In this work, we exploit HHG in a noble gas to merge the azimuthally twisted wavefront of a vortex beam and the spatially varying polarization of a vector beam, yielding EUV vector-vortex beams (VVB) that are tailored simultaneously in their SAM and OAM. Employing a high-resolution EUV Hartmann wavefront sensor (EUV HASO, Imagine Optic), we perform the complete spatial intensity and wavefront characterization of the vertical polarization component of the 25th harmonic beam centered at a wavelength of 32.6 nm. By driving the HHG using IR VVB, we show that HHG enables the production of EUV VVB exhibiting radial, azimuthal, or even intermediate polarization distribution. Furthermore, the wavefront characterization allows for the unambiguous confirmation of the topological charge and OAM helicity of the upconverted harmonic VVB. Notably, our work reveals that HHG provides a means for the synchronous and controlled manipulation of SAM and OAM. The production of ultrafast EUV VVB with high OAM and adjustable polarization distributions opens up promising prospects for their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source.
High-order harmonic generation (HHG) is an instrumental process enabling the transfer of short infrared pulse coherence properties into the Extreme Ultraviolet (EUV) spectral range. This phenomenon has opened the way to ultrafast pump-probe experiments at the nanoscale level. Recently, HHG has provided a straightforward approach to frequency upconvert beams structured in their phase and/or polarization. An emblematic example is the optical vortex beam, which is characterized by an azimuthally twisting wavefront. From a fundamental point of view, such a beam exhibits a phase singularity on the propagation axis and is carrying orbital angular momentum (OAM). Vector beams denote another structured beam family, exhibiting a spatially varying polarization.
In this paper, we will present our recent results on the generation and characterization of EUV vortex beams exhibiting very high topological charges (up to 100). Besides, using a similar HHG up-conversion scheme, we will show the production of so-called EUV vector-vortex beams that present the combined characteristics of the vortex and vector beams. Finally, progress on plasma-based soft x-ray laser amplification of such structured beams will be outlined,
High-order harmonic generation (HHG) has been recently proven to produce harmonic vortices carrying orbital angular momentum (OAM) in the extreme-ultraviolet (XUV) region from the nonlinear up-conversion of infrared vortex beams. In this work we present two methods to control and extend the OAM content of the harmonic vortices. First, we show that when a driver combination of different vortex modes is used, HHG leads to the production of harmonic vortices with a broad OAM content due to its nonperturbative nature. Second, we show that harmonic vortices with two discrete OAM contributions –so called fractional OAM modes– are generated when HHG is driven by conical refraction beams. Our work offers the possibility of generating tunable OAM beams in the XUV regime, potentially extensible to the soft x rays, overcoming the state of the art limitations for the generation of OAM beams far from the visible domain.
Extreme-ultraviolet (EUV) attosecond vortices carrying orbital angular momentum (OAM) are produced through high-order harmonic generation (HHG) from the nonlinear conversion of infrared twisted beams. While previous works demonstrated a linear scaling law of the vortex OAM content with the harmonic order, an unexpectedly rich scenario for the OAM buildup appears when HHG is driven by a vortex combination. The non-perturbative nature of HHG increases the OAM content of the attosecond vortices when the driving field presents an azimuthally varying intensity profile. We theoretically explore the underlying mechanisms for this diversity and disentangle the perturbative and non-perturbative nature in the generation of EUV attosecond twisted through numerical simulations.
Non linear propagation of ultra short pulses in air is studied. By preparing an initial field distribution by an amplitude mask we can obtain a Townes soliton[1] (self similar channel of coherent radiation) in air. Experimental observation can be described accurately by the numerical integration of the Non Linear Schroedinger Equation (NLSE) and allow us to explain the origin of the remarkable stability of this soliton as a balance between diffraction and Kerr effect. We further explore on the role of coherence by revisiting the two slit Young's experiment but now in the non linear regime.
It is known that solitons of Bose-Einstein condensates with positive scattering lengths can be produced in optical
lattices. The key of this process is the negative sign of the effective mass of the wavepacket as it comes closer
to the edge of the first Brillouin zone. However, the underlying assumption of the effective mass approach is
the slow variation of the wavefunction envelope between consecutive lattice cells. Previous experiments and
computations on this type of solitons show that this is not the usual case. In this contribution we will go beyond
the slowly varying assumption to demonstrate that the equations governing the dynamics of gap solitons contain
a third derivative in the dispersion. As a result, the dynamics of the stabilized wavepackets is found different that
the soliton's. In particular, a radiative component is present, and the stability under collisions is only partially
achieved.
We report the observation of self-guided propagation of 120 fs, 0.56 mJ infrared pulse in air for distances greater
than a meter (more than thirty Rayleigh Lengths). The numerical simulations demonstrates the this localized
structure corresponds to a Townes soliton, specially stable under these conditions.
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