Atomic X-ray Laser Research Articles

Study of population dynamics toward the development of atomic X-ray lasers in the 3–5 keV photon energies

X-ray free-electron lasers (XFELs) are powerful tools for characterizing and probing the properties of matter at atomic resolution on the ultrafast timescale. However, they have certain limitations such as spectral fluctuation and poor temporal coherence. Atomic X-ray lasers offer the narrow bandwidth, longitudinal coherence, and spectral stability that can overcome these limitations. In this paper, we study the interaction of inner-shell vacancy states with high-intensity XFEL pulses. We show that it is possible to achieve population inversion between K-shell and L-shell vacancy states in calcium and titanium when pumped by high-intensity XFEL pulses. These states can be used to generate atomic X-ray laser emission in the 3–5 keV photon energy range.

Transient-gain photoionization x-ray laser

We present a generalized theory based on one-dimensional Maxwell-Bloch equations to study the amplification process of an inner-shell photoionization-pumped atomic x-ray laser. Focusing an x-ray free-electron laser beam in an elongated neon-gas target results in a strong exponential amplification of $K\ensuremath{\alpha}$ fluorescence, as recently demonstrated [N. Rohringer et al., Nature (London) 481, 488 (2012); C. Weninger et al., Phys. Rev. Lett. 111, 233902 (2013)]. Here, we present an in-depth theoretical study of the amplification process that goes beyond the previous theory based on a rate-equation approach. We study the evolution of the pulse characteristics during the amplification process for transform-limited Gaussian and broadband self-amplified spontaneous-emission pump pulses. We discuss the impact of the gain-dependent group velocity on the emitted x-ray radiation and the resulting gain-guiding effects. A thorough analysis of the spectral and temporal properties of the emitted radiation is presented, including higher-order field-correlation functions, to characterize the ensemble of emitted x-ray pulses.

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

Even harder X-rays

With the laser just over half a century old, another dream of the pioneers of this light source has been fulfilled. An atomic X-ray laser with unprecedentedly high photon energy has been demonstrated. See Letter p.488 Achieving lasing at increasingly shorter wavelengths is a long-standing goal. This paper demonstrates an X-ray laser operating on an inner-shell transition in singly ionized neon — the first atomic laser in the 1 kiloelectronvolt regime. It is pumped by X-ray free electron laser (XFEL) pulses from the Linac Coherent Light Source at the SLAC National Accelerator Laboratory in Menlo Park, California. The atomic laser's high-intensity, femtosecond X-ray pulse emission has significantly improved wavelength stability, monochromaticity and temporal coherence compared with XFEL radiation. Atomic X-ray lasers could have applications in high-resolution X-ray spectroscopy and nonlinear X-ray studies.

An atomic inner-shell laser pumped with an x-ray free-electron laser

Focusing an x-ray free electron laser (XFEL) pulse into a gas target, a plasma of transiently core-excited ions can be created within a few femtoseconds, building a pathway to an inner-shell keV atomic x-ray laser. Varying the XFEL parameters, a wide variety of pulse structures can be created with comparable peak-intensities to XFELs: isolated pulses of sub-femtosecond duration, trains of pulses with increased temporal coherence, and trains of femtosecond pulses of different wavelengths. We present self-consistent gain and amplification calculations, tailored to predict first experiments on lasing on neon pumped by the Linac Coherent Light Source at Stanford.