Extreme nonlinear optical processes with beams carrying orbital angular momentum

Abstract

Summary form only given. Light beams carrying an isolated point singularity with a screw-type phase distribution are called an optical vortex (OV) (Fig 1). The fact that in free space the Poynting vector of the beam gives the momentum flow leads to an orbital angular momentum (OAM) of the photons in such a singular beam, independent on the spin angular momentum [1]. There are many applications of optical OAM shown in literature that would benefit from the availability of optical vortex beams in all spectral regions. For example it was shown that transitions forbidden by selection rules in dipole approximation appear allowed when using photons with the additional degree of freedom of optical OAM [2]. However, the common techniques of producing new light frequencies by nonlinear optical processes seem problematic in conserving the optical vortex when the nonlinearity becomes large.We show that with the extremely nonlinear process of High Harmonic Generation (HHG) it is possible to transfer OVs from the near-infrared to the extreme ultraviolet (XUV) [3] at wavelengths down to ~30 nm.We use a conventional HHG setup, where Harmonics down to the 27th are produced in Argon gas by exposing it to ultrashort 30 fs laser pulses at 800 nm wavelength. In order to impose a helical phase front onto these driving pulses, we used a spatial light modulator (Fig 2). The observed light of several plateau harmonics was examined spatially and spectrally, while also two different tools to investigate the phase structure were employed. The spatial profile showed the expected singular behaviour, a dark region in the centre, although two separate "lobes" of intensity were observed. A phase feature that showed a shift of on opposing sides of the beam profile was found with a wavefront splitting technique. A screw-like phase evolution around the profile was also verified by employing a Hartmann type measurement.The generated spectrum revealed that in all Harmonic orders an OV was present. The profile however looked the same in all orders, indicating identical topological charge, which runs counterintuitive to the assumption that the phase of is multiplied by the order of the nonlinearity.