Simultaneous lasing of Ni Kα and Cu Kα lasers from an alloy foil irradiated with an intense X-ray free-electron laser pulse

We report the simultaneous lasing of two distinct Kα emissions at photon energies of 7.48 keV (Ni Kα₁) and 8.05 keV (Cu Kα₁). This was achieved by a population inversion induced through intense X-ray free-electron laser (XFEL) irradiation of a Cu–Ni alloy foil. This demonstration of multi-color X-ray lasing using a single XFEL source is expected to contribute significantly to the future development of X-ray lasers and their applications.

1. Introduction

Brilliant femtosecond X-ray free-electron laser (XFEL) pulses (Emma et al., 2010; Ishikawa et al., 2012) have played an important role in opening up new fields of X-ray science (Chapman et al., 2011; Suga et al., 2015; Kim et al., 2020). Lasing via amplified spontaneous emission (ASE) of Kα emission has been successfully achieved by generating population inversion through high-intensity XFEL irradiation of various materials (Rohringer et al., 2012; Yoneda et al., 2015; Kroll et al., 2018; Kroll et al., 2020; Zhang et al., 2022; Doyle et al., 2023; Linker et al., 2025). If ASE lasing at different wavelengths can be simultaneously induced using a single XFEL pulse, multi-color X-ray lasers could be realized. Such a capability is expected to greatly advance the development of X-ray laser systems and enable novel applications, including laser oscillators (Halavanau et al., 2020) on multi-color X-rays and nonlinear X-ray spectroscopic techniques (Tanaka & Mukamel, 2002; Sun et al., 2010), which require synchronized multi-color X-ray sources.

When a material is irradiated with X-rays, various characteristic emissions such as Kα₁, Kα₂ and Kβ lines are simultaneously generated. In spontaneous emission, the intensity of Kα₁ is typically about twice that of Kα₂. However, in the case of stimulated emission, such as in ASE X-ray lasers, the transition from the 2p_3/2 to the 1s state (which has a significantly larger stimulated emission cross section than the 2p_1/2 to 1s transition responsible for Kα₂) dominates. As a result, only Kα₁ typically undergoes lasing. When excited by extremely intense X-rays, Kα₂ lasing may occur very weakly like a satellite of Kα₁ (Yoneda et al., 2015). Similarly, Kβ ASE lasing is nearly impossible due to strong competition with Kα transitions. These constraints make the generation of multi-color ASE X-ray lasers from a single-element sample extremely challenging. To overcome this, we propose using a sample composed of different atomic species to achieve simultaneous multi-color ASE lasing. Since ASE lasing occurs independently in each atomic species, simultaneous oscillation of distinct Kα₁ lines becomes possible. In this study, we report the simultaneous generation of 7.48 keV Ni Kα₁ and 8.05 keV Cu Kα₁ ASE lasers using a foil of Ni–Cu alloy.

2. Experiment and discussion

The experiment was conducted at SACLA BL2 EH3. A schematic of the experimental setup is shown in Fig. 1. XFEL pulses with a photon energy of 10 keV were focused using Kirkpatrick–Baez (KB) optics (Inubushi et al., 2025). The beam sizes were measured to be 150 nm (horizontal) and 220 nm (vertical) in full width at half-maximum (FWHM). Typically, a pulse energy of 110 µJ was achieved at the focal point. The pulse duration was 7 fs (Inubushi et al., 2012; Inubushi et al., 2017), resulting in an intensity of 2.5 × 10¹⁹ W cm⁻². The XFEL pulses were focused onto a 20 µm-thick constantan foil, an alloy composed of 45% Ni and 55% Cu. Given that the K-absorption edges of Ni and Cu are 8.33 keV and 8.98 keV, respectively, the 10 keV XFEL pulses were capable of ionizing the K-shell electrons of both elements. The constantan foil, placed in air, was moved after each shot to ensure irradiation on a fresh surface. The XFEL intensity was varied by adjusting the position of the foil along the beam path. In this context, the position where the maximum XFEL intensity was achieved is referred to as the `optimal position'.

Figure 1 Schematic of the experimental setup. Focused 10 keV XFEL pulses irradiated a Ni–Cu alloy (constantan) foil. The resulting Kα1 lasers were simultaneously measured in a single-shot manner using a dispersive spectrometer consisting of a convex bent Si(220) analyzer crystal and an MPCCD detector. The central photon energy for the observation was set to 7.8 keV, corresponding to a Bragg angle of 24.5°. An energy observation range of 1.2 keV was achieved, with an energy resolution of 1.5 eV per pixel.

To verify the simultaneous lasing of two Kα₁ lines, a single-shot measurement with an energy observation range exceeding 600 eV is necessary, as the photon energies of Ni Kα₁ and Cu Kα₁ emissions are 7.48 keV and 8.05 keV, respectively. Here, we propose the combined use of a divergent X-ray beam and a bent crystal (Zhu et al., 2012) to achieve a significantly broader energy observation range. In our experiment, a convex bent Si(220) crystal with a curvature radius of 250 mm was employed. A ray-tracing simulation was conducted to design and optimize the spectrometer configuration. Fig. 1 shows the experimental setup, including the parameters used in the ray-tracing simulation. As a result, an energy observation range of 1.2 keV was achieved. The MPCCD detector (Kameshima et al., 2014) provided an energy resolution of 1.5 eV per pixel.

Fig. 2(a) shows the single-shot spectra measured at the `optimal position'. Distinct peaks are observed at 7.48 keV, corresponding to the Ni Kα₁ line, and at 8.05 keV for the Cu Kα₁ line, with variations in intensity. As illustrated in Fig. 2(b), both peaks exhibit nonlinear increases in signal intensity as a function of XFEL intensity, indicating successful simultaneous lasing of the Ni Kα₁ and Cu Kα₁ lines. At higher XFEL intensities, the Cu Kα₁ laser becomes more intense than the Ni Kα₁ laser. This is likely due to the higher Cu content in the alloy, the greater absorption coefficient of Cu for 10 keV X-rays and the higher transmittance of the Cu Kα₁ emission through the sample. Furthermore, as shown in Fig. 2(c), a statistically significant positive correlation was observed between the Ni Kα₁ and Cu Kα₁ laser intensities (correlation coefficient r = 0.63, p value p < 0.001), indicating that the two lasing processes are not mutually competitive. This implies that increasing the XFEL beam intensity could lead to the simultaneous enhancement of both lasers, until saturation.

Figure 2 (a) Single-shot spectra observed at the optimal position. (b) Averaged intensities of the Ni Kα1 and Cu Kα1 lasers as functions of XFEL intensity. Error bars represent standard errors. (c) Correlation between the intensities of the Ni Kα1 and Cu Kα1 lasers at the optimal position.

3. Summary and future perspectives

We successfully demonstrated the simultaneous generation of two Kα₁ lasers by irradiating an alloy foil with an intense XFEL pulse. This achievement suggests the potential for producing multi-color X-ray laser pulses by incorporating multiple atomic species – not limited to alloys, but also including, for example, layered sample and mixed-atom droplets. Such multi-color X-ray lasers are expected to pave the way for future advancements in X-ray laser technology and enable novel applications across various scientific fields.