Seeded stimulated X-ray emission at 5.9 keV

Abstract

X-ray free-electron lasers (XFELs) provide intense pulses that can generate stimulated X-ray emission, a phenomenon that has been observed and studied in materials ranging from neon to copper. Two schemes have been employed: amplified spontaneous emission (ASE) and seeded stimulated emission (SSE), where a second color XFEL pulse provides the seed. Both phenomena are currently explored for coherent X-ray laser sources and spectroscopy. Here, we report measurements of ASE and SSE of the 5.9 keV Mn Kα₁ fluorescence line from a 3.9 molar NaMnO₄ solution, pumped with 7 femtosecond FWHM XFEL pulses at 6.6 keV. We observed ASE at a pump pulse intensity of 1.7 × 10¹⁹ W/cm², consistent with earlier findings. We observed SSE at dramatically reduced pump pulse intensities down to 1.1 × 10¹⁷ W/cm². These intensities are well within the range of many existing XFEL instruments, which supports the experimental feasibility of SSE as a tool to generate coherent X-ray pulses, spectroscopic studies of transition metal complexes, and other applications.

Fig. 1.

(a) Experimental layout (not to scale): SACLA XFEL radiation containing the seed and pump pulses is focused with a KB mirror system onto a liquid jet of NaMnO₄. 2% of the seed pulse is outcoupled and registered in the seed spectrometer. The resulting stimulated emission is dispersed onto the Kα spectrometer with a Si (220) crystal analyzer. (b) The level diagram and concept behind SSE. The pump pulse (yellow), which is tuned above the Mn K edge, creates a population inverted 1s core hole excited state. The seed pulse (blue) is tuned to the Kα emission line to seed the SSE signal (red). (c) Schematic showing how the pump and seed pulse intensity is varied by moving the gain medium to various distances out of focus along the beam direction.

Fig. 2.

(a) Selected single-shot ASE spectrum collected at a pump intensity of 1.7 × 10¹⁹ W/cm²; single-shot SSE spectrum (center) and seed pulse (right vertical line) at (b) δE = 7 eV above Kα₁ and (d) δE = 0 eV at a pump intensity of 1.1 × 10¹⁸ W/cm²; (c) detuning scan of Kα₁ SSE at a pump intensity of 1.1 × 10¹⁸ W/cm², in which each horizontal row corresponds to the average spectrum for a given seed pulse energy collected across 1924 shots and each seed is indicated by a square box on the diagonal; (e) 1D version of (c), showing Kα₁ SSE spectra (solid red lines), in which the center of the seed pulse energy is indicated by a blue vertical line. Each 2D spectrum shown was collected at the Kα spectrometer, and is peak normalized. Each 1D spectrum in (e) is normalized to the peak pixel value from (c).

Fig. 3.

Seed pulse spectrum obtained from averaging 1924 single shots, and the corresponding best fit from Eq. (1). The left inset shows the shot-by-shot seed intensity, which exhibits 100% fluctuation and has an estimated average value of 7.8 × 10⁶ (black broken line). The average spectrum and corresponding fit were shifted to δE = 7 eV from Kα₁ for demonstration purposes. Next, two regions were selected from the shifted fit: (a) δE = 0 eV and (b) δE = 7 eV. In region (a) 2.0 × 10⁴ photons were observed within $\Delta E_{\text{K}{\alpha }_1}=2.1\text{eV}$ FWHM. In region (b) 8.2 × 10⁶ photons were found in a bandwidth of $\Delta E_{\text{K}{\alpha }_1}=2.1\text{eV}$ FWHM. Integrated photon counts were computed from the fit. The right inset shows the single shot spectrum corresponding to Fig. 2(b) to illustrate how, even in the case of a detuned seed, enough photons from the low-energy tail can stimulate emission.

Fig. 4.

Left: The FWHM, f, of the Mn Kα₁ line as a function of the seed pulse detuning at different pump intensities. Lines are drawn for reference. Right: Example of single-shot spectra, in which the seed is δE = 0 eV from the Mn Kα₁ line, to further illustrate spectral narrowing.

Fig. 5.

(a) Seeding efficiency as a function of seed photon counts that fall within the spectral range of Kα₁ FWHM for two different pump intensities. Shots are displayed for cases where the seed is in the range of δE = [0, + 7] eV for the case of 150 μm out of focus, and δE = [0, + 2] eV for the case of 400 μm out of focus. (b) Photon count distribution of Kα₁ and seed pulse photons from a seeded scan at 150 μm out of focus, at a pump pulse intensity of 1.1 × 10¹⁸ W/cm².