Intense x-ray radiation changes how light and matter interact
When light is not particularly bright, it interacts with matter in a linear fashion. However, as the peak intensity of light increases the interaction with matter diverges from the linear behavior. This is true for all the photon energies from microwaves to X-rays. So far, interaction of intense X-rays with matter has been particularly challenging to explore because high intensity X-ray sources with well controlled properties are rare. Here, we employ the soft X-ray single-shot high fluence achievable at the EIS-TIMEX beamline of FERMI to monitor the transmission of thin films of graphite under high intensity conditions thus reaching a nonlinear light-matter interaction regime.
Investigating the interaction of high intensity light with matter is rather complicated because two competing effects become prominent. The first is saturable absorption (SA), which leads to an increase in the transmission of light with respect to a linear response. SA occurs because, as a light pulse travels through a material, there is increasingly less matter left that has not been perturbed. The leading part of the pulse pre-excites electrons in a larger number of atoms of the material, which results in the change of the material before the pulse tail arrives. In X-ray excited materials, core electrons are promoted in the conduction band, therefore there are just a small number of electrons left that can absorb photons from the light pulse leading to a nominal increase of transmission compared to a linear response. However, at high intensities a second effect called two photon absorption (TPA) also occurs that exhibits the opposite effect of decreasing transmission of light through a sample. TPA results from a quantum mechanical second order process that drives the absorption of two photons at the same time when the electromagnetic field is high enough. This leads to the opposite effect of SA and the overall effect in practice depends on how bright the light is as well as how many electrons are available for a given transition energy. Intense soft X-ray pulses from the FERMI free-electron laser permit to study with remarkable accuracy the direct effects of both SA and TPA in the same experiment. By transmitting X-ray pulses through graphite and monitoring the amount of light transmitted as a function of light intensity, SA and TPA can be disentangled.
To interpret the results, detailed calculations have been carried out. In a simple case without effects like TPA and SA, as the intensity is increased, transmission would occur in a linear fashion. As shown in Fig. 1a and 1b, experimental soft X-ray transmission does not exhibit a linear trend with the FEL intensity. Interestingly, the effect is strongly dependent on the photon energy used to excite the graphite sample. Low lying excitations (photon energy below the C K-edge) see more TPA effects at lower power, leading to a deviation below the expected linear transmission, while with a high photon energy (above the C K-edge) the opposite occurs leading to higher transmission. Our theoretical model (Fig. 1c and 1d) suggests that TPA becomes more dominant for larger intensities (>1014 W/cm2) no matter of what intensity is used to excite the sample. This experiment demonstrates that the interaction between materials and intense soft X-rays becomes strongly nonlinear thus making inadequate the usual physical laws valid for the low intensity regime.
Figure 1: Experimental and simulated transmission for resonant excitation from graphite ground state into a low lying and high lying excitation (below and above the C K-edge, respectively). (a, b) Experimental transmitted intensity for low lying (285.7 eV) and high lying excitations (309.2 eV) for incoming FEL intensities in the regions shown in (c) and (d) as dotted lines. Plot (a) shows an increase in absorption, while plot (b) shows a decrease in absorption relative to a linear absorption model valid at low fluences. Linear fits to the first three data points highlight the nonlinear behavior. (c, d) Simulated transmission vs intensity for low lying and high lying excitations including two-photon absorption (TPA, purple dashed line). Comparison with a model which does not consider TPA (green line) shows that TPA becomes dominant at high intensities. The high lying excitation behaves differently due to a stronger TPA response. Gray dotted lines indicate the intensity range that was measured in the experiment. Note that (a) and (b) show the total transmitted intensity, while (c) and (d) show the transmitted intensity as a fraction of the incoming intensity.
This research was conducted by the following research team:
Lars Hoffmann1,2, Sasawat Jamnuch3, Craig P. Schwartz4,5, Tobias Helk6, Sumana L. Raj1, Hikaru Mizuno1,4, Riccardo Mincigrucci7, Laura Foglia7, Emiliano Principi7, Richard J. Saykally1,4, Walter S. Drisdell1,8, Shervin Fatehi9, Tod A. Pascal3, and Michael Zuerch1,2,10
1 Department of Chemistry, University of California, Berkeley, California, USA
2 Fritz Haber Institute, Berlin, Germany
3 ATLAS Materials Science Laboratory, Department of Nanoengineering and Chemical Engineering, University of California, San Diego, California, USA
4 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
5 Nevada Extreme Conditions Laboratory, University of Nevada, Las Vegas, Nevada, USA
6 Institute of Optics and Quantum Electronics, Friedrich-Schiller University, Jena, Germany
7 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
8 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California, USA
9 Department of Chemistry, University of Texas, Rio Grande Valley, Edinburg, TX, USA
10 Chemical Sciences Division and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
