Abstract

Time-domain spectroscopy (TDS) systems are the workhorses in terahertz labs around the world, enabling applications from fundamental research to industrial non-destructive testing. While early systems filled entire optical tables and required intricate setup, state-of-the-art all fiber-coupled terahertz spectrometers have come a long way towards reliable “plug-and-play” solutions requiring little more than a few rack units of space.

However, the use of such systems outside of research labs is still impeded by their comparative bulk, mechanical sensitivity, and high cost. The main culprits are the femtosecond fiber laser and the free-space optical delay line. For more than 20 years, different alternatives based on semiconductor lasers have been investigated. Besides frequency-domain spectroscopy (FDS) systems based on a pair of single-mode lasers, so-called “cross-correlation” spectrometers (CCS) using simple multimode laser diodes or superluminescent diodes have been experimentally demonstrated.

More recently, a promising new concept using monolithic mode-locked laser diodes (MLLDs) was proposed. Since MLLDs emit stable pulse trains with sub-picosecond pulse durations and a repetition rate of a few dozen gigahertz, this approach is typically named “ultra-high repetition rate terahertz time-domain spectroscopy” (UHRR-THz-TDS). These compact, robust, and efficient systems consist mostly of standard telecom components for the 1550 nm band. They exhibit bandwidths of up to 1.5 THz and a peak dynamic range of more than 130 dB.

In this lecture, we start with a brief historical outline and a thorough look at the state of the art of optoelectronic terahertz spectroscopy. We discuss different concepts and develop an instructive classification of different available systems. We take an in-depth look at UHRR-THz-TDS and develop a simple yet accurate mathematical model. Based on that knowledge, we explore novel system concepts, current trends, and promising applications.

Lecturer’s bio

Kevin Kolpatzeck received the B.S., M.S., and Dr.-Ing. degrees in electrical engineering and information technology from the University of Duisburg-Essen (UDE), Duisburg, Germany, in 2013, 2016, and 2022, respectively, where he investigated terahertz time-domain spectroscopy systems driven by monolithic mode-locked laser diodes.

He is currently a postdoctoral researcher with the Chair of Communication Systems (NTS), UDE. His current research interests include terahertz photonics, photonic radar, beamforming at terahertz frequencies, and the use of terahertz spectroscopy for non-destructive testing.

Abstract

Spectroscopy in the THz range provides access to unique absorption features with relevance for threat detection, environmental monitoring, or search for counterfeit drugs. Whereas conventional THz optoelectronic sources like semiconductor devices or photoconductive antennas are unmatched in terms of convenience, their optical bandwidth is typically limited from tens of GHz to a few THz. This allows one to cover only a few relevant spectral features, which hinder the ability to identify the analyte. During the lecture, a different approach to THz generation and detection will be discussed. Recent advances in molecular engineering have rendered organic crystals with tailored optical nonlinearities for room-temperature THz spectroscopy with 30 THz bandwidths (up to 10 µm of wavelength). Organic crystals compatible with popular telecommunication wavelength mode-locked lasers are expected to bridge the THz range with the mid-infrared in many scenarios.

Lecturer’s bio

Łukasz A. Sterczewski is an assistant professor in the faculty of Electronics, Photonics, and Microsystems at Wroclaw University of Science and Technology in Poland. In 2023, he received the European Research Council (ERC) Starting grant, focusing on room temperature generation and detection of terahertz frequency combs emitted by chip-scale semiconductor laser structures. Between 2015–2018 Dr. Sterczewski pioneered work on semiconductor laser frequency combs and their application to broadband, high-resolution spectroscopy at Princeton University. Between 2019–2021 he was a postdoctoral researcher at NASA Jet Propulsion Laboratory working on battery-operated mid-infrared comb generators. In 2021 he returned to Wroclaw, Poland as a Marie Sklodowska-Curie fellow to explore the field of organic nonlinear optical crystals offering an ease of fabrication and spectral coverage from the long-wave infrared to the terahertz when pumped by near-infrared mode-locked laser sources.

Abstract

The terahertz (THz) frequency range (1–10 THz) holds immense potential for spectroscopy and sensing applications, as many molecules exhibit unique absorption fingerprints within this spectrum. Despite its promise for high-speed chemical imaging, disease detection, and environmental monitoring, the practical utilization of the THz range has been limited by the lack of compact, powerful, and broadband sources.

In this lecture, I will discuss recent work addressing these challenges through advancements in terahertz quantum cascade lasers (QCLs) and QCL-based frequency comb technology. By exploiting the intrinsically broad gain spectra of QCLs—engineered to cover substantial bandwidths—compact THz frequency combs emitting high powers across wide spectral ranges can be created. Central to this progress is our discovery of a new fundamental comb state that acts as the phase equivalent of passive mode-locking. This universal phenomenon manifests in various lasers across different wavelengths, enabling the phase-locking of cavity modes without external modulation and bridging the gap between narrowband lasers and broadband incoherent sources.

Building on this foundation, I will discuss how these THz QCL frequency combs enable advanced spectroscopic techniques. Specifically, I will highlight dual-comb spectroscopy and ptychoscopy as powerful applications that leverage the coherence and broad spectral coverage of these combs. Dual-comb spectroscopy allows for high-resolution, rapid, and broadband spectral measurements without moving parts, enhancing the detection of molecular signatures. Ptychoscopy, another recently developed sensing modality, enables ultra-precise optical spectrum measurements by characterizing remote signals with quantum-limited frequency resolution over the entire bandwidth of a comb. Together, these techniques could significantly improve the capabilities of THz spectroscopic systems.

Lecturer’s bio

David Burghoff is an Assistant Professor in the Chandra Department of Electrical and Computer Engineering at UT Austin, where his lab blends photonics and quantum science to develop novel sensing and computing modalities. Prior to this, he was an assistant professor at Notre Dame and a research scientist at MIT (where he also received his Ph.D.). His awards include the IRMMW-THz Society Young Scientist Award, Young Investigator Awards from the ONR, AFOSR, and NSF, the Gordon and Betty Moore Foundation’s Inventor’s Fellowship, and the J.A. Kong Award for MIT’s Best Electrical Engineering Thesis. He is also the lead investigator of the PRISM project, a Multidisciplinary University Research Initiative (MURI) project that aims to expand the limits of precision radiometry.

Abstract

The field spintronics aims at utilizing the electron’s spin degree of freedom to process information. As some conventional electronics already achieve operational speeds beyond the gigahertz range, also spintronic functionalities should ideally scale up to terahertz (THz) rates to be compatible and competitive.
In my talk, I will discuss our recent efforts to drive and detect ultrafast spin (S) currents in magnetic thin film structures. To this end, femtosecond laser pulses launch terahertz S currents from a ferromagnetic into a paramagnetic material. Subsequently, different conversion processes lead to the formation of ultrafast charge currents radiating a THz pulse [1].
I will show how laser-induced S currents allow for a rapid spintronic material characterization, detailed insights into the coupling between spins, electrons and the lattice [2], and eventually even allow one to build efficient spintronic THz emitters and detectors [3].

[1] Seifert, T., et al. “Efficient metallic spintronic emitters of ultrabroadband terahertz radiation.” Nature Photonics 10 (2016).
[2] Rouzegar, R., et al. “Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization.” Physical Review B 106 (2022).
[3] Chekhov, A. L., et al. “Broadband spintronic detection of the absolute field strength of terahertz electromagnetic pulses.” Physical Review Applied 20.3 (2023): 034037.

Lecturer’s bio

PhD 2018 Fritz-Haber Institute of the Max Planck Society; 2018-2020 Postdoc at ETH Zurich (P. Gambardella Lab); Since 2020: Senior scientist at Freie Universität Berlin (T. Kampfrath Lab) and Cofounder/CEO of TeraSpinTec GmbH

Abstract

Terahertz frequency range is still referred as being the one of the least exploited spectrum range of electromagnetic radiation. It is located between the millimeter waves and infrared with a loosely defined span from about 300 GHz to 10 THz. During the last two decades, there was a noticeable scientific interest devoted for the development of THz devices and techniques which resulted in the invention of novel sources and detectors.

The lecture will give an overview of different, novel as well as well-established detection schemes which are used to detect THz radiation and will address to the underlying physical principles, their potentials, fields of applications and limitations. It will start from the introduction to a thermal detection principle and how this principle can be applied to implement detectors operating at room temperature as well as at cryogenic temperatures and to be “trimmed” to be able to detect just few THz photons. Special attention will be given to detectors based on electric nonlinear characteristics such as Schottky diodes and field-effect-transistor based detectors (TeraFETs). Furthermore, there will be addressed the main differences between the power and amplitude detection and the state-of-the-art devices used for detection of single THz photons.

Lecturer’s bio

Abstract

Plasmons are charge density waves similar to these of sound ( air density waves) or waves on the water. Using plasmon waves in semiconductors for THz manipulation is a dream of physicists and engineers old more than 50 years  old. Some of these dreams have been realized like plasma wave THz detectors or filters. THz amplification or THz generation by plasmons is a still very challenging task.

In this lecture we will present  the basic ideas followed by an overview of most important results and current research activities on THz plasmonic devices. We will talk also on recent extensive study of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures, which enable an electrical control of periodic charge carrier density profile. We will show how  main terahertz (THz) plasmonic resonances in these structures can be explained only within the framework of the plasmonic crystal model. We will talk also on  the pioneer work on THz amplification by graphene grating gates the plasma resonances.  

Finally we will present the International research Agenda Project and Laboratory CENTERA encouraging young engineers and scientists to join one of the strongest EU team in Terahertz Science and Technology operating since a few years  in Warsaw Technical University – CEZAMAT.

Lecturer’s bio

W. Knap obtained his master and PhD degrees from Faculty of Physics – Warsaw University Poland  working in  Experimental Solid State Physics Department on Terahertz (Far infrared) properties of narrow gap semiconductors HgTe and InSb.  In 1987 he left to France and worked at University of Montpellier, Grenoble High Magnetic Field Laboratory, Toulouse Pulsed High Magnetic Field Laboratory and Montpellier  University. Between 1999 and 2001 he worked at– Rensselaer Polytechnic Institute USA and between 2007 and 2008 at  Tohoku University Japan. He currently leads International Research Agenda -CENTERA project and ERC advanced grant on THZ plasmonics – TERAPLASM  associated  with Warsaw Technical University and  Polish Academy of Sciences respectively. His main scientific  interests are : i) Fair Infrared-FIR (Terahertz – THz) properties of semiconductors, and ii)Terahertz Plasma excitations in nanostructures.

Abstract

In recent years, the increased research has been conducted in the field of terahertz (THz) optics and imaging. The description that explores the landscape of THz optics, focusing on its achievements, current challenges, and prospects is given in this lecture. THz radiation, characterized by wavelengths considerably longer than visible light, induces substantial diffraction effects, profoundly impacting its behavior and imaging capabilities with optical elements. Moreover, the high coherence exhibited by various THz sources facilitates precise wave manipulation. However, it also introduces unwanted interference effects, which are challenging to suppress. Moreover, in many cases, THz optical systems operate within the near-field diffraction zone, which has its peculiarities.

The advancement of THz optics is closely related to exploring various materials and manufacturing techniques. Different materials, ranging from dielectrics to semiconductors, exhibit excellent optical properties in the THz range. Furthermore, innovative manufacturing methods such as lithography, additive manufacturing, and metamaterial engineering play crucial roles in developing novel THz optics.

This lecture highlights various achievements, current challenges, and promising avenues in the field of THz optics. Emphasizing its versatile applications and the role of material science and manufacturing innovation underscores the transformative potential of THz technology in shaping future advancements.

Lecturer’s bio

Abstract

Room temperature terahertz (THz) imaging is a powerful tool for a non-destructive inspection for different types of dielectrical materials, security checks, or medical applications [1]. However, for practical implementation in real environment conditions THz imaging still experiences challenges because of low powers of THz emitters, reliability of sensitive THz detectors and compact solutions in the design of passive optical components. A particular focus needs to be attributed to the development of compact imaging systems entailing enhanced functionality, reduced power consumption, and increased convenience in use [1].

The given lecture covers possible routes for rational design of compact and effective THz multispectral imaging. Principles of room temperature semiconductors-based THz emitters and detectors will be described; silicon diffractive optics-based THz light engineering, enabling thus both compact focusing, extended focus geometry and structured light application in THz imaging will be considered [2]. Special attention will be attributed to lensless nonparaxial THz photonic setups and their features, peculiarities in design and operational advantages. Possible technological challenges as well as extrapolations of possible further evolution in compact THz imaging will be given as well.

[1] G. Valušis, A. Lisauskas, H. Yuan, W. Knap, and H. G. Roskos, Roadmap of Terahertz Imaging 2021, Sensors 21, 4092 (2021).
[2] R. Ivaškevičiūtė-Povilauskienė, P. Kizevičius, E. Nacius, D. Jokubauskis, K. Ikamas, A. Lisauskas, N. Alexeeva, I. Matulaitienė, V. Jukna, S. Orlov, L. Minkevičius, G. Valušis, Light: Science & Applications 11, 1 326 (2022).
[3] S. Orlov, R. Ivaškevičiūtė‐Povilauskienė, K. Mundrys, P. Kizevičius, E. Nacius, D. Jokubauskis, K. Ikamas, A. Lisauskas, L. Minkevičius, and G. Valušis, Laser Photon. Rev. 18, 2301197 (2024).

Lecturer’s bio