Attosecond Extreme Ultraviolet Vortices from High-Order Harmonic Generation

Abstract

We present a theoretical study of high-order harmonic generation (HHG) and propagation driven by an infrared field carrying orbital angular momentum (OAM). Our calculations unveil the following relevant phenomena: extreme-ultraviolet harmonic vortices are generated and survive to the propagation effects, vortices transport high-OAM multiple of the corresponding OAM of the driving field and, finally, the different harmonic vortices are emitted with similar divergence. We also show the possibility of combining OAM and HHG phase-locking to produce attosecond pulses with helical pulse structure.PACS numbers: 42.50. Tx, 42.65.Ky, 32.30Rj Helical phased beams, also called optical vortices, are structures of the electromagnetic field with a spiral phase ramp about a point-phase singularity. The phase wind imprints an orbital angular momentum (OAM) to the beam, in addition to the intrinsic angular momentum associated with the polarization [1][2][3]. As light-matter interaction is inherently connected with the exchange of momentum, OAM can also be transferred to atoms [4][5][6], molecules [7,8], or an ensemble of atoms [9][10][11][12][13]. Optical vortices have potential technological applications in optical communication [14,15], micromanipulation [16], phase-contrast microscopy [17,18], and others [19]. In the recent years an interest has burgeoned in imprinting phase singularities in the XUV/X-ray light generated both in synchrotron and X-ray free-electron laser (XFEL) facilities [20][21][22]. In this short-wavelength regime one can drastically reduce the diffraction limit, as well as exploit the selectively site-specific excitation, with an important impact in microscopy [23,24] and spectroscopy [25].Phase singularities can also be imprinted to shorterwavelength light using high-order harmonic generation (HHG), as it was recently demonstrated by Zürch and coworkers producing vortices in the extreme ultraviolet (XUV) [26]. The confluence of OAM and HHG constitutes an extraordinary promising perspective. For instance, the phase twist is not imprinted directly to the short-wavelength radiation, but to the fundamental field. Therefore, it requires a single setup (diffractive mask, for instance) to imprint OAM to the fundamental field, that will be subsequently transferred to the rainbow of harmonic wavelengths by non-linear conversion. On the other hand, high-order harmonics have extraordinary temporal coherence qualities [27,28], a unique feature that has triggered a revolutionary metrology tool for the temporal characterization of ultrafast processes at the atomic scale (both spatially and temporally) [29,30]. Two decades ago, it was demonstrated that the higher part of the HHG spectra can be used to synthesize short * carloshergar@usal.es XUV pulses of attosecond duration [31][32][33][34]. It is, therefore, an appealing possibility if such attosecond pulses can be synthesized with OAM. In addition, the new HHG generation schemes, based on the present development of mid-infrared (mid-IR) laser sources, show that keV...

Generation of Coherent Extreme-Ultraviolet Radiation Carrying Orbital Angular Momentum

“…With our setup the generation of well-defined vortex beams with unit topological charge could be extended into the so-called linear propagation regime [46], where the propagation instabilities which lead to formation of low-order vortices [47] are not important. Even though the transverse mode selection process is absent in HHG, the propagation effects alone could result in a well-defined dominant l = 1 OAM mode in far field.…”

Probing the energy flow in Bessel light beams using atomic photoionization

“…This zero intensity line is called the vortex line of the field and it embodies a phase singularity. In the XUV frequency region, twisted light beams have been generated recently by means of undulators [40,41] or by using high-harmonic generation [42][43][44]. Experimentally, such twisted beams can be prepared in different modes with regard to the (components of the) angular momenta that are conserved for some given beam.…”

Generation and Applications of Extreme-Ultraviolet Vortices

“…In 2013, a theoretical approach, which considered both the generation of HHG by a single atom and the macroscopic superposition, predicted the production of high-OAM order vortices via HHG [18]. In particular, if the non-linear propagation of the driver is negligible-as in standard low density HHG experiments-each harmonic is generated with a topological charge of q (q being the harmonic order and the topological charge of the driving field), see Figure 1.…”

“…Two theoretical approaches were described in Refs. [18,24] in order to describe the nonlinear process of HHG with a vortex-structure driving beam. The first method includes a complete quantum description both at the microscopic (quantum single-atom) and 3D macroscopic (phase-matching) levels [25].…”