M. Billardon

Free electron laser experiment at Orsay: A review

In this paper we review the results obtained in the last two years with the Orsay free electron laser.

Surface states and space charge layer dynamics on Si(111)2×1: A free electron laser-synchrotron radiation study

Combining the use of a UV storage ring free electron laser and of synchrotron radiation, a time resolved core level spectroscopy study has been performed on photoexcited Si(111)2×1 surfaces with subnanosecond resolution. This enabled us to measure band bending fluctuations, caused by surface carrier dynamics, during the first nanosecond after photoexcitation; differences in the Si2p core level lineshape, dependent on the pump-probe time delay, were also observed. The presence of defects was found to reduce the fluctuations and make the carrier recombination process faster.

Diagnostic techniques and UV-induced degradation of the mirrors used in the Orsay storage ring free-electron laser.

Due to its low gain, the Orsay storage ring free-electron laser necessitates the use of high reflectivity mirrors. Three techniques for measuring the mirror losses are presented, based on cavity decay time measurements using either an external laser, the synchrotron radiation stored in the cavity, or the free-electron laser itself. The high signal-to-noise ratio allowed the detection of loss variations as low as 10(-7)/sec(1/2). From these diagnostics three distinct processes of UV-induced degradation of TiO2/SiO2 dielectric mirrors were identified. One was a surface absorption of the upper SiO2-air interface; it was not affected by annealing. The other two corresponded to a volume absorption of the layers which completely recovered after annealing.

A longitudinal feedback for the Super-ACO free electron laser stability in the UV

Abstract To take complete benefit of the FELs features, longitudinal feedback has been developed for operating the FEL at perfect tuning without jitter. The laser micropulse temporal position is detected and compared to a reference position corresponding to perfect tuning. The time difference is converted into a correcting voltage, which is applied to the RF frequency pilot to adjust the synchronism condition. When the feedback is established, the micropulse jitter is reduced, the intensity fluctuations on the ms scale are damped and wavelength drift is suppressed. It has thus significantly improved the FEL operation at perfect tuning, and provides a stable laser with all its parameters optimized for the users.

First Production of Vacuum-Ultraviolet Coherent Light by Frequency Multiplication in a Relativistic Electron Beam

We report the first generation of vacuum-ultraviolet harmonics of an external laser focused on an ultrarelativistic electron bunch circulating in a periodic magnetic field. The amplification of the undulator emission at the 3rd (at 1773 A) and 5th harmonics (at 1064 A) of a doubled neodymium-YAG laser (at 5320 A) has been observed in our optical klystron installed on the electron storage ring ACO at Orsay.

Operation of the super-ACO free-electron laser in the UV range at 800 MeV

The Super-ACO Free-Electron Laser (FEL) has been operated around 350 nm first at 600 MeV and more recently in June 1992 at 700 and 800 MeV, which is the highest energy for an FEL. Because of the energy increase, the laser lifetime is greater than ten hours and the output power has been enhanced by two orders of magnitude. In addition, with its good temporal stability, it appears to be a possible tool for users.

Surface photovoltage in semiconductors under pulsed optical excitation, and its relevance to synchrotron radiation spectroscopy

Abstract We used the SuperACO Free Electron Laser (FEL) to excite photocarriers in semiconductors at room temperature, and synchrotron radiation photoemission spectroscopy to measure the consequently induced surface photovoltage (SPV). The natural synchronization between the two sources allowed us to study with sub-nanosecond resolution the temporal dependence of the non-equilibrium charge carrier distribution for two prototype systems, Ag/GaAs(110) and Si(111)2×1. We found that to correctly interpret the effects of the SPV on the band position, the interplay between instrumental factors (pulse duration and repetition rate) and semiconductor parameters (such as surface and bulk recombination rates) must be taken into account. Since the FEL has the typical temporal structure of a synchrotron radiation source and a flux comparable to the one obtainable at third generation storage rings, these results are of relevance to the most advanced spectroscopic techniques used to study the electronic structure of semiconductors; therefore, in this paper we discuss the possible effects of intense synchrotron radiation beams on position and lineshape of photoemission features at semiconductor surfaces.

Ultraviolet coherent generation from an optical klystron

The optical klystron, previously successfully optimized for the realization of a FEL on the ACO storage ring at Orsay, has been used as a nonlinear medium for producing harmonics of an external 1.06 μm Nd:YAG laser focused into it. Detailed experimental data are reported. A comparison with theoretical calculations shows very good qualitative and quantitative agreement between the experimental data and the theory.

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.

Short–Wave-Length Operation of a Storage Ring Free-Electron Laser

The Orsay storage ring free-electron laser has been operated in the short–wave-length side (blue) of the visible spectral range from 4630 A to 4860 A. General conclusions on storage ring free-electron laser are derived after our successive operations of a tunable laser over almost the entire visible spectral range.