Joseph Nilsen

Laser with dynamic holographic intracavity distortion correction capability

We report here a novel laser resonator with the ability to correct for intracavity phase distortions. The optical cavity employs a passive (self-pumped) phase conjugate reflector to provide this capability.

Nondegenerate four-wave mixing in a Doppler-broadened resonant medium

Third-order susceptibilities are calculated for a Doppler-broadened two-level system. The dependence of four-wave mixing on the angle θ between the pump and signal fields is studied, and the reflection coefficient is shown to have a narrow field of view that is inversely proportional to the ratio of the Doppler width to the homogeneous linewidth. For the collinear geometry, θ = 0°, the frequency dependence of the nondegenerate case is analyzed and shown capable of yielding a real-time narrow-bandwidth optical filter whose bandwidth is limited by the homogeneous linewidth.

Picosecond resolution soft x-ray laser plasma interferometry

We describe a soft-x-ray laser interferometry technique that allows two-dimensional diagnosis of plasma electron density with picosecond time resolution. It consists of the combination of a robust high-throughput amplitude-division interferometer and a 14.7-nm transient-inversion soft-x-ray laser that produces approximately 5-ps pulses. Because of its picosecond resolution and short-wavelength scalability, this technique has the potential for extending the high inherent precision of soft-x-ray laser interferometry to the study of very dense plasmas of significant fundamental and practical interest, such as those investigated for inertial confinement fusion. Results of its use in the diagnostics of dense large-scale laser-created plasmas are presented.

Longitudinal coherence measurements of a transient collisional x-ray laser

We present what is to our knowledge the first longitudinal coherence measurement of a transient inversion collisional x-ray laser. We investigated the picosecond output of a Ni-like Pd x-ray laser at 14.68 nm generated by the COMET laser facility at the Lawrence Livermore National Laboratory. Interference fringes were generated with a Michelson interferometer setup in which a thin multilayer membrane was used as a beam splitter. We determined the longitudinal coherence for the 4dS01→4pP11 lasing transition to be ~400μm (1/e half-width) by changing the length of one interferometer arm and measuring the resultant variation in fringe visibility. The inferred gain-narrowed linewidth of ~0.29pm is a factor of 4 less than previously measured in quasi-steady-state x-ray laser schemes.

Demonstration of transient gain x-ray lasers near 20??nm for nickellike yttrium, zirconium, niobium, and molybdenum

We demonstrate strong lasing on the Ni-like 4d(1)S(0)?4p(1)P(1) transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17-26cm (-1) are determined on the 4d?4p transition, with overall gain-length products gL of 11-12. Lasing is observed and gain is measured on the 4f(1)P(1)?4d(1)P(1) transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time.

Wavelengths of neon-like 3p → 3s X-ray laser transitions

This paper presents the wavelengths of the neon-like 3p → 3s laser lines which have been observed in both lasing and nonlasing plasmas and compares the experimental wavelengths with relativistic multi-configuration Hartree-Fock calculations in order to estimate the wavelengths for these lines in all neon-like ions between S6+ and Xe44+. The seven 3p → 3s transitions which have been observed to lase in some neon-like ions are considered in this work.

First observation of lasing at 231 Å in neon-like nickel using the prepulse technique

We report lasing for the first time in nickel on the neon-like J=0→1, 3p→3s transition at 231 Å as well as several weaker transitions including the J=2»1 lines at 298 Å and 304 Å. Amplification is seen only when the prepulse technique of using a low intensity prepulse before the main optical drive pulse is used to illuminate the nickel target. The prepulse technique is also shown to produce lasing in copper and dramatically improve the output of the germanium laser.

Traveling wave excitation and amplification of neon-like germanium 3p–3s transitions

Abstract We report on experiments using traveling wave excitation of neon-like germanium to produce amplification of 3p–3s soft X-ray transitions. The laser pulse consisted of three 100 ps Gaussian laser pulses separated by 400 ps. Targets were 3 cm long germanium slabs. Traveling wave excitation was employed by tilting the wave front of the driving laser by 45 degrees to match the propagation speed of the X-ray laser photons along the length of the target. We compare cases with the traveling wave, with no traveling wave, and against the traveling wave. Significant enhancement of the X-ray laser intensities is observed using the traveling wave technique due to the short gain duration of these plasmas. Gain was inferred from line intensity measurements at two lengths. Our results are compared to a numerical model.

Sensitivity of lasing in neonlike zinc at 21.2 nm to the use of the prepulse technique.

We report strong lasing in zinc on the neonlike 3p → 3s, J = 0 → 1 transition at 21.2 nm when the prepulse technique of using a low-intensity prepulse before the main optical drive pulse is used with the 1.315-μm Asterix laser to illuminate the zinc target. Without the use of the prepulse, this line is not visible. The usual J = 2 → 1 lines at 26.2 and 26.7 nm are observed weakly both with and without the use of the prepulse. By variation of the prepulse energy, we demonstrate that even a very weak prepulse with less than 0.1% of the main pulse energy can enable the zinc to lase at 21.2 nm.

Damage and ablation of large bandgap dielectrics induced by a 46.9 nm laser beam

We applied a 0.3 mJ, 1.7 ns, 46.9 nm soft-x-ray argon laser to ablate the surface of large bandgap dielectrics: CaF2 and LiF crystals. We studied the ablation versus the fluence of the soft-x-ray beam, varying the fluence in the range 0.05-3 J/cm2. Ablation thresholds of 0.06 and 0.1 J/cm2 and ablation depths of 14 and 20 nm were found for CaF2 and LiF, respectively. These results define new ablation conditions for these large bandgap dielectrics that can be of interest for the fine processing of these materials.