Generation and Applications of Femtosecond Optical Vortices

An optical vortex is a singularity point in a (scalar) electric field where the amplitude vanishes and the phase is undetermined. Laguerre-Gaussian modes are examples of modes containing an optical vortex. Our interest in vortex modes stems from the fact that their photons possess optical orbital angular momentum (OAM).$^{2}$ Our goal is to make strong ultrashort pulses with a vortex, so we can study the influence of optical OAM on intense-field ionization. Our motivation is the role of the photon's \textit{spin} angular momentum: in its manifestation as polarization, this affects intense-field ionization. Notable are electron recollision processes, central to many schemes to generate attosecond pulses. What role optical \textit{orbital} angular momentum plays in intense-field processes is to the best of our knowledge experimentally unexplored territory. In 2005, we were the first to report the generation of a pure femtosecond vortex.$^{3}$ Our setup uses holographic diffraction and properly deals with bandwidth (tens of nm). We now use a programmable hologram.$^{4}$ We are currently increasing the intensity of our fs vortices to reach ionization levels so we can image focused vortices with our spatially-resolved ion detector. Recent progress will be discussed. Refs: $^{2}$Allen L \textit{et al.} 2003 \textit{Optical Angular Momentum} (Bristol: IoP Publ.); $^{3}$Mariyenko I \textit{et al.} 2005 \textit{Opt. Expr.} \textbf{13} 7599; $^{4}$Strohaber J \textit{et al.} 2006 \textit{J. Phys B}. subm.