Angular momentum is an intrinsic property of light, with spin angular momentum (SAM) of h (h is the Planck’s constant) per photon carried by circularly polarized (CP) light. It has been demonstrated that light beams with helical phase-fronts (optical vortices) carry an orbital angular momentum (OAM) equivalent to lh per photon. XUV/X-ray pulses with OAM are of particular interest for certain experiments, preferably in combination with pulse durations in the attosecond regime. To date, high-order harmonics in the XUV with OAM have been observed in laser-atom interactions, operating at a moderate intensity level. In the relativistic regime, the proposed methods to generate intense XUV pulses with OAM mainly utilize vortex laser pulses as the drivers, which suffer from a limitation for the driving intensity.
Recently, a research team from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS), in collaboration with Helmholtz Institute Jena, showed that the SAM of circularly polarized, high-power laser could be transferred to the harmonics carrying OAM via the relativistic oscillating mirror (ROM) mechanism, resulting in a single attosecond pulse with OAM when a few-cycle laser pulse was employed. The result was published in Nature Communications.
In demonstrating the principle of SAM to OAM conversion in figure (a), an intense left-handed CP Gaussian laser pulse impinged a plane target from the left side. In such a geometry, it was believed that no harmonics would be generated because the plasma surface was not efficiently oscillated.
However, the radiation pressure along the target normal resulted in rapid target deformation for sufficiently high intensities. This broke the symmetry of the interaction and resulted in the laser becoming increasingly obliquely incident away from laser axis and therefore the generation of harmonics in the outer parts of the focal spot, while the suppression of harmonic generation on the axis remained.
Further experiment presented the phase structure of the third-order harmonic, as seen in figure (b). They found the isosurface had a helical structure. The number of intertwined helices depended on the order of the harmonics, e.g., the number of helices h is h=n-1 where n was the harmonic order. The intensities distribution of the harmonic was doughnut-like as expected for a laser beam with OAM. The transverse distributions of the harmonic phase indicated that the phases were azimuthally-angle dependent.
For a target that dynamically deformed in response to the laser pressure, very few harmonics were produced in the leading edge of the laser pulse. Researchers therefore considered the possibility of employing a pre-dented target. Figure (c) presented the generation of a pulse and one could see a spiral structure when harmonics were most efficiently generated.
This findings may open exciting applications opportunities of employing intense XUV attosecond pulses carrying OAM. For example, the near-relativistic intensity of the vortex harmonics make it possible transferring OAM of light to atoms and generating twisted gamma photons by Compton scattering. Besides, the attosecond duration of the isolated OAM pulse makes it an ideal tool for probing the chiral interactions on the sub-femtosecond timescale, the ultrafast dynamics of spin and orbital moments in magnetic materials.
This work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences President’s International Fellowship Initiative and the Strategic Priority Research Program (B).
Figure (a) Principle of OAM harmonic generation.
(b) Phase structure, intensity distribution and phase distribution of the third harmonic.
(c) A single attosecond pulse with OAM. (Images by SIOM)
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS