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The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling between its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (~14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provide insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials.

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.

This dataset is raw streak camera images for nanowire, PL spectra, python code for fitting.

This data set presents the optical pump-probe traces of Fe3Sn2 as function of temperature and excitation fluence. It also shows initial data analysis with exponential fits, relaxation times and FFT calculations.

The elastic accommodation of lattice mismatch in core/shell nanowires presents unique features because both the shell and the core can be strained depending on their volume ratio. This allows the realization of heterostructure combinations with large lattice mismatch, as well as the extensive strain-mediated tailoring of their electronic properties [1]. Owing to the large stresses involved, any shell thickness asymmetry around the core may cause the bending of the nanowires, forming shapes like circular arcs, which can be a desired effect or not, depending on the targeted application. Here, we monitor the asymmetric shell growth, the asymmetric strain, and the resulting bending during the growth of GaAs/In0.4Al0.6As core/shell nanowires [2]. We show how the profile of the elemental beams in a conventional MBE system accounts for the asymmetric growth rate of the shell around the core, in spite of the substrate rotation during growth. The resulting asymmetric stresses around the core cause the bending of the nanowires to the side of the thinner shell, which, in turn, enhances further the growth rate and stress asymmetry, and thus the bending itself, in a self-feeding circle. Nevertheless, after a critical minimum shell thickness on all core sides has been reached, the bending is reversed and the nanowires straighten up. Analyzing cross sections normal to the axis of such straightened up nanowires by STEM, we developed a method to reconstruct, for the first time, the evolution of the shell thickness on every core side and the corresponding bending angle during growth. Furthermore, geometric phase analysis revealed the spatial distribution of strain in the core and the shell with respect to the uneven shell thicknesses around the core. Our results shine a light on the growth of lattice-mismatched core/shell nanowires and could serve as a guide to design deliberate curved structures or, instead, strategies to suppress the bending. Figure 1. (a) Side-view SEM image of as

The FELBE free-electron laser delivers intense THz and mid infrared pulses that are ideal to excite various low-energy quasipartcles in solids and to study their ultrafast dynamics [1]. We present the capabilities in the FELBE laboratories for degenerate and two-color pump-probe experiments, four-wave mixing and time-resolved near-field experiments. As an example, we show how nonlinear intraband transport in bilayer graphene manifests itself in polarization resolved pump-probe experiments [2]. Polarization resolved pump-probe experiments furthermore shed light into the microscopic scattering mechanisms, allowing us to disentangle Coulomb and phonon-related scattering processes [3]. [1] M. Helm, S. Winnerl, A. Pashkin, J. M. Klopf, J.-C. Deinert, S. Kovalev, P. Evtushenko, U. Lehnert, R. Xiang, A. Arnold, A. Wagner, S. M. Schmidt, U. Schramm, T. Cowan & P. Michel, Eur. Phys. J. Plus 138, 158 (2023). [2] A. Seidl, R. Anvari, M. M. Dignam, P. Richter, T. Seyller, H. Schneider, M. Helm, and S. Winnerl Phys. Rev. B 105, 085404 (2022). [3] J. C. König-Otto, M. Mittendorff, T. Winzer, F. Kadi, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl, Phys. Rev. Lett. 117, 087401 (2016).

Population inversion and optical gain are often difficult to measure in systems that do not exhibit lasing. For example, two-color pump-probe experiments targeting gain require precise reference measurements in order to distinguish gain from simple absorption bleaching due to Pauli blocking. In the mid-infrared (MIR) and far-infrared (FIR) spectral range such experiments are even more difficult as free-carrier absorption complicates the analysis. Here we utilize a three-pulse technique [1], which has been employed to find evidence for gain and spectral hole burning in the near-infrared (NIR), to study the dynamics of the MIR population inversion in optically pumped intrinsic graphene. Graphene is an interesting material to apply this technique since there are on one hand many reports on ultrafast thermalization, excluding inversion, but on the other hand many suggestions to realize gain, in particular in the THz range. The principle of the technique is sketched in Fig. 1a. A strong NIR “gain” pulse (photon energy 1.55 eV) excites interband transitions in an epitaxial multilayer graphene sample on SiC. The majority of graphene layers is almost intrinsic. The low-energy carrier dynamics is monitored by measuring the differential transmission change in a degenerate MIR (photon energy 250 meV) pump-probe experiment. This differential transmission generally is positive, corresponding to bleaching via Pauli blocking by carriers that are photoexcited by the MIR pump pulse. If, however, the gain pulse is strong enough to induce an inverted population at 250 meV, the situation is qualitatively different: Now the mid-infrared pulse causes stimulated emission from the inverted population, thus decreasing the number of carriers in the conduction band at the probed energy. Consequently, the differential transmission with regard to the MIR pump pulse changes from positive to negative (cf. Fig.1b) In addition to the NIR fluence dependence shown in Fig. 1b we also present for the g

The Kagome lattice is a two-dimensional network of corner-sharing triangles that is known to combine linear bands hosting massless Dirac fermions and dispersionless flat bands featuring massive localized electrons, both arising due to its geometry. FeSn binary compounds and the RM$_6$Sn$_6$(R=Tb,Gd and Y) series are commonly studied magnetic Kagome metals, which possess different magnetic ground states and interlayer Kagome coupling. Several steady-state experimental techniques have been used to study the magnetic and electronic structure of these materials and the effects of magnetism on the band structure. However, the ultrafast dynamics and the interplay of these unusual features have not yet been widely explored in the scope of time-domain spectroscopy. Here we present temperature- and fluence-dependent carrier dynamics of various magnetic Kagome metals studied using the optical pump-probe technique. Distinct carrier relaxations have been observed, and they can be partially attributed to the simple two-temperature model, as these are highly metallic compounds.

Photoconductive emitters excited by fs-lasers are the most commonly used emitters in THz-TDS systems, enabling record high dynamic range and wide bandwidths [1,2]. So far, these emitters have mostly optimized for operation with low average power, low-cost fs-laser systems and have a typical dimension of a few µm^2 for the photoconductive gap [3]. In parallel, large-area emitters with mm^2 size have been explored for scaling fluence and obtaining high fields [4]. However, so far, no attempts have been made of using high average power laser systems as excitation sources, as a possible path to increase the THz source average power and reach even higher dynamic range than current reported records. In this work, we present our first results in this direction and show THz emission from a large area photoconductive emitter (LAE) based on SI-GaAs with a 1×1 cm^2 active area, excited by a frequency-doubled home-built high average power ultrafast oscillator, capable of 92 fs at a centre frequency 515 nm with 90 MHz repetition rate and 17 W of average power. We show here preliminary results exciting the LAE with 1 W from this laser system, which is to the best of our knowledge the highest average power applied to such an emitter, without any apparent sign of thermal saturation. 1. R. B. Kohlhaas, S. Breuer, L. Liebermeister, S. Nellen, M. Deumer, M. Schell, M. P. Semtsiv, W. T. Masselink, and B. Globisch, Appl. Phys. Lett. 117, 131105 (2020). 2. U. Nandi, K. Dutzi, A. Deninger, H. Lu, J. Norman, A. C. Gossard, N. Vieweg, and S. Preu, Opt. Lett. 45, 2812 (2020). 3. S. Winnerl, J Infrared Milli Terahz Waves 33, 431 (2012). 4. M. Beck, H. Schäfer, G. Klatt, J. Demsar, S. Winnerl, M. Helm, and T. Dekorsy, Opt. Express 18, 9251 (2010).

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.

We present a broadband optical study of non-magnetic kagome metals AV3Sb5 (A = K, Rb, Cs) down to 10 K. Different contributions to the optical spectra have been discussed and compared with the DFT calculations in normal and charge density wave (CDW) states. Spectra reflect the response of the 2D Dirac fermions and are frequency-independent in a broad energy range. Low energies are governed by the itinerant and localized charge carriers that show a spectral weight redistribution below the CDW transition. Our results show that the CDW gaps evolve systematically between the siblings (K

When lowering the dimensions of semiconductor heterostructures, controlled compositional changes become paramount owing to the increasing role of the heterointerfaces in the electronic properties of the heterostructures. A common issue faced in fabricating axial heterostructures in vapor-liquid-solid (VLS)-grown nanowires is the compositional grading, which is caused by the involved growth mechanisms and broadens the interfaces. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow AlxGa1-xAs axial insertions in self-catalyzed GaAs nanowires. Some examples showcasing the capability of DCAPE in growing systems of complex, abrupt heterostructures are shown in Fig. 1a, where AlxGa1-xAs is revealed as horizontal dark grey lines. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and a semi-empirical growth model [2] are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the total Al content [2]. We show that pre-filling the Ga droplet with Al before the growth of an insertion is enough to sharpen the GaAs-to-AlxGa1-xAs interface at any TG. On the other hand, though, sharper AlxGa1-xAs-to-GaAs interfaces are obtained only at lower TG and/or smaller RNW, where the so-called reservoir effect is markedly reduced. In the best case, symmetric AlxGa1-xAs insertions with interface widths of only 2– 3 monolayers (comparable to, or better than, state-of-the-art thin film heterostructures) are achieved for the full range of x (Fig. 1b), proving the strength of VLS in the fabrication of sharp axial heterostructures. Figure 1: (a) HAADF-STEM images with 5 examples of GaAs/AlxGa1-xAs heterostructures grown in self-catalyzed GaAs na

In this study, we present optical pump-probe measurements on a magnetic kagome metal, Fe3Sn2, under different temperatures down to 10 K. The obtained spectra can be fitted with a double exponential decay, indicating that the system has two distinct relaxation processes. Additionally, some unexpected and pronounced oscillations are dominating the spectra, giving evidence of a strong electron-phonon coupling in Fe3Sn2, at least in this ultra-fast regime. The frequency of this coupled phonon is determined to be around 2.5 THz. Finally, we will discuss the temperature and pump fluence dependence of the observed phonon coupling and the distinct relaxation dynamics in this material.

We discuss the basics and modern developements regarding generation and detection of pulsed and cw THz radiation with photoconductive antennas.

We discuss basic physical properties of graphene and other 2D materials, in particular TMDCs and their heterostructures with a focus on light-matter interaction.

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. Although, as the physical dimensions of such devices approach the atomic scale, managing the compositional grading effect of the constituent materials at the heterointerface becomes critical and can be particularly challenging in nanowires grown in vapor-liquid-solid mode. We grow self-catalyzed GaAs nanowires with AlxGa1-xAs axial insertions on Si substrates using our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1]. Here, nanowires are grown using alternating short beam pulses as opposed to the conventional molecular beam epitaxy analog where material is continuously supplied. With this approach, precise control over the axial growth rate, droplet composition and contact angle is granted along with the unique opportunity to grow nanowire heterostructures at CMOS compatible temperatures. To examine the compositional grading mechanism of Al in our AlxGa1-xAs axial insertions we employ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Quantitative AlxGa1-xAs profiles are extracted from atomically resolved images to study in detail the profile’s interface sharpness and symmetry in relation to the profiles total Al content, heterostructure growth temperature and nanowire diameter. Additionally, building on a previously existing heterostructure growth model that describes the AlxGa1-xAs-to-GaAs interface while considering the solid-liquid thermodynamics and the so-called reservoir effect [2], we derive our own semi-empirical model that describes remarkably well the AlxGa1-xAs profile as a whole and allows for a rigorous analysis of both the profile’s lea

III-V semiconductor heterostructures have contributed to a wealth of studies in solid state physics, as well as to applied research and technology in electronics and photonics. More recently, the nanowire geometry introduced new possibilities, such as one-dimensional quantum transport, formation of Majorana modes, enhanced light-matter coupling, photon entanglement, as well as monolithic integration in Si-CMOS platforms for the realization of more-than-Moore hybrid systems. This talk will be focusing on the bottom-up fabrication and the structural and electronic properties of III-As nanowire heterostructures. The first type of heterostructures will be radial ones, comprising a GaAs core and a lattice-mismatched InxAl1-xAs or InxGa1-xAs shell. Molecular beam epitaxy and a combination of vapor-liquid-solid and vapor-solid growth modes are employed to grow free-standing nanowires on Si substrates [1, 2]. Owing to its high surface-to-volume ratio and the peculiar geometry, the thin core can be hydrostatically tensile strained to extremely high levels, depending on the In content x and the thickness of the shell. As a welcome effect, the bandgap of GaAs can be tuned to be anywhere between the strain-free value of 1.4 and 0.8 eV, allowing for potential applications in telecom photonics [3]. Furthermore, the electron mobility in the GaAs core is increased with increasing the tensile strain, as a result of a corresponding decrease in electron effective mass [4]. This is of major importance for the realization of transistors with high speed and low-power consumption. The second type of heterostructures will be axial ones, where the composition is modulated from GaAs to AlxGa1-x¬As and back to GaAs along the nanowire axis. Here, we develop a pulsed-growth technique [5], which grants precise control over the axial growth rate and droplet composition. Using advanced transmission electron microscopy methods and a thermodynamic equilibrium model, it becomes possible to quantitati

Over six decades ago, the potential of relativistic electrons to emit electromagnetic radiation was initially reported [1]. The discovery of graphene with a band structure resembling massless Dirac fer-mions revitalized the search for Landau-level lasing schemes [2, 3] that started earlier on non-parabolic semiconductors. The realization of a maser based on this concept, with tuning through changes in magnetic field, holds significant appeal, especially for applications within the terahertz frequency range. The initial steps towards reaching optical gain are selective pumping of individual Landau levels (LLs) and characterizing the lifetimes in these levels. The carrier dynamics in the three-level system LL-1, LL0 and LL1 of the non-equidistant Landau ladder in graphene was investigated by pump-probe ex-periments. Circularly polarized radiation was utilized to selectively pump and probe the energetically degenerate transitions LL-1 → LL0 and LL0 → LL1 [4]. The experiments were carried out at a photon energy of 75 meV (wavelength 16.5 µm) using the free-electron laser FELBE as a source for intense ps pulses [5]. Our findings indicate, fast Auger scattering rapidly redistributes the carriers [4]. Since several Landau laser schemes [2, 3] involve the levels LL1 and LL2, we performed pump-probe experiments on the transitions LL-1 → LL2 and LL-2 → LL1. Pumping and probing with co-polarized radiation results in higher pump-probe signals as compared to counter-polarized configurations (cf. Fig. 1). Note that in the absence of scattering, the coun-ter-polarized configuration would provide no signal as it probes levels that are not directly affected by optical pumping. The results show that within the temporal resolution of the experiment, a non-equilibrium distribution is achieved. However, it rapidly thermalizes as indicated by the induced transmission in counter-polarized configurations that reaches a maximum a few ps later than the signals in co-polarized configuration.

We explore the carrier dynamics in graphene and gapless bulk Hg0.83Cd0.17Te under Landau quantization. To this end, individual levels of the non-equidistant Landau ladders are pumped and probed by circularly polarized mid-infrared radiation pulses. While Auger scattering is very efficient in graphene, resulting in carrier redistribution on a ps timescale, this process is strongly suppressed in Hg0.83Cd0.17Te, yielding two orders of magnitude longer lifetimes.

Strain engineering is a powerful tool for designing nanowires with tailored properties for a variety of applications. By carefully controlling the built-in strain in nanowires, it is possible to tune their bandgap to the near-infrared region, making them ideal for applications in telecommunication and imaging. In our previous work, we demonstrated that in GaAs/In x Al 1-x As core/shell nanowires, the bandgap of the core can be narrowed by up to 40%, for x up to 0.54, via strain due to the lattice mismatch between the shell [1]. Here, we explored the upper end of the lattice mismatch regime, extending the same concept to the contents of the shell towards x = 1, achieving unusually high strain values. The strain in the core and its effect on band structure are studied by a combination of spectroscopic methods and high-resolution transmission and scanning-transmission electron microscopy (HR(S)TEM). Raman spectroscopy showed that the tensile strain in the GaAs core increased linearly with increasing the In content in the shell (Fig. 1a), following the trend we reported in the past for lower values of x [1]. This behavior suggests the absence of plastic relaxation despite the very large lattice mismatch between the core and the shell. Using cross-sectional and longitudinal HR(S)TEM observations, we assessed the strain distribution normal and along the nanowire axis (Figs. 1b to 1d), which was found to be in good agreement with finite element and molecular dynamics simulations. Above a critical x value, plastic relaxation sets in via dislocations (Fig. 1b). We also correlated the photoluminescence emission properties with the strain distribution in the core and the shell, and the corresponding band alignment via band structure simulations. All in all, our results identified the limits of a coherent core and shell heterostructures and the potential application of tensile-strained GaAs nanowires for C- and O-band telecom photonics.

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong (~1°) ultrafast Faraday rotation (~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.

Placing quantum materials into optical cavities can provide a unique platform to control quantum cooperative properties of matter, in the regimes of both weak and strong light-matter coupling. Here we show the first experimental evidence of the reversible touchless control of a metal-to-insulator phase transition in the charge density wave material 1T-TaS2 embedded in a tunable THz optical cavity. The switch between conductive and insulating behavior, which is obtained by mechanically tuning the distance between the cavity mirrors and their alignment, is related to a significant renormalization of the sample temperature. This suggests a Purcell-like scenario in which the spectral profile of the cavity modifies the energy exchange between the material and the external electromagnetic field. Our findings uncover a new path to control the thermodynamics and macroscopic transport properties of quantum materials by engineering their electromagnetic environment.

We study THz-driven condensate dynamics in epitaxial thin films of MgB2, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature—unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near ≈0.7 Tc, is attributed to THz-driven, long-lived, nonthermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair breaking.

BaFe2As2 is the parent compound for a family of iron-based high-temperature superconductors as well as a prototypical example of the spin-density wave (SDW) system. In this study, we perform an optical pump-probe study of this compound to systematically investigate the SDW order across the pressure-temperature phase diagram. The suppression of the SDW order by pressure manifests itself by the increase of relaxation time together with the decrease of the pump-probe signal and the pump energy necessary for complete vaporization of the SDW condensate. We have found that the pressure-driven suppression of the SDW order at low temperature occurs gradually in contrast to the thermally-induced SDW transition. Our results suggest that the pressure-driven quantum phase transition in BaFe2As2 (and probably other iron pnictides) is continuous and it is caused by the gradual worsening of the Fermi-surface nesting conditions.

Nonlinear phenomena in the THz spectral domain are important for the understanding of optoelectronic properties of quantum systems and provide a basis for modern information technologies. Here, we report a giant THz nonlinearity in high-mobility 2D topological insulators based on HgTe quantum wells, which manifests itself in a highly efficient third harmonic generation. We observe a third harmonic THz susceptibility several times higher than in bare graphene and many orders of magnitude higher than in trivial quantum well structures based on other materials. To explain the strong nonlinearity of HgTe-based heterostructures at the THz frequencies, we consider the acceleration of free carriers with high mobility and variable dispersion. This acceleration model, for which the non-parabolicity of the band dispersion is key, in combination with independently measured scattering time and conductivity is in good agreement with our experimental data in a wide temperature range for THz fields below the saturation. Our approach provides a route to material engineering for THz applications based on frequency conversion.

High-pressure single-crystal x-ray diffraction experiments reveal that the superconducting kagome metal CsV₃Sb₅ transforms from hexagonal (P6/mmm) to monoclinic (C2/m) symmetry above 10 GPa if nonhydrostatic pressure conditions are created in a diamond anvil cell with silicon oil as pressure-transmitting medium. This is contrary to the behavior of CsV₃Sb₅ under quasi-hydrostatic conditions in neon, with the hexagonal symmetry retained up to at least 20 GPa. Monoclinic distortion leaves the kagome planes almost unchanged but deforms honeycomb nets of the Sb atoms. Using ab initio density-functional calculations, we show that this distortion facilitates the pressure-induced shift of van Hove singularities away from the Fermi level and preserves the overall scenario of the Fermi surface reconstruction caused by the formation of interlayer Sb-Sb bonds.

We present two types of pump-probe spectroscopy on single core-shell III-V nanowires: while the pump is either interband or intraband, a broad-band mid-infrared beam is used as probe. This provides interesting insight into carrier heating and relaxation.

Raw pump-probe data for two Ba-122 DAC fillings used in the paper, photos of the samples inside the DAC, Matlab script for constructing the p-T plot of the SDW order

streak camera data, meta data

This upload represents the data used for publication <> including datasets, images and programming code. 1. Raw_data.rar contains raw data files obtained during transport measurements, two-colour pump-probe experiments(FELBE) and third harmonic generation experiments. 2. Drude_fit.rar contains the result of fitting the complex change in conductivity with Drude fit. 3. Band_structure_calculation.rar contains the result of the fermi energy and dispersion calculations based on kp-method. 4. Theoretical_model_calculation.rar contains the code of the program for the theoretical model for THG and fitting it with experimental data and its result 5. Presentation_Sample_QC0600.pptx contains the information about used sample.

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 –

We demonstrate a broadband photoconductive THz emitter compatible with femtosecond fiber lasers operating at wavelengths of 1.1 and 1.55 m. The emitted 1.5-cycle transient covers the spectral range up to 70 THz.

In our study we employ a powerful method, namely a three-pulse pump-probe technique, that was first suggested by Kim et al.1, to explore the possibility to achieve transient gain photon energies below the optical phonon energy (∼ 200 meV) in graphene. Intriguingly, this technique is not widely established and to our knowledge has never been used in the mid- or far-infrared spectral range. The principle behind this method relies on the effect of a strong pre-pump pulse of 1.55 eV photons, which can cause a transient population inversion at lower energies. This population inversion is evidenced by a sign flip of the mid-infrared (86 meV photon energy) pump-probe signal that is related to either absorption or stimulated emission of mid-infrared photons of the pump beam. We present the results on multilayer graphene obtained under various experimental configurations. Our findings shed light into the completion of rapid thermalization via Coulomb scattering and carrier cooling via optical phonons. 1. Kim, K.; Urayama, J.; Norris, T.; Singh, J.; Phillips, J.; Bhattacharya, P. Appl. Phys. Lett., 2002, 81, 670-672.