High-Order-Harmonic Generation and Superradiance in a Seeded Free-Electron Laser

Abstract

Higher order harmonic generation in a free-electron laser amplifier operating in the superradiant regime [R. H. Dicke, Phys. Rev. 93, 99 (1954).] has been observed. Superradiance has been induced by seeding a single-pass amplifier with the second harmonic of a Ti:sapphire laser, generated in a -Barium borate crystal, at seed intensities comparable to the free-electron laser saturation intensity. Pulse energy and spectral distributions of the harmonics up to the 11th order have been measured and compared with simulations. DOI: 10.1103/PhysRevLett.108.164801 PACS numbers: 41.60.Cr, 41.50.+h, 42.55.Vc Nonlinear harmonic generation is widely used to extend the operation of optical lasers to the UV-vacuum ultra violet (UV-VUV) spectral region, where research methods for the investigation of matter [1-4] require ultrashort coherent pulses. Frequency up-conversion to the 10-100 nm range may be accomplished with high harmonics generated in gas (HHG) [5,6], where the active medium is a low density noble gas. The emission of high-energy photons, however, is inherently coupled with ionization, and the use of a nonlinear optical medium poses limitations to the conversion efficiency at the shortest wavelengths. In the spectral region where ionization processes are dominant, harmonic generation may still be obtained in freeelectron lasers (FELs). The mechanism of frequency up-conversion is based on the nonlinear density modulation of an electron beam at a given seed wavelength, seed . The frequency components of the modulated beam enforce the collective emission process at the resonant wavelength r ¼ u ð1 þ K 2 =2Þ=2 2 % seed and its harmonics [ is the Lorentz factor of the electrons, u is the period, and K ¼ eB u u =ð2 m e cÞ is the deflection parameter of the undulator] [7]. The seed transfers its longitudinal coherence properties to the FEL pulse [8], and schemes based on this principle were proposed to improve the longitudinal coherence of FELs operating in a self amplified spontaneous emission mode [9][10][11], where the temporal pulse structure is dominated by stochastic fluctuations associated with the initial shot noise [12]. The seed amplification, combined with the generation of coherent harmonics, has been demonstrated first in the midinfrared [8,13] and then in the UV-VUV range [14][15][16][17]. User facilities based on the frequency up-conversion of a seed laser in the VUV soft-x-ray region of the spectrum are now in operation and provide radiation with unprecedented properties of longitudinal coherence [18]. Coherent harmonics have been observed when seeding a single-pass FEL amplifier, and include the 3rd, 5th [19], and 7th harmonics [20]. The process of harmonic generation is expected to extend to higher orders when the FEL operates in the regime of super-radiance [21], in which a short optical pulse slips over the electron beam and increases its energy while maintaining a self-similar shape [22,23]. In this regime, the radiation pulse has a peak power that increases with the square of the distance z along...

Superradiant Cascade in a Seeded Free-Electron Laser

“…An energy scaling vs the longitudinal position along the undulator of the type E / z 3=2 has been measured, in satisfactory agreement with simulations. Sidebands in the spectrum appear after three sections of the radiator and the emission of higher order harmonics as expected in superradiance [29] has been confirmed.…”

Characterization of the THz radiation source at the Frascati linear accelerator

“…THz, [31][32][33] Compton, etc., to novel schemes for generation of FEL radiation, [34][35][36][37] to plasma acceleration experiments. 38,39 In the following, we will focus on the activity concerning the generation, characterization and transport of both a high brightness single pulse electron beam and a multi-pulses train through the dogleg beamline down to the THz station, for the development of high peak power, both broadband and narrowband THz radiation, whose experimental layout is described in Sec.…”

Demonstration of harmonic interaction in an undulator up to the 15th order

“…There are many methods for studying high harmonics in an undulator. A conceptually straightforward technique is to measure the photon flux at various harmonics for fixed beam and undulator parameters, and compare the results with expectations [31]. However, this requires one to accurately measure the spectrum over a wide frequency range, extending in our case from the IR to the extreme ultraviolet.…”