Extreme Ultraviolet Fractional Orbital Angular Momentum Beams from High Harmonic Generation

Alex Turpin,Julio San Román,Laura Rego,Antonio Picón,Carlos Hernández-García

doi:10.1038/srep43888

Abstract

We investigate theoretically the generation of extreme-ultraviolet (EUV) beams carrying fractional orbital angular momentum. To this end, we drive high-order harmonic generation with infrared conical refraction (CR) beams. We show that the high-order harmonic beams emitted in the EUV/soft x-ray regime preserve the characteristic signatures of the driving beam, namely ringlike transverse intensity profile and CR-like polarization distribution. As a result, through orbital and spin angular momentum conservation, harmonic beams are emitted with fractional orbital angular momentum, and they can be synthesized into structured attosecond helical beams –or “structured attosecond light springs”– with rotating linear polarization along the azimuth. Our proposal overcomes the state of the art limitations for the generation of light beams far from the visible domain carrying non-integer orbital angular momentum and could be applied in fields such as diffraction imaging, EUV lithography, particle trapping, and super-resolution imaging.

Highlights

Light beams exhibit two intrinsic degrees of freedom associated to angular momentum: spin angular momentum (SAM) or polarization, indicating the direction in which the field oscillates; and orbital angular momentum (OAM) related to the spatial profile of the phase of the electric wave
Regarding the far-field EUV yield of the conical refraction (CR)-like harmonic beams, our simulation results indicate that the efficiency is similar to that presented in High-order harmonic generation (HHG) driven by Gaussian beams, where up-conversion efficiencies up to 10−5 can be achieved if phase-matching conditions are met[23,49,50,51]
Angular momentum conservation implies that the 17th harmonic exhibits fractional-OAM of 17 = 17/2, and the 23rd 23 = 23/2, as predicted above. This result shows that OAM and SAM are not exchanged in CR-driven HHG, but conservation of both quantities separately leads to the generation of fractional-OAM harmonics

Summary

Introduction

Light beams exhibit two intrinsic degrees of freedom associated to angular momentum: spin angular momentum (SAM) or polarization, indicating the direction in which the field oscillates; and orbital angular momentum (OAM) related to the spatial profile of the phase of the electric wave. It has been recently shown that light beams can carry fractional or non-integer OAM, i.e., photons can exhibit a half-integer angular momentum[3,4,5,6,7,8,9,10] In this sense, conical refraction (CR) produced with optically biaxial crystals has been demonstrated to be an efficient method to generate half-integer OAM beams[11,12,13,14,15]. Perturbative harmonic generation does not up-convert the CR beam structure into higher frequencies, preventing the extension of half-integer OAM beams into shorter wavelength regimes[18]. High-order harmonic generation (HHG) is known as a unique non-perturbative frequency up-conversion process for the generation of coherent EUV and soft x-ray radiation, emitted in the form of attosecond bursts[19,20]

Methods

Results

Conclusion