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,
A new polarization state generator (PSG) is presented based on two geometric-phase diffraction gratings, useful to generate partially polarized light in an efficient and controlled way. The PSG is used to experimentally analyze the generation of vector and vortex beams which lie, respectively, inside the generalized higher-order (HOPS) and the orbital angular momentum (OAMPS) Poincaré spheres. This way the mapping relations between polarization states, vector beams and vortex beams are analyzed and the classical concept of degree of polarization (DoP) is extended to these other Poincaré spheres. Experimental results that validate the theory are presented.
Using multiplexed broadband ptychography (MBP), we characterize the EUV light in high-order harmonic generation (HHG). MBP can measure spectrally resolved complex beam profiles for different harmonic outputs without spectral dispersion. Through a simple change to the experimental setup, we also characterize the driving laser for the process. The experimentally measured driving laser profile is used in an SFA+ with dipole approximation simulation of high harmonic generation. The simulated results are compared to the experimentally measured high harmonics produced with the characterized driving laser. By characterizing the input and output in HHG we can further understand the generation mechanics to better control the process. We also can investigate how control of the fundamental laser can lead to control of the output harmonics.
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.
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