Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge

Lars Hoffmann; Sasawat Jamnuch; Craig P Schwartz; Tobias Helk; Sumana L Raj; Hikaru Mizuno; Riccardo Mincigrucci; Laura Foglia; Emiliano Principi; Richard J Saykally; Walter S Drisdell; Shervin Fatehi; Tod A Pascal; Michael Zuerch

doi:10.1021/acs.jpclett.2c01020

Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge

J Phys Chem Lett. 2022 Oct 6;13(39):8963-8970. doi: 10.1021/acs.jpclett.2c01020. Epub 2022 Sep 27.

Authors

Lars Hoffmann^{1

2

3}, Sasawat Jamnuch⁴, Craig P Schwartz^{2

5}, Tobias Helk^{6

7}, Sumana L Raj¹, Hikaru Mizuno^{1

2}, Riccardo Mincigrucci⁸, Laura Foglia⁸, Emiliano Principi⁸, Richard J Saykally^{1

2}, Walter S Drisdell^{2

9}, Shervin Fatehi¹⁰, Tod A Pascal^{4

11

12}, Michael Zuerch^{1

2

3

13}

Affiliations

¹ Department of Chemistry, University of California, Berkeley, California 94720, United States.
² Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
³ Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany.
⁴ ATLAS Materials Science Laboratory, Department of Nano Engineering and Chemical Engineering, University of California San Diego, La Jolla, California 92023, United States.
⁵ Nevada Extreme Conditions Laboratory, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, United States.
⁶ Institute of Optics and Quantum Electronics, Abbe Center of Photonics, Friedrich-Schiller University, 07743 Jena, Germany.
⁷ Helmholtz Institute Jena, 07743 Jena, Germany.
⁸ Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, 34149 Trieste, Italy.
⁹ Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
¹⁰ Department of Chemistry, The University of Texas Rio Grande Valley, Edinburg, Texas 78539, United States.
¹¹ Materials Science and Engineering, University of California San Diego, La Jolla, California 92023, United States.
¹² Sustainable Power and Energy Center, University of California San Diego, La Jolla, California 92023, United States.
¹³ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Abstract

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>10¹⁴ W/cm²). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.