Circular polarization made to order in the extreme UV

Attosecond pulses in the extreme UV (XUV) frequency regime can detect the ultrafast movement of electrons in atoms, molecules, and materials (see Physics Today, January 2018, page 18 , and June 2018, page 20 ). Circularly polarized visible light has a different diagnostic power. It can distinguish between the mirror-image forms of asymmetric, or chiral, structures—a category that includes almost all biomolecules and many of the substances that interact with them (see Physics Today, July 2018, page 14 ). Now a team of researchers led by Nirit Dudovich of the Weizmann Institute of Science in Rehovot, Israel, has demonstrated a conceptually simple scheme for creating attosecond XUV pulses with any polarization: linear, circular, or any of the continuum of elliptical polarization states in between.

Attosecond researchers generate their pulses using a process called recollision. An intense, linearly polarized IR pulse focused in a target gas pulls electrons away from their parent atoms; the electrons and atoms then recollide to produce a short burst of linearly polarized XUV photons. But using a circularly polarized IR wave doesn’t yield circularly polarized XUV—or indeed any XUV: The electrons embark on circular orbits and fail to recollide with their parent atoms. It’s possible to generate circularly polarized XUV by superposing circularly polarized IR and visible light so that the net electric field oscillates in a trefoil pattern, but the strong-field nature of the interaction means the output polarization is difficult to fully tune.

Dudovich and her colleagues use a completely different approach—one that’s similar, in fact, to how researchers produce circularly and elliptically polarized light in the visible regime. With different portions of a single IR starting wave, they generate two copropagating linearly polarized XUV pulse trains. By independently adjusting the XUV electric field vectors E₁ and E₂ (shown in the figure as orthogonal, although they don’t have to be) and the time delay Δt between the two waves, they can straightforwardly dial up any desired XUV polarization state. Chiral effects in real-life samples are often small, and one can detect them with greater sensitivity by modulating the incident polarization and looking for a signal oscillation at the same frequency. The new scheme enables researchers to rapidly modulate Δt with attosecond precision and thus switch between right and left circular polarization, thereby making such homodyne detection possible in the XUV. (D. Azoury et al., Nat. Photon. 13, 198, 2019 .)