Search Results (9 total)

At the 1.7-GeV electron storage ring BESSY II, a first source of synchrotron radiation with 100 fs pulse duration, variable (linear and circular) polarization, tunable photon energy (300 to 1400 eV), and excellent signal-to-background ratio was constructed and is now in routine operation.

Femtosecond far-infrared radiation pulses in the THz spectral range were observed as a consequence of the energy modulation of 1.7 GeV electrons by femtosecond laser pulses in the BESSY storage ring in order to generate femtosecond x-ray pulses (”femtoslicing”). In addition to being crucial for diagnostics of the laser-electron interaction, the THz radiation itself is useful for experiments where intense ultrashort THz pulses of well-defined temporal and spectral characteristics are required.

The energy modulation of 1.7-GeV electrons by femtosecond laser pulses was studied at the BESSY II “femtoslicing” source, a facility commissioned in 2004 for the purpose of producing sub-100 fs x-ray pulses. As a test case for future seeded free-electron lasers, the laser-electron interaction was investigated as function of various laser and electron beam parameters using different experimental methods.

We present a model describing high power stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), which enhances higher frequency coherent emission, and limits to stable emission due to an instability excited by the SR wakefield. It gives a quantitative explanation of several features of the recent observations of CSR at the BESSY II storage ring. We also use this model to optimize the performance of a source for stable CSR emission.

Infrared (IR) spectroscopy has been employed to investigate the c axis reflectivity of Bi₂Sr₂CaCu₂O₈ in the sub-THz frequency region. In order to reach this challenging frequency range a synchrotron source has been employed. Working in a special low-momentum compaction mode of operation where the electron bunch shape is significantly shortened and distorted, stable broadband coherent (superradiant) very far-infrared radiation is produced with orders of magnitude more intensity than conventional thermal and synchrotron sources. Using this source for reflectivity measurements we have been able to observe the Josephson plasma resonance (JPR) in optimally doped Bi₂Sr₂CaCu₂O₈. Evidence is found for an inhomogenous distribution of superfluid density within the sample. This source allows us to investigate charge dynamics in this extremely anisotropic superconductor, and opens up the possibility to study other highly correlated systems in this critical low-energy region.

A new technology for generating steady state, brilliant, broadband, coherent, far-infrared (FIR) radiation in electron storage rings is presented, suitable for FIR spectroscopy. An FIR power increase of up to 100 000 compared to the normal, incoherent synchrotron radiation in the range of ∼5 to ∼40  cm^-1 could be achieved. The source is up to 1000 times more brillant compared to a standard Hg arc lamp. The coherent synchrotron radiation is produced in a “low alpha” optics mode of the synchrotron light source BESSY, by bunch shortening and non-Gaussian bunch deformation.

At BESSY II it is demonstrated that far-infrared coherent synchrotron radiation (CSR) can be generated by a controlled, steady-state process at storage rings. As an indication for coherent emission, the radiated power grows with the square of the beam current. The spectrum was analyzed by an interferometer in the 1-mm to 0.3-mm wavelength range. The CSR was enhanced more than 3000 times above background; the incoherent radiation remained below the background level. Steady-state and bursting CSR were discriminated by time resolved analysis from μ seconds to seconds.

We report x-ray-absorption spectra at the oxygen K edge for LaNi_1-xM_xO₃ (M=Mn, Fe, and Co) for values of x spanning the entire composition range which exhibit metal-insulator transitions at critical values of x. This study clearly shows that the metal-insulator transition in the homovalent substituted series is achieved by transferring hole states from near E_F to an energy position above E_F due to the potential of the substituent. This is in strong contrast to hole doping, e.g., as in La_1-xSr_xMnO₃, where states are formed within the band gap of the underlying electronic structure. The sensitivity of the O K-edge spectra to substitution confirms the large oxygen 2p admixture to states at the Fermi level due to a substantial hopping interaction strength between the transition-metal 3d and oxygen 2p derived states in these perovskite oxides. The present results suggest that the electronic structures of these perovskite oxides are dominated by local interactions.

The electronic structure of crystalline CoSi₂ produced by ion-beam synthesis has been studied. Using fluorescence spectroscopies, we have taken advantage of the large photon penetration depth to obtain information from silicide layers, buried several hundred Ångströms deep in Si wafers, prepared by ion-beam synthesis. The unoccupied local Co d density of states (DOS) was determined via the Co L₃ near-edge absorption spectrum, measured in the fluorescence-yield mode. The occupied local Co d DOS was determined via the Co L₃ emission spectrum, excited both with photons and with high-energy electrons. The results show that the buried layers have the electronic structure of crystalline CoSi₂ and that the states in the vicinity of the Fermi level have nonbonding character.