An optical-field-ionization soft x-ray laser using a clustered gas jet was demonstrated. Pd-like xenon lasing at 41.8 nm with 95-nJ pulse energy and 5.2-mrad divergence was achieved, indicating near-saturation amplification. By using a prepulse to control the expansion of ionized clusters, it was found that the microscopic uniformity of the plasma is essential for efficient lasing. In addition, the optimal atom density for maximum lasing reported previously is verified to result from the tradeoff between large gain coefficient and short gain length due to ionization-induced refraction.

By using deflectometry of a longitudinal probe pulse and reflective interferometry of a transverse probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials. Many important issues crucial to x-ray lasing such as electron density distribution, electron temperature, and the optimal timing between pumping pulses can be resolved with these methods.

By utilizing the intensity- and duration-dependent heating and expansion rate of nanoplasma to generate a transient transverse gradient of the refractive index, prepulse controlled laser-beam propagation is demonstrated. The dynamical response of the macroscopic optical refractive index is traced back to the microscopic polarizability of nanoplasmas experimentally, in accordance with hydrodynamic nanoplasma models. In particular, the delay between the prepulse and the main pulse for maximum Rayleigh scattering is found to be longer than that for maximum x-ray emission, supporting the more refined one-dimensional self-consistent hydrodynamic nanoplasma model.

By using an axicon lens in conjunction with the ignitor-heater scheme, a 1.2-cm-long high-quality plasma waveguide is generated efficiently, which can extend the range of laser-plasma interaction much beyond the limit of Rayleigh range