A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.

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Specular reflectivity of p-polarized light near the iron L\\ and L\u absorption edges was measured from a single-crystal iron film with an external magnetic field perpendicular to the scattering plane. Large changes in reflectivity were observed upon reversal of the direction of the magnetic field. We attribute this resonant magnetization-sensitive effect to the interference between magnetic and nonmagnetic contribution to the resonant scattering. Similar effects can be expected in other ferromagnetic and ferrimagnetic systems. PACS numbers: 78.70.Ck, 75.25,+z, 76.90.+d X-ray scattering has become an important experimental technique in the investigation of magnetic materials. ] Of particular importance is the recently observed large resonant enhancement of the magnetic scattering cross section near an absorption threshold. 2,3 In favorable cases the magnetic scattering cross section can be comparable to that of charge scattering. For example, measurements in holmium by Gibbs et al. 1 have shown an enhancement of over fifty in the magnetic satellite intensities as the photon energy is tuned across the L\\\ edge (2/73/2 to 5d, 8.067 keV). Similar experimental studies at the Af iv and My edges of UAs (3d 3 /i to 5/, 3.728 keV and 3^5/2 to 5/, 3.552 keV, respectively) by Isaacs et al. 3 have demonstrated even larger resonant enhancements of magnetic Bragg reflections. At the M\\ edge the scattering intensity of the (0,0, f ) reflection is 1% of the intensity of the (0,0,2) charge reflection, which is an enhancement of 7 orders of magnitude over the magnetic scattering intensity measured far from the edge. These observations can all be explained by resonant exchange scattering, 4 in which large resonant enhancements result from strong electric multipole transitions from atomic core levels to unoccupied states above the Fermi level, with the magnetic sensitivity arising both from the spin polarization of the partially occupied states and from exchange splitting of the unoccupied states.The previous work 2,3 on Ho and UAs studied the energy dependence of Bragg peaks in these antiferromagnetic structures which are purely magnetic in origin. For the 3d transition metals large resonant enhancements are predicted for the Lnjn edges. 4,5 In these materials, however, the transitions all occur at wavelengths which are outside the limiting sphere for Bragg reflections. However, measurement of the specular reflectivity (which is not restricted in wavelength) from ferromagnetic or ferrimagnetic samples should also be sensitive to resonant exchange scattering. In this work, we measure the specular reflectivity utilizing a linearly polarized photon beam with its polarization vector in the scattering plane (p-polarized light), and an external magnetic field perpendicular to the scattering plane.The effect of resonant exchange scattering is manifested as changes in the specular reflectivity upon reversal of the direction of the magnetic field. 6 To illustrate the effect better, an asymmetry ratio R defined as was derived as a functi...

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We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.