Relativistic positron creation using ultraintense short pulse lasers.

We measure up to 2x10;{10} positrons per steradian ejected out the back of approximately mm thick gold targets when illuminated with short ( approximately 1 ps) ultraintense ( approximately 1x10;{20} W/cm;{2}) laser pulses. Positrons are produced predominately by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic. Modeling based on the measurements indicate the positron density to be approximately 10;{16} positrons/cm;{3}, the highest ever created in the laboratory.

Demonstration of coherent addition of multiple gratings for high-energy chirped-pulse-amplified lasers.

Petawatt solid-state lasers require meter-sized gratings to reach multiple-kilojoule energy levels without laser-induced damage. As an alternative to large single gratings, we demonstrate that smaller, coherently added (tiled) gratings can be used for subpicosecond-pulse compression. A Fourier-transform-limited, 650-fs chirped-pulse-amplified laser pulse is maintained by replacing a single compression grating with a tiled-grating assembly. Grating tiling provides a means to scale the energy and irradiance of short-pulse lasers.