We report temporal coherence measurement of solid-target plasma-based soft X-ray laser (XRL) in amplified spontaneous emission (ASE) mode. By changing the XRL pumping angle, we generate lasing at two-times higher electron density than the routine condition. A relatively shorter coherence time at a higher pumping angle indicates a clear spectral signature of higher electron density in the gain region. We probe the amplification dynamics of XRL in routine, and high electron density conditions to confirm gain-duration reduction resulting from ionization gating in the latter case. We also present recent results on the seeding of a vortex beam carrying orbital angular momentum (OAM) in XRL plasma. A small part of the high topological charge extreme ultraviolet (EUV) vortex is injected in XRL. These preliminary results suggest that the vortex seed indeed can be efficiently amplified. In the end, we propose a pathway towards the seeding of the complete vortex beam and wavefront characterization of the amplified beam.

We investigate the coherence of plasma-based soft X-ray laser (XRL) for different conditions that can alter the electron density in the gain region. We first measure the source temporal coherence in amplified spontaneous emission (ASE) mode. We develop a data analysis procedure to extract both its spectral width and pulse duration. These findings are in agreement with the spectral line shape simulations and seeded operation experimental results. Utilizing the deduced spectral width and pulse duration in a one-dimensional Bloch-Maxwell code, we reproduce the experimental temporal coherence properties of the seeded-XRL. Finally, we demonstrate efficient lasing in ASE and seeded mode at an electron density two times higher than the routine conditions. In this regime, using Bloch-Maxwell modeling, we predict the pulse duration of the seeded XRL to be ∼500fs.

We present a quantitative analysis and interpretation of the presence of shoulder features at delays larger than the coherence time in the linear autocorrelation traces of partially coherent, ultrashort pulse laser sources.

A quantitative interpretation method is described for experiments involving the linear autocorrelation of partially coherent extreme-ultraviolet (XUV) pulses, generated by either x-ray free-electron lasers or plasma-based XUV lasers. A recently published modeling method for partially coherent pulses is numerically implemented in that specific case. Analytical expressions for the statistical root-mean-square average of the fringe visibility are derived. The method yields unambiguous information on both the coherence time and the pulse duration, providing a valuable data interpretation tool.

We study how the degree of temporal coherence of plasma-based extreme ultraviolet lasers operated in the amplification of the spontaneous emission mode is encoded in the shape of the linear autocorrelation function, which is obtained from the variation of the fringe visibility while varying the delay in a variable path-difference interferometer. We discuss the implications of this effect when the technique is used to infer the spectral properties of the source. Our numerical simulations, based on a partial coherence model developed by other authors for x-ray free electron lasers, are in good agreement with previously reported sets of measurements, illustrating similar statistical properties for both sources.

We review our recent work devoted to the experimental characterization and numerical simulations of XUV laser pulses generated from hot and dense plasmas. We discuss the specific properties exhibited by few-picosecond, partially coherent pulses arising from Amplification of Spontaneous Emission. We describe our ongoing effort to develop a single-shot time-resolved measurement tool adapted to shorter, femtosecond pulses, which are foreseen with advanced seeded XUV lasers developed in several laboratories.

Development of efficient soft x-ray laser plasma amplifiers adapted to seeded operation, requires a better control over amplifier transverse spatial extent, brilliance control and gain lifetime. Here it is shown that pumping the plasma amplifier with one long and two short pump pulses (1L2S) provides advantages in terms of control for the specified parameters in the case of Ni-like Ag x-ray laser. Also, significant tunability of the gain lifetime in the 1L2S pumping scheme for Ne-like Ti x-ray laser is observed. Direct harmonics seeding and chirped harmonics seeding amplification approaches may benefit from the control of the gain lifetime, in terms of better use of the pump energy and as a way to reduce the amplified spontaneous emission in x-ray lasers. (C) 2016 Optical Society of America

We present a refined measurement of the temporal coherence of the zinc X-ray laser generated at PALS, using a wavefront-division interferometer. The corresponding bandwidth of the lasing line is shown (i) to be broader than those of other types of X-ray lasers, previously characterized with the same instrument, and (ii) is compatible with the amplification of subpicosecond pulses. Similar measurements were also performed for shorter amplification lengths and the small-signal gain was carefully characterized.

Development of efficient soft x-ray laser plasma amplifiers adapted to seeded operation, requires a better control over amplifier transverse spatial extent, brilliance control and gain lifetime. Here it is shown that pumping the plasma amplifier with one long and two short pump pulses (1L2S) provides advantages in terms of control for the specified parameters in the case of Ni-like Ag x-ray laser. Also, significant tunability of the gain lifetime in the 1L2S pumping scheme for Ne-like Ti x-ray laser is observed. Direct harmonics seeding and chirped harmonics seeding amplification approaches may benefit from the control of the gain lifetime, in terms of better use of the pump energy and as a way to reduce the amplified spontaneous emission in x-ray lasers.

In this paper we report on development of the secondary X-ray sources at the PALS Centre and discuss the plan for the ELI Beamlines project. The spatial and temporal coherence of the most energetic quasi-steady state Ne-like Zn X-ray laser, which is operated at PALS Centre as standard user beamline, was examined proving that amplification of coherent EUV pulses with duration below 1 ps will be possible. Meanwhile, the first transient lasing at PALS Center was achieved using 10 Hz Ti:Sapphire laser chain with peak power of 20 TW as a driver. Finally, we discuss the recent design of laser driven secondary sources generating short coherent or incoherent EUV/X-ray pulses within the ELI Beamlines project.

We discuss the role of the spectral properties of XUV lasers in the amplification of femtosecond pulses in two different regimes. We present an overview of our recent spectral characterization of the four types of collisionally-pumped systems, also including other measurements from the literature. This is used to assess the potential of the different types of XUV lasers to amplify femtosecond pulses, either in the adiabatic or in the dynamic regime.

Harmonic seeded operation of a neon-like titanium plasma-based soft x-ray laser is described. The plasma amplifier is pumped with a variation of the grazing incidence technique involving a fast and localized ionization step. We discuss its effect on gain dynamics by measuring the amplifying factor as a function of the delay between pump pulse and harmonic seed. Two different regimes are pointed out, following the pumping scheme used. For one of them, a delay in the gain generation compared with the pumping laser pulse is observed.

X-ray laser simulations based on Ehybrid code have shown that enhanced plasma x-ray laser emission can be achieved mastering the ionization dynamics and plasma temperature using one long and two short pulses (Ursescu and Ionel, J Optoelectron Adv Mat 12:48–51, 2010). In parallel, two simple methods to generate multiple short pulses for pumping x-ray lasers were reported in conjunction with x-ray laser developments. Five to ten fold enhancement in the emission of the silver x-ray laser was demonstrated using the newly developed pump methods, when compared with the traditional one, based on a long pulse followed by one short pump pulse. An overview of these recent experiments will be presented. The possible implementation of these novel x-ray laser pumping methods at Extreme Light Infrastructure—Nuclear Physics facility will be discussed.

We have carried out a detailed characterization of the temporal coherence and spectral bandwidth of the Ne-like Zn XUV laser, collisionally pumped in the quasi-steady-state (QSS) regime and operated at saturation. The measured coherence time tau(c) = 680 +/- 70 fs is significantly shorter than previously observed with other types of collisional XUV lasers, using the same measurement technique. Additional measurements performed below saturation were used to investigate gain narrowing of the spectral width of the lasing line. Our experimental data are well reproduced by calculations performed with a one-dimensional radiative transfer numerical code, taking the measured gain as an input. We show that our results are consistent with spectral measurements of QSS XUV lasers reported by different authors in the 1990s, although they were based on slightly different pumping conditions and measurement techniques.

Seeded operation of a Neon-like Titanium plasma-based soft x-ray laser is described. The plasma amplifier is pumped using a variation of the classical grazing incidence pumping technique, combining a long low energy pulse followed by a main short pulse. Because the preformed plasma is underionized, a part of the main short pulse energy is used to ionized it to the lasing stage. Consequences of this feature on seeded laser properties are discussed.

An expression is proposed for a threshold discriminating between classical quasi-steady-state amplification and a strong pumping regime where a new dynamic behavior shows up in plasma-based extreme-ultraviolet (XUV) lasers. The criterion derived is applied to laser systems currently investigated in various laboratories. It is found that only high-gain XUV laser systems can be driven in that dynamic regime, associated with Rabi oscillations. For those systems, the small-signal dynamic gain accounting for the exponential amplification of the laser intensity is shown to be significantly smaller than the adiabatic value usually inferred from numerical simulations.

The results of development and applications of the secondary sources at PALS Center will be presented. Currently the iodine system and the Ti: Sapphire system are operating at the PALS Center as driving lasers for generation of secondary sources. The iodine system with net energy of 1kJ is used for QSS X-ray lasing schemes. The most robust and most energetic QSS scheme with this driver is the Ne-like Zn X-ray laser, which is working here as standard user beamline for diverse applications. Recent experiment devoted to temporal coherence measurement shows possibility to amplify short duration X-rays. The second system with high rep rate is Ti: Sapphire laser chain with peak power 20TW. This laser system is used for generation high order harmonics and transient collisionally excited X-ray lasers.

One promising way to achieve ultra-short pulse amplification in XUV laser amplifiers rests on nonlinear processes involving phenomena such as Rabi oscillations. A criterion for such behavior to occur in currently available XUV lasers is derived. Two-dimensionalMaxwell-Bloch simulations of short pulse amplification are presented in the case of Ni-like Ag pumped in grazing incidence and seeded by a high-order harmonics pulse. From numerical seed parameter optimization, it is shown that an ultrashort, high intensity, quasi-pi XUV pulse can be produced in that system.