Intense picosecond x rays from structured targets

grantor: University of TorontoLaser plasmas from a new type of nanostructured surface are studied. This nanowire surface is especially useful as a very high absorption target for high power (1 TW) subpicosecond laser-matter interaction. The ensemble of oriented 10-200 nm metallic fibers of this material have linear absorption of 1 m m light greater than 95%. When irradiated by 1 ps pulses at intensities up to 1017 W/cm-2, these targets produce a pulse of x-rays (average energy > 1.5 keV) 30 times more efficiently than do uniform solid targets, while preserving several-picosecond emission times. X-ray conversion efficiencies from the nanowire target and from previously investigated grating and 'smoke' targets are compared to those of flat targets for various angles of incidence and polarizations. Streak camera results show that a bright picosecond measure x-ray pulse is emitted from the near-solid density plasma created from the nanowire target. The measured x-ray pulse length is resolution-limited ( 6.5 ps) for emission above 1.5 keV and is 20-25 ps for soft x rays. An absorbing uniaxial material model of the nanowire target yields an effective dielectric constant for the structured layer. This model helps to optimize structure parameters for: (1) minimum Fresnel losses on the vacuum/structured-target interface; and, (2) highest light absorption in the structured layer in the small-signal regime. A possible extension of this model into the plasma absorption regime is discussed. As part of this research a state-of-the-art 1 TW chirped pulse amplification (CPA) laser system was constructed. The Nd:glass system is based on a feedback-controlled mode-locked oscillator capable of providing high contrast 1 ps pulses at a wavelength of 1.054 m m, with energies up to 5 m J/pulse. The characteristics of the CPA system are described. A new detector was developed to measure absolute x-ray yield, in various spectral ranges, radiated from the solid-density plasma. A calibrated silicon PIN photodiode connected to a charge-sensitive amplifier circuit allows x-ray yield measurements from picosecond pulses. If properly filtered the detector can measure the amount of x-rays radiated in a narrow spectral range. The electrical and spectral characteristics of the PIN photodiode detectors are given.Ph.D