Self-amplification of spontaneous XUV emission suppresses helium autoionization
Amplified Spontaneous Emission (ASE) is a phenomenon observed in lasers and optical devices. It starts with a spontaneous emission in an active gain medium, such as a crystal or a gas. Emitted photons stimulate radiative decay of other atoms in the medium that are still in the excited state. The process results in emission of additional photons with the same energy, phase, and direction, and thus lead to amplification of the spontaneous emission signal.
Since the invention of optical lasers in the 1960s, laser-based technologies have emerged as cornerstones of numerous research methods and indispensable tools for various applications in everyday life. However, widespread lasers have a fundamental limit: their wavelength is limited to specific ranges: the discrete energy levels of the gain medium, whose excitation leads to ASE, as well as the characteristics of the optical cavity, dictate the frequency of light that can be emitted. The laser wavelengths are thus confined to the infrared and visible spectral ranges, with an addition of a few plasma-based stripped ionic systems that may emit amplified light in the extreme ultraviolet (XUV) range. This limitation impedes the flexibility needed for comprehensive exploration across various spectroscopic and imaging domains, underscoring the significance of alternative technologies, like free-electron lasers (FELs), in overcoming these constraints.
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Figure 1: The dismounted upper cover of the helium (He) target holder reveals the open-end micro gas capillary (entrance and exit diameters: 100 µm) carved into the fused silica glass..
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FELs are capable of producing ultra-fast light pulses with tunable wavelength, and have empowered researchers to investigate matter with unparalleled sensitivity and resolution in both spatial and temporal dimensions. One of the intriguing activities in this new research field deals with generalization of nonlinear spectroscopies from an optical to the short wavelength region. ASE involving inner-shell transitions in a solid or diluted medium emerges as a powerful approach to nonlinear x-ray spectroscopy because it can enable acquisition of significantly amplified signals from weak overlapping valence-to-core transitions that hold a wealth of chemical information. However, XUV ASE from short-lived quantum states above the first ionization threshold was yet to be witnessed. This is primarily due to the swift nonradiative decay processes, such as autoionization or Auger electron emission, which take place on the femtosecond temporal scale. We have overcome the problem thanks to an open-end gas cell produced at the Istituto di Fotonica e Nanotecnologie (IFN-CNR, Milano), by which a ten-millimeter-long column of He gas at tens of mbar pressure was submitted to the focused XUV light pulse (Fig. 1). |
At the EIS-TIMEX beamline of the FERMI FEL facility in Trieste, Italy, we were able to trigger, observe and quantify XUV ASE from an autoionizing resonance in helium that emits at 30.4 nm. Despite 1600-times more probable autoionization with respect to the XUV emission, the tuned pump pulse was intense enough to create a sufficiently large population of helium atoms in short enough time to initiate the ASE process from a short-lived doubly excited state. Such an ASE based approach to the nonlinear XUV spectroscopy resulted in eight orders of magnitude amplification of a weak spontaneous emission signal in the forward direction on account of high directionality and radiative to non-radiative branching ratio redistribution (Fig. 2). The measurements were performed under well-controlled experimental conditions and allowed for an absolute comparison with the state-of-art ASE theory for the simplest non-trivial atomic system. The new results are important in showing the necessity to incorporate the effects of inelastic electron scattering and transient molecular configurations in the modelling.
Figure 2: The ASE amplification factor of the strongest spontaneous emission channel of doubly excited state in He at 40 mbar gas pressure at different FEL pump pulse energies I0.
This work represents a link between the two existing research lines: the advanced ASE studies in the x-ray domain and the impulsive XUV superfuorescence studies below the first ionization threshold. The nonlinear XUV spectroscopy of gases is expected to have a significant impact on XUV spectroscopy in general because it promises orders of magnitude higher efficiency, higher chemical sensitivity and upper-state control. Our results may motivate machine physicists to provide, simultaneously with the pump, an XUV light pulse with incommensurate wavelength to selectively seed and amplify secondary weak transitions.
This research was conducted by the following research team:
J. Turnšek1,2, Š. Krušič1, A. Mihelič1,2, K. Bučar1,2, L. Foglia3, R. Mincigrucci3, M. Krstulović3, M. Coreno3, G. Bonano3, K. C. Prince3,4, C. Callegari3, A. Simoncig3, Z. Ebrahimpour3, E. Paltanin3, A. Benediktovitch5, R. Osellame6, A. G. Ciriolo6, R. Martínez Vázquez6, C. Vozzi6, E. Principi3 and M. Žitnik1,2
1 Jožef Stefan Institute, Ljubljana, Slovenia
2 Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia
3 Elettra-Sincrotrone Trieste S.C.p.A, Basovizza, Trieste, Italy
4 Department of Surface and Plasma Science, Charles University, Prague, Czech Republic
5 Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
6 National Research Council (CNR) — Institute for Photonics and Nanotechnologies, Milano, Italy
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Reference
J. Turnšek et al. “Amplification of Spontaneous Emission from Doubly Excited He Atoms", Phys. Rev. Lett. 135, 093001 (2025). DOI: 10.1103/mg5s-xvct .
