title: Giant narrowband twin-beam generation along the pump-energy propagation direction.
abstract: Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions. While transverse walk-off can be eliminated by proper medium engineering, longitudinal walk-off is harder to avoid. In particular, ultrafast twin-beam generation via pulsed parametric down-conversion and four-wave mixing is only possible in short crystals or fibres. Here we show that in high-gain parametric down-conversion, one can overcome the destructive role of both effects and even turn them into useful tools for shaping the emission. In our experiment, one of the twin beams is emitted along the pump Poynting vector or its group velocity matches that of the pump. The result is markedly enhanced generation of both twin beams, with the simultaneous narrowing of angular and frequency spectrum. The effect will enable efficient generation of ultrafast twin photons and beams in cavities, waveguides and whispering-gallery mode resonators.
authors.affiliation: Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bau 24, Erlangen 91058, Germany.|Institute of Optics, Information and Photonics, University of Erlangen-Nürnberg, Staudtstrasse 7/B2, Erlangen 91058, Germany.
authors.forename: Angela M
authors.initials: AM
authors.lastname: Pérez
authors.affiliation: Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bau 24, Erlangen 91058, Germany.|Institute of Optics, Information and Photonics, University of Erlangen-Nürnberg, Staudtstrasse 7/B2, Erlangen 91058, Germany.|Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia.
authors.forename: Kirill Yu
authors.initials: KY
authors.lastname: Spasibko
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia.
authors.forename: Polina R
authors.initials: PR
authors.lastname: Sharapova
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia.
authors.forename: Olga V
authors.initials: OV
authors.lastname: Tikhonova
authors.affiliation: Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bau 24, Erlangen 91058, Germany.|Institute of Optics, Information and Photonics, University of Erlangen-Nürnberg, Staudtstrasse 7/B2, Erlangen 91058, Germany.
authors.forename: Gerd
authors.initials: G
authors.lastname: Leuchs
authors.affiliation: Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bau 24, Erlangen 91058, Germany.|Institute of Optics, Information and Photonics, University of Erlangen-Nürnberg, Staudtstrasse 7/B2, Erlangen 91058, Germany.|Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia.
authors.forename: Maria V
authors.initials: MV
authors.lastname: Chekhova