We report on generation of relativistic electron beams in the wake of a relativistically intense laser pulse traversing a 1.7 mm long atmospheric density helium gas jet. The plasma wake structure is recovered using a Frequency-Domain Holography (FDH) and Frequency-Domain Shadowgraphy (FDS). As the gas density changes, the accelerated electron beams show variations in cross-section area, divergence, total charge, and peak energy. FDH phase reconstruction shows discontinuities and large phase jumps due to plasma electrons blown out by the pump pulse, probe pulse refraction, and nonlinear propagation in plasma. However, FDS amplitude reconstruction shows bright spots that yield information about bubble formation and evolution.
We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of mid-IR radiation. Reflectance measurements demonstrate critical coupling by a double scan of wavelength and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and the substrate, resulting in maximal electric field enhancement.
Nonlinear focusing of a bi-color laser in plasma can be controlled by varying the difference frequency Ω. The driven electron density perturbation forms a co-moving periodic focusing (de-focusing) channel if Ω is below (above) the electron Langmuir frequency ω p . Hence, the beam focusing is enhanced for Ω < ω p and is suppressed otherwise. In particular, a catastrophic relativistic self-focusing of a high-power laser beam can be prevented all-optically by a second, much weaker, co-propagating beam shifted in frequency by Ω > ω p . A bi-envelope equation describing the early stage of the mutual de-focusing is derived and analyzed. Later stages, characterized by a well-developed electromagnetic cascade, are investigated numerically. Stable propagation of the over-critical laser pulse over several Rayleigh lengths is predicted. The non-resonant plasma beat wave (Ω ≠ ω p ) can accelerate pre-injected electrons above 100 MeV with low energy spread.
Recent experiments (Kopp et al 2007 J. Opt. Soc. Am. B 24 A48) have demonstrated that the polarization sensitivity of chiral fiber gratings depends strongly on the grating symmetry: double-helix fibers are polarization sensitive while single-helix fibers are not. A coupled-mode perturbation theory is developed and used to explain the polarization properties of chiral fiber gratings. Features of the transmission spectrum such as multiple dips in the spectrum and circular dichroism are also derived and attributed to chiral Bragg scattering of the core modes into the cladding modes of the fiber.
In this review we describe the challenges and opportunities for creating magnetically active metamaterials in the optical part of the spectrum. The emphasis is on the sub-wavelength periodic metamaterials whose unit cell is much smaller than the optical wavelength. The conceptual differences between microwave and optical metamaterials are demonstrated. We also describe several theoretical techniques used for calculating the effective parameters of plasmonic metamaterials: the effective dielectric permittivity ϵ eff ( ω ) and magnetic permeability μ eff ( ω ) . Several examples of negative permittivity and negative permeability plasmonic metamaterials are used to illustrate the theory.