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Thin films of antiferromagnetic insulators (Cr2O3, NiO etc.) are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A standard micromagnetic approach for the description of such thin films relies on the effective parameters being homogeneously distributed along the film thickness. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the magnetomechanical coupling promises technological advantages: the cross-coupling between elastic, magnetic and electric subsystems opens additional degrees of freedom in the control of the respective order parameters [1, 2, 3]. In this work, we discover the presence of flexomagnetic effects in epitaxial Cr2O3[4]. We demonstrate that a gradient of mechanical strain affect the order-disorder magnetic phase transition resulting in the distribution of the Neel temperature along the thickness of a Cr2O3 film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the strained films can persist up to 100°C, rendering Cr2O3 as a prospective material for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

We investigate the coherent coupling of metamaterial resonators with hydrogen-like boron acceptors in Si at cryogenic temperatures. When the resonance frequency of the metamaterial, chosen to be in the range 7–9 THz, superimposes the transition frequency from the ground state of the acceptor to an excited state, Rabi splitting as large as 0.4 THz is observed. The coherent coupling shows a feature of cooperative interaction, where the Rabi splitting is proportional to the square root of the density of the acceptors. Our experiments may help to open a possible route for the investigation of quantum information processes employing strong coupling of dopants in cavities.

We observe third-harmonic generation (THG) and fifth-harmonic generation (FHG) of free holes in the valence band of bulk Si:B at cryogenic temperature upon irradiation with intense terahertz pulses from a free-electron laser, polarized along the Γ−X direction. The intensities of both signals increase as a function of pulse energy following power laws with respective exponents of 4.2 and 6.2, larger than the exponents of 3 and 5 expected for xi(3) and xi(5) processes with constant susceptibilities and a fixed number of holes. The larger values are attributed to the increase in the density of mobile holes, which results from impact ionization of boron dopants by thermally activated holes in the electric field of the terahertz pulses as already observed in our studies of THG in Si:B reported in Phys. Rev. B 102, 075205 (2020). We apply Monte Carlo simulations of the nonlinear heavy- and light-hole field response, coupled with a finite-difference time-domain treatment of the pump-pulse propagation in the sample, which reproduce the experimental THG:FHG intensity ratio reasonably well. An analysis of the local response demonstrates that, in our pump regime, the harmonic generation is dominated by the band anharmonicity as opposed to the energy-dependent momentum scattering rates or interband scattering. Comparison with the results of a simple one-dimensional model and scrutiny of the three-dimensional band structure shows that one must account for the extent of the carrier distribution transverse to the Γ−X axis as the anharmonicity grows rapidly away from this axis.

We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current and may lead to device breakdown. We manage to increase the maximum bias field before device breakdown by a factor of 3 under illuminated conditions. For a laser system with 250-kHz repetition rate, the terahertz emission amplitude increases linearly with applied bias field up to 120 kV/cm bias field, which results in 3 times higher signal as compared to the standard device. Furthermore, we have also achieved this expanded breakdown prevention at 78-MHz repetition rate, where an integrated on-chip resistance leads to an enhancement of the terahertz field amplitude by 70%. This simple technique can increase the performance of almost all photoconductive terahertz emitters by using appropriate resistances according to the emitter capacitance and laser repetition rate.

Here we report a pronounced redshift of the trion resonance in MoSe 2 monolayer induced by picosecond THz pulses. The kinetic energy gained by hot carriers due to absorption of THz radiation is transferred during the formation of the trion, leading to an effective redshift of its optical transition. By performing time resolved measurements, we have determined the electron cooling time (τ = 70 ps) and estimated the absorption at 7.7 THz (α = 0.3%). A quantitative model based on the Heisenberg equation of motion explains the experimental observations and reproduces the data with good accuracy.

We have studied the THz generation in DSTMS using a quasi-3D model that takes into account high-order nonlinear effects beyond the χ(2) nonlinearity. The nonlinear refractive index (Kerr effect) and three-photon absorption (3PA) coefficients for the wavelength of 1.4 μm utilized in the model were determined by the Z-scan method. The simulation results are in good agreement with the experimentally measured THz and near-infrared spectra, confirming the validity of the proposed model. Our analysis shows that the 3PA process mainly limits THz generation in DSTMS at high pump fluences. The cascaded optical rectification further decreases THz conversion efficiency due to the spectral broadening of the excitation pulse.

We observe third-harmonic generation (THG) and fifth-harmonic generation (FHG) of terahertz free-electron laser (FEL) pulses in Si:B at cryogenic temperatures. The intensities of the harmonics increase as a function of pulse energy following power laws with exponents of 4.2 and 6.2, respectively. We attribute the generation of the fifth harmonic mainly to the valence band non-parabolicity. The values of χ(3) and χ(5) increase with the density of mobile carriers, which are excited via impact ionization of the boron dopants under irradiation by the intense FEL pulses.

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 report the emission of high-field terahertz pulses from a GaAs large-area photoconductive emitter pumped with a Ti:Sapphire amplifier laser system at 800 nm wavelength and 1 kHz repetition rate. The maximum estimated terahertz electric field at the focus is ≳ 230 kV/cm. We also demonstrate the capability of the terahertz field to cause a non-linear effect, which usually requires high-field terahertz pulses generated through optical rectification or an air plasma. A significant drop in the optical conductivity of optically pumped GaAs due to Γ-L inter-valley scattering of free electrons caused by the strong THz field is found.

We investigate the optical Kerr nonlinearity and multi-photon absorption (MPA) properties of DSTMS excited by femtosecond pulses at a wavelengths of 1.43 {\mu}m, which is optimal for terahertz generation via difference frequency mixing. The MPA and the optical Kerr coefficients of DSTMS at 1.43 {\mu}m are strongly anisotropic indicating a dominating contribution from cascaded 2nd-order nonlinearity. These results suggest that the saturation of the THz generation efficiency is mainly related to the MPA process and to a spectral broadening caused by cascaded 2nd-order frequency mixing within DSTMS, Comment: 5pages, 4 figures

Mechanical-strain-gated switches are cornerstone components of material-embedded circuits that perform logic operations without using conventional electronics. This technology requires a single material system to exhibit three distinct functionalities: strain-invariant conductivity and an increase or decrease of conductivity upon mechanical deformation. Herein, mechanical-strain-gated electric switches based on a thin-film architecture that features an insulator-to-conductor transition when mechanically stretched are demonstrated. The conductivity changes by nine orders of magnitude over a wide range of tunable working strains (as high as 130%). The approach relies on a nanometer-scale sandwiched bilayer Au thin film with an ultrathin poly(dimethylsiloxane) elastomeric barrier layer; applied strain alters the electron tunneling currents through the barrier. Mechanical-force-controlled electric logic circuits are achieved by realizing strain-controlled basic (AND and OR) and universal (NAND and NOR) logic gates in a single system. The proposed material system can be used to fabricate material-embedded logics of arbitrary complexity for a wide range of applications including soft robotics, wearable/implantable electronics, human-machine interfaces, and Internet of Things. Funding Agencies|Alexander von Humboldt postdoc fellowshipAlexander von Humboldt Foundation; German Science FoundationGerman Research Foundation (DFG) [404941515, SPP 2100]; German Research Foundation (DFG)German Research Foundation (DFG) [MA 5144/9-1, MA 5144/13-1, MA 5144/28-1]; Helmholtz Association of German Research CentresHelmholtz Association [HIL-A04]; Projekt DEAL

We report nonlinear charge-carrier response in GaAs/InGaAs core/shell nanowires that are driven by intense THz pulses. In the first experiment, half-cycle THz pulses emitted from an organic DSTMS crystal lead to a red-shift of the plasmon peak indicating intervalley transfer of the electrons. In the second experiment, a single, highly electron doped nanowire is investigated by scattering near-field infrared microscopy using intense free-electron laser (FEL) pulses. Here the observed red shift of the mid-infrared plasma resonance depends on the pulse energy and can be explained by heating the electron system in the nonparabolic conduction band.

The efficiency of photoconductive terahertz emitters is improved by increasing the operating bias field and preventing the breakdown. We can apply ~ 120 kV/cm bias field by using external resistances in series to the emitter.

Piezoresistive soft composite materials are widely used in strain sensing and typically exhibit a decrease in conductivity upon elongation—the so-called positive gauge effect. We demonstrate a thin-film architecture that features the inverse behavior: a strain-induced transition from insulating to metallic conductivity, spanning nine orders of magnitude in conductivity. Our approach is based on a nanometer-scale sandwiched bilayer Au thin film with a polydimethylsiloxane elastomeric barrier layer. Upon application of strain, the thickness of the thin soft barrier decreases because of the strain governed by the Poisson effect, followed by electron-tunneling currents through the barrier, forming an interconnected bilayer metal electrode. An extremely high on–off electrical conductivity ratio (~ 109) is observed over a wide range of working strains (as high as 130%), which mimics the ideal features of a mechanical-force-controlled electric transistor. This conceptual design strategy is expected to benefit a wide range of applications in which operation under minimal standby power could be an essential feature, such as in implantable soft strain sensors and in prosthetic long-term monitoring systems for detecting sudden a swelling/volume expansion of human body organs or blood vessels, thereby helping to avoid acute and severe syndromes.

Electron-optical phonon interaction is the dominant energy-loss mechanism in low-dimensional Ge/SiGe heterostructures and represents a key parameter for the design and realization of electronic and optoelectronic devices based on this material system compatible with the mainstream Si complementary metal-oxide semiconductor technology. Here we investigate the intersubband relaxation dynamics of $n$-type Ge/SiGe multiquantum wells with different symmetry and design by means of single-color pump-probe spectroscopy. By comparing the experimental differential transmittance data as a function of the pump-probe delay with numerical calculations based on an energy-balance rate-equation model, we could quantify an effective value for the optical phonon deformation potential describing the electron-phonon coupling in two-dimensional Ge-based systems. We found nonradiative relaxation times longer than 20 ps even in samples having intersubband energy separations larger than the optical phonon energy, evidencing the presence of a less effective electron-phonon coupling with respect to that estimated in bulk Ge.

We observe third-harmonic generation (THG) in boron-doped silicon (Si:B) upon pumping with picosecond 1.56-THz pulses from a free-electron laser with a peak electric field strength of up to 12 kV/cm. The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 4. The observations can be explained by THz emission by free holes accelerated in the nonparabolic valence band, under the assumption that the density of free holes increases with the pump intensity. A quantitative treatment supports that these carriers are generated by impact ionization, initiated by the population of thermally ionized carriers, as opposed to direct tunneling ionization. In addition, we also observe intracavity THG by embedding the Si:B in a one-dimensional photonic crystal cavity. The THG efficiency is increased by a factor of eight due to the field enhancement in the cavity, with the potential to reach a factor of more than 100 for pump pulses with a spectrum narrower than the linewidth of the cavity resonance.

Germanium is a nonpolar semiconductor with missing Reststrahlen band. In spite of other promising properties including low bandgap and small effective mass, its long, µs-scale recombination time has been prohibitive for applications as photoconductive THz emitters. Using Au-implantated Ge, with recombination times reduced to sub-ns values, we have demonstrated a broadband photoconductive THz emitter compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 µm and with pulse repetition rates of 78 MHz. Reaching up to 70 THz bandwidth, this approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.

We probe the electron transport properties in the shell of GaAs/In0.2Ga0.8As core/shell nanowires at high electric fields using optical pump / THz probe spectroscopy with broadband THz pulses and peak electric fields up to 0.6 MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a systematic redshift and a suppression of its spectral weight for THz driving fields exceeding 0.4 MV/cm. This behavior is attributed to the intervalley electron scattering resulting in the increase of the average electron effective mass and the corresponding decrease of the electron mobility by about 2 times at the highest fields. We demonstrate that the increase of the effective mass is non-uniform along the nanowires and takes place mainly in their middle part, leading to a spatially inhomogeneous carrier response. Our results quantify the nonlinear transport regime in GaAs-based nanowires and show their high potential for development of nano-devices operating at THz frequencies., includes supporting information

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron&ndash, phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1&rarr, 3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters.

Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy – reaches only up to 7 THz in the regular transmission mode due to absorption by infrared-active optical phonons. Here, we present ultrabroadband (extending up to 70 THz) THz emission from an Au-implanted Ge emitter that is compatible with mode-locked fibre lasers operating at wavelengths of 1.1 and 1.55 μm with pulse repetition rates of 10 and 20 MHz, respectively. This result opens up the possibility for the development of compact THz photonic devices operating up to multi-THz frequencies that are compatible with Si CMOS technology., Doping germanium for more terahertz A negligible absorption of far-infrared radiation together with high electron mobility makes germanium a perfect material for photoconductive terahertz emitters. However, a very long ‘lifetime’ of photoexcited carriers is a serious disadvantage – electrons and holes excited by a laser pulse do not vanish until the next pulse arrives, they accumulate leading to a dramatic device degradation. The research team at the Helmholtz-Zentrum Dresden-Rossendorf in Germany has managed to overcome this limitation by introducing gold impurities into germanium, thus reducing the carrier lifetime by more than thousand times - down to the sub-nanosecond level. In cooperation with colleagues at the University of Konstanz (Germany) they demonstrated a germanium-based photoconductive antenna emitting terahertz pulses of “unprecedented” bandwidth of 70 THz using a femtosecond fibre laser. Further work on germanium-based THz devices is expected to revolutionize terahertz technologies, with applications in security, communication, and pharmaceuticals.

We show nonlinear plasmonic response in GaAs/In0.2Ga0.8As nanowires using high-field terahertz pulses. With increasing THz field, plasmon resonance redshifts and its spectral weight decreases indicating a spatially inhomogeneous intervalley electron scattering.

Photoconductive terahertz (THz) emitters have been fulfilling many demands required for table-top THz time-domain spectroscopy up to 3–4 THz. In contrast to the widely used photoconductive materials such as GaAs and InGaAs, Ge is a nonpolar semiconductor characterized by a gapless transmission in the THz region due to absence of one-phonon absorption. We present here the realization of a Ge-based photoconductive THz emitter with a smooth broadband spectrum extending up to 13 THz and compare its performance with a GaAs-based analogue. We show that the spectral bandwidth of the Ge emitter is limited mainly by the laser pulse width (65 fs) and, thus, can be potentially extended to even much higher THz frequencies.

We have investigated the plasmonic response of GaAs/In 0.20 Ga 0.80 As core/shell nanowires driven resonantly by strong THz fields with the amplitude of few MV/cm. The plasmon mode exhibits a systematic redshift with the suppression of the spectral weight with the increase of the driving THz field. Interestingly, the scaling of the plasmon parameters does not follow the usual quadratic behavior, indicating an inhomogeneous intervalley electron scattering across the nanowire.

We explore saturable absorption and terahertz photoluminescence emission in a set of n-doped Ge/SiGe asymmetric coupled quantum wells, designed as three-level systems (i.e., quantum fountain emitter). We generate a non-equilibrium population by optical pumping at the 1→3 transition energy using picosecond pulses from a free-electron laser and characterize this effect by measuring absorption as a function of the pump intensity. In the emission experiment we observe weak emission peaks in the 14–25 meV range (3–6 THz) corresponding to the two intermediate intersubband transition energies. The results represent a step towards silicon-based integrated terahertz emitters.

Niobium dioxide (NbO 2 ) is an isovalent counterpart of VO 2 with a considerably higher transition temperature (T C =1080 K). We have performed time-resolved optical pump – THz probe measurements on a NbO 2 epitaxial thin film at room temperature. Notably, the pump energy required for the switching into a metastable metallic state is smaller than the energy necessary for heating NbO 2 upto T C providing strong evidence for the non-thermal character of the photoinduced insulator-to-metal transition.

We report third-harmonic generation (THG) of terahertz free-electron laser (FEL) pulses in Si:B at cryogenic temperatures. The physical mechanism of THG is attributed to the free-carrier χ(3) nonlinearity due to the non-parabolicity of the valence band. The value of χ(3) increases as a function of the carrier density, which are generated via impact ionization of the boron dopants under irradiation by the FEL pulses. By positioning the Si:B in a one-dimensional photonic crystal (1D PC) cavity, the measured THG intensity increases by a factor of about 200.

We present Ge as a promising material for THz photoconductive emitters to push forward the limit of a gapless broadband spectrum from 6.5 THz to 13 THz. Maximum THz electric field reported so far from photoconductive technique is 36 kV/cm using a large-area interdigitated electrode based emitter. We also present here a better electrode design to enhance the THz electric field amplitude emitted from large-area interdigitated electrode emitters.

We present the electrical properties of GaAs/In

We present the ultrafast THz response of VO2 under high pressures. A clear anomaly is observed around 8 GPa indicating a pressure-induced phase transition. Our observations can be interpreted in terms of a bandwidth-controlled Mott-Hubbard transition.

THz radiation is a perfect tool for probing electrical properties of semiconductor nanostructures in a contactless way. When applied to semiconductor nanowires, THz probe pulses can drive the oscillations of photoexcited electrons and holes in the form of localized surface plasmon. We used optical pump – THz probe spectroscopy to study plasmonic response of charge carriers in GaAs/In x Ga 1-x As core/shell nanowires. The carrier lifetimes are about 80-100 ps, depending on the shell composition and the photoexcitation level, while the extracted mobilities reach 3700 cm2/Vs at room temperature.

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 “off ” 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.

The efficient integration of electronic and optoelectronic functions on a single chip within a CMOS-compatible material and technology platform is witnessing the next disruptive innovation in the microelectronics industry. Among the proposed approaches to “siliconize” photonics, the most ambitious is to develop a monolithic Si-based light source. The main challenge is the indirect bandgap of Si which hinders efficient light emission. A number of different solutions has been suggested to break down this material limitation, and the optimal strategy depends on the specific spectral range of application. In the THz range, quantum cascade laser (QCL) architectures based on the Group-IV material system have been predicted as viable potential sources up to room temperature1-3, by exploiting intersubband (ISB) transitions in the conduction band of n-type, Ge-rich Ge/SiGe multi-quantum well (MQW) heterostructures3. In this paper, we will show that the interface quality and threading dislocation density4 in this material system have finally reached the level required for possibly enabling, within a wide temperature range, a material gain larger than the cavity losses5. From the structural standpoint, we report on the growth by ultra-high-vacuum chemical vapor deposition (UHV-CVD) of strain-compensated QCL stacks with a thickness up to 10 µm, demonstrating by X-ray diffraction and scanning transmission electron microscopy their high crystalline quality and remarkable growth reproducibility [Fig. 1(a)]. We will also describe optimized designs obtained in waveguide modelling with which waveguide losses comparable to III-V architectures are achieved6. In the model system of n-type Ge-rich asymmetric coupled quantum wells (ACQWs)7,8, being the building block of a QCL structure, we will prove a high degree of control on the engineering at the nanoscale the intersubband electronic spectrum and the wavefunctions relevant for tunneling processes, which allowed us the observation of photoluminescence at 4 THz after optical excitation using a free-electron laser (FEL)9. In addition, we will present a systematic calibration study of the material parameters controlling the interface roughness (IFR) and electron-phonon scatterings in n-type Ge/SiGe MQWs by combining experimental pump-probe data with a numerical model of ISB carrier dynamics including inelastic and elastic scattering channels. The time evolution of subband populations after FEL optical pumping is investigated as a function of the MQW design geometry, comparing symmetric and stepwise configurations, and analyzed by varying the subband energy spacing changing the well width. As a matter of fact, these material parameters critically affect the outcomes of the non-equilibrium Green’s function calculations widely used for predicting quantum transport and, ultimately optical gain of QCL devices10,11 and so to guide the optimization of proposed device designs. Hitherto, however, in most cases, the adopted values for these material parameters were not calibrated against experiments. For a proper calibration of the deformation potentials, we studied the non-radiative lifetimes as a function of the subband spacing on a set of symmetric MQWs of different Ge well widths. The corresponding α2D absorption spectra are reported in the left panels Fig. 1(b-d). Differential transmission spectra from single-color pump-probe experiments and model simulations indicate [Fig. 1(e-g)] that the electron-phonon coupling is much less effective than that expected from the bulk deformation potentials reported by Jacoboni et al.12. In particular, we found that the relaxation mediated by the emission of intervalley OPs is suppressed in low-dimensional multilayer structures while the intravalley OP scattering is described by a reduced effective deformation potential, thus explaining the observation of relaxation times larger than 10 ps above both the OP energy thresholds in single-color pump probe spectroscopy experiments. These results are crucial for developing adequate simulation frameworks and specific design approaches for achieving optical gain in silicon-compatible quantum cascade emitters. [1] K. Driscoll et al., Appl. Phys. Lett. 89, 191110 (2006). [2] K. Driscoll et al., J. Appl. Phys. 102, 093103 (2007). [3] D. J. Paul, Laser & Photonics Reviews 4, 610 (2010). [4] T. Grange et al., Physical Review Applied 13, 044062 (2020). [5] T. Grange et al., Appl. Phys. Lett. 114, 111102 (2019). [6] K. Gallacher et al., Opt. Express 28, 4786 (2020). [7] C. Ciano et al., Physical Review Applied 11, 014003 (2019). [8] L. Persichetti et al., Crystals 10, 179 (2020). [9] C. Ciano et al., Opt. Express 28, 7245 (2020). [10] A. Valavanis et al., Phys. Rev. B 83, 195321 (2011). [11] C. Jirauschek et al., Applied Physics Reviews 1, 011307 (2014). [12] C. Jacoboni et al., Phys. Rev. B 24, 1014 (1981). Figure 1

We report a strong shift of the plasma resonance in highly-doped GaAs/InGaAs core/shell nanowires (NWs) for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied NWs show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous wave excitation by a CO2 laser. Probing the same NWs with the pulsed free-electron laser with peak electric field strengths up to several 10 kV cm-1 reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.

The FELBE User Facility at the ELBE Center for High Power Radiation Sources offers two FELs dedicated to optical studies of materials over a wide range of the THz and IR spectrum. In 2017, the installation and commissioning of a new U37 undulator was completed, resulting in enhanced performance over the spectral range of 7.5 – 60 THz (5 – 40 um), Operation of the new FEL for User operation began during the 2nd half of 2017. Results of the commissioning and early User runs are reported here.

Phonons in graphene and interexcitonic transition in transition metal dichalcogenides are examples for low-energy excitations in 2D materials. Free-electron lasers such as FELBE deliver tunable short mid-infrared pulses that are ideally suited to study the carrier dynamics in 2D materials in the energy range of these low-energy excitations. We present results on the electron dynamics in graphene and the exciton dynamics in MoSe 2 .

The present study introduces scattering-type scanning near-field infrared optical nanospectroscopy (s-SNIM) as a valuable and well-suited tool for spectrally fingerprinting n-butyl xanthate (KBX) molecules adsorbed to chalcopyrite (CCP) sample surfaces. The collector KBX is well known to float CCP and is used in beneficiation. We thus identified KBX reaction products both by IR optical far- and near-field techniques, applying attenuated total internal reflection Fourier-transform infrared spectroscopy (ATR FT-IR) in comparison to s-SNIM, respectively. The major KBX band around 880 cm−1 was probed in s-SNIM using both the tunable free-electron laser FELBE at the Helmholtz-Zentrum Dresden-Rossendorf facility, Germany, and table-top CO2 laser illumination. We then were able to monitor the KBX agglomeration in patches

We present nanoscopic infrared-optical investigations on highly n-type doped GaAs-based nanowires, revealing interesting nonlinear phenomena such as a pronounced redshift of the plasma resonance by the strong THz fields of a free-electron laser.

We study the photoinduced metallization process in niobium dioxide ${\mathrm{NbO}}_{2}$. This compound undergoes the thermal insulator-to-metal transition at the remarkably high temperature of 1080 K. Our optical pump-terahertz probe measurements reveal the ultrafast switching of the film on a subpicosecond timescale and the formation of a metastable metallic phase when the incident pump fluence exceeds the threshold of $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$. Remarkably, this threshold value corresponds to the deposited energy which is capable of heating ${\mathrm{NbO}}_{2}$ only up to 790 K, thus, evidencing the nonthermal character of the photoinduced insulator-to-metal transition. We also observe an enhanced formation of the metallic phase above the second threshold of $\ensuremath{\sim}17.5\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$ which corresponds to the onset of the thermal switching. The transient optical conductivity in the metastable phase can be modeled using the Drude-Smith model confirming its metallic character. The present observation of nonthermal transition evidences a leading role played by electronic correlations in ${\mathrm{NbO}}_{2}$ and can serve as an important test bed for understanding photoinduced phenomena in strongly correlated oxides.