Search

Organic photovoltaic devices operate by absorbing light and generating current. These two processes are governed by the optical and transport properties of the organic semiconductor. Despite their common microscopic origin—the electronic structure—disclosing their dynamical interplay is far from trivial. Here we address this issue by time-resolved photoemission to directly investigate the correlation between the optical and transport response in organic materials. We reveal that optical generation of non-interacting excitons in a fullerene film results in a substantial redistribution of all transport levels (within 0.4 eV) of the non-excited molecules. As all observed dynamics evolve on identical timescales, we conclude that optical and transport properties are completely interlinked. This finding paves the way for developing novel concepts for transport level engineering on ultrafast time scales that could lead to novel functional optoelectronic devices., Correlating the optoelectronic properties in organic semiconductors is vital to realizing devices with high performance and new functionalities. Here, the authors report the role of optically excited charge transfer excitons on energy level alignment of the transport levels in organic thin films.

Significance Electron–electron interactions are among the fastest processes in materials that determine their fascinating properties, occurring on attosecond timescales on up (1 as = 10 −18 s). The recent development of attosecond angle-resolved photoemission spectroscopy (atto-ARPES) using high harmonic generation has opened up the possibility of probing electron–electron interactions in real time. In this paper, we distinguish electron–electron screening and charge scattering in the time domain in individual energy bands within a solid. These results open up new possibilities for probing fundamental electron–electron interactions in a host of materials including magnetic, superconducting, and advanced quantum materials.

Using spin- and time-resolved XUV photoemission, researchers monitor the band structure evolution of Co during its phase transition., The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.

We reveal an explicit strategy to design the magneto-optic response of a magneto-plasmonic crystal by correlating near- and far-fields effects. We use photoemission electron microscopy to map the spatial distribution of the electric near-field on a nanopatterned magnetic surface that supports plasmon polaritons. By using different photon energies and polarization states of the incident light we reveal that the electric near-field is either concentrated in spots forming a hexagonal lattice with the same symmetry as the Ni nanopattern or in stripes oriented along the Γ−K direction of the lattice and perpendicular to the polarization direction. We show that the polarization-dependent near-field enhancement on the patterned surface is directly correlated to both the excitation of surface plasmon polaritons on the patterned surface as well as the enhancement of the polar magneto-optical Kerr effect. We obtain a relationship between the size of the enhanced magnetooptical behavior and the polarization and wavelength of optical excitation. The engineering of the magneto-optic response based on the plasmon-induced modification of the optical properties introduces the concept of a magneto-plasmonic meta-structure., E.Th.P. acknowledges Carl Zeiss Foundation for financial support. A.G.-M. acknowledges the Spanish Ministry of Economy and Competitiveness (Contract No. MAT2014- 58860-P) and the Comunidad de Madrid (Contract No. S2013/MIT-2740). P.T. acknowledges financial support through the Excellence Initiative (DFG/GSC 266). V.K. acknowledges financial support from the Knut and Alice Wallenberg Foundation. We thank Piotr Patoka for the preparation of the polystyrene bead template. We gratefully acknowledge the Deutsche Forschungsgemeinschaft program SFB/TRR 173: SPIN+X.

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; We report magnetic domain imaging of a terbium cobalt (TbCo) alloy thin film with a high perpendicular magnetic anisotropy in one- and two-photon photoemission electron microscopy (PEEM). Both photoemission processes deliver a clear magnetic circular dichroism (MCD) whose strength is strongly energy dependent. Comparing the energy dependence of the MCD signal in one- and two-photon photoemission, we conclude that the magnetic contrast is mainly an initial state effect. Our results ultimately show that MCD contrast can be obtained in PEEM in valence band photoemission from a material supporting all-optical magnetization switching. This opens the way for the investigation of the all-optical switching process with simultaneous ultrahigh temporal and spatial resolution.