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- ItemAn investigation on THz yield from laser-produced solid density plasmas at relativistic laser intensities([London] : IOP, 2018) Herzer, S.; Woldegeorgis, A.; Polz, J.; Reinhard, A.; Almassarani, M.; Beleites, B.; Ronneberger, F.; Grosse, R.; Paulus, G.G.; Hübner, U.; May, T.; Gopal, A.We experimentally characterize the generation of high-power terahertz radiation (THz) at the rear surface of a target irradiated by multiple laser pulses. A detailed dependence of the THz yield as a function of laser pulse duration, energy, target material and thickness is presented. We studied the THz radiation emitted mainly in two directions from the target rear surface, namely target normal (acceptance angle 0.87 sr) and non-collinear direction (perpendicular to the target normal direction—acceptance angle 4.12 sr). Independent measurements based on electro-optic diagnostics and pyroelectric detector were employed to estimate the THz yield. Most of the energy is emitted at large angles relative to the target normal direction. THz yield increases with incident laser intensity and thinner targets are better emitters of THz radiation compared to thicker ones.
- ItemBright optical centre in diamond with narrow, highly polarised and nearly phonon-free fluorescence at room temperature([London] : IOP, 2017) John, Roger; Lehnert, Jan; Mensing, Michael; Spemann, Daniel; Pezzagna, Sébastien; Meijer, JanUsing shallow implantation of ions and molecules with masses centred at 27 atomic mass units (amu) in diamond, a new artificial optical centre with unique properties has been created. The centre shows a linearly polarised fluorescence with a main narrow emission line mostly found at 582 nm, together with a weak vibronic sideband at room temperature. The fluorescence lifetime is ∼2 ns and the brightest centres are more than three times brighter than the nitrogen-vacancy centres. A majority of the centres shows stable fluorescence whereas some others present a blinking behaviour, at faster or slower rates. Furthermore, a second kind of optical centre has been simultaneously created in the same diamond sample, within the same ion implantation run. This centre has a narrow zero-phonon line (ZPL) at ∼546 nm and a broad phonon sideband at room temperature. Interestingly, optically detected magnetic resonance (ODMR) has been measured on several single 546 nm centres and two resonance peaks are found at 0.99 and 1.27 GHz. In view of their very similar ODMR and optical spectra, the 546 nm centre is likely to coincide with the ST1 centre, reported once (with a ZPL at 550 nm), but of still unknown nature. These new kinds of centres are promising for quantum information processing, sub-diffraction optical imaging or use as single-photon sources.
- ItemQuasi-linearly polarized hybrid modes in tapered and metal-coated tips with circular apertures: understanding the functionality of aperture tips([London] : IOP, 2017) Tugchin, B.N.; Janunts, N.; Steinert, M.; Dietrich, K.; Kley, E.B.; Tünnermann, A.; Pertsch, T.In this study, we investigate analytically and experimentally the roles of quasi-linearly polarized (LP), hybrid, plasmonic and photonic modes in optical detection and excitation with aperture tips in scanning near-field optical microscopy. Aperture tips are tapered and metal-coated optical fibers where small circular apertures are made at the apex. In aperture tips, there exist plasmonic modes that are bound at the interface of the metal cladding to the inner dielectric fiber and photonic modes that are guided in the area of the increased index in the dielectric fiber core. The fundamental photonic mode, although excited by the free-space Gaussian beam, experiences cutoff and turns into an evanescent mode. The photonic mode also becomes lossier than the plasmonic mode toward the tip aperture, and its power decay due to absorption and reflection is expected to be at least 10−9. In contrast, the fundamental plasmonic mode has no cutoff and thus reaches all the way to the tip aperture. Due to the non-adiabaticity of both modes' propagations through the taper below a core radius of 600 nm, there occurs coupling between the modes. The transmission efficiency of the plasmonic mode, including the coupling efficiency and the propagation loss, is expected to be about 10−6 that is at least 3 orders of magnitude larger than that of the photonic mode. Toward the tip aperture, the longitudinal field of the photonic mode becomes stronger than the transverse ones while the transverse fields always dominate for the plasmonic mode. Experimentally, we obtain polarization resolved images of the near-field at the tip aperture and compare with the x- and y-components of the fundamental quasi-LP plasmonic and photonic modes. The results show that not only the pattern but also the intensity ratios of the x- and y-components of the aperture near-field match with that of the fundamental plasmonic mode. Consequently, we conclude that only the plasmonic mode reaches the tip aperture and thus governs the near-field interaction outside the tip aperture. Our conclusion remains valid for all aperture tips regardless of the cladding metal type that mainly influences the total transmission efficiency of the aperture tip.
- ItemTwinned-domain-induced magnonic modes in epitaxial LSMO/STO films([London] : IOP, 2017) Wahlström, Erik; Macià, Ferran; Boschker, Jos E; Monsen, Åsmund; Nordblad, Per; Mathieu, Roland; Kent, Andrew D; Tybell, ThomasThe use of periodic magnetic structures to control the magneto-dynamic properties of materials-Magnonics-is a rapidly developing field. In the last decade, a number of studies have shown that metallic films can be patterned or combined in patterns that give rise to well-defined magnetization modes, which are formed due to band folding or band gap effects. To explore and utilize these effects in a wide frequency range, it is necessary to pattern samples at the sub-micrometer scale. However, it is still a major challenge to produce low-loss magnonic structures with periodicities at such length scales. Here, we show that for a prototypical perovskite, La0.7 Sr0.3MnO3, the twinned structural order can be used to induce a magnetic modulation with a period smaller than 100 nm, demonstrating a bottomup approach for magnonic crystal growth. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
- ItemEnsemble analysis of complex network properties—an MCMC approach([London] : IOP, 2022) Pfeffer, Oskar; Molkenthin, Nora; Hellmann, FrankWhat do generic networks that have certain properties look like? We use relative canonical network ensembles as the ensembles that realize a property R while being as indistinguishable as possible from a background network ensemble. This allows us to study the most generic features of the networks giving rise to the property under investigation. To test the approach we apply it to study properties thought to characterize ‘small-world networks’. We consider two different defining properties, the ‘small-world-ness’ of Humphries and Gurney, as well as a geometric variant. Studying them in the context of Erdős-Rényi and Watts-Strogatz ensembles we find that all ensembles studied exhibit phase transitions to systems with large hubs and in some cases cliques. Such features are not present in common examples of small-world networks, indicating that these properties do not robustly capture the notion of small-world networks. We expect the overall approach to have wide applicability for understanding network properties of real world interest, such as optimal ride-sharing designs, the vulnerability of networks to cascades, the performance of communication topologies in coordinating fluctuation response or the ability of social distancing measures to suppress disease spreading.
- ItemDiffraction imaging of light induced dynamics in xenon-doped helium nanodroplets([London] : IOP, 2022-11-30) Langbehn, B.; Ovcharenko, Y.; Clark, A.; Coreno, M.; Cucini, R.; Demidovich, A.; Drabbels, M.; Finetti, P.; Di Fraia, M.; Giannessi, L.; Grazioli, C.; Iablonskyi, D.; LaForge, A.C.; Nishiyama, T.; Oliver Álvarez de Lara, V.; Peltz, C.; Piseri, P.; Plekan, O.; Sander, K.; Ueda, K.; Fennel, T.; Prince, K.C.; Stienkemeier, F.; Callegari, C.; Möller, T.; Rupp, D.We explore the light induced dynamics in superfluid helium nanodroplets with wide-angle scattering in a pump–probe measurement scheme. The droplets are doped with xenon atoms to facilitate the ignition of a nanoplasma through irradiation with near-infrared laser pulses. After a variable time delay of up to 800 ps, we image the subsequent dynamics using intense extreme ultraviolet pulses from the FERMI free-electron laser. The recorded scattering images exhibit complex intensity fluctuations that are categorized based on their characteristic features. Systematic simulations of wide-angle diffraction patterns are performed, which can qualitatively explain the observed features by employing model shapes with both randomly distributed as well as structured, symmetric distortions. This points to a connection between the dynamics and the positions of the dopants in the droplets. In particular, the structured fluctuations might be governed by an underlying array of quantized vortices in the superfluid droplet as has been observed in previous small-angle diffraction experiments. Our results provide a basis for further investigations of dopant–droplet interactions and associated heating mechanisms.
- ItemFemtosecond XUV–IR induced photodynamics in the methyl iodide cation([London] : IOP, 2021) Murillo-Sánchez, Marta L.; Reitsma, Geert; Poullain, Sonia Marggi; Fernández-Milán, Pedro; González-Vázquez, Jesús; de Nalda, Rebeca; Martín, Fernando; Vrakking, Marc J. J.; Kornilov, Oleg; Bañares, LuisThe time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV-IR pump-probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited state potential energy surface. Photoelectrons and photofragment ions - and I+ - are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state, considering the presence of the IR pulse. The and I+ pump-probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the state, mainly through the coupling to the ground state and to the excited state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of fragments after the cation is transferred to the ground states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
- ItemDielectric function decomposition by dipole interaction distribution: Application to triclinic K2Cr2O7([London] : IOP, 2020) Sturm, C.; Höfer, S.; Hingerl, K.; Mayerhöfe, T.G.; Grundmann, M.Here we present a general approach for the description for the frequency dependent dielectric tensor coefficients for optically anisotropic materials. Based on symmetry arguments we show that the components of the dielectric tensor are in general not independent of each other. For each excitation there exists an eigensystem, where its contribution to the dielectric tensor can be described by a diagonal susceptibility tensor. From the orientation of the eigensystem and the relative magnitude of the tensor elements, the dipole interaction distribution in real space can be deduced. In the limiting cases, the oriented dipole approach as well as the tensor of isotropic and uniaxial materials are obtained. The application of this model is demonstrated exemplarily on triclinic K2Cr2O7 and the orientation and directional distribution of the corresponding dipole moments in real space are determined. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
- ItemLaser stripping of hydrogen atoms by direct ionization([London] : IOP, 2015) Brunetti, E.; Becker, W.; Bryant, H.C.; Jaroszynski, D.A.; Chou, W.Direct ionization of hydrogen atoms by laser irradiation is investigated as a potential new scheme to generate proton beams without stripping foils. The time-dependent Schrödinger equation describing the atom-radiation interaction is numerically solved obtaining accurate ionization cross-sections for a broad range of laser wavelengths, durations and energies. Parameters are identified where the Doppler frequency up-shift of radiation colliding with relativistic particles can lead to efficient ionization over large volumes and broad bandwidths using currently available lasers.
- ItemCorrigendum: Generation of high-quality GeV-class electron beams utilizing attosecond ionization injection (2021 New J. Phys. 23 043016)([London] : IOP, 2021) Lécz, Zsolt; Andreev, Alexander; Kamperidis, C.; Hafz, NasrAcceleration of electrons in laser-driven plasma wakefields has been extended up to the ∼8 GeV energy within a distance of tens of centimeters. However, in applications, requiring small energy spread within the electron bunch, only a small portion of the bunch can be used and often the low-energy electrons represent undesired background in the spectrum. We present a compact and tunable scheme providing clean and mono-energetic electron bunches with less than one percent energy spread and with central energy on the GeV level. It is a two-step process consisting of ionization injection with attosecond pulses and acceleration in a capillary plasma wave-guide. Semi-analytical theory and particle-in-cell simulations are used to accurately model the injection and acceleration steps.