Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Dynamical study of remanent magnetization in nanoparticles

For collections of noninteracting nanoparticles, we study the reduced static remanent magnetization, m_R, produced by the removal of a saturating magnetic field. We show that, except for special cases such as easy uniaxial anisotropy, m_R depends on both the ramp-down rate of the field and the energy dissipation rate of the spin dynamics. Using the Landau–Lifshitz equation, we illustrate this result with explicit dynamical calculations of m_R for cubic and for mixed cubic–uniaxial anisotropies.     

Time resolved spectroscopy of CdSe,using monopulsed picosecond excitation

We have studied at 4.2 K, the time resolved luminescence of CdSe platelets, using two photons absorption and monopulsed excitation. From the analysis of the A-LO excitonic lineshape, we have obtained the relaxation kinetics of the exciton temperature during the first 1.3 nsec. We deduce the loss rate d<E>/dt of the mean exciton energy. We report also at higher excitation, the occurence of new lines in the luminescence spectrum.     

Picosecond ultrasonics time resolved spectroscopy using a photonic crystal fiber

We present a broadband picosecond ultrasonics time resolved spectroscopy. Detection of picosecond coherent acoustic phonons using a wavelength continuum generation in a photonic crystal fiber (PCF) with femtosecond laser pulses is developed. Measurements are performed for selected wavelengths of a broad wavelength probe pulse within a bandwidth of 250 nm with an 825 nm center wavelength on two samples made of tungsten and of gallium arsenide.