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Controlling the properties of mid- and far-infrared radiation can provide a means to transiently alter the properties of materials for novel applications. However, a limited number of optical elements are available to control its polarization state. Here we show that a 15-µm thick liquid crystal cell containing 8CB (4-octyl-4′-cyanobiphenyl) in the ordered, smectic A phase can be used as a phase retarder or wave plate. This was tested using the bright, short-pulsed (∼1 ps) radiation centered at 16.5 µm (18.15 THz) that is emitted by a free electron laser at high repetition rate (13 MHz). These results demonstrate a possible tool for the exploration of the mid- and far-infrared range and could be used to develop novel metamaterials or extend multidimensional spectroscopy to this portion of the electromagnetic spectrum.

Field-effect transistors are capable of detecting electromagnetic radiation from less than 100 GHz up to very high frequencies reaching well into the infrared spectral range. Here, we report on frequency coverage of up to 30THz, thus reaching the technologically important frequency regime of CO2 lasers, using GaAs/AlGaAs high-electron-mobility transistors. A detailed study of the speed and polarization dependence of the responsivity allows us to identify a cross over of the dominant detection mechanism from ultrafast non-quasistatic rectification at low Terahertz frequencies to slow rectification based on a combination of the Seebeck and bolometric effects at high frequencies, occurring at about the boundary between the Terahertz frequency range and the infrared at 10THz.

The free-electron laser (FEL) is an ideal source of high-power coherent THz radiation for many applications. The FEL provides continuously tunable THz radiation, typically in ultrashort transform limited pulses with very high peak power. One limitation though is that the carrier envelope phase (CEP) of the FEL pulses is not fixed, making measurements of coherent THz-driven processes extremely difficult, if not impossible. We present here, a novel technique that enables shot-to-shot measurement of the CEP of every FEL pulse up to very high repetition rates. This powerful technique enables phase-resolved measurements of the FEL pulses, which opens the door for the study of coherent THz-driven phenomena as well as the advanced FEL diagnostics needed in the pursuit of CEP stable operation of an FEL.

The coherent infrared and THz sources driven by the superconducting electron accelerator ELBE are described. The present status of the facility is summarized and a few scientific highlights are mentioned. Finally plans for a successor facility (Dresden Advanced Light Infrastructure, DALI) are outlined along with the most important scientific and technological challenges.

This presentation will provide an overview of the coherent THz sources and the user activities at the ELBE Radiation Source. Ideas for new types of measurements and materials to study will be discussed, followed by a look toward DALI, the concept for a new facility at HZDR for advanced accelerator-based THz sources.

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the average charge carrier concentration and mobility in NWs is enabled by terahertz time domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photoexcited carriers has been investigated with 10 nm and 10 fs spatial and temporal resolution [2]. Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by THz radiation via intraband excitation. Here we report on THz-pump MIR-probe s SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband THz radiation from the free-electron laser (FEL) FELBE. The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and an 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies, these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate. The experiment was carried out with an s-SNOM setup from Neaspec GmbH equipped with a broadband difference-frequency generation (DFG) source (5 – 15 µm; 20 – 60 THz). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to the FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppresses the scattered THz FEL radiation from the nano-FTIR unit (Fig 1a). In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon intraband pumping with 13THz FEL radiation (pulse duration 2 –

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the charge carrier concentration and mobility in NWs is enabled by terahertz time-domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering-type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photogenerated carriers has been investigated on the 10 nm and 10 fs scale [2]. Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by far-infrared (FIR) radiation via free-carrier absorption. Here we report on FIR-pump MIR-probe s-SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband FIR radiation from the free-electron laser (FEL) FELBE. The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and a 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate. The experiment was carried out with the s-SNOM setup from Neaspec GmbH equipped with difference-frequency generation (DFG) source (5 – 15 µm; 83 – 248 meV). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppress the scattered FIR radiation from FELBE going into the nano-FTIR unit (Fig 1,a). In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon below-bandgap pumping with 23 µm FEL radiation (pulse duration 2 – 5 ps, averag

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the average charge carrier concentration and mobility in NWs is enabled by terahertz time domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photoexcited carriers has been investigated with 10 nm and 10 fs spatial and temporal resolution [2]. Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by THz radiation via intraband excitation. Here we report on THz-pump MIR-probe s SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband THz radiation from the free-electron laser (FEL) FELBE. The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and an 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies, these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate. The experiment was carried out with an s-SNOM setup from Neaspec GmbH equipped with a broadband difference-frequency generation (DFG) source (5 – 15 µm; 20 – 60 THz). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to the FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppresses the scattered THz FEL radiation from the nano-FTIR unit (Fig 1a). In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon intraband pumping with 13THz FEL radiation (pulse duration 2 –

Radiation sources with a stable carrier-envelope phase (CEP) are highly demanded tools for field-resolved studies of light-matter interaction, providing access both to the amplitude and phase information of dynamical processes. At the same time, many coherent light sources, including those with outstanding power and spectral characteristics lack CEP stability, and so far could not be used for this type of research. In this work, we present a method enabling linear and non-linear phase-resolved terahertz (THz) -pump laser-probe experiments with CEP-unstable THz sources. THz CEP information for each pulse is extracted using a specially designed electro-optical detection scheme. The method correlates the extracted CEP value for each pulse with the THz-induced response in the parallel pump-probe experiment to obtain an absolute phase-resolved response after proper sorting and averaging. As a proof-of-concept, we demonstrate experimentally field-resolved THz time-domain spectroscopy with sub-cycle temporal resolution using the pulsed radiation of a CEP-unstable infrared free-electron laser (IR-FEL) operating at 13 MHz repetition rate. In spite of the long history of IR-FELs and their unique operational characteristics, no successful realization of CEP-stable operation has been demonstrated yet. Being CEP-unstable, IR-FEL radiation has so far only been used in non-coherent measurements without phase resolution. The technique demonstrated here is robust, operates easily at high-repetition rates and for short THz pulses, and enables common sequential field-resolved time-domain experiments. The implementation of such a technique at IR-FEL user end-stations will facilitate a new class of linear and non-linear experiments for studying coherent light-driven phenomena with increased signal-to-noise ratio.

The dynamical complexity of the hydrogen-bonded water network can be investigated with intense Terahertz (THz) spectroscopy, which can drive the liquid into the nonlinear response regime and probe anharmonicity effects. Here we report single-color and polarization-dependent pump-probe experiments at 12.3 THz on liquid water, exciting the librational mode. By comparing results obtained on a static sample and a free-flowing water jet, we are able to disentangle the distinct contributions by thermal, acoustic, and nonlinear optical effects. We show that the transient transmission by the static water layer on a time scale of hundreds of microseconds can be described by thermal (slow) and acoustic (temperature-dependent) effects. In addition, during pump probe overlap we observe an anisotropic nonlinear optical response. This nonlinear signal is more prominent in the liquid jet than in the static cell, where temperature and density perturbations are more pronounced. Our measurements confirm that the THz excitation resonates with the rotationally-damped motion of water molecules, resulting in enhanced transient anisotropy. This model can be used to explain the non-linear response of water in the frequency range between about 1 and 20 THz.

The FELBE User Facility at the ELBE Center for High-Power Radiation Sources offers a pair of FELs that deliver beam to eight different user labs. The FELs are driven by a two-stage Superconducting RF (SRF) linac, which produces a quasi-CW beam (13 MHz/1 mA) at an energy of up to 36 MeV. The tuning range spanned by the two FELs extends from the mid IR to THz (5 – 250 m). The spectral range and ultrashort pulse width (p ≈ 0.7 – 25 ps) are ideal for time-resolved measurements of many types of transient processes in low-dimensional materials [1], quantum structures [2], and correlated systems [3]. The high pulse energy can also drive nonlinear phenomena [4] and strong coupling [5] in light-matter interactions. The FELBE User Labs are equipped with instrumentation and synchronized ultrashort table-top lasers (i.e. Ti:Sa oscillators, regens, OPAs, SFG/DFG) which facilitate various classes of degenerate (single-color), and non-degenerate (two-color) pump-probe experiments. Optical cryostats and an 8 T split coil magnet are also available for low temperature and magnetic field dependent studies. Furthermore, the FELBE beamline extends into the adjacent High Field Magnet Lab (HLD) for performing magneto-optical spectroscopy measurements at fields up to 70 T [6]. The high repetition rate and tunability of the FELBE beam has uniquely enabled revolutionary methods in scattering-Scanning Nearfield Optical Microscopy (s­SNOM) to image novel light-matter interactions with resolution far below the diffraction limit [7]. Proposals for beamtime on FELBE and the other secondary sources at ELBE are invited from users twice a year. (https://www.hzdr.de/FELBE). [1] T. Venanzi, et al., ACS Photonics 8, 2931-2939 (2021). [2] J. Schmidt, et al., Optics Express 28, 25358-25370 (2020). [3] M. M. Jadidi, et al., Phys. Rev. B 102, 245123 (2020). [4] F. Meng, et al., Phys. Rev. B 102, 075205 (2020). [5] B. Piętka, et al., Phys. Rev. Lett. 119, 077403 (2017). [6] M. Ozerov, et al., Phys. Rev. L

The heart of the ELBE Center for High Power Radiation Sources is a superconducting RF (SRF) linac, which accelerates electrons up to 35 MeV for driving a diverse set of secondary radiation sources. The ELBE linac is particularly unique in the capability of accelerating a continuous beam of ultrashort bunches of electrons at very high repetition rates (up to 26 MHz). This extremely high-power electron beam is selectively directed into specially designed beamlines to drive secondary radiation sources for THz/IR photons (FELBE and TELBE), positrons (pELBE), neutrons (nELBE), and gamma radiation (ELBE), each with dedicated laboratories and instrumentation. Users from all over the world utilize the advanced radiation sources at ELBE for a wide array of both fundamental and applied studies of matter, health, energy, and technology. The ELBE accelerator was first commissioned in 2001 in the form of a grid-pulsed 250 kV thermionic gun followed by two stages of RF bunching to inject beam into a linac comprised of two SRF cryomodules, each containing two 9-cell DESY TTF-type niobium accelerating cavities. Through continuous development and research, the ELBE facility has achieved many major advancements in accelerator technology, the most important being the ELBE SRF Gun program, which has designed, built, and commissioned several prototype SRF electron guns for high bunch charge, high average current, and low emittance. The ELBE SRF Gun-II is the first and only electron source of its type to deliver electron beam to a user experiment, and is now in routine operation for the TELBE and pELBE beamlines. An overview of the performance parameters of the ELBE accelerator and secondary sources will be presented in this talk along with a summary of the experimental capabilities available to users. Highlights of recent user results will also be presented to help illustrate the great potential ELBE provides for a diverse scientific community. Beamtime proposals are accepted and revie

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the charge carrier concentration and mobility in NWs is enabled by terahertz time-domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering-type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photogenerated carriers has been investigated on the 10 nm and 10 fs scale [2]. Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by far-infrared (FIR) radiation via free-carrier absorption. Here we report on FIR-pump MIR-probe s-SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband FIR radiation from the free-electron laser (FEL) FELBE. The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and a 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate. The experiment was carried out with the s-SNOM setup from Neaspec GmbH equipped with difference-frequency generation (DFG) source (5 – 15 µm; 83 – 248 meV). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppress the scattered FIR radiation from FELBE going into the nano-FTIR unit (Fig 1,a). In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon below-bandgap pumping with 23 µm FEL radiation (pulse duration 2 – 5 ps, averag

We demonstrate an optical switch based on gold implanted germanium (Ge:Au) suitable for cavity dumping of a free-electron laser (FEL). We achieve a switching contrast of more than 50 % in a broad range of FEL wavelengths from 6 to 90 µm. A linear relationship between the switching fluence and the frequency of the FEL optical field supported by our simulation highlights the role of a photoinduced finite sub-µm thickness of the reflecting plasma layer. The plasma switch exhibits negligible absorption of the FEL radiation in the ʻoffʼ state and requires only a moderate thermoelectric cooling at incident FEL power of several Watts.

We present a plasma switch based on gold implanted germanium (Ge:Au) as a potential candidate for efficient cavity dumping of a free-electron laser (FEL). Ge:Au has a sub-nanosecond carrier lifetime – much shorter than the FEL pulsing period of 77 ns – and demonstrates a high photoinduced reflectivity in a broad range of infrared wavelengths from 6 to 90 µm. The Ge:Au plasma switch exhibits negligible absorption of the FEL radiation in the ʻoff ʼ state and requires only moderate thermoelectric cooling for incident FEL power of several Watts. A reflectivity level of more than 50 % in the ‘on’ state is achieved over the entire spectral range of this study. The corresponding optical pump fluence exhibits a linear relationship with the FEL frequency. This scaling is corroborated by our simulations highlighting the role of a finite sub-µm thickness of the photoinduced reflecting plasma layer. The demonstrated device is promising for the realization of the FEL cavity dumping for experiments that simultaneously require higher pulse energy and lower average power.

Hyperbolic phonon polaritons have recently attracted considerable attention in nanophotonics mostly due to their intrinsic strong electromagnetic field confinement, ultraslow polariton group velocities, and long lifetimes. Here we introduce tin oxide (SnO2) nanobelts as a photonic platform for the transport of surface and volume phonon polaritons in the mid- to far-infrared frequency range. This report brings a comprehensive description of the polaritonic properties of SnO2 as a nanometer-sized dielectric and also as an engineered material in the form of a waveguide. By combining accelerator-based IR-THz sources (synchrotron and free-electron laser) with s-SNOM, we employed nanoscale far-infrared hyperspectral-imaging to uncover a Fabry–Perot cavity mechanism in SnO2 nanobelts via direct detection of phonon-polariton standing waves. Our experimental findings are accurately supported by notable convergence between theory and numerical simulations. Thus, the SnO2 is confirmed as a natural hyperbolic material with unique photonic properties essential for future applications involving subdiffractional light traffic and detection in the farinfrared range.

We present a plasma switch based on gold implanted germanium (Ge:Au) as a potential candidate for efficient cavity dumping of a free-electron laser (FEL). Ge:Au has a sub-nanosecond carrier lifetime – much shorter than the FEL pulsing period of 77 ns – and demonstrates a high photoinduced reflectivity in a broad range of infrared wavelengths from 6 to 90 µm. The Ge:Au plasma switch exhibits negligible absorption of the FEL radiation in the ʻoff ʼ state and requires only moderate thermoelectric cooling for incident FEL power of several Watts. A reflectivity level of more than 50 % in the ‘on’ state is achieved over the entire spectral range of this study. The corresponding optical pump fluence exhibits a linear relationship with the FEL frequency. This scaling is corroborated by our simulations highlighting the role of a finite sub-µm thickness of the photoinduced reflecting plasma layer. The demonstrated device is promising for the realization of the FEL cavity dumping for experiments that simultaneously require higher pulse energy and lower average power.

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge3Sb2Te6 (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we employ that far-infrared (THz) SNOM is more sensitive to free charge carriers compared to mid-infrared SNOM and prove that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

Measurements were carried out in ELBE 113 c lab. Free electron laser (FEL) pulses with a wavelength ranging from 6 to 90 µm from the FELBE laser operating at its maximum power level were used. Photoswitching of Ge and Ge:Au samples were carried out using a 1 kHz Ti:Sapphire Amplifier system at 800 nm. The data used in the paper is arranged in subfolders 1, 2, and 3. The description is as follows 1. Scheme of the experiment (used in Figure 1 and shows the idea of the experiment) 2. FEL pulse picking traces from photoswitched Ge and Ge: Au samples were recorded using an oscilloscope with 1 GHz bandwidth (Keysight InfiniiVision DSOX4104A). Relevance: This data is used in Figures 2 and 3. This measurement showed the photoswitching attributes in both reflection and transmission geometry for the Ge and Ge: Au samples and suitability of Ge:Au sample for efficient cavity dumping. 3. Photoinduced reflectivity measurements for the Ge: Au sample as a function of the time delay between FEL pulses and the pulses from a kHz amplifier system is recorded. The fluence required to couple out FEL wavelengths with an efficiency of at least 50 % reflectivity and Matlab scripts used for simulation of photoinduced reflectivity for the same FEL wavelengths. Relevance: This data is used in Figure 4 and shows the fluence requirements of the Ge:Au photoswitch and also a faster recovery time of sub-ns, much shorter than the FEL pulsing period of 77ns.

Multiferroic BiFeO3 (BFO) shows several phonon modes at infrared (IR) to THz energies, which are expected to carry information on any sample property coupled to crystal lattice vibrations. While macroscopic IR studies of BFO are often limited by single-crystal size, scattering-type scanning near-field optical microscopy (s-SNOM) allows for IR thin film spectroscopy of nanoscopic probing volumes with negligible direct substrate contribution to the optical signal. In fact, polaritons such as phonon polaritons of BFO introduce a resonant tip–sample coupling in s-SNOM, leading to both stronger signals and enhanced sensitivity to local material properties. Here, we explore the near-field response of BFO thin films at three consecutive resonances (centered around 5 THz, 13 THz, and 16 THz), by combining s-SNOM with a free-electron laser. We study the dependence of these near-field resonances on both the wavelength and tip–sample distance. Enabled by the broad spectral range of the measurement, we probe phonon modes connected to the predominant motion of either the bismuth or oxygen ions. Therefore, we propose s-SNOM at multiple near-field resonances as a versatile and very sensitive tool for the simultaneous investigation of various sample properties.

The results of experiments aimed at the observation of split 1s states in Mg-doped Si are reported. From the results, it is possible to determine the chemical shift and exchange interaction energy of a neutral Mg donor in Si. The position of the 1s(E), 1s(T2), and 2s(A1) parastates determines the possibility for attaining population inversion and the specific mechanism of stimulated Raman scattering. The energy of the 1s(T2) parastate is determined from the position of the Fano resonances in the photoconductivity spectrum of Si:Mg at T = 4 K, and the energies of the 1s(T2) and 1s(E) orthostates from the transmittance spectra at elevated temperatures. On the basis of the experimental data, the relaxation rates are estimated, and the possible mechanisms of stimulated emission are analyzed.

The results of experiments aimed at the observation of split 1s states in Mg-doped Si are reported. From the results, it is possible to determine the chemical shift and exchange interaction energy of a neutral Mg donor in Si. The position of the 1s(E), 1s(T2), and 2s(A1) parastates determines the possibility for attaining population inversion and the specific mechanism of stimulated Raman scattering. The energy of the 1s(T2) parastate is determined from the position of the Fano resonances in the photoconductivity spectrum of Si:Mg at T = 4 K, and the energies of the 1s(T2) and 1s(E) orthostates from the transmittance spectra at elevated temperatures. On the basis of the experimental data, the relaxation rates are estimated, and the possible mechanisms of stimulated emission are analyzed.