and similar combs have yet been demonstrated up to 1 THz [6], however, we are limited by the frequency coverage of the available electronic source to the low end of the THz domain. Fig. 1b illustrates the spectrum of the signal under test at a frequency of 100 GHz and a THz signal strength of +2.2 dBm. The data shows Lorentzian characteristics (orange line) with a linewidth of 1.7±1.0 Hz. The two close side-peaks at a distance of 90 Hz are an artifact of the EO THz comb.



Superstrate Enhanced Frequency Control in CMOS-Based Terahertz Sources and Detectors

Domantas Vizbaras1, Kestutis Ikamas1, Jakob Holstein2, Hartumt G. Roskos2, Alvydas Lisauskas1,2

1Vilnius University, Lithuania; 2Goethe University Frankfurt, Germany

This study presents a comprehensive investigation into the performance optimization and frequency tuning of integrated terahertz (THz) sources and detectors through the use of chip-level superstrate coatings. Utilizing a commercial 65-nm CMOS process, we develop harmonic voltage-controlled oscillators (VCOs) and monolithically integrated patch-antenna-coupled field-effect transistor (FET) detectors operating across the 243–294 GHz and 580 GHz–2 THz frequency ranges, respectively. The THz source employs a differential Colpitts configuration with an on-chip integrated antenna, allowing frequency tuning via bias control by approximately 6 GHz. Additionally, we demonstrate post-fabrication fine-tuning of the emission frequency by applying dielectric coatings on the CMOS chip surface, achieving an additional 3.5% downward shift in center frequency (by approximately 10 GHz). For detector modules, different superstrate layers topped by a hyper-hemispherical silicon lenses are used to enhance optical coupling. For a patch antenna-coupled detector with resonant frequency of 580 GHz, this method enable resonance frequency modification by 60 GHz and potentially can reach up to100 GHz shift. These superstrate-based techniques offer a practical and scalable method for tailoring the operating frequencies of CMOS-based THz devices to match specific spectral lines, with potential applications in gas spectroscopy and biosensing.



Intense Multi-Terahertz Fields for Time-Domain Quantum Optics from an Optical Parametric Oscillator

Elias Hils, Hannes Kempf, Alfred Leitenstorfer

University of Konstanz, Germany

The vacuum fluctuations of the electromagnetic field represent an upper limit for the precision of any electric field measurement. It was only in 2015 that highly sensitive electro-optic sampling (EOS) in the mid-infrared enabled the first direct detection of the vacuum fluctuations of the electric field in the time domain [1]. The time-domain detection of squeezed vacua with EOS provides a way to surpass the vacuum noise limit but requires intensive and phase-stable pump pulses in the mid infrared.

Our setup (Fig. 1a) is based on a passively phase-stable Er:fiber comb at 193 THz that is split into a probe branch generating few-fs near-infrared pulses and a pump branch that synthesizes a phase-stable pulse train centered at 154 THz. In a synchronously pumped optical parametric oscillator (OPO), an orientation-patterned (OP) GaAs crystal generates and reamplifies the subharmonic at 77 THz. The emitted frequency comb of the degenerate OPO is phase-locked to the pump and therefore phase-stable as well. This feature enables the first direct field-resolved detection of an OPO output with EOS (blue in Fig. 1a-b) [2].

In addition, we derive explicit expressions for squeezed states resulting from spontaneous parametric downconversion (SPDC). Our derivation takes into account the spectral, temporal and spatial structure of these states, providing a realistic description of squeezed vacuum states in the time domain. Fig. 2b-c) shows the calculated noise patterns assuming a GaAs crystal with adequate patterning and the OPO output field as input parameters as well as the optimizing crystal length L and pump spot radius w0. For the average OPO output power of 1 mW currently available, the calculations show a quantum squeezing of the vacuum fluctuations by 1.2% (Fig. 2b). For 100 mW output power we are aiming at in an extended setup, a squeezing factor of 5.5% and a large asymmetry between anti-squeezing and squeezing is expected (Fig. 2c).

References:

[1] C. Riek et al. Science 350, 6259 (2015)

[2] H. Kempf, A. Muraviev, F. Breuning, P. G. Schunemann, R. Tenne, A. Leitenstorfer and K. Vodopyanov, APL Photonics 9, 036111 (2024)



 
Contact and Legal Notice · Contact Address:
Privacy Statement · Conference: FGTC 2025
Conference Software: ConfTool Pro 2.8.106
© 2001–2025 by Dr. H. Weinreich, Hamburg, Germany