Hanbury Brown–Twiss Interferometry at a Free-Electron Laser

Abstract

We present measurements of second- and higher-order intensity correlation functions (so-called Hanbury Brown-Twiss experiment) performed at the free-electron laser (FEL) FLASH in the non-linear regime of its operation. We demonstrate the high transverse coherence properties of the FEL beam with a degree of transverse coherence of about 80% and degeneracy parameter of the order 10(9) that makes it similar to laser sources. Intensity correlation measurements in spatial and frequency domain gave an estimate of the FEL average pulse duration of 50 fs. Our measurements of the higher-order correlation functions indicate that FEL radiation obeys Gaussian statistics, which is characteristic to chaotic sources.

Radiation characteristics of extreme UV and soft X-ray sources

“…Existing coherence studies pose simplifying assumptions on the FEL beam [23,24,25], e.g., that it can be described by the Gaussian Schell-model. Here, the four-dimensional mutual coherence function (MCF) is determined for the photon beam of FLASH without theoretical restrictions.…”

“…The spatial coherence properties of free-electron lasers are addressed in many publications employing Young's double pinhole experiment [23], speckle statistics [24] or Hanbury Brown-Twiss interferometry [25]. However, in these studies a comprehensive characterization of the four-dimensional mutual coherence function is only possible if the FEL beam follows the Gaussian Schell-model.…”

Quantum imaging with incoherently scattered light from a free-electron laser

“…In this way interference fringes show up even in the complete absence of first-order coherence, allowing the extraction of structural information from incoherently emitting objects. This has been applied in Earth-bound stellar interferometry to measure the angular diameter of stars with 100-fold increased resolution 13 or to reveal the spatial and statistical properties of pulsed FEL sources 20,21 .Extending this concept to arbitrary arrangements of incoherently scattering emitters enables one to use intensity correlations for imaging applications. This has been demonstrated recently for one-dimensional arrays of emitters in the visible range of the spectrum [22][23][24] , where a spatial resolution even below the canonical Abbe limit has been achieved.…”

“…In this way interference fringes show up even in the complete absence of first-order coherence, allowing the extraction of structural information from incoherently emitting objects. This has been applied in Earth-bound stellar interferometry to measure the angular diameter of stars with 100-fold increased resolution 13 or to reveal the spatial and statistical properties of pulsed FEL sources 20,21 .…”

“…been applied in Earth-bound stellar interferometry to measure the angular diameter of stars with 100-fold increased resolution 13 or to reveal the spatial and statistical properties of pulsed FEL sources 20,21 .…”