Laurent Nahon

Mirror degradation and heating in storage ring FELs

Abstract Storage Ring Free Electrons Lasers (SRFELs) in the UV are now utilized as laser facilities for time-resolved fluorescence experiments or two-colour experiments with synchronized FEL and synchrotron radiations. Such experiments set new requirements for the laser and, consequently, for the optics such as long duration lasing, high laser power and short wavelengths. As these features imply operation of the storage ring at higher energies, cavity mirrors are subjected to radiation from undulator harmonics under more drastic conditions: increased power leading to heating of the front mirror, and degradation induced by undulator radiation extending towards X-rays. Also, for short λ operation, new difficulties arise: some materials become more absorbant and scattering losses increase.

Optical design and performance of the IR microscope beamline at SUPERACO-France

Infrared microspectrometry, using a synchrotron radiation source, has been developed at Super-ACO (LURE-France). In order to accommodate for constrained horizontal (45 mrad) and vertical (18 mrad) collection angles, a particular care has been devoted to the design and making of the extraction optics, in order to achieve the highest brightness as possible, for small area illumination. Experimentally, a net gain of one hundred in Signal to Noise Ratio has been measured for an upper aperture of 3 X 3 micrometers 2. Several applications are currently underway, and some of them, related to Biomedical Science are reported in this paper.

Dynamic behavior of the Super-ACO FEL micropulse

Abstract Recently, we have conducted temporal and spectral studies of the Super-ACO FEL micropulse. These studies are indispensable to supply a stable and well characterized light source for application experiments. The result of the temporal study has already been presented. In this paper, we present the spectral features of the FEL micropulse, including spectral widths, spectral evolution and the relation between temporal width and spectral width of the laser micropulse.

Applications of UV-storage ring free electron lasers: the case of super-ACO

Abstract The potential of UV-storage ring free electron lasers (SRFELs) for the performance of original application experiments is shown with a special emphasis concerning their combination with the naturally synchronized synchrotron radiation (SR). The first two-color FEL+SR experiment, performed in surface science at Super-ACO is reported. The experimental parameters found to be the most important as gathered from the acquired experience, are underlined and discussed. Finally, future prospects for the scientific program of the Super-ACO FEL are presented with two-color experiments combining the FEL with SR undulator-based XUV and VUV beamlines as well as with a SR white light bending magnet beamline emiting in the IR-UV ( 20 μm – 0.25 μm ) .

Two-color experiments combining the UV storage ring free-electron laser and the SA5 IR beamline at Super-ACO

The UV-storage ring Free Electron Laser (FEL) operating at Super-ACO is a tunable, coherent and intense (up to 300 mW) photon source in the near-UV range (300 - 350 nm). Besides, it has the unique feature to be synchronized in a one-to-one shot ratio with the Synchrotron Radiation (SR) at the high repetition rate of 8.32 MHz. This FEL + SR combination appears to be very powerful for the performance of pump- probe time-resolved and/or frequency-resolved experiments on the sub-ns and ns time-scales. In particular, there is a strong scientific case for the combination of the recently- commissioned SA5 Infra-Red Synchrotron Radiation beamline with the UV-FEL, for the performance of transient IR- absorption spectroscopy on FEL-excited samples with a Fourier-transform spectrometer coupled with a microscope allowing high spectral and spatial resolution. The principle and interest of the two-color combination altogether with the description of both the FEL and the SA5 IR beamline are presented. The first synchronization signal between the IR and the UV beams is shown. The correct spatial overlap between the UV (FEL) and the IR (SR) photon beams is demonstrated by monitoring via IR-spectro-microscopy the time evolution of a single mineral particulate (kaolinite) under UV-FEL irradiation.

Transient absorption spectroscopy in biology using the Super-ACO storage ring FEL and the synchrotron radiation combination

The Super-ACO storage ring FEL, covering the UV range down to 300 nm with a high average power (300 mW at 350 nm) together with a high stability and long lifetime, is a unique tool for the performance of users applications. We present here the first pump-probe two color experiments on biological species using a storage ring FEL coupled to the synchrotron radiation. The intense UV pulse of the Super-ACO FEL is used to prepare a high initial concentration of chromophores in their first singlet electronic excited state. The nearby bending magnet synchrotron radiation provides, on the other hand a pulsed, white light continuum (UV–IR), naturally synchronized with the FEL pulses and used to probe the photochemical subsequent events and the associated transient species. We have demonstrated the feasibility with a dye molecule (POPOP) observing a two-color effect, signature of excited state absorption and a temporal signature with Acridine. Applications on various chromophores of biological interest are carried out, such as the time-resolved absorption study of the first excited state of Acridine.

Transient absorption spectroscopy using the super-ACO storage ring FEL

The Super-ACO storage ring FEL is operating with a high average power in the UV range (300 mW at 350 nm), and recently at wavelengths down to 300 nm. In addition this source exhibits high stability and long lifetime which makes it a unique tool for user applications. The coupling of the FEL with other synchrotron based sources (bending magnet and undulator) opens many unexplored possibilities for various types of two-color time-resolved spectroscopies. Presently, we are developing a two-color experiment where we study the sub-nanosecond time-resolved absorption of different chromophoric compounds. In this type of pump-probe experiments, the intense UV pulse of the Super-ACO FEL is used to prepare a high initial concentration of chromophores in their first singlet electronic excited state. The nearby bending magnet synchrotron radiation provides on the other hand a pulsed, white light continuum ranging from UV to IR, which is naturally synchronized with the FEL pulses and can be used to probe the photochemical subsequent events and the transient species. With a dye molecule (POPOP), we have obtained a two-color effect which demonstrates the feasibility of the experiment in terms of flux. Applications on various chromophores of biological interest are planned.

Applications in biology with the super-ACO FEL and future prospects

The first applications of a storage ring Free Electron Laser started in 1993 on the Super-ACO FEL with the study of the anisotropy decay of a coenzyme, NADH, allowing to understand the thermodynamical equilibrium of the different conformational states of the molecule and their hydrodynamical volume in solution. After this first one- color experiment using the time-resolved fluorescence technique, a transient absorption experiment was developed in which the system is excited with the UV FEL and is probed by Visible-UV absorption using synchrotron radiation. First results on the dynamical behavior were obtained for the acrinide molecule.

The super-ACO free electron laser source in the UV and its applications

Abstract Free electron lasers (FELs) are new sources of tuneable coherent radiation, based on the interaction of a relativistic electron beam with a permanent magnetic field. The Super-ACO FEL operates in the UV (down to 300 nm ) at 800 MeV , the nominal energy of the electron beam, with a high repetition rate (8 MHz ) . It presents a high average extracted power (up to 300 mW ), short pulses (15–50xa0ps FWHM) and small bandwidth (3×10 −2 nm ) . Taking advantage of these characteristics, we demonstrated for the first time the possibility of using a storage ring FEL as a coherent source of radiation for scientific applications. In particular, the tuneable Super-ACO FEL source, combined with synchrotron radiation covering the X-ray to infra-red range, is a unique tool for the time-dependent studies of excited states. Such analysis benefits from the natural synchronisation of both sources at a high repetition rate, their mutual tunability, high intensity and coherence. Several experimental set-ups are now under operation.

A VUV-EUV High Resolution Fourier Transform Spectrometer without a Beam Splitter

FTS can be performed in the VUV-EUV by using wavefront division interferometers. The first tests of a dedicated instrument, for high resolution absorption FTS down to 60 nm, shows nominal performances at 200 nm.