The total angular momentum (TAM) of light can be separated into spin and orbital parts of the angular momentum (SAM and OAM). The SAM is associated with the circularly polarized state of light. Each photon carries a SAM of +1 or -1. Previous researches indicate that a circularly polarized (CP) laser pulse normally irradiating a plasma target does not generate harmonics because its constant ponderomotive force cannot excite an oscillating current on the plasma surface.
More generally, the physical reason why the harmonics vanish for the CP light lies in the constraint from the dynamical symmetry. The polarization-dependent rotational symmetry leads to the SAM conservation for the CP light, which restricts the harmonics to the first order, so the harmonic generation is forbidden.
In a recent study, researchers at Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, have revealed a new harmonic generation mechanism via the interaction of an intense circularly polarized (CP) laser pulse with a solid density plasma, in which an interesting optical spin-to-orbital angular momentum conversion (STOC) was achieved. The study was published in New Journal of Physics.
In this study, researchers presented a new physical mechanism to break the dynamical symmetry for the CP light, which was based on inducing a longitudinal electric field by focusing the laser focal spot to a small size. That longitudinal electric field broke the polarization-dependent symmetry (related to the SAM conservation) and the spatially dependent symmetry (related to the OAM conservation) individually, and thus the harmonic generation was allowed.
In the laser-plasma interaction, the transverse and longitudinal fields of the CP laser pulse drove an oscillating current vortex on the plasma surface, and then the harmonics with nontrivial angular momentum content were emitted by the current vortex.
However, the combined dynamical symmetry (polarization-dependent symmetry together with spatially dependent symmetry) still held when the laser pulse was tight focused, so the TAM was conserved in the harmonic generation process. The TAM conservation caused the STOC. Consequently, the generated harmonics must come out with vortex structures carrying the definite OAM.
Furthermore, they pointed out that the OAM of the generated harmonics could become more tunable and controllable if a spiral phase plate (SPP) was used. The SPP is an optical element with a helical surface and its thickness can increase smoothly with azimuthal position.
When a plane wave laser was reflected on the SPP, it could be converted into a vortex mode with a helical phase in azimuth and acquire a definite OAM. By changing the SPP parameters, the OAM values of the harmonics would become higher or lower.
This harmonic generation and STOC scheme provide a new method to generate the high-frequency vortex light beams, with potential applications in extreme ultraviolet optical vortex, optical manipulation and quantum information.
The new symmetry breaking mechanism for CP light is only dependent on the longitudinal field of the laser pulse itself; therefore, it has universal generalization and can be applied to various CP harmonic processes, including solid density plasma harmonics, gas plasma harmonics, as well as neutral atomic or molecular harmonics.
As the angular momentum conversion is an importantly fundamental problem in physics, this study opens a new window in the topic of spin-orbital interaction and conversion in optics.
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