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Ultra-intense laser-matter interaction experiments (>10$^{18}$ W/cm$^{2}$)with dense targets are highly sensitive to the effect of laser "noise" (in theform of pre-pulses) preceding the main ultra-intense pulse. Thesesystem-dependent pre-pulses in the nanosecond and/or picosecond regimes areoften intense enough to modify the target significantly by ionizing and forminga plasma layer in front of the target. Time resolved interferometry offers arobust way to characterize the expanding plasma during this period. We havedeveloped a novel pump-probe interferometry system for an ultra-intense laserexperiment that uses two short-pulse amplifiers synchronized by one ultra-fastseed oscillator to achieve 40-femtosecond time resolution over hundreds ofnanoseconds, using a variable delay line and other techniques. The first ofthese amplifiers acts as the pump and delivers maximal energy to theinteraction region. The second amplifier is frequency shifted and thenfrequency doubled to generate the femtosecond probe pulse. After passingthrough the laser-target interaction region, the probe pulse is split andrecombined in a laterally sheared Michelson interferometer. Importantly, thefrequency shift in the probe allows strong plasma self-emission at the secondharmonic of the pump to be filtered out, allowing plasma expansion near thecritical surface and elsewhere to be clearly visible in the interferograms.Three-phase reconstructions of the electron densities are then inferred by Abelinversion. This interferometric system delivers precise measurements ofpre-plasma expansion that can identify the condition of the target at themoment that the ultra-intense pulse arrives. Such measurements areindispensable for correlating laser pre-pulse measurements with instantaneousplasma profiles and for enabling realistic Particle-in-Cell simulations of theultra-intense laser-matter interaction.

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A Novel Femtosecond-Gated, High-Resolution, Frequency-Shifted Shearing  Interferometry Technique for Probing Pre-Plasma Expansion in Ultra-Intense  Laser Experiments