FTh4C.6.pdf CLEO:2015 © OSA 2015 Direct Observation of Rescattering from Strong Field Ionization by Two-Color Circularly Polarized Laser Fields Chris Mancuso 1 , Daniel D. Hickstein 1 , Patrik Grychtol 1 , Ronny Knut 1 , Ofer Kfir 2 , Xiao-Min Tong 3 , Franklin Dollar 1 , Dmitriy Zusin 1 , Maithreyi Gopalakrishnan 1 , Christian Gentry 1 , Emrah Turgut 1 , Jennifer L. Ellis 1 , Ming-Chang Chen 4 , Avner Fleischer 2,5 , Oren Cohen 2 , Henry C. Kapteyn 1 , and Margaret M. Murnane 1 1 JILA and Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309-0440 USA, 2 Solid State Institute and Physics Department, Technion, Haifa, 32000, Israel, 3 Division of Material Science, Faculty of Pure and Applied Science, University of Tsukuba, Ibaraki 305-8573, Japan, 4 Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan, and 5 Department of Physics and Optical Engineering, Ort Braude College, Karmiel 21982, Israel christopher.mancuso@jila.colorado.edu Abstract: The photoelectron distribution from strong field ionization by a two-color counter- rotating circularly polarized laser field uniquely exhibits distinct low-energy features. These can be attributed to electron-ion Coulomb rescattering, providing a new window into this process. OCIS codes: (020.2649) Strong field laser physics; (320.7110) Ultrafast nonlinear optics The interaction of intense laser fields (10 14 Wcm -2 ) with atoms and molecules is of great scientific and technological interest because of two related phenomena: high harmonic generation (HHG) and strong field ionization (SFI). Both HHG and SFI begin with the tunnel-ionization of an electron from an atom or molecule, after which the free electron is accelerated in the laser field. HHG occurs when an electron that is driven back to the parent ion recombines, releasing its kinetic energy by emitting an extreme ultraviolet (EUV) or soft X-ray photon. The SFI photoelectron angular distributions result from the electrons that do not recombine. However, they may still re-encounter their parent ion and scatter. These two processes allow for the study of a number of diverse phenomena including: capturing chemical reactions in real time, uncovering correlated charge/spin/phonon dynamics in materials, coherent imaging on the nanometer scale, and the study of dynamic orbital and molecular structure. The fact that both recombination (i.e. HHG) and rescattering are strongly suppressed in elliptically or circularly polarized driving laser fields means that past studies have generally used linearly polarized light to drive HHG and SFI processes. Fortunately, in two-color circularly polarized fields in which the two-colors have opposite helicities (i.e., counter-rotating), electrons move in a two-dimensional (2D) plane and certain trajectories return to the parent ion. Recently, it has been demonstrated that by driving the HHG process with these counter-rotating two-color light fields, bright EUV circularly polarized harmonics can be generated [1]. This new ability enables powerful spectroscopies such as magnetic circular dichroism studies of magnetic materials [2] - to now be implemented using a table-top setup. Furthermore, the 2D electron trajectories resulting from ionization in these counter-rotating two-color fields allows for the electron to be driven back towards the parent ion from a very different direction than from where it was originally ionized. Especially in molecules, this separate control of the tunneling and recombination directions could help overcome a long-standing limitation of HHG and SFI spectroscopies, as both techniques struggle to deconvolve distinct molecular structural information encoded in each of these two (i.e. ionization, re-encounter) steps. In this paper, we use tomographic techniques and a velocity map imaging (VMI) spectrometer to reconstruct the three-dimensional (3D) photoelectron angular distribution resulting from strong field ionization using such two-color Fig 1. (a) The combined laser electric field  and final drift momentum of tunnel-ionized electrons , where the dots indicate time zero for  and . (b) Experimental photoelectron distributions recorded on the velocity map imaging spectrometer. (c) Normalized 1D projections of the experimental photoelectron distributions plotted as a function of time delay between the 790 and 395 nm laser pulses, which reveal oscillations due to the rotation of the photoelectron distribution with a period of 1.3 fs (one cycle of the 395 nm field). (d) Theoretical photoelectron distributions using the strong field approximation.