Schemes for generation of isolated attosecond pulses of pure circular polarization

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Schemes for generation of isolated attosecond pulses of pure circular polarization

C. Hernández-García, C. G. Durfee, D. D. Hickstein, T. Popmintchev, A. Meier, M. M. Murnane, H. C. Kapteyn, I. J. Sola, A. Jaron-Becker, and A. Becker

Phys. Rev. A 93, 043855 – Published 28 April 2016

Abstract

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.

Received 3 February 2016

DOI:https://doi.org/10.1103/PhysRevA.93.043855

Physics Subject Headings (PhySH)

High-order harmonic generation Ultrafast phenomena X-ray beams & optics

Atomic gases X-ray lasers

Atomic orbital Bound states Density functional theory development Dipole approximation Nonperturbative methods Schroedinger equation Strong-field approximation

Atomic, Molecular & OpticalNonlinear Dynamics

Authors & Affiliations

C. Hernández-García^1,2,*, C. G. Durfee^1,3, D. D. Hickstein¹, T. Popmintchev¹, A. Meier³, M. M. Murnane^1,4, H. C. Kapteyn^1,4, I. J. Sola², A. Jaron-Becker^1,4, and A. Becker^1,4

¹JILA, University of Colorado at Boulder, Boulder, Colorado 80309-0440, USA
²Grupo de Investigación en Aplicaciones del Láser y Fotónica, University of Salamanca, E-37008 Salamanca, Spain
³Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA
⁴Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309-0440, USA

^*carloshergar@usal.es

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Issue

Vol. 93, Iss. 4 — April 2016

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Images

Figure 1
Scheme of noncollinear circularly polarized, high-harmonic generation (NCP-HHG): Two counter-rotating circularly polarized femtosecond laser pulses are focused into a gas to produce both left and right circularly polarized harmonic beams. At the focal plane, the two circularly polarized beams sum to yield an electric field that exhibits linear polarization, which rotates as a function of the transverse position across the laser focus (inset). The locally linear nature of the polarization enables the application of schemes to isolate a single attosecond pulse, known from linearly polarized harmonic and attosecond pulse generation.
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Figure 2
Comparison of circularly polarized attosecond pulses obtained through NCP-HHG and linearly polarized pulses driven by collinear two-color beams. Results are shown for relatively long (upper row) and ultrashort (lower row) driver pulses. Harmonic spectra [panels (a) and (b)] and circularly polarized attosecond pulses [panels (c) and (d)] for 800-nm counter-rotating noncollinear mixing with $θ_{NC} = 32$ mrad and laser pulse durations $t_{FWHM} = 5.8$ cycles (15.4 fs) and $t_{FWHM} = 1.1$ cycles (2.9 fs), respectively. Panels (e) and (f) show the linearly polarized attosecond pulses obtained using the collinear bichromatic (800 and 400 nm) scheme, for $t_{FWHM, 1} = t_{FWHM, 2} = 11.5$ -fs driver laser pulses and $t_{FWHM, 1} = t_{FWHM, 2} = 2$ -fs driver laser pulses, respectively.
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Figure 3
Spatial selection of isolated circularly polarized attosecond pulses with different frequencies. Shown are the HHG spectrum (first column), time-frequency analysis (second column), and attosecond pulse (third column) obtained at three different divergence angles: 1 mrad (a), 1.5 mrad (b), and 2 mrad (c), corresponding to the results presented in Fig. 2.
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Figure 4
Compensation of transversal temporal walk-off using angularly chirped noncollinear beams. Shown are schematics of the wave fronts due to the transverse walk-off (a) and after compensation using pulse-front-tilted beams, with opposite angular chirp rate (b). In panel (c) a comparison of the harmonic spectra at 1.5 mrad with (dashed red line) and without (light blue line) transverse walk-off effect, as well as after compensation with angular chirp (dark blue line) are shown. Laser parameters are given in the text.
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Figure 5
Time-gating mechanism for generating an isolated circularly polarized attosecond pulse with longer driving pulses. Ellipticity (a) and intensity (b) of the resulting laser pulse at the focus position as a function of the time delay $t_{d}$ between the two noncollinear pulses. Attosecond pulses (left, linear scale, amplitude normalized to 1 in each panel) and HHG spectra (right, logarithmic scale) obtained in a NCP-HHG scheme for (c) $t_{d} = 0$ , (d) $t_{d} = 0.5 t_{FWHM}$ , (e) $t_{d} = 0.75 t_{FWHM}$ , and (f) $t_{d} = t_{FWHM}$ . Laser parameters are as in Figs. 2 and 2, except that the pulse duration is doubled to 5.8 fs.
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