Amplified spontaneous emission in the extreme ultraviolet by expanding xenon clusters

Abstract

Focused short-wavelength free-electron laser (FEL) pulses interacting with gas phase samples can induce by inner-shell ionization a short-lived population inversion, followed by coherent collective emission of directed, short, and strong radiation bursts. We extend our studies into the warm-dense matter (WDM) regime by investigating the nanoplasmas produced in an ensemble of nanometer-sized clusters by FEL irradiation. Here, additional pathways can also lead to strong, laserlike emission: Electron-ion collisions can yield a long-lived population inversion, and subsequent amplified spontaneous emission. We observe amplified spontaneous emission (ASE) in the extreme ultraviolet in xenon clusters excited by soft x-ray FEL pulses, we diagnose the generated nanoplasmas by fluorescence spectroscopy, and we study under various cluster and FEL parameters the directed ASE from the Xe 2+ 65 nm line. We show its exponential increase as a function of FEL irradiation power, and an accompanying collisional broadening of the emission spectra. These findings are corroborated by extensive numerical simulations based on theory, combining detailed hydrodynamic and kinetic simulations with time-dependent calculations of radiation transport, amplification, and collective emission in the WDM nanoplasma. Our theoretical findings underline that population inversion is due to electron-ion collisions and that the observed decoherence processes can be empirically characterized by a phenomenological decoherence time in the range of 100-200 fs.

“Amplified Spontaneous Emission” in Micro- and Nanolasers

“…At the macroscopic scale, ASE is responsible for astrophysical laser sources [8,17], on the one hand, and has been exploited to strongly improve the efficiency of the Free Electron Laser, using a self-amplification regime [18][19][20], or for UV amplification [21]. ASE, exploited as a biocompatible nonlinear, scattering induced amplifier [22], is the natural precursor of random lasing [23,24] and is predicted to appear in concomitance with a reduction in the number of degrees of freedom in such systems [25].…”

Maxwell-Bloch modeling of an x-ray pulse amplification in a 1D photonic crystal

“…Besides this, it has been proved that XFEL sources have the potential to create large population inversions in gases [4], clusters [5], solids [6][7][8] and liquids [9], resulting in the creation of an x-ray amplifying medium. These approaches based on lasing in atomic media have an important potential to obtain useful short and coherent x-ray pulses.…”