Strong-field physics with singular light beams

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 information8 and high-resolution 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.