Plasmonic structures like ribbons or disks can strongly enhance the light-matter interaction of two-dimensional electron gases at resonance. This can be exploited to fabricate devices with improved performance. In addition, also the optical nonlinear properties of the material is enhanced. Here we present a series of pump-probe measurements on various samples, mostly based on graphene, to disentangle the different effects. Besides the optical nonlienarities, circular plasmons can be excited with circularly polarized radiation, resulting in a uni-polar magnetic field in the vicinity of the plasmonic element, due to the inverse Faraday effect. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project No. 278162697-SFB 1242.

We report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and demonstrate that these hybrid THz cavities combine high quality factors of up to Q∼37 with a deep-subwavelength mode volume of V=3.2x10−4λ3. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

Bismuth chalcogenides have emerged as an intriguing option in nanophotonics due to their unique properties, including a significant dielectric function and topologically protected surface states in the terahertz (THz) range. Our study focuses on the optical response of Bi2Se3 topological-insulator nanospheres, where topological properties arise, influencing electromagnetic modes and field enhancement. Within a fully electrodynamic picture, we explore the emergence of unreported magnetic modes induced by Dirac plasmon polaritons resulting from terahertz photon and Dirac electron interactions. We observe their profound impact on electric and magnetic transitions of quantum emitters near Bi2Se3 nanospheres, resulting in record - high Purcell factors. Our research highlights the rich optical response of Bi2Se3 nanospheres, involving contributions from both phonon polaritons and topologically protected surface states. Our findings confirm the emergence of topological optical modes in Bi2Se3 spherical TIs, positioning them as promising candidates for advancements in strong light-matter interactions in topological nanophotonics and THz technologies.

In terahertz system design, the integration of reconfigurable components, and especially switches, is indispensable for routing terahertz signal flow. While on-off switching in the terahertz range has been demonstrated in various topologies, the realization of 1-to-N switching has remained unexplored. This significant gap in dynamically reconfigurable routing capabilities has fundamentally limited the achievable complexity of terahertz systems. To address this, we present 1-to-N switches using cascaded disk resonators in an integrated substrateless silicon waveguide platform. These switches are individually controlled through photoexcitation, achieved by directing a low-power 658-nm laser onto the disk resonator. This action suppresses the resonance and impedes signal coupling into a specific output port. Furthermore, we integrate a 1-to-3 switch monolithically together with a Luneburg lens to realize beam switching. This integration is a major step in terahertz technology, as it is the first demonstration of dynamic reconfigurable beam control in a low-loss terahertz integration platform, opening up promising possibilities for sensing, imaging, and communications.

Terahertz lightwave scanning tunneling microscopy (THz-STM) enables the imaging of ultrafast dynamics of elementary excitations on surfaces with angstrom spatial and THz-sub-cycle temporal resolution. Spintronic THz emitters (STE) are attractive sources for THz-STM because they allow the generation of ultra-broadband single-cycle THz pulses with easy polarity and polarization control by magnetic fields. However, the ultrathin metal film of the STE makes it highly sensitive to laser-induced heating, limiting the THz fields achievable with multi-watt laser systems operating at MHz repetition rates as required for THz-STM. We show that average power heating can be effectively reduced by rotating the STE at sub-kHz speed. The rotating STE can be operated at optimal excitation fluences of ~0.1-1 mJ/cm2 using laser pulses of a few tens of µJ energy and up to 18 W power at MHz repetition rates collimated in a pump spot of a few mm diameter, resulting in power densities that would normally cause immediate thermal damage to the STE. Finally, we highlight the ability of the STE-driven THz-STM to drive plasmonic STM-luminescence and to probe ultrafast electron and phonon dynamics in 1T-TaS2.

We present photoconductive emitters for broadband THz emission based on Ge and GeSn. Ge is attractive for this purpose due to its high carrier mobility and because of the absence of infrared-active phonons that suppress the THz emission. Emitters based on Ge, with Au trapping centers for carrier lifetime reduction, excited with 11 fs near-infrared pulses emit gapless THz spectra extending up to 70 THz. For excitation at the telecom wavelength 1550 nm a slight reduction of the bandgap of Ge is desirable. We show that this can be achieved in layers of GeSn with 2 % of Sn, grown by molecular beam epitaxy on Si. THz devices on a silicon platform offer the perspective to develop CMOS compatible THz systems with photonic integration both for the near-infrared radiation and the broadband THz radiation.

Spintronic THz emitters, consisting of Ta/Co/Pt trilayers patterned into rectangles of size 10 µm, have been integrated in planar electromagnetic antennas of various types (dipole, bow-tie, spiral). Antenna dimensions and shapes have been optimized with the help of electromagnetic simulations so as to maximize antenna efficiency in both narrow-band and broad-band geometries at/around 1 THz. The THz emission has been characterized using a pump-probe free space electro-optic sampling set-up, both for a single emitter geometry and for arrays of emitters. An increase of the detected THz signal is observed for all antenna geometries, with an enhancement ratio up to a factor of ten, together with changes of the emission bandwidth consistent with simulated characteristics.

In this work, we performed temperature-dependent studies of the THz transient amplitude FeCo waveformes, from a FeCo/graphene nanobilayer sample, triggered by fs pulsed laser in the 80–400 K range. We show that a due-twofold extension, in the range 80–300 K the amplitude increases with temperature and tends to saturate above this range. This dependence contrasts sharply with the temperature dependence of the FeCo film's magnetization, which shows a typical ferromagnetic (FM) trend with Curie temperature well above 400 K. We explain this discrepancy, as the presence of an antiferromagnetic (AFM) at the FeCo/graphene interface, which is associated with the native oxide formed at the FeCo surface. The Angle Resolved X-ray Photoelectron Spectroscopy studies of a bare FeCo film revealed coexistence of the metallic [Co(0), Fe(0)] and antiferromagnetic [Co(II)O and the Fe(III)2O3] phases, at the sample surface. The observation of the exchange bias in our magnetization hysteresis loop of a FeCo film confirms presence of an FM/AFM interface layer at the FeCo surface. In summary, we conclude that the temperature dependence of the THz transient amplitude is governed by the AFM phase.

THz photonics-based sources are attractive as they offer room-temperature solutions that rely on mature photonics technology and provide broadband tunability and large modulation bandwidth to address specific THz applications such as high-data-rate communications or spectroscopy. We will present an overview of our recent results on coherent and structured light emitted from III-V semiconductor lasers and we will focus on THz generation based on these original near-infrared lasers operating at 1064 nm. Vertical external-cavity surface-emitting lasers that exploit parity symmetry breaking together with integrated meta-surfaces can generate unconventional light states such as vortex light, spatially modeless laser, transverse multiplexing, non-linear structured light... Coherent THz emission has been obtained from a dual-mode laser, that operates simultaneously on two Laguerre-Gauss transverse modes, using either uni-traveling-carrier photodiodes and plasmonic photo-conductive antennas. We will discuss the ongoing work towards multiplex structured coherent photonic sources that offer high potential for powerful THz emission.

Structures, dynamics, and phenomena in the chemical and biological sciences are based on equilibrium descriptions in the absence of an external stimulus. However, an ever-growing swathe of processes involving energetic stimuli – for example, light excitation in photocatalysis or strong-field THz radiation –drive these systems out of equilibrium to render new states or behavioral dynamics along with new properties in health-related applications. In this talk, we will anchor ultrafast terahertz excitation as a generalized approach to theoretically and experimentally study basic nonequilibrium states of water and water-based systems to orchestrate solvation (bio)chemistry to capable of tackling real-world societal challenges, such as paving the path to water remediation of environmental pollutants and understanding biological function of proteins under external stimuli for personalized medicine.

As continuous wave (cw) THz spectroscopy advances rapidly, its high potential for sensing and non-destructive testing is becoming increasingly apparent. This is demonstrated in two recent developments: First, I will present our novel terahertz receivers for frequency-domain spectroscopy based on rhodium-doped InGaAs grown by molecular beam epitaxy. These new devices provide a peak dynamic range of 130 dB, an 18 dB improvement over the state of the art. Secondly, I will present a compact terahertz spectroscopy system with a measurement rate of 1 kHz, made possible by a single photonic integrated circuit that acts as the optical driver engine. These new results demonstrate the excellent performance and flexibility of frequency-domain spectroscopy, paving the way for compact and task-specific terahertz systems for science and industry.

Overcoming technical performance limitations in the detection and characterization of terahertz (THz) radiation will enable ground-breaking scientific advances from the study of fast and non-reproducible phenomena to enabling THz quantum applications that require single-photon sensitivity. Electro-optic sampling techniques intrinsically rely on the acquisition of multiple data points to reconstruct the full THz waveform, which leads to long data acquisition times, and prevents the detection of single photons. We have developed two distinct and highly sensitive detection techniques for pulsed THz radiation: i) a single-pulse measurement which employs chirped-pulse spectral encoding and a dispersive Fourier transform method for time-resolved THz spectroscopy at a demonstrated rate of 50 kHz; and ii) a single-THz-photon detection technique based on parametric frequency conversion and single-photon counting technology capable of detecting THz pulses at the zeptojoule level. These extreme detection schemes will lay the foundation for THz applications in the single-pulse and single-photon regimes.

Over the past three decades, science community has seen development and emergence of terahertz technologies. This frequency range which is halfway between optical and radio wavelength is very interesting since most of materials show transparency in this band. Furthermore, emission et detection techniques of such rays allow to get amplitude and phase of the signal. In this paper we propose to exploit these properties with a terahertz bench that can acquire signals reflected and transmitted from a laminated sample with different incident angles.

Terahertz non-destructive testing offers a highly attractive solution for inline testing of electrode thicknesses in battery production for electric vehicles. Measuring systems with high spectral bandwidths are required to address the thin layers of typically less than 100 μm. In addition, multiple measuring heads are desired at different location on a production line to ensure adequate control at high throughput. We solve this by means of a highly scalable photonic terahertz radar. Its measuring principle is based on frequency-modulated continuous wave technology in conjunction with two-color laser radiation. The number of measuring heads can be easily scaled through the use of laser amplifiers. Another advantage of a photonic continuous wave system is the simple possibility of distributing fiber-coupled measuring heads over long fiber lengths of even more than 100 m. In this article, we show the potential of the system concept by the implementation of an 8-channel system and demonstration of relevant thickness measurements.

Terahertz time-domain spectroscopy (THz-TDS) systems enable a wide range of applications from fundamental research to industrial non-destructive testing. However, the comparative bulk, mechanical sensitivity, and high cost have hindered their widespread use in applications outside of research labs. Solutions for constructing more compact and lower cost THz-TDS systems based on semiconductor lasers have been around for more than 20 years. However, their performance has lagged far behind conventional systems. A type of light source that promises to fill this gap are monolithic mode-locked laser diodes (MLLDs). These sources emit stable pulse trains in the 1550 nm telecom band with sub-picosecond pulse durations and a repetition rate of a few dozen gigahertz. In this contribution, we review the theoretical background of THz-TDS using MLLDs and the steps that are necessary for achieving good system performance. We discuss promising applications and present first results as well as suitable data evaluation techniques.

We present the application of THz monocycles to the Atom Probe Tomography (APT), an analytical microscope that allows the three-dimensional mapping of chemical heterogeneities in a material at the atomic scale. When a positive electric field of several volts per angstrom is applied to the surface of a material, the surface atoms evaporate as ions even at cryogenic temperatures, this is the work principle of APT. We prove that THz transient can induce the controlled evaporation of surface atoms due to the strong increase in the THz field in the near field of the sample. In addition, the use of THz pulses reduces the thermal effects reported when using laser pulses in the visible or near ultraviolet domain. We are also studying the effect of the THz pulses on the energy of the evaporated ions.