Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-Pérot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillatordriven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost. IntroductionThe terahertz (THz) window, loosely defined as the frequency range from 0.3 to 30 THz in the electromagnetic spectrum, is located between the realms of electronics and optics 1,2 . As this region coincides with many fundamental resonances of materials, THz radiation enables very selective spectroscopic insights into all phases of matter with high temporal 3,4 and spatial 5,6,7,8 resolution. Consequently, numerous applications in basic research 3,4 , imaging 5 and quality control 8 have emerged.To fully exploit the potential of THz radiation, energy-efficient and low-cost sources of ultrashort THz pulses are required. Most broadband table-top emitters are driven by femtosecond laser pulses that generate the required THz charge current by appropriately mixing the various optical frequencies 9,10 . Sources made from solids usually consist of semiconducting or insulating structures with naturally or artificially broken inversion symmetry. When the incident photon energy is below the semiconductor band gap, optical rectification causes a charge displacement that follows the intensity envelope of the incident pump pulse 9,10,11,12,13,14,15,16,17 . For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.The so far most promising sources covering the full THz window are photocurrents in transient gas plasmas 9,10,25,26,27,28,29 . The downside of this appealing approach is that the underlying ionization process usually requires amplified laser pulses with high threshold energies on the order of 0....

Ultrafast charge and spin excitations in the elusive terahertz regime1, 2 of the electromagnetic spectrum play a pivotal role in condensed matter3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The electric field of free-space terahertz pulses has provided a direct gateway to manipulating the motion of charges on the femtosecond timescale6, 7, 8, 9. Here, we complement this process by showing that the magnetic component of intense terahertz transients enables ultrafast control of the spin degree of freedom. Single-cycle terahertz pulses switch on and off coherent spin waves in antiferromagnetic NiO at frequencies as high as 1 THz. An optical probe pulse with a duration of 8 fs follows the terahertz-induced magnetic dynamics directly in the time domain and verifies that the terahertz field addresses spins selectively by means of the Zeeman interaction. This concept provides a universal ultrafast means to control previously inaccessible magnetic excitations in the electronic ground state

In spin-based electronics, information is encoded by the spin state of electron bunches 1,2,3,4 . Processing this information requires the controlled transport of spin angular momentum through a solid 5,6 , preferably at frequencies reaching the so far unexplored terahertz (THz) regime 7,8,9 . Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin-current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is employed to drive spins 10,11,12 from a ferromagnetic Fe thin film into a nonmagnetic cap layer that has either low (Ru) or high (Au) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter 13 based on the inverse spin Hall effect 14,15 that converts the spin flow into a THz electromagnetic pulse. We find that the Ru cap layer yields a considerably longer spin-current pulse because electrons are injected in Ru d states that have a much smaller mobility than Au sp states 16 . Thus, spin current pulses and the resulting THz transients can be shaped by tailoring magnetic heterostructures, which opens the door for engineering high-speed spintronic devices as well as broadband THz emitters 7,8,9 , in particular covering the elusive range from 5 to 10THz.Contemporary electronics is based on the electron charge as information carrier whose presence or absence encodes the value of a bit. Much more efficient devices for low-power data storage and processing could be realized if the spin degree of freedom were used in addition 1,2,3,4 . The spintronics approach requires the generation and control of spin currents, that is, the transport of spin angular momentum through space 5,6 . Spintronic operations should be performed at a pace exceeding that of today's computers, which ultimately requires the generation of spin current pulses with terahertz (1 THz = 10 12 Hz) bandwidths 7,8 as well as the possibility to manipulate them in novel structures 17,18 . To date, femtosecond spin-current pulses have been successfully launched by optically exciting electrons in semiconductors 10 or ferromagnetic metals 11,12 . However, to enable ultrafast basic operations on these transients (such as buffering or delaying), their shape and propagation have to be controlled on subpicosecond time scales.Here, we employ magnetic heterostructures containing an optimally chosen nonmagnetic metallic layer whose electron mobility allows us either to trap or to transmit electrons and, thus, to engineer ultrafast spin pulses. The spin flow is probed in a contactless manner using the inverse spin Hall effect 14,15 (ISHE) that converts the spin current into a detectable THz electromagnetic pulse 13 . Our findings open up a route to device-oriented femtosecond spintronics as well as novel broadband emitters of THz radiation 7,8,9 .Our idea is illustrated in Fig. 1a, which shows a schematic of a ferromagnetic Fe film capped by a thin layer of Ru or Au. Absorption of a femtosecond laser pulse (photon energy 1...

CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.

Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.

Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Time-resolved photoelectron spectroscopy is employed to study the dynamics of photoexcited electrons in optimally doped Bi{2}Sr{2}CaCu{2}O{8+delta} (Bi-2212). Hot electrons thermalize in less than 50 fs and dissipate their energy on two distinct time scales (110 fs and 2 ps). These are attributed to the generation and subsequent decay of nonequilibrium phonons, respectively. We conclude that 20% of the total lattice modes dominate the coupling strength and estimate the second momentum of the Eliashberg coupling function lambdaOmega{0}{2}=360+/-30 meV{2}. For the typical phonon energy of copper-oxygen bonds (Omega{0} approximately 40-70 meV), this results in an average electron-phonon coupling lambda<0.25.

Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.