Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1^{-1} from a metallic spintronic emitter

To explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields, we excite a W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range and on the sub-picosecond time scale.Comment: 7 pages, 4 figure

Fabrication of epitaxial CoSi₂ nanowires

We have developed a method for fabricatingepitaxialCoSi₂nanowires using only conventional optical lithography and standard silicon processing steps. This method was successfully applied to ultrathin epitaxialCoSi₂ layers grown on Si(100) and silicon-on-insulator substrates. A nitride mask induces a stress field near its edges into the CoSi₂/Siheterostructure and leads to the separation of the CoSi₂ layer in this region during a rapid thermal oxidation step. A subsequent etching step and a second oxidation generate highly homogenous silicide wires with dimensions down to 50 nm

Efficient Auger scattering in Landau-quantized graphene

We present an analytical expression for the differential transmission of a delta-shaped light field in Landauquantized graphene. This enables a direct comparison of experimental spectra to theoretical calculations reflecting the carrier dynamics including all relevant scattering channels. In particular, the relation is used to provide evidence for strong Auger scattering in Landau-quantized graphene

Observation of Coulomb-Assisted Dipole-Forbidden Intraexciton Transitions in Semiconductors

We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum-well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the $1s$ exciton emission, which we attribute to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced enhancement of the $2s$-exciton emission is observed, despite the $1s$-to-$2s$ transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the 2$s$ and 2$p$ states, yielding an effective terahertz transition between the 1$s$ and 2$s$ populations.Comment: 5 pages, 3 figure

Terahertz two-photon quantum well infrared photodetector.

A two-photon detector based on intersubband transitions in GaAs/AlGaAs quantum wells operating in the Terahertz regime below the Reststrahlenband is reported. Resonantly enhanced optical nonlinearities enables sensitive quadratic detection at pJ pulse energies. We demonstrate its use in a quadratic autocorrelator for far-infrared picosecond pulses at around 7 THz

Coherent phonon and unconventional carriers in the magnetic kagome metal Fe3_3Sn2_2

Temperature- and fluence-dependent carrier dynamics of the magnetic Kagome metal Fe$_3$Sn$_2$ were studied using the ultrafast optical pump-probe technique. Two carrier relaxation processes ($\tau_1$ and $\tau_2$) and a laser induced coherent optical phonon were observed. By using the two-temperature model for metals, we ascribe the shorter relaxation $\tau_1$ (~1 ps) to hot electrons transferring their energy to the crystal lattice via electron-phonon scattering. $\tau_2$ (~25 ps), on the other hand, cannot be explained as a conventional process and is attributed to the unconventional (localized) carriers in the material. The observed coherent oscillation is assigned to be a totally symmetric A$_{1g}$ optical phonon dominated by Sn displacements out of the Kagome planes, and possesses a prominently large amplitude, on the order of 10$^{-3}$, comparable to the maximum of the reflectivity change ($\Delta$R/R). This amplitude is equivalent to charge-density-wave (CDW) systems, although no signs of such an instability were hitherto reported in Fe$_3$Sn$_2$. Our results set an unexpected connection between Fe$_3$Sn$_2$ and kagome metals with CDW instabilities, and suggest a unique interplay between phonon and electron dynamics in this compound.Comment: 12 pages, 14 figure

Ultra-fast transistor-based detectors for precise timing of near infrared and THz signals

A whole class of two-color experiments involves intense, short Terahertz radiation pulses. A fast and moderately sensitive detector capable to resolve both near-infrared and Terahertz pulses at the same time is highly desirable. Here we present the first detector of this kind. The detector element is a GaAs-based field effect transistor operated at room temperature. THz detection is successfully demonstrated at frequencies up to 4.9 THz. The THz detection time constant is shorter than 30 ps, the optical time constant is 150 ps. This detector is ideally suited for precise, simultaneous resolution of optical and THz pulses and for pulse characterization of high-power THz pulses up to tens of kW peak power levels. The dynamic range of the detector is as large as 65±3dB/Hz−−−√, enabling applications in a large variety of experiments and setups, also including table-top systems

Slow Noncollinear Coulomb Scattering in the Vicinity of the Dirac Point in Graphene

The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower. For low photon energies, carrier-optical-phonon processes are strongly suppressed and Coulomb mediated noncollinear scattering is remarkably slow, namely on a ps time scale. This effect is very promising for infrared and THz devices based on hot carrier effects