We report results and analysis of time-resolved photoinduced reflectivity experiments on the cuprate superconductor YBa 2 Cu 3 O 6.5 . The sample, which has T c = 45 K, was characterized by a high degree of purity and Ortho II ordering. The change in reflectivity ⌬R was induced and probed using pulses of 100 femtosecond duration and photon energy 1.55 eV from a Ti:Sapphire laser. We provide a detailed picture of the decay rate ␥ of ⌬R as a function of temperature T and pump intensity I. At low T, ␥ decreases linearly with decreasing I, extrapolating to nearly zero in the limit that I tends to zero. At higher temperature ␥ has the same linear dependence, but with nonzero limit as I → 0. In the interpretation of these results we assume that ⌬R is proportional to the nonequilibrium quasiparticle density created by the laser. From an analysis of the ␥ vs I we estimate ␤, the coefficient of proportionality relating the quasiparticle decay rate to the density. The intercept of ␥ vs I yields the thermal equilibrium quasiparticle decay rate. In a discussion section, we argue that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone. We explain the divergence of the lifetime of these particles as T and I both tend to zero as a consequence of momentum and energy conservation in electron-electron scattering. Next, we discuss the significance of the measured value of ␤, which is Ϸ0.1 cm 2 s −1 . We point out that the natural unit for ␤ in a two-dimensional superconductor is ប / m * , and define a dimensionless constant C such that ␤ ϵ Cប / m * . If the decay process is one in which quasiparticles return to the condensate with emission of a phonon, then C is a measure of the electron-phonon interaction. Alternatively, expressing the marginal Fermi liquid scattering in the normal state in terms of an effective ␤ implies C =1/, which is in excellent agreement with the experimentally determined value in the superconducting state.

The conversion efficiency of spectral emission from laser-irradiated solid targets was investigated for short wavelength source development. The plasma brightness was quantified using absolutely calibrated detectors for 20 materials and spectra were obtained between 50 and 200 Å. Laser parameters such as wavelength, pulse length, intensity, and spot size were systematically varied to establish a comprehensive database for source optimization. Qualitative differences in the underlying dominant emission features as a function of atomic number and laser wavelength were observed that accounted for the relatively high spectral conversion efficiencies produced. In the specific case of Sn, a conversion efficiency greater than 0.8%/eV has been observed in the technologically important region of λ=134.0 Å using a laser intensity of 1–2×1011 W/cm2.

X-ray production in the region ˜ 13 nm from laser-produced plasmas has been investigated as a source for projection x-ray lithography. The dependence of x-ray conversion efficiency on target material, intensity, and pulse length has been studied by using a 0.53-µm laser with a maximum of 0.3 J. A conversion efficiency of 1% into a 0.3-nm bandwidth has been demonstrated for Sn targets at intensities of ˜ 10(11) W/cm(2) by using a 7.5-ns pulse. Intensity scaling suggests that laser spot size and two-dimensional expansion are important for optimizing x-ray production at these low-irradiation intensities.

Laser-produced plasmas are investigated as a source for soft x-ray projection lithography. The dependence of conversion efficiency on target material, intensity, wavelength, and pulse width is determined using absolutely calibrated detectors. Conversion efficiency greater than 1% into a 2.2 eV bandwidth is demonstrated for Sn targets, fulfilling the system source requirements.