Conference Agenda

Photonic concepts offer a distinct advantage in generating high-quality signals, particularly in the THz regime, due to their ability to achieve ultra-low phase noise and high stability. This capability is crucial for test and measurement applications, where precise and reliable signal generation is essential for accurate characterization and performance evaluation of various systems

In this presentation, we will provide an overview of the ADLANTIK project, in which R&S is collaborating with TOPTICA Photonics AG, Fraunhofer HHI and other partners. The focus of the ADLANTIK project is the development and integration of an ultra-stable tunable THz system for 6G wireless communication and test and measurement applications. The central component of the investigated systems is a unique photonic signal source developed by TOPTICA Photonics AG. This system can generate highly stable RF signals, with its tunability extending from a few GHz up to several THz. Such a versatile source enables extensive application possibilities. Here, we will specifically focus on its use in a broadband VNA system, which allows for the characterization of devices under test (DUTs) over a large frequency range. This innovative approach has the potential to significantly accelerate broadband S-parameter measurements compared to state-of-the-art VNA systems.

Comment: I would prefer to present this contribution in form of a poster

Frequency combs are of great interest as not only are the position of the modes precisely known they also have a particularly narrow linewidth. The analysis of gases by THz spectroscopy is able to provide a high degree of molecular discrimination. The use of two frequency combs together in ASynchronous Optical Sampling (ASOPS) or Dual Comb Spectroscopy is a compelling solution as it allows the entire frequency band to be obtained simultaneously. Nevertheless, the useable resolution achieved is generally limited to the laser repetition rate, indeed the frequency resolution of a single point is certainly better than 0.1 MHz but subsequent datapoints are separated by the repetition rate which can typically be 100 MHz for fibre lasers.

We have developed an alternative approach consisting of a THz Frequency Comb (FC) and a heterodyne detection. The advantage being that a uniform high-resolution can be obtained by applying small frequency steps to the laser repetition rate. The Intermediate Frequency (IF) signal produced by the heterodyne contains a large number of FC modes. In excess of 80 modes are simultaneously measured by using an eXtended Fast Fourier Transform Spectrometer (XFFTS) to analyse the IF signal [1]. The final system is able to record a spectrum some 7.5 GHz in length with a resolution of 76 kHz with an acquisition time of 20 minutes. The retuning of the local oscillator frequency allows direct access to any frequency in the 500 to 750 GHz range. The instrument has been evaluated by measuring the spectrum of methanol at 723 GHz [2].

Reference:

[1] B. Klein, S. Hochgürtel, I. Krämer, A. Bell, K. Meyer, and R. Güsten, ‘High-resolution wide-band fast Fourier transform spectrometers’, A&A, vol. 542, p. L3, Jun. 2012, doi: 10.1051/0004-6361/201218864.

[2] F. Hindle et al."Terahertz Frequency Comb High-Resolution Heterodyne Spectrometer", submitted to IEEE Transactions on Terahertz Science and Technology

Hollow-core circular waveguides (HCCW) have enabled significant advances in the transmission of terahertz (THz) radiation. These waveguides are characterized by mode analysis, attenuation coefficients, and spatial intensity profiles captured using imaging techniques [1]. In HCCWs, radiation is coupled from free space into the air core via a coupling element to ensure efficient transmission. These coupling elements may include an auxiliary waveguide, a coupling lens, or a direct THz source such as a photoconductive antenna [2-3]. The transmission properties of the waveguide vary depending on the coupling conditions [4]. A straightforward coupling method involves using a lens to focus the THz beam from free space into the waveguide core.

Although coupling is relatively straightforward, the use of broadband THz pulses introduces complexity in the mode propagation dynamics within the waveguide. In this study, we characterized the fundamental modes of HCCWs using a Teflon lens with a focal length of f = 75 mm, along with a diaphragm positioned upstream to control the beam spot size at the waveguide entrance. Characterization was performed using a THz time-domain spectroscopy (THz-TDS) system - specifically, the TeraFlash Pro model from Toptica - covering the 0.1 to 2 THz frequency range.

Multimode interference was simulated following Ito's approach [3], with the addition of an overlap integral to evaluate the coupling coefficients. We observed that a beam diameter of 50 mm at the lens aperture (D) leads to coupling of the fundamental TE₁₁ mode with higher-order modes, resulting in pronounced interference above 0.2 THz (Fig. 1(a)). When D is reduced to 25 mm, the coupling is dominated by the TE₁₁ mode at low frequencies, in good agreement with simulation results (Fig. 1(b-c)). A further reduction of D to 12 mm enables efficient excitation of the TE₁₁ mode over an extended frequency range from 0.2 to 0.7 THz (Fig. 1(c)).

In conclusion, we have characterized a 10 cm-long copper waveguide using a coupling scheme based on a lens and an adjustable diaphragm. By reducing the beam size before the lens, the majority of the energy is efficiently coupled into the fundamental TE₁₁ mode. This approach minimizes multimode interference and supports low-loss propagation of the fundamental mode across a broad frequency range.

In this work we compare two methods for THz gas sensing with CW THz photomixing: a multi-pass cell and a substrate-integrated hollow waveguide (iHWG). Compared with multi-pass cells, the iHWG has the advantage that it requires much less gas volumn thus suitable for instant gas detection.

Urolithiasis or the formation of urinary stones has emerged as a global health crisis in recent times. The standard treatment procedure involves endoscopic detection and in situ laser-based or ultrasonic fragmentation of the stones. This results in the disintegration of the calculi and the nucleation site. Hence, during treatment essential details on the morphology and mineralogy are lost that would be instrumental in view of post-operative protocols and prevention of recurrence.

Here we report a THz time-domain-spectroscopy-based approach to investigate several types of urinary stones and characterize them based on the extracted optical constants. Extracted urinary stones are separated as per their morphology and composition into four different species, namely, type 1A (calcium-oxalate-monohydrate), type 3A (uric acid), type 4C (struvite), and, type 5A or cystine-based (amino-acid) structures. A transfer matrix-based algorithm is deployed to extract the associated optical parameters that evaluate a theoretical transfer function (TF) considering the possible propagation modes inside the sample, and minimizes the deviation relative to the experimental TF. Extracted absorption coefficients are compared.

Optical rectification has been a widely used technique for THz generation, one of the most used crystals is the LiNbO₃ because of its strong nonlinear coefficients. In this work, we study the polarization state of THz waves generated in LiNbO₃ polycrystalline powder. Numerical simulations have been realised as well as experimental evidence of the phenomenon.

The growing demand for sustainable agriculture and environmental safety is fostering the development of non-invasive diagnostic technologies capable of identifying internal or non-visible contaminants. Terahertz (THz) spectroscopic imaging emerges as a powerful tool in this context, enabling label-free and contactless analysis with both molecular and structural sensitivity. This work demonstrates the versatility of THz imaging through two case studies in agri-food quality control and environmental monitoring.
Measurements were carried out using the TeraASOPS spectrometer by Menlo Systems, which employs asynchronous optical sampling (ASOPS) with two femtosecond lasers connected via optical fibers to THz emitter and receiver antennas. The system is integrated with a two-dimensional scanning stage capable of imaging areas up to 30 × 30 cm², with lateral and depth resolutions of ~1.5 mm and ~60 µm, respectively. It provides a spectral bandwidth of 4 THz over a 10 ns time-domain scan window and achieves a signal-to-noise ratio exceeding 70 dB under optimal acquisition settings.
In the first case study, THz hyperspectral imaging is used to detect fungal infections in chestnuts, even when the contamination lies beneath the outer shell. Unsupervised learning techniques are applied to the full spectral dataset of each sample. Despite relying on different mathematical principles, all methods consistently distinguish infected from healthy tissue, enabling reliable, non-destructive internal quality assessment. The clustering results were also found to be stable across variations in hyperparameters, further confirming the robustness of the approach. Moreover, the identified spectral features correlate with biochemical changes induced by fungal activity, highlighting the sensitivity of THz imaging to early-stage degradation.
The second case focuses on detecting polyethylene (PE) microplastics dispersed in soil. Samples with increasing PE content (1%, 5%, 10%) were analyzed. As a first step, the average spectra—obtained by averaging all the spectra associated with the hyperspectral imaging of each sample—were examined, revealing a progressive attenuation of the characteristic absorption peak at 0.4 THz, along with a narrowing of the overall spectral profile at higher concentrations. To further assess spatial localization, unsupervised learning methods were employed to cluster the spectral images. This enabled the identification and mapping of plastic-rich regions within the heterogeneous soil matrix, validating the method’s effectiveness in environmental microplastic detection.
In conclusion, these results demonstrate the strong potential of THz spectroscopic imaging as a non-destructive, label-free diagnostic platform for a broad range of applications. When combined with unsupervised machine learning, THz imaging offers a robust and scalable solution for the internal inspection of complex, optically opaque materials in both agricultural and environmental sectors.

The terahertz frequency range is of growing interest for ultrafast communications and photonics, but widespread adoption is limited by the high cost and complexity of existing materials like low-temperature-grown GaAs or InGaAs. Germanium is a promising alternative, offering compatibility with standard fabrication and potential for ultrafast carrier dynamics when defects are introduced.

We present a cost-effective platform for THz devices based on evaporated Ge thin films, grown on semi-insulating GaAs via electron-beam physical evaporation at substrate temperatures ranging from room temperature to 400 °C. Using optical pump THz-probe (OPTP) spectroscopy at 800 nm, we observe sub-picosecond carrier lifetimes in amorphous Ge, attributed to trap-assisted recombination (Fig. 1). At higher deposition temperatures, lifetimes extend due to reduced trap density and increasing crystallinity.

Photoconductive switches fabricated from these films emit THz pulses that propagates along a transmission line before being detected (see inset of Fig. 2). Typical pulse traces with durations around 2 ps are shown in Fig. 2. Higher signal amplitudes are obtained for Ge grown at moderate temperatures, reflecting enhanced carrier mobility without compromising recombination speed. These results are consistent with previously reported THz performance in Ge-based photoconductive antennas.

Our findings demonstrate that evaporated Ge enables THz functionality without relying on epitaxial growth, offering an accessible route toward scalable, high-frequency optoelectronic systems.

In this work, we present the results of electron wave packet generation in a 2D GaAs/ AlGaAs material using THz optoelectronic circuits. The LTG-GaAs-based circuit consists of two photoconductive switches for the generation and detection of THz pulses propagating along a transmission line connected to the 2D material. The electronwavepacket are generated using short (1- 2 ps) electrical pulses. In addition, by modifying the dispersion of the femtosecond laser pulses used to generate the electrical pulses, we are also demonstrating the circuit's ability to deliver electrical pulses with longer and variable duration (2- 15 ps). These pulses will be used to dynamically control the propagation of the electron wave packet in a micrometer-sized circuit for quantum applications.

We explore the suitability of a photoconductor, made from InGaAlAs doped with iron (Fe) and rhodium (Rh), for the 1030 nm wavelength range. As expected, our quaternary material shows an order of magnitude higher resistance for 1030 nm compared to ternary materials for 1550 nm excitation wavelength. At the same time, it exhibits exceptional electron mobility of >1500 cm²/Vs and possesses sub-picosecond chargecarrier lifetimes. Consequently, it holds significant promise for future applications in photoconductive antennas. The use of 1030 nm allows for semiconductors with a higher band gap, thus enabling more efficient photoconductive antennas with greater breakthrough field strengths for emitters and lower noise levels for receivers. A key challenge is the epitaxial growth of ultrafast photoconductors, due to the higher complexity of quaternary growth compared to ternary structures. The InGaAlAs epitaxial layers were grown on InP:Fe substrates using gas-source molecular beam epitaxy. Precise control of the group-III component fluxes is crucial for achieving the target composition of In0.53Ga0.19Al0.28As. Hall effect measurements indicate mobilities of 1520 cm²/Vs (Fe) and 1760 cm²/Vs (Rh), and an order of magnitude higher resistance compared to the ternary materials (60.9 kΩ·cm (Fe) and 14 kΩ·cm (Rh)), see Fig. 1 a). Optical pumpterahertz probe (OPTP) measurements reveal sub-picosecond carrier lifetimes of 0.73 ps (Fe) and 0.58 ps (Rh) (see Fig. 1 b).), highlighting the potential of this material for THz antennas.

THz imaging is a powerful technique for analyzing the chemical and material compositions of various samples. However, existing THz imaging methodologies face substantial challenges, notably the inherent trade-off between spatial resolution and field-of-view [1]. Such a limitation reduces the attainable space-bandwidth product, impeding the precise examination of complex structural details within complex media. Several approaches have been proposed to overcome resolution constraints, including digital holography [2], ptychography [3], near-field imaging [4], and wavefront shaping [5–7]. Despite yielding notable improvements in spatial resolution, these techniques encounter practical challenges: holography typically implemented on monochromatic source illumination, hence lacking broadband imaging capabilities; wavefront shaping requires elaborate and bulky experimental configurations due to limited modulation technologies available at THz frequencies; and near-field THz imaging, suffers from experimental complexity and prolonged image acquisition times.

As an alternative, synthetic aperture imaging presents a promising solution by coherently synthesizing multiple diffraction-limited acquisitions, each capturing distinct spatial frequency subsets, thereby effectively expanding the spatial bandwidth of the imaging system [8]. In this vision, we propose a spatiotemporally coherent synthetic aperture imaging framework in the THz regime capable of surpassing conventional resolution constraints. Our approach involves illuminating the sample with broadband THz pulses from multiple incident angles, effectively scanning an expanded spatial frequency bandpass across Fourier space [1]. The resulting time-resolved sample response acquired through THz-TDS is subsequently processed using a computational routine to reconstruct the complex field response, encompassing both amplitude and phase information.

In our approach, an imaging object modulates the incident THz field illuminated from multiple angles. The resulting diffraction-limited, low-resolution spatiotemporal images are subsequently acquired using state-of-the-art THz-TDS at the imaging plane. As a result, the reconstructed phase images underline the high fidelity and precision achievable by our time-resolved reconstruction method, where we show the capability of our method to accurately resolve temporal dynamics at individual pixels.

As a next demonstration, we extend our methodology for the application to characterize structurally complex and heterogeneous samples. Specifically, we simulate a semi-transparent target consisting of three distinct materials -Teflon, Topas, and HDPE- each with a thickness of Δ=100 μm. Conventional diffraction-limited THz imaging achieves a distorted refractive index distribution, resulting in limited material discrimination. In comparison, our proposed reconstruction algorithm produces a hyperspectral image exhibiting significantly improved spatial resolution and distinct delineation of the constituent materials. Consequently, our imaging framework offers novel insights into the optical properties and structural complexity of samples, thus opening promising avenues in diverse research fields, including chemical sensing, biomedical imaging, and materials science utilizing THz radiation.

Reference:

[1] V. Kumar, P. Mukherjee, L. Valzania, A. Badon, P. Mounaix, and S. Gigan, "Fourier Synthetic Aperture-based Time-resolved Terahertz Imaging," Photonics Research 13, 407–416 (2024).

[2] M. Humphreys, J. P. Grant, I. Escorcia-Carranza, C. Accarino, M. Kenney, Y. D. Shah, K. G. Rew, and D. R. S. Cumming, "Video-rate terahertz digital holographic imaging system," Opt. Express, OE 26, 25805–25813 (2018).

[3] L. Valzania, T. Feurer, P. Zolliker, and E. Hack, "Terahertz ptychography," Opt. Lett., OL 43, 543–546 (2018).

[4] L. L. Hale, T. Siday, and O. Mitrofanov, "Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited]," Opt. Mater. Express, OME 13, 3068–3086 (2023).

[5] V. Kumar, V. Cecconi, L. Peters, J. Bertolotti, A. Pasquazi, J. S. Totero Gongora, and M. Peccianti, "Deterministic Terahertz Wave Control in Scattering Media," ACS Photonics 9, 2634–2642 (2022).

[6] V. Cecconi, V. Kumar, J. Bertolotti, L. Peters, A. Cutrona, L. Olivieri, A. Pasquazi, J. S. Totero Gongora, and M. Peccianti, "Terahertz Spatiotemporal Wave Synthesis in Random Systems," ACS Photonics 11, 362–368 (2024).

[7] V. Kumar, V. Cecconi, A. Cutrona, L. Peters, L. Olivieri, J. S. Totero Gongora, A. Pasquazi, and M. Peccianti, "Terahertz microscopy through complex media," Sci Rep 15, 11706 (2025).

[8] J. W. Goodman, Statistical Optics (John Wiley & Sons, 2015).

Terahertz (THz) imaging modalities offer several advantages over other imaging methods. THz radiation is noninvasive, nonionizing and does not require physical contact with the sample, making it highly suitable for nondestructive testing of materials such as polymers. While techniques like X-ray computed tomography (CT) and ultrasound are commonly used for such purposes, THz CT presents a promising alternative due to its unique properties. Most existing THz CT approaches primarily utilize the intensity of the transmitted or reflected THz radiation [1]. In contrast to traditional X-ray CT, many Terahertz systems are capable of measuring the phase of the received signal. As THz waves propagate through a sample, they experience phase delays due to variations in the refractive index of the medium. This phase information can be exploited to reconstruct the refractive index distribution, that can reveal features more clearly than intensity based reconstructions [2].

In this study we investigated a polymer pipe (Fig. 1a) using a bistatic frequency modulated continuous wave (FMCW) system operating between 230 and 320 GHz. The THz beam was collimated and focused at the sample's center. A 2 mm diameter hole—comparable to the beam spot size—was drilled into the pipe and covered with a 0.15 mm thick PVC band. The sample was scanned with a 0.5 mm translational step and a 2° rotational step. Both amplitude and phase of the received signals were recorded and used for image reconstruction using filtered backprojection. While the intensity image (Fig. 1b) failed to reveal the defect, it was clearly discernible in the corresponding phase image (Fig. 1c).

We demonstrate that THz phase tomography can reveal subtle features that are missed by tomography based on intensity measurements, emphasizing the significant potential of phase information for continuous wave THz CT.

Optoelectronic concepts feature properties that make them attractive for the realization of radar systems for the terahertz frequency range. These include frequency tunability and the ability to construct a coherent system of spatially distributed transmitters and receivers. We have demonstrated concepts for optoelectronic correlation radar that are based on modulating the optical signals within a conventional terahertz frequency-domain spectroscopy (THz-FDS) system with so-called maximum-length sequences (MLS).

In one approach, we modulate the amplitude of the terahertz signal generated by the photodiode-based transmitter by on-off keying (OOK) the light in the transmit branch with an MLS. We demodulate the received terahertz signal by measuring the photocurrent generated by the photoconductive receiver as it is driven with light that is modulated with delayed versions of the same sequence. In another approach, we adapt this concept by replacing OOK with binary phase-shift keying (BPSK). While requiring a more complex setup, BPSK provides better correlation properties for the MLS than OOK and thus facilitates the differentiation of multiple targets, especially under high path-loss conditions.

All experiments with the BPSK approach are conducted at a carrier frequency of 300 GHz with a modulation chip rate of 2.5 GChips/s. While the straightforward approach involves delaying the sequence in the receive branch by integer multiples of a chip period T_C, it is also possible to measure the photocurrent at the output of the terahertz receiver for delays that are fractions of a chip period. The measured data indicates a pulse width of 360 ps. This number is in good agreement with the output bandwidth of the arbitrary waveform generator generating the modulation signal.

This work presents a compact, wideband 90° polarization rotation transition within the gap waveguide framework, enabling conversion from a full-mode E-plane groove gap waveguide (GGW) to a half-mode H-plane GGW (HM-GGW). The structure leverages a high-impedance surface (HIS) formed by periodic pins to guide the wave and suppress leakage. In the transition, a stepwise introduction of the top groove forms the HM-GGW, where the pins act as an artificial magnetic conductor. Simulation results demonstrate low return loss, high transmission across 110–170 GHz, and clear 90° electric field rotation. This design offers a compact, fabrication-tolerant solution for polarization control in THz gap waveguide systems.