Strong-field physics with singular light beams (original) (raw)

Light beams carrying a point singularity with a screw-type phase distribution are associated with an optical vortex. The corresponding momentum flow leads to an orbital angular momentum of the photons 1-3 . The study of optical vortices has led to applications such as particle micro-manipulation 4,5 , imaging 6 , interferometry 7 , quantum information 8 and highresolution microscopy and lithography 9 . Recent analyses showed that transitions forbidden by selection rules seem to be allowed when using optical vortex beams 10 . To exploit these intriguing new applications, it is often necessary to shorten the wavelength by nonlinear frequency conversion. However, during the conversion the optical vortices tend to break up 11-13 . Here we show that optical vortices can be generated in the extreme ultraviolet (XUV) region using high-harmonic generation 14,15 . The singularity impressed on the fundamental beam survives the highly nonlinear process. Vortices in the XUV region have the same phase distribution as the driving field, which is in contradiction to previous findings 16 , where multiplication of the momentum by the harmonic order is expected. This approach opens the way for several applications based on vortex beams in the XUV region.