Reflectivity of plasmas created by high-intensity, ultra-short laser pulses

Abstract

Experiments were performed to characterize the creation and evolution of high-temperature (T e~100eV), high-density (ne>1022cm-3) plasmas created with intense (~1012-1016W/cm2), ultra-short (130fs) laser pulses. The principle diagnostic was plasma reflectivity at optical wavelengths (614nm). An array of target materials (Al, Au, Si, SiO2) with widely differing electronic properties tested plasma behavior over a large set of initial states. Time-integrated plasma reflectivity was measured as a function of laser intensity. Space- and time-resolved reflectivity, transmission and scatter were measured with a spatial resolution of ~3μm and a temporal resolution of 130fs. An amplified, mode-locked dye laser system was designed to produce ~3.5mJ, ~130fs laser pulses to create and nonintrusively probe the plasmas. Laser prepulse was carefully controlled to suppress preionization and give unambiguous, high-density plasma results. In metals (Al and Au), it is shown analytically that linear and nonlinear inverse Bremsstrahlung absorption, resonance absorption, and vacuum heating explain time-integrated reflectivity at intensities near 1016W/cm2. In the insulator, SiO2, a non-equilibrium plasma reflectivity model using tunneling ionization, Helmholtz equations, and Drude conductivity agrees with time-integrated reflectivity measurements. Moreover, a comparison of ionization and Saha equilibration rates shows that plasma formed by intense, ultra-short pulses can exist with a transient, non-equilibrium distribution of ionization states. All targets are shown to approach a common reflectivity at intensities ~1016W/cm2, indicating a material-independent state insensitive to atomic or solid-state details.