Nonlinear phenomena in X-ray fluorescence from single nanoparticles under extreme conditions

Abstract

Materials exposed to intense femtosecond X-ray pulses with energies above their K-shell absorption edge can enter an extremely ionized state, which could give rise to nonlinear phenomena, such as saturable absorption and reverse saturable absorption. In this work, we investigate these effects on single copper nanoparticles irradiated by an X-ray free-electron laser pulse. We study the properties of the K$α$ fluorescence for two different short pulse durations and three X-ray incident energies below and above the K-shell absorption edge, and correlate these with incident fluence estimates based on coherent diffraction. We observe that the incident fluence of the pulse and not its duration, is the main factor that modulates the nonlinear response, which leads to an effective shortening of the fluorescence emission. Our findings have implications for fluorescence-based methods for imaging single particles using transiently coherent fluorescence, or diffractive imaging through transient resonances.

Figures (6)

• Figure 1: Schematic illustration of the experimental setup. The spectrum of each incident XFEL pulse is recorded by a hard X-ray single-shot spectrometer hirex using the Gotthard detector amozzanica_gotthard_2012. The copper nanocubes are aerosolized and delivered to the interaction region, where they are intercepted by the X-rays. Both scattering and fluorescence are recorded in the forward region by the AGIPD detector becker_single_2012. The Nickel filter allows coherent diffraction at low angles because of an opening around the direct beam, which can be used for size selection, orientation determination, and classification wollweber2025. At high angles, elastic photons are filtered out, and K$\alpha$ fluorescence is predominant on the detector (right panel).
• Figure 2: Signature of saturable absorption on single copper nanoparticles for 25fs (long pulses, left column) and 2.5fs (short pulses, right column) pulses as a function of incident fluence for 9015eV and 9170eV photon energies. (a,b) Aggregate of single-shot fluorescence over the incident fluence values, determined by the total intensity on the region shadowed by the nickel filter and the region exposed by the foil cutout, respectively. We trace a linear fit (solid line) for each photon energy, which reveals a crossover between the two energies. (c,d) Fluorescence cross sections based on the fluorescence intensity, from single particle measurements. The horizontal lines show the tabulated cross section.
• Figure 3: Simulated copper fluorescence cross section as a function of incident X-ray fluence, for the (a) 25, and (b) 2.5 X-ray pulses. The cross sections are based on pulse-weighted changes to the linear absorption coefficient at 9015eV and 9170eV.
• Figure 4: Signature of reverse saturable absorption on single copper nanoparticles. Experimental and theoretical copper K$\alpha$ fluorescence from (a) 25fs and (b) 2.5fs duration incident X-ray pulses with 8910eV photon energies, below the copper K-shell absorption edge.
• Figure 5: Estimation of the short pulse duration. (a) Exemplary hard X-ray single-shot spectrum. (b) Second order correlation function $g^{(2)}(\Delta E)$ for all spectra with a photon energy of 9170eV. The full width at half maximum of the central feature is inversely proportional to the pulse duration and of the broad feature (fit not shown) to the averaged single SASE spike duration.