"Matter in Coherent Light"
While the coherence properties of visible light lasers are nowadays not only used for scientific experiments but are meanwhile applied also in every day life, the exploitation of coherent photon beams at higher photon energies, especially in the X-ray regime, has been hampered by the availability of suitable sources. However, new third generation synchrotron radiation sources provide a considerable flux of coherent photons also in the X-ray regime. This is due to their small source sizes which are in the 10 to 100 micrometer range and the large source to sample distances of typically about 40 to 100 m. The corresponding transverse coherence lengths are already in the 10 micrometer range. A totally new world in terms of coherence properties for photon beams in the VUV/XUV and X-ray energy range has been opened up by free electron lasers (FELs) which are almost completely coherent in the transverse direction. The FLASH FEL Facility is already operating in the VUV/XUV range (up to 200eV photon energy) at DESY, Hamburg. Free electron lasers providing coherent beams also in the keV photon energy range are under construction (LCLS at SLAC, Stanford, SCCS at Spring8, Japan) or in the final planning stage (European XFEL at DESY Hamburg). In addition to the coherence properties, these new FELs combine some other radiation properties like extremely short and intense photon pulses. The measured photon pulse length at the FLASH facility is about 20-30 fs at 13 nm wavelength. Generally speaking, such a FEL source provides roughly the same number of photons in one 20-100 fs pulse as a modern synchrotron radiation source does in one second. These properties will enable a wealth of new experiments to explore the static and dynamic properties of matter at both very short time scales and atomic resolution.
Widely applied at new third generations synchrotron radiation sources are X-ray correlation spectroscopy (XPCS) techniques to study the dynamic properties of matter on the nanoscale in a time – Q-vector range that is not accessible by other methods. The study of fast (t << 1 microsecond) dynamics (at large momentum transfers Q) was up to now restricted to the energy domain (inelastic) techniques. With the new FEL sources one will be able to study fast dynamics in the time domain which is of outmost importance for a variety of phenomena e.g. for non-equilibrium dynamics. One will start in the short term with existing synchrotron sources but then focus on the use of PETRA III (from 2009), LCLS from 2010 and the XFEL from 2012/13. The expected high degree of coherence will give access furthermore to non-gaussian dynamics in complex fluids and other phenomena. Novel, coherence preserving optics coupled with fast 2-D detectors will be mandatory for the success of these projects.
The properties of these sources are particularly well suited to unravel the structure of nanoscopic objects via lensless and other imaging techniques requiring coherent radiation. In principle, the achievable resolution of the newly developing lensless imaging techniques is only limited by the maximum achievable Q-vector modulus. Since the method works in the far field, mechanical stability issues of the set up are less critical than in other imaging methods. However, in order to solve the phase problem upon inversion, the diffraction image has to be sufficiently over-sampled. This technique is rapidly developing at present and there are still a number of critical questions that are under discussion. Other imaging techniques exploiting the coherence properties of X-ray beams like phase contrast microtomography or holo-tomography are more developed but on several aspects active research is also still onging in this field. Compared to complementary imaging techniques like electron microscopy or light microscopy, coherent X-ray imaging has the advantage of being able to provide true three-dimensional information also of opaque objects thicker than several micrometers due to the penetration properties of the radiation. The ultimate long-term goal for diffraction imaging is “Single Particle Imaging”, e.g. the structural investigation of single non-translation periodic biological nano-objects using coherent X-rays from future XFELs. The route to such a structure determination at highest possible spatial resolution is difficult and needs to address complex issues like handling and orientation of nano objects, control of radiation damage, achievement of short (<100 fs) pulse lengths, improved reconstruction mechanisms to overcome the phase-problem, etc., which only can be solved in a multi disciplinary approach. In the near future these issues will be addressed in a modular fashion in the short and medium term e.g. by operating with larger objects at novel synchrotron radiation sources (e.g. PETRA III) and at longer wavelengths at FLASH Facilty.
Wilhelm und Else Heraeus Stiftung
While the coherence properties of visible light lasers are nowadays not only used for scientific experiments but are meanwhile applied also in every day life, the exploitation of coherent photon beams at higher photon energies, especially in the X-ray regime, has been hampered by the availability of suitable sources. However, new third generation synchrotron radiation sources provide a considerable flux of coherent photons also in the X-ray regime. This is due to their small source sizes which are in the 10 to 100 micrometer range and the large source to sample distances of typically about 40 to 100 m. The corresponding transverse coherence lengths are already in the 10 micrometer range. A totally new world in terms of coherence properties for photon beams in the VUV/XUV and X-ray energy range has been opened up by free electron lasers (FELs) which are almost completely coherent in the transverse direction. The FLASH FEL Facility is already operating in the VUV/XUV range (up to 200eV photon energy) at DESY, Hamburg. Free electron lasers providing coherent beams also in the keV photon energy range are under construction (LCLS at SLAC, Stanford, SCCS at Spring8, Japan) or in the final planning stage (European XFEL at DESY Hamburg). In addition to the coherence properties, these new FELs combine some other radiation properties like extremely short and intense photon pulses. The measured photon pulse length at the FLASH facility is about 20-30 fs at 13 nm wavelength. Generally speaking, such a FEL source provides roughly the same number of photons in one 20-100 fs pulse as a modern synchrotron radiation source does in one second. These properties will enable a wealth of new experiments to explore the static and dynamic properties of matter at both very short time scales and atomic resolution.
Widely applied at new third generations synchrotron radiation sources are X-ray correlation spectroscopy (XPCS) techniques to study the dynamic properties of matter on the nanoscale in a time – Q-vector range that is not accessible by other methods. The study of fast (t << 1 microsecond) dynamics (at large momentum transfers Q) was up to now restricted to the energy domain (inelastic) techniques. With the new FEL sources one will be able to study fast dynamics in the time domain which is of outmost importance for a variety of phenomena e.g. for non-equilibrium dynamics. One will start in the short term with existing synchrotron sources but then focus on the use of PETRA III (from 2009), LCLS from 2010 and the XFEL from 2012/13. The expected high degree of coherence will give access furthermore to non-gaussian dynamics in complex fluids and other phenomena. Novel, coherence preserving optics coupled with fast 2-D detectors will be mandatory for the success of these projects.
The properties of these sources are particularly well suited to unravel the structure of nanoscopic objects via lensless and other imaging techniques requiring coherent radiation. In principle, the achievable resolution of the newly developing lensless imaging techniques is only limited by the maximum achievable Q-vector modulus. Since the method works in the far field, mechanical stability issues of the set up are less critical than in other imaging methods. However, in order to solve the phase problem upon inversion, the diffraction image has to be sufficiently over-sampled. This technique is rapidly developing at present and there are still a number of critical questions that are under discussion. Other imaging techniques exploiting the coherence properties of X-ray beams like phase contrast microtomography or holo-tomography are more developed but on several aspects active research is also still onging in this field. Compared to complementary imaging techniques like electron microscopy or light microscopy, coherent X-ray imaging has the advantage of being able to provide true three-dimensional information also of opaque objects thicker than several micrometers due to the penetration properties of the radiation. The ultimate long-term goal for diffraction imaging is “Single Particle Imaging”, e.g. the structural investigation of single non-translation periodic biological nano-objects using coherent X-rays from future XFELs. The route to such a structure determination at highest possible spatial resolution is difficult and needs to address complex issues like handling and orientation of nano objects, control of radiation damage, achievement of short (<100 fs) pulse lengths, improved reconstruction mechanisms to overcome the phase-problem, etc., which only can be solved in a multi disciplinary approach. In the near future these issues will be addressed in a modular fashion in the short and medium term e.g. by operating with larger objects at novel synchrotron radiation sources (e.g. PETRA III) and at longer wavelengths at FLASH Facilty.
Wilhelm und Else Heraeus Stiftung