Compact X-ray laser amplifier in the "Water Window".

Spectroscopy and microscopy in the so-called "water-window" is a holy grail of modern molecular biology. A pulsed source of coherent X-rays within this spectral window, falling between 2.3 nm and 4.4 nm, provides a unique tool for time-resolved imaging of bio-systems in their naturally water-rich state. Within this spectral range, water is mostly transparent, while proteins are mostly opaque. This results in a high-contrast image on the sub-cellular level. Here we present, for the first time, generation of a very high gain of G≈ 60/cm in He-like CV ions via transitions to the ground state at 4.03 nm in a table-top device.

Soft x-ray lasers and their applications.

The emerging technology of soft x-ray lasers has novel applications to microscopy, lithography, and other fields. This article describes the status of soft x-ray laser research with the aim of bringing the rapid developments in this field to the attention of potential users in other disciplines. The different techniques for generating a population inversion and producing a soft x-ray laser are reviewed. The status of current research in the field and the near-term prospects are described. It is expected that the range of potential applications of soft x-ray lasers will increase as their performance improves. Work aimed at increasing the output power and progressing to shorter wavelengths with these devices is also reviewed.

Backward Raman amplification in a partially ionized gas.

Compressing laser pulses to extremely high intensities through backward Raman amplification might be accomplished in a plasma medium. While the theory is relatively straightforward for homogeneous fully ionized plasma, a number of important effects enter when the plasma is not fully ionized. In particular, when a mixture of gases is employed to accomplish the coupling, there can be several thresholds for incremental ionization. The refraction of both the pump and the seed is then strongly affected by the plasma ionization. Moreover, in the case of Raman backscattering in partially ionized plasma, the degree of plasma ionization is particularly sensitive to the counterpropagating geometry. This idea is examined in light of data for a recent experiment on a Raman amplifier.

Raman backscattering and amplification in a gas jet plasma.

Raman backscattering (RBS) of a picosecond 800-nm laser pulse from a gas jet plasma has been observed. The frequency shifts are in agreement with independent interferometric density measurements and the band width of the RBS is consistent with the linear growth rate. Raman amplification of a subpicosecond seed pulse, provided by an optical parametric oscillator and tuned to the spectral range of the RBS, has been obtained. The Raman resonance is confirmed by simultaneous plasma density measurements.

Numerical investigation of recombination gain in the Li III transition to ground state.

We present a numerical investigation of the parameters characterizing the 2-->1 transition recombination gain in Li III ions ( 13.5 nm ). The numerical model includes the initial optical field ionization of the plasma by an intense 100 fs laser pulse, taking into account above threshold ionization heating, particle collisions, and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools. We show that by taking into account the non-Maxwellian nature of the electron distribution function in the plasma and its spatial distribution, high gain in the 2-->1 transition of Li III is feasible under certain initial conditions, even though initial estimates based on the energy absorption during the ionization predict very low gain. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions.

Raman amplification of ultrashort laser pulses in microcapillary plasmas.

Experimental evidences of Raman amplification of ultrashort pulses in microcapillary plasmas are presented. The amplification of 100-500 fs pulses was investigated in microcapillaries with different lengths. The experimental data, together with simulation results, indicate that the resonance condition for Raman amplification in high-density plasma, n(e) approximately 1-3x10(20) cm(-3), existed only in a very short plasma column. Such an assumption makes it possible to reconcile the experimental results and theoretical predictions. Investigations in very short microcapillaries (0.2-0.5 mm) with a broadband seed pulse further support this hypothesis and the amplification factor is in agreement with the linear growth rate.

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses.

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.

Gain measurements at 18.22 nm in C VI generated by a Nd:glass laser.

We present recent gain measurements in C VI at 18.22 nm for a soft-x-ray amplifier produced by a line-focused glass laser (1.053 microm) on a solid carbon target. The maximum gain measured was 8 +/- 2 cm(-1) in the recombining plasma column, with additional radiation cooling, with additional radiation cooling by iron impurities.

Demonstration of ultrashort laser pulse amplification in plasmas by a counterpropagating pumping beam

We experimentally demonstrated amplification of ultrashort laser pulses (200 fs pulses) in microcapillary plasmas by a counterpropagating pumping beam. Energy amplification as large as a factor of 5 was observed. Importantly, a nonlinear pump depletion regime must have been accessed, since the parameters support not only the amplification of the pulse, but an absolutely growing instability if the pump is not depleted. Further indication of accessing the nonlinear depletion is indicated by the relative insensitivity to the initial pulse energy.