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The study of ultrafast dynamics in magnetic materials provides rich opportunities for greater fundamental understanding of correlated phenomena in solid-state matter, because many of the basic microscopic mechanisms involved are as-yet unclear and are still being uncovered. Recently, two different possible mechanisms have been proposed to explain ultrafast laser induced magnetization dynamics: spin currents and spin-flip scattering. In this work, we use multilayers of Fe and Ni with different metals and insulators as the spacer material to conclusively show that spin currents can have a significant contribution to optically induced magnetization dynamics, in addition to spin-flip scattering processes. Moreover, we can control the competition between these two processes, and in some cases completely suppress interlayer spin currents as a sample undergoes rapid demagnetization. Finally, by reversing the order of the Fe/Ni layers, we experimentally show that spin currents are directional in our samples, predominantly flowing from the top to the bottom layer.

The spatiotemporal evolution of a SPP wave packet with femtosecond duration is experimentally investigated in two different plasmonic focusing structures. A two-dimensional reconstruction of the plasmonic field in space and time is possible by the numerical analysis of interferometric time-resolved photoemission electron microscopy data. We show that the time-integrated and time-resolved view onto the wave packet dynamics allow one to characterize and compare the capabilities of two-dimensional components for use in plasmonic devices operating with ultrafast pulses.

The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.

Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro- or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.

We introduce a spectroscopic method that determines nonlinear quantum mechanical response functions beyond the optical diffraction limit and allows direct imaging of nanoscale coherence. In established coherent two-dimensional (2D) spectroscopy, four-wave-mixing responses are measured using three ingoing waves and one outgoing wave; thus, the method is diffraction-limited in spatial resolution. In coherent 2D nanoscopy, we use four ingoing waves and detect the final state via photoemission electron microscopy, which has 50-nanometer spatial resolution. We recorded local nanospectra from a corrugated silver surface and observed subwavelength 2D line shape variations. Plasmonic phase coherence of localized excitations persisted for about 100 femtoseconds and exhibited coherent beats. The observations are best explained by a model in which coupled oscillators lead to Fano-like resonances in the hybridized dark- and bright-mode response.

We investigate the ultrafast demagnetization for two Heusler alloys (Co₂Mn(1-x)FexSi) with a different lineup of the minority band gap and the Fermi level. Even though electronic spin-flip transitions are partially blocked by the band gap in one compound, the respective magnetization dynamics, as measured by the time-resolved Kerr effect, are remarkably similar. Based on a dynamical model that includes momentum and spin-dependent carrier scattering, we show that the magnetization dynamics are dominated by hole spin-flip processes, which are not influenced by the gap.

We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration.

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the light-matter interaction in spectroscopic methods as well as in nanophotonic applications.

We use few-femtosecond soft x-ray pulses from high-harmonic generation to extract element-specific demagnetization dynamics and hysteresis loops of a compound material for the first time. Using a geometry where high-harmonic beams are reflected from a magnetized Permalloy grating, large changes in the reflected intensity of up to 6% at the M absorption edges of Fe and Ni are observed when the magnetization is reversed. A short pump pulse is used to destroy the magnetic alignment, which allows us to measure the fastest, elementally specific demagnetization dynamics, with 55 fs time resolution. The use of high harmonics for probing magnetic materials promises to combine nanometer spatial resolution, elemental specificity, and femtosecond-to-attosecond time resolution, making it possible to address important fundamental questions in magnetism.

Optically active surfaces that can provide strong localization of electromagnetic fields at predefined points are desired for applications that require high spatial resolution and high sensitivity. Here, we examine the geometric influences on, and polarization dependencies of, electromagnetic near fields on the surface of an array of tailor designed, mesoscalic, silver-coated structures with threefold symmetry characteristics. For spatially resolved mapping of the electromagnetic near fields and examining the influence of polarization, we use a photoelectron emission microscope. We find that the investigated structures not only provide an increase of the near-field intensity at their boundaries, but also that the symmetry centers of the structures focus energy in a polarization dependent manner. Changing the polarization of the incident light enables the localization of near-field intensities without displacing the excitation. Hence we show that breaking of symmetry can provide controllable centers of "hot spots" for the basis of an improved design to gain more efficient surface structures.

A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt-CuPc interface. We report a spin injection efficiency of 85-90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10-30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss < or = 200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.

This Letter presents a theoretical investigation of ultrafast spin-dependent carrier dynamics in semiconductors due to strong spin-orbit coupling using holes in bulk GaAs as a model system. By computing the microscopic carrier dynamics in the anisotropic hole-band structure including spin-orbit coupling, we obtain spin-relaxation times in quantitative agreement with measured hole-spin relaxation times [Phys. Rev. Lett. 89, 146601 (2002)10.1103/PhysRevLett.89.146601]. We show that different optical techniques for the measurement of hole-spin dynamics yield different results, in contrast to the case of electron-spin dynamics.

Human vocalizations (HV), as well as environmental sounds, convey a wide range of information, including emotional expressions. The latter have been relatively rarely investigated, and, in particular, it is unclear if duration-controlled non-linguistic HV sequences can reliably convey both positive and negative emotional information. The aims of the present psychophysical study were: (i) to generate a battery of duration-controlled and acoustically controlled extreme valence stimuli, and (ii) to compare the emotional impact of HV with that of other environmental sounds. A set of 144 HV and other environmental sounds was selected to cover emotionally positive, negative, and neutral values. Sequences of 2 s duration were rated on Likert scales by 16 listeners along three emotional dimensions (arousal, intensity, and valence) and two non-emotional dimensions (confidence in identifying the sound source and perceived loudness). The 2 s stimuli were reliably perceived as emotionally positive, negative or neutral. We observed a linear relationship between intensity and arousal ratings and a "boomerang-shaped" intensity-valence distribution, as previously reported for longer, duration-variable stimuli. In addition, the emotional intensity ratings for HV were higher than for other environmental sounds, suggesting that HV constitute a characteristic class of emotional auditory stimuli. In addition, emotionally positive HV were more readily identified than other sounds, and emotionally negative stimuli, irrespective of their source, were perceived as louder than their positive and neutral counterparts. In conclusion, HV are a distinct emotional category of environmental sounds and they retain this emotional pre-eminence even when presented for brief periods.

A custom-made Omicron-compatible sample holder for time-resolved photoelectron emission microscopy experiments is presented. It comprises a sample plate with four contacts that hosts a chip carrier where the semiconductor substrate is mounted. Covering the sample holder, a 6 mm diameter mask protects electrostatically the sample from the extractor lens voltage while keeping the imaging quality unperturbed. The improvements are a greater sample lifetime and the ability to withstand much higher currents in the stripline that provides the magnetic pulse to the magnetic microstructure.

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by two-photon photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.