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.
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Image: Figure 1 (c & d – click link above to view full figure): 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.