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

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 (>10 14 W/cm 2 ). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Progress and Perspectives of Spectroscopic Studies on Carbon K-Edge Using Novel Soft X-ray Pulsed Sources

“…Observations and theoretical calculations both suggest that the depletion of the core by an intense FEL pulse drives the SA effect, depending on the rate of the Auger decay, which is ~7 fs in graphite. At higher intensities, TPA dominates at both photon energies with a different onset [30]. These results are useful to better understand soft X-ray absorption and scattering data at high intensities.…”

“…The observation of nonlinear effects in graphite at the C K-edge was achieved at the EIS-TIMEX end station at the FERMI FEL-2 facility in different experiments [28][29][30]. 0001) due to optical laser irradiation, CO molecules rotate and shift toward higher coordinated sites from on-top sites.…”

“…The observation of nonlinear effects in graphite at the C K-edge was achieved at the EIS-TIMEX end station at the FERMI FEL-2 facility in different experiments [28][29][30]. Graphite samples 100-700 nm thick were exposed to a tunable soft X-ray seeded FEL pulse 25 fs (FWHM) long and with a spot size of 350 µm 2 at different photon energies: 260.5 eV (non-resonant), 284.18 eV or 285.7 eV (on-resonance), and 307 eV and 309.2 eV (above resonance).…”

“…The results show that TPA is a bulk probe, possible in centrosymmetric systems, at soft X-ray energies, is element selective and, similar to SHG, is significantly enhanced for energies of the incident photon near the absorption edge of an atom of the material. It is also worth mentioning that the interpretation of TPA's strength can be hampered by FEL's energy fluctuations [28,30].…”

Transient Absorption of Warm Dense Matter Created by an X-Ray Free-Electron Laser

“…The interaction between intense X-rays and matter is a new area of science accessible through the development of X-ray free-electron lasers (XFEL). These brilliant and ultrafast sources, in conjunction with strongly focusing optics, are capable of driving solids out of equilibrium, inducing nonlinear effects such as saturable absorption (SA), through which, e.g., a metal transiently becomes transparent [1][2][3][4] in specific spectral regions, or its counterpart, reverse saturable absorption (RSA), where its absorption increases with increasing excitation level [5]. SA and RSA are routinely used in the optical regime for applications such as mode-locking, pulse-shaping or two-photon microscopy [6][7][8], with possible transi-tions between these regimes [9,10].…”