Polarization shaping of ultrashort extreme-ultraviolet light pulses
Conventional lasers produce light with a well-defined, time-independent polarization. Two common examples are linear polarization, where the electric field oscillates in a certain direction in the plane perpendicular to the direction of light propagation, and circular polarization, where the electric field rotates clockwise (right circular) or counter-clockwise (left circular) about the propagation direction. Recently, however, the generation of pulsed laser light whose polarization is varying on a femtosecond timescale, has attracted significant attention. Such polarization-shaped pulses have been used in a number of applications ranging from manipulation of electron wave packets to improving the sensitivity of advanced spectroscopic techniques.
In the visible, a time-dependent polarization is accomplished using a pulse shaper. On the other hand, lack of efficient optical elements and greater difficulties in controlling the propagation of light at short wavelengths significantly restrain pulse shaping in the extreme ultraviolet (XUV) and x-ray spectral regions. We show here that the externally seeded free-electron laser (FEL) FERMI provides a solution to the problem of tailoring the polarization profile of short and intense XUV pulses.
In our study, FERMI was operated in a special split-undulator configuration where the electron beam emitted a right-circularly (RC) polarized FEL pulse in the first part of the radiator (R1), was then delayed with respect to this FEL pulse using a magnetic chicane and then generated a left-circularly (LC) polarized FEL pulse in the second part of the radiator (R2). A phase shifter PS (small chicane) just before R2 was used to fine tune the relative phase between the two counter-rotating FEL sub-pulses, see Figure 1a.
Such a configuration produced a composite pulse whose electric field is shown in the top panel of Figure 1b for a sub-pulse delay of 60 fs and relative phase of π/4. The bottom panel shows that the Stokes parameters (a set of values that describes the polarization state of light) of the composite pulse are time-dependent and that the polarization evolves from RC in the pulse head (t < −50 fs), to linear in the pulse center (t = 0), to LC in the pulse tail (t > 50 fs). By modifying the relative phase between the sub-pulses, we could control the polarization of the composite pulse, in particular the direction of the linear polarization at t = 0.
Figure 1: The scheme for generating an XUV FEL pulse with time-dependent polarization by combining two counter-rotating FEL sub-pulses. (b) Schematic output of the setup shown in (a) for a separation between the sub-pulse envelopes equal to their FWHM durations (60 fs) and a relative phase (set by PS before R2 in (a)) equal to π/4. Top: components of the total electric field and total intensity. The FEL wavelength is exaggerated to visualize oscillations of the fields. Bottom: temporal profiles of the intensity-normalized Stokes parameters. (c) VMI images obtained from photoionization of helium atoms for a zero delay between the sub-pulse envelopes as a function of the relative phase: the polarization varies from almost pure horizontal (phase = 0, left), to diagonal (phase = π /2, middle), to almost pure vertical (phase = π, right). (d) Intensity-normalized, time-integrated Stokes parameter S1 as a function of the relative phase for zero (left) and 30 fs (right) delay between the sub-pulse envelopes.
We confirmed the time-varying polarization profile by measuring photoelectron distributions from helium atoms excited by such FEL pulses using a velocity map imaging (VMI) detector at the Low-Density Matter (LDM) beamline. By decomposing VMI images into a weighted sum of images corresponding to linear horizontal and vertical polarizations (Figure 1c), we extracted the time-integrated Stokes parameter S1, and found that it oscillates as a function of the relative phase between the FEL sub-pulses (Figure 1d). The measurements were performed for 0 and 30 fs delay. The decrease of the oscillation amplitude of S1/S0 when going from 0 to 30 fs with the sub-pulse delay is direct evidence that we produced two delayed FEL pulses that generated the FEL output shown in Figure 1b. The temporal polarization profile of FEL pulses was further verified using above-threshold ionization in the presence of an optical dressing field (not shown).
Because FELs are accelerator based, electron-beam energy fluctuations introduce unwanted phase fluctuations between the sub-pulses, leading to a fluctuating output polarization. This shows up in our measurements as increased error bars in Figure 1d, especially when going to larger delays. However, a clear oscillation of S1/S0 for a delay of 30 fs indicates that such fluctuations are at a level that will not compromise potential experiments
Potential applications of our scheme in atomic and molecular physics include coherent control of electron wave packets, dichroic spectroscopy of molecules, and observing photoelectron circular dichroism within a single measurement. We also envisage applications in condensed matter, especially in the field of ultrafast magnetism, where magnetic dynamics may proceed on a femtosecond time scale. Having XUV sources generating polarization-shaped pulses on such time scales opens the door to pump-probe studies of coherent spin-dependent processes with element selectivity.
This research was conducted by the following research team:
G. Perosa1,2, J. Wätzel3, D. Garzella1, E. Allaria1, M. Bonanomi4,5, M. B. Danailov1, A. Brynes1, C. Callegari1, G. De Ninno1,6, A. Demidovich1, M. Di Fraia1,7, S. Di Mitri1,2, L. Giannessi1,8, M. Manfredda1, L. Novinec1, N. Pal1, G. Penco1, O. Plekan1, K. C. Prince1, A. Simoncig1 , S. Spampinati1, C. Spezzani1, M. Zangrando1,7, J. Berakdar3, R. Feifel9, R. J. Squibb9, R. Coffee10, E. Hemsing10, E. Roussel11, G. Sansone12, B. W. J. McNeil13,14,15 and P. R. Ribič1
1 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
2 Physics Department, University of Trieste, Trieste, Italy
3 Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany
4 Politecnico di Milano, Milano, Italy
5 Istituto di Fotonica e Nanotecnologie, Milano, Italy
6 Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
7 CNR-IOM, Trieste, Italy
8 ENEA C.R. Frascati, Frascati (Roma), Italy
9 Department of Physics, University of Gothenburg, Gothenburg, Sweden
10 SLAC National Accelerator Laboratory, Menlo Park, California, USA
11 Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, Lille, France
12 Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
13 University of Strathclyde (SUPA), Glasgow, United Kingdom
14 Cockcroft Institute, Warrington, United Kingdom
15 ASTeC, STFC Daresbury Laboratory, Warrington, United Kingdom
Reference
G. Perosa et al., “Femtosecond Polarization Shaping of Free-Electron Laser Pulses", Phys. Rev. Lett. 131, 045001 (2023); DOI:10.1103/PhysRevLett.131.045001.
