Ultrafast dynamics and sub-wavelength periodic structure formation following irradiation of GaAs with femtosecond laser pulses

A theoretical investigation of the ultrafast processes and dynamics of the excited carriers upon irradiation of GaAs with femtosecond (fs) pulsed lasers is performed in conditions that induce material damage and eventually surface modification of the heated solid. A parametric study is followed to correlate the produced transient carrier density with the damage threshold for various pulse duration values {\tau}p (it increases as ~ at relatively small values of {\tau}p while it drops for pulse durations of the order of some picoseconds) based on the investigation of the fundamental multiscale physical processes following fs-laser irradiation. Moreover, fluence values for which the originally semiconducting material demonstrates a metallic behaviour are estimated. It is shown that a sufficient number of carriers in the conduction band are produced to excite Surface Plasmon (SP) waves that upon coupling with the incident beam and a fluid-based surface modification mechanism lead to the formation of sub-wavelength periodic structures orientated perpendicularly to the laser beam polarization. Experimental results for the damage threshold and the frequencies of induced periodic structures show a good agreement with the theoretical predictions.Comment: 11 color pages To appear in the Physical Review

"What You Get" - Orbit and tune measurements and feedback

The Large Hadron Collider (LHC) has now been operated successfully for 3 years relying on the good performance of its beam instrumentation, including position and tune monitors and their respective feedback systems. This contribution gives an overview of the performance and limitations of the current orbit and tune systems. The major hardware and software modifications performed in 2012 are presented as well as the developments and improvements planned during the Long Shutdown 1 (LS1). The last part of the paper discusses the expected performances of both systems with 6.5TeV beams after LS1