Helicity-selective phase-matching and quasi-phase matching of circularly polarized high-order harmonics: Towards chiral attosecond pulses

Phase matching of circularly polarized high-order harmonics driven by counter-rotating bi-chromatic lasers was recently predicted theoretically and demonstrated experimentally. In that work, phase matching was analyzed by assuming that the total energy, spin angular momentum and linear momentum of the photons participating in the process are conserved. Here we propose a new perspective on phase matching of circularly polarized high harmonics. We derive an extended phase matching condition by requiring a new propagation matching condition between the classical vectorial bi-chromatic laser pump and harmonics fields. This allows us to include the influence of the laser pulse envelopes on phase matching. We find that the helicity dependent phase matching facilitates generation of high harmonics beams with a high degree of chirality. Indeed, we present an experimentally measured chiral spectrum that can support a train of attosecond pulses with a high degree of circular polarization. Moreover, while the degree of circularity of the most intense pulse approaches unity, all other pulses exhibit reduced circularity. This feature suggests the possibility of using a train of attosecond pulses as an isolated attosecond probe for chiral-sensitive experiments

Picosecond ionization dynamics in femtosecond filaments at high pressures

We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse. Our study suggests that this picosecond evolution can be attributed to collisional ionization including Penning and associative ionizations and electron-impact ionization of excited atoms generated during the pulse. The dominance of excited atoms over ionized atoms at the end of the pulse also indicates an intrapulse inhibition of avalanche ionization. This delayed ionization dynamics provides evidence for diagnosing atomic and molecular excitation and ionization in intense laser interaction with high-pressure gases

Electron correlation and interference effects in strong-field processes

Several correlation and interference effects in strong-field physics are investigated. We show that the interference of continuum wave packets can be the dominant mechanism of high-harmonic generation (HHG) in the over-the-barrier regime. Next, we combine HHG with resonant x-ray excitation to force the recolliding continuum electron to recombine with a core hole rather than the valence hole from that it was previously tunnel ionized. The scheme opens up perspectives for nonlinear xuv physics, attosecond x-ray pulses, and spectroscopy of core orbitals. Then, a method is proposed to generate attochirp-free harmonic pulses by engineering the appropriate electron wave packet. Finally, resonant photoionization mechanisms involving two atoms are discussed which can dominate over the direct single-atom ionization channel at interatomic distances in the nanometer range.Comment: to be published in Springer Proceedings "Multiphoton Processes and Attosecond Physics

Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers

High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo–electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.The experimental work was funded by a National Security Science and Engineering Faculty Fellowship, and the NSF Center for EUV Science and Technology. A.G., A.J.-B., M.M.M., H.C.K. and A. Becker acknowledge support for theory from the U.S. Air Force Office of Scientific Research (grant no. FA9550-10-1-0561); A. Baltuška acknowledges support from Austrian Science Fund (FWF, grant no. U33-16) and the Austrian Research Promotion Agency (FFG, Project 820831 UPLIT); and C.H.-G. and L.P. acknowledge support from Junta de Castilla y León, Spanish MINECO (CSD2007-00013 and FIS2009-09522), and from Centro de Láseres Pulsados, CLPU. T.P., M.-C.C., A. Bahabad, M.M.M. and H.C.K. have filed for a patent on “Method for phase-matched generation of coherent soft and hard X-rays using IR lasers,” U.S. patent application 61171783 (2008)

High flux coherent supercontinuum soft X-ray source driven by a single-stage 10 mJ, kHz, Ti:sapphire laser amplifier

We demonstrate the highest flux tabletop source of coherent soft X-rays to date, driven by a single-stage 10 mJ Ti:sapphire regenerative amplifier at 1 kHz. We first down-convert the laser to 1.3 um using a parametric amplifier, before up-converting it to soft X-rays using high harmonic generation in a high-pressure, phase matched, hollow waveguide geometry. The resulting optimally phase matched broadband spectrum extends to 200 eV, with a soft X-ray photon flux of > 10^6 photons/pulse/1% bandwidth at 1 kHz, corresponding to > 10^9 photons/s/1% bandwidth, or approximately a three order-of-magnitude increase compared with past work. Finally, using this broad bandwidth X-ray source, we demonstrate X-ray absorption spectroscopy of multiple elements and transitions in molecules in a single spectrum, with a spectral resolution of 0.25 eV, and with the ability to resolve the near edge fine structure.Comment: 9 pages, 3 figures, under Optics Express peer revie

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas

High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers.We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma.We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.The experimental work was done at JILA, supported by Army Research Office grant WN11NF-13-1-0259, an NSF PFI AIR award, and U.S. Department of Energy (DOE) grant DE-SC0008803 (M.M.M., T.P., and H.C.K.). Theory was supported by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013) under REA grant agreement 328334 (C.H.-G.); Junta de Castilla y León (SA116U13, UIC016) and MINECO (FIS2013-44174-P) (C.H.-G. and L.P.); NSF grants PHY-1125844 and PHY-1068706 and AFOSR MURI “Mathematical Modeling and Experimental Validation of Ultrafast Light-Matter Coupling associated with Filamentation in Transparent Media” grant FA9550-10-1-0561 (A.J.-B., R.J.L., X.G., A.L.G., M.M.M., and H.C.K.); Ministry of Science and Technology, Taiwan, grant 102-2112-M-007-025-MY3 (M.-C.C.); U.S. Department of Energy, Division of Chemical Sciences, Atomic, Molecular and Optical Sciences Program (A.B.); and DOE Office of Fusion Energy, HED Laboratory Plasmas program, grant AT5015033 (S.B.L., M.F., and J.A.G.). Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security LLC for DOE, National Nuclear Security Administration, under contract DE-AC52-07NA27344, LLNL-JRNL-676693. T.P., D.P., M.M.M., and H.C.K. have filed a patent on “Generation of VUV, EUV, X-ray Light Using VUV-UV-VIS Lasers,” U.S. patent application 61873794 (2013)/US 20150063385 (2015)

Schemes for generation of isolated attosecond pulses of pure circular polarization

We propose and analyze two schemes capable of generating isolated attosecond pulses of pure circular polarization, based on results of numerical simulations. Both schemes utilize the generation of circularly polarized high-order-harmonics by crossing two circularly polarized counter-rotating pulses in a noncollinear geometry. Our results show that in this setup isolation of a single attosecond pulse can be achieved either by restricting the driver pulse duration to a few cycles or by temporally delaying the two crossed driver pulses. We further propose to compensate the temporal walk-off between the pulses across the focal spot and increasing the conversion efficiency by using angular spatial chirp to provide perfectly matched pulse fronts. The isolation of pure circularly polarized attosecond pulses, along with the opportunity to select their central energy and helicity in the noncollinear technique, opens new perspectives from which to study ultrafast dynamics in chiral systems and magnetic materials.The authors acknowledge Luis Plaja for valuable discussions. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA Grant Agreement No. 328334. C.H.-G. and I.J.S. acknowledge support from Junta de Castilla y León (Project SA116U13, UIC016) and MINECO (Grants No. FIS2013-44174-P and No. FIS2015-71933-REDT). A.J.-B. was supported by grants from the U.S. National Science Foundation (Grants No. PHY-1125844 and No. PHY-1068706). D.H. was supported via a grant from the Department of Energy. M.M.M., H.C.K., C.G.D., and A.B. acknowledge support by a MURI grant from Air Force Office of Scientific Research under Award Number FA9550-16-1- 0121. This work utilized the Janus supercomputer, which is supported by the U.S. National Science Foundation (Grant No. CNS-0821794) and the University of Colorado Boulder