L. A. Johnson

Plasma lenses for ultrashort multi-petawatt laser pulses

An ideal plasma lens can provide the focusing power of a small f-number, solid-state focusing optic at a fraction of the diameter. An ideal plasma lens, however, relies on a steady-state, linear laser pulse-plasma interaction. Ultrashort multi-petawatt (MPW) pulses possess broad bandwidths and extreme intensities, and, as a result, their interaction with the plasma lens is neither steady state nor linear. Here, we examine nonlinear and time-dependent modifications to plasma lens focusing, and show that these result in chromatic and phase aberrations and amplitude distortion. We find that a plasma lens can provide enhanced focusing for 30 fs pulses with peak power up to ∼1 PW. The performance degrades through the MPW regime, until finally a focusing penalty is incurred at ∼10 PW.

Guiding supersonic projectiles using optically generated air density channels

We investigate the feasibility of using optically generated channels of reduced air density to provide trajectory correction (guiding) for a supersonic projectile. It is shown that the projectile experiences a force perpendicular to its direction of motion as one side of the projectile passes through a channel of reduced air density. A single channel of reduced air density can be generated by the energy deposited from filamentation of an intense laser pulse. We propose changing the laser pulse energy from shot-to-shot to build longer effective channels. Current femtosecond laser systems with multi-millijoule pulses could provide trajectory correction of several meters on 5 km trajectories for sub-kilogram projectiles traveling at Mach 3.

Backward Raman amplification in the long-wavelength infrared

The wealth of work in backward Raman amplification in plasma has focused on the extreme intensity limit; however, backward Raman amplification may also provide an effective and practical mechanism for generating intense, broad bandwidth, long-wavelength infrared radiation (LWIR). An electromagnetic simulation coupled with a relativistic cold fluid plasma model is used to demonstrate the generation of picosecond pulses at a wavelength of 10 μm with terawatt powers through backward Raman amplification. The effects of collisional damping, Landau damping, pump depletion, and wave breaking are examined, as well as the resulting design considerations for an LWIR Raman amplifier.

Pulse splitting of stimulated Raman backscattering with a chirped pump

Raman amplified seed splitting was demonstrated in the backscattering scheme in a plasma, when a significant chirp was introduced into the pump. Particle-in-cell simulations have shown that a single seed laser pulse gradually splits into two, self-compressed pulses in the nonlinear amplification regime. This is in difference to previous studies of the pump chirp, which mainly have focused on its compensation for the plasma density gradient, in order to maximize the resonant amplification. The splitting, as revealed by the spectral evolution of the amplified seed, is attributed to resonance slipping, which is the result of the spatio-temporal distribution of the gain of the chirped pump.

Reciprocity breaking during nonlinear propagation of adapted beams through random media

Adaptive optics (AO) systems rely on the principle of reciprocity, or symmetry with respect to the interchange of point sources and receivers. These systems use the light received from a low power emitter on or near a target to compensate phase aberrations acquired by a laser beam during linear propagation through random media. If, however, the laser beam propagates nonlinearly, reciprocity is broken, potentially undermining AO correction. Here we examine the consequences of this breakdown, providing the first analysis of AO applied to high peak power laser beams. While discussed for general random and nonlinear media, we consider specific examples of Kerr-nonlinear, turbulent atmosphere.

Synchrotron radiation from a curved plasma channel laser wakefield accelerator

A laser pulse guided in a curved plasma channel can excite wakefields that steer electrons along an arched trajectory. As the electrons are accelerated along the curved channel, they emit synchrotron radiation. We present simple analytical models and simulations examining laser pulse guiding, wakefield generation, electron steering, and synchrotron emission in curved plasma channels. For experimentally realizable parameters, a ∼2 GeV electron emits 0.1 photons per cm with an average photon energy of multiple keV.A laser pulse guided in a curved plasma channel can excite wakefields that steer electrons along an arched trajectory. As the electrons are accelerated along the curved channel, they emit synchrotron radiation. We present simple analytical models and simulations examining laser pulse guiding, wakefield generation, electron steering, and synchrotron emission in curved plasma channels. For experimentally realizable parameters, a ∼2 GeV electron emits 0.1 photons per cm with an average photon energy of multiple keV.

Stimulated Raman and Brillouin scattering, nonlinear focusing, thermal blooming, and optical breakdown of a laser beam propagating in water

The physical processes associated with propagation of a high-power laser beam in a dielectric include self-focusing, stimulated Raman scattering, stimulated Brillouin scattering, thermal blooming, and multiphoton and collisional ionization. The interplay between these processes is analyzed using a reduced model consisting of a few differential equations that can be readily solved, enabling rapid variation of parameters and the development of theoretical results for guiding new experiments. The presentation in this paper is limited to propagation of the pump, the Stokes Raman, and the Brillouin pulses, ignoring the anti-Stokes Raman. Consistent with experimental results in the literature, it is found that self-focusing has a dramatic effect on the propagation of high-power laser beams in water. A significant portion of the pump laser energy is transferred to Stokes Raman forward scatter along with a smaller portion to Brillouin backscatter.

Ideal form of optical plasma lenses

The canonical form of an optical plasma lens is a parabolic density channel. This form suffers from spherical aberrations, among others. Spherical aberration is partially corrected by adding a quartic term to the radial density profile. Ideal forms which lead to perfect focusing or imaging are obtained. The fields at the focus of a strong lens are computed with high accuracy and efficiency using a combination of eikonal and full Maxwell descriptions of the radiation propagation. The calculations are performed using a new computer propagation code, SeaRay, which is designed to transition between various solution methods as the beam propagates through different spatial regions. The calculations produce the full Maxwell vector fields in the focal region.

Intense underwater laser propagation, ionization and heating for remote shaped plasma generation

Summary form only given. Intense underwater laser propagation, filamentation, and ionization are beingIntense underwater laser propagation, filamentation, and ionization are being investigated at NRL for many Navy applications, including remote undersea laser acoustic generation for low-frequency long-range sonar, as well as advanced micromachining using underwater laser-guided discharges. The key to low-frequency laser acoustic generation is shaping of the heated underwater volume, with more elongated volumes producing longer-duration acoustic pulses with more energy at low-frequencies. A patented scheme for generating elongated, meter-scale, high energy density underwater plasmas1 is under study, in which a filamenting laser pulse could serve as a target for a second energetic “heater” laser pulse. Early experiments suggest improved ionization efficiency using a ps filament pulse at either 266 nm or 355 nm, together with a 532 nm ns heater pulse. Time-resolved absorption spectroscopy of intense underwater propagation and ionization enabled measurement of hydrated electron density of 5.4 × 1018 cm-3 and lifetime of 350 ps. High-resolution fluorescence imaging of ns underwater laser propagation, using twophoton absorbing dye, confirmed previous measurements of 100 μm diameter filament structures2. Intense underwater laser propagation can involve both strong forwardpropagating stimulated Raman scattering (SRS) and backward-propagating stimulated Brillouin scattering (SBS), motivating our construction of a new lab-frame nonlinear laser propagation code. 1D and 2D analytical calculations and numerical simulations are underway to predict beam envelope propagation, filamentation, and early photoionization and plasma heating behavior. Analytical modeling revealed that cross-phase modulation by SRS can dominate the pump beam envelope dynamics for some conditions3. Results from recent experiments and modeling will be presented.