Session C03: Elliptically Polarized High-Harmonics and Applications

Synthetic chiral light for extremely efficient laser-controlled chiral discrimination.

Distinguishing left- and right-handed molecular enantiomers is challenging, especially on ultrafast time scale. Traditionally one uses a chiral reagent, an object of known handedness, to determine the unknown handedness of a chiral molecule. In optics, one uses the spatial helix formed by circularly polarized light as a ``chiral photonic reagent''. However, in optical domain the pitch of this helix -- the light wavelength -- does not match the size of the molecule, leading to very small chiral signals. In dipole approximation, which neglects the size of the molecule with respect to light wavelength, circularly polarized light is not chiral. Indeed, the Lissajous figure drawn by the tip of the electric field vector is confined to a plane: the dipole approximation turns chiral helix of light into a circle. Since the dominant optical response arises in the dipole approximation, it is destined to be the same in opposite molecular enantiomers. I will introduce a new concept of synthetic chiral light [1], which is chiral already in the dipole approximation. Compared to the inefficient chiral reagent -- the light helix in space, here the helix is in time. In synthetic chiral light the electric field vector draws a three dimensional chiral Lissajous figure, at every point in space. The key point is that this chiral structure will appear already in the dipole approximation. I will show how this chiral photonic reagent can be tuned to ``react'' with the desired enantiomer of a chiral molecule and not with its mirror twin, achieving the ultimate limit in efficiency of chiral discrimination. The simplicity of generating synthetic chiral light in a laboratory opens a broad field of shaping and controlling chiral matter with light. [1] David Ayuso et al, Locally and globally chiral fields for ultimate control of chiral light matter interaction, https://arxiv.org/abs/1809.01632   

Attosecond pulses and high-order harmonics with controllable spin and orbital angular momentum.

Angular momentum can be routinely transferred to visible/infrared light beams using waveplates, or spatial light modulators, among other techniques. However, it becomes a lot harder in the extreme-ultraviolet (EUV) and x-ray regimes, where those techniques are inefficient. This challenging goal is very much worth the effort: imprinting spin (SAM) and/or orbital (OAM) angular momentum into the EUV/x-ray regimes will bring the applications of structured light down to the nanometric and ultrafast scales. The extreme nonlinear frequency upconversion of an intense IR femtosecond laser pulse through high harmonic generation (HHG) has become a powerful technique to imprint SAM and OAM onto the EUV regime. This talk reviews our recent work in the generation of coherent, EUV high-harmonic beams and attosecond pulses with full, simultaneous control of both spin angular momentum (SAM) and orbital angular momentum (OAM). By harnessing the quantum coherence of HHG, we uncover a new form of all-optical SAM-OAM interplay showing, experimentally and theoretically, that this phenomenon allows for unprecedented control over the polarization and vortex charge of attosecond EUV vortex beams. This work paves the way to perform ultrafast studies of magnetic materials and chiral systems at the subfemtosecond and nanometer scales.   

Symmetries and Symmetry-Breaking in High Harmonic Generation: Controlled Polarization and Ultrafast Spectroscopy

The analysis of symmetries and their associated selection rules is extremely useful in all fields of science. The field of nonlinear optics is no exception. In the early days of nonlinear optics symmetries were used to derive whether particular nonlinear processes are allowed or forbidden according to the medium’s point-group  [1] . This approach (which is believed to be complete) is regularly taught in graduate classes and relies on reducing the nonlinear optical coefficient tensor to its minimal representation, where zeroed-out elements indicate a forbidden process (e.g. no second harmonic generation in centrosymmetric media)  [1] . However, this derivation is based on a perturbative expansion that is inappropriate in extremely nonlinear processes such as high harmonic generation (HHG). Moreover, it fails to take into account the symmetries of the driving pump field (dynamical symmetries, DS), or the symmetries of the wave equations, which may manifest over several length scales (both on microscopic and macroscopic scales). While some selection rules were derived for HHG in the microscopic/macroscopic regimes, these were restricted to ad-hoc cases and a general theory has not been formulated. In particular, no theory has addressed combining these two regimes.\\ \\ I will present a general, closed-form, group-theory based analysis for the role of dynamical symmetries (DS) in harmonic generation. This approach is used to derive novel symmetries and selection rules for any light-field interacting with any type of medium (gas, solid, or liquid). We experimentally explore several of these new DSs in harmonic generation for the first time, including a multi-scale macroscopic-microscopic DS, and an elliptical DS  [2–4] , allowing polarization control over the emitted XUV radiation. I will also discuss the role of symmetry breaking in ultrafast spectroscopy. Specifically, I will focus on DS- breaking-based detection of chiral degrees of freedom  [5,6] , leading to all-optical electric-dipole based chiral-signals, including preliminary experiments in chiral limonene liquid showing a huge chiral discrimination of 163\%.\\ \\ 1. R. W. Boyd, Nonlinear Optics, 3rd ed. (2003). 2. O. Neufeld et al., Nat. Comm. 10, 405 (2019). 3. O. Neufeld et al., New J. Phys. 19, 23051 (2017). 4. O. Neufeld et al., Photonics 4, 31 (2017). 5. O. Neufeld and O. Cohen, arXiv1807.02630 (2018). 6. D. Ayuso et al., arXiv1809.01632 (2018).   

Dynamical Electron Vortices in Attosecond Double Photoionization of H$_2$$^*$

\textit{Kinematical} electron matter-wave vortices have been predicted in both single~[1,~2] and double~[3,~4] ionization of He. They have been observed experimentally in multiphoton single ionization of K~[5,~6]. We present in this talk results of a study of a new kind of \textit{dynamical} electron vortices~[7]. Specifically, we study electron momentum vortices in single-photon double ionization of H$_2$ by time-delayed, counter-rotating, elliptically-polarized attosecond pulses propagating either parallel or perpendicular to the molecular axis. For the parallel configuration, \textit{kinematical} vortices occur similar to those found for He~[1-4] and K~[5,~6]. For the perpendicular configuration, we find \textit{dynamical} vortex structures originating from an ellipticity-dependent interplay of $^1\Sigma^+_u$ and $^1\Pi^+_u$ continuum amplitudes. We propose a complete experiment to determine the magnitudes and relative phase of these amplitudes by varying the pulse ellipticities and time delays.\\ \\ $[1]$~J.M. Ngoko Djiokap \textit{et al.}, Phys. Rev. Lett. \textbf{115}, 113004 (2015).\\ $[2]$~J.M. Ngoko Djiokap \textit{et al.}, Phys. Rev. A \textbf{94}, 013408 (2016).\\ $[3]$~J.M. Ngoko Djiokap \textit{et al.}, Phys. Rev. A \textbf{96}, 013405 (2017).\\ $[4]$~J.M. Ngoko Djiokap and A.F. Starace J. Opt. \textbf{19}, 124003 (2017).\\ $[5]$~D. Pengel, S. Kerbstadt, D. Johannmeyer, L. Englert, T. Bayer, and M. Wollenhaupt, Phys. Rev. Lett. \textbf{118}, 053003 (2017).\\ $[6]$~D. Pengel, S. Kerbstadt, L. Englert, T. Bayer, and M. Wollenhaupt, Phys. Rev. A \textbf{96}, 043426 (2017).\\ $[7]$~J.M. Ngoko Djiokap, A.V. Meremianin, N.L. Manakov, L.B. Madsen, S.X. Hu, and A.F. Starace, Phys. Rev. A \textbf{98}, 063407 (2018).\\ \\ $^*$Research supported in part by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under Award No. DE-FG03-96ER14646.