Coherent synchrotron radiation

M. Dohlus and T. Limberg, Calculation of coherent synchrotron radiation in the ttf-fel bunch compressor magnet chicanes. Nucl. Instrum. Methods Phys. Res. Sect. A 407(1-3), 278-284 (1998). 

Synchrotron infrared beamlines have been used for studying a number of exciting condensed matter systems from high temperature superconductors, to VO2, to semiconductors, to graphene. Here we overview some recent work about the fascinating optical properties of graphene and the use of coherent synchrotron radiation as a novel and powerful far-infrared source. 

Coherent Synchrotron Radiation (CSR) has opened up the ability to perform measurements within the so-called THz-gap using this novel high-power THz and sub-THz source [13, 14]. Several facilities now offer a few days a year where the accelerator is run in a mode to specifically produce the short electron bunches required for CSR. One scientific area where this has been specifically exploited is the low-frequency study of novel superconductors. A high-flux synchrotron source allows a high accuracy in the detection of small effects in the reflectivity spectra and overcomes the problem of the very low intensity transmitted by these systems. 

Figure 3.17 Measured far-IR intensity for the BESSY coherent synchrotron radiation (CSR) source, compared with mercury arc and Globar conventional thermal sources. In the picture the turn-on of the CSR source below 2cm is a real effect of the CSR emission process, while the drop off at the low frequency end is due to a combination of diffraction losses in the optical path of the beamline and to contributions of optical components in the interferometer. (Reprinted from ref. 102.)...

Schade, U., Ortolani, M., and Lee, J. (2007) THz experiments with coherent synchrotron radiation from BESSY 11. Syrwhrotron Rad. News, 20,17. 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

Leupold et al. were the first to report on coherent nuclear resonant scattering of synchrotron radiation from the 67.41 keV level of Ni. The time evolution of the forward scattering was recorded by employing the so-called nuclear lighthouse... 

The spin state of a paramagnetic system with total spin S wiU lift its (25 + l)-fold degeneracy under the influence of ligand fields (zero-field interaction) and applied fields (Zeeman interaction). The magnetic hyperfine field sensed by the iron nuclei is different for the 25 + 1 spin states in magnitude and direction. Therefore, the absorption pattern of a particular iron nucleus for the incoming synchrotron radiation and consequently, the coherently scattered forward radiation depends on how the electronic states are occupied at a certain temperature. 

Aside from its effects in high-energy experimentation, synchrotron radiation is of interest and value primarily as a source of tunable coherent x-rays. As summarized in a 1985 paper by Atwood, Halbach, and Kim... 

Snigerev, A., Snigireva, I., Kohn, V., Kuznetsov, S., and Schelokov, I. 1995. On the possibilities of X-ray phase-contrast microimaging by coherent high-energy synchrotron radiation. Rev. Sci. Instrum. 66, 5486-5492. 

Sandy, A. R., Lurio, L. B., Mochrie, S. G. J., Malik, A., Stephenson, G. B., Pelletier, J. F., and Sutton, M. (1999). Design and characterization of an undulator beamline optimized for small-angle coherent X-ray scattering at the advanced photon source. J. Synchrotron Radiat. 6, 1174-1184. 

An alternative track which has been pursued recently is time-resolved X-ray diffraction. An important goal is to improve the time resolution such that direct observation of the dynamics of the chemical bond is possible, corresponding to the observation of the time-dependent distribution of atomic positions. Pulsed X-rays are, e.g., obtained from synchrotron radiation or plasma sources. The temporal duration of these pulses is currently in the range of 100 ps-100 fs [4], Very recently, the first free-electron laser has produced short 100-10 fs coherent and highly intense bursts of X-rays [4—6]. 

First, we consider the situation where the coherence time, i.e., the time during which Cp(t) is substantially different from zero, is so short (sub-fs) that we can ignore all of the dynamics in the material system during this time. This is a relevant limit for X-rays from incoherent synchrotron radiation and laser plasma sources [4]. The time-dependent material system is created by the pump UV-laser pulse and it, therefore, contains amplitude on the electronic ground state as well as on one (or several) excited electronic state(s). Adopting a density-operator description, we expand the state of the material system in the adiabatic electronic basis,... 

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