Changes In Angular Momentum Of Vortex Light After Reflection From A Plasma Flying Mirror

Owing to the development of laser technology(e.g.,chirped pulse amplification),lasers with focal intensities above 10²² W/cm² and durations shorter than 10 fs have been developed.When an ultra-short and ultra-strong laser pulse impinges on a solid foil,the laser radiation pressure drives the electrons in the target and even the entire solid target(including electrons and protons)to move relativistically and produce an electron or plasma flying mirror.The relativistic electron layer or plasma flying mirror reflects another incoming laser,resulting in a change in the laser parameters and new physical phenomena,such as enhancing the laser pulse intensity and generating an ultra-short pulse.Light reflection at a plane interface is one of the most basic optical processes.Usually,light is reflected at the plane of incidence,and its angle of reflection is equal to the incident angle.This is the well-known optical reflection law in the stationary frame of reference.The optical reflection law in the stationary frame of reference is not applicable when the reflecting plane moves relativistically.Owing to the relativistic Doppler shift,which can be derived from the Lorentz transformation,the angle of reflection is not equal to the angle of incidence.In 2009,H.-C.Wu et al.studied the process of Gaussian beams obliquely impinging on a relativistic electron layer and the Doppler shift formula was verified.According to Maxwell's theory,both energy and momentum are carried by electromagnetic waves.The momentum carried by photons includes both linear and angular contributions.The angular momentum(AM)has a spin component associated with the polarization of light and an orbital component associated with the spatial distribution.As there are only two polarization states,namely,the right and left circular polarizations,the average spin angular momentum(SAM)carried by a photon is fundamentally limited to two values:?or-?,where?is the reduced Planck constant.As an intrinsic quantity,the direction of the SAM is expected to be parallel to the linear momentum of light.In 1992,Allen et.al.recognized that light beams with an azimuthal phase dependence of exp(4)7))can carry an orbital angular momentum(OAM).In the case of oblique incidence and static reflecting plane,the direction of the AM changes after being reflected because the AM is even when it is under space inversion as a pseudovector.The AM is still collinear with the linear momentum carried by the reflected beam but in opposite directions.This reflection phenomenon of vortex light on stationary target surface has been reported by Zhang et al in 2016,and new physical phenomena caused by OAM carried by vortex light have also been found.Inspired by the flying mirror and the generation of relativistic vortex beams with a large OAM,in this study,we re-examined the behavior of an intense vortex beam reflected by a relativistic flying mirror in the case of oblique incidence based on Three-dimensional PIC simulation.We observe that in addition to the frequency and emission angle of the reflected beam,the OAM changes.In this case,the OAM of the reflected beam is no longer noncollinear with the linear momentum,which is different from that of the SAM.The change of angular momentum direction is related to the incident angle of laser and the motion of plasma.