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.
Graphene has been recently reported to have a damage threshold high enough to allow for the interaction with ultrashort laser pulses of intensities above 1013 W/cm2. It is natural to explore if this situation to what extend the laser pulse is able to induce the highly non-linear dynamics that gives rise to high harmonic generation. We perform the exact numerical integration of the set of coupled two-level equations that describe the valence-to-conduction band transitions by a laser pulse, at any point in the reciprocal space. We analyze the dynamics of the excitation to the conduction band, and the spectra of the harmonics produced. We show that harmonic radiation is produced by interband as well as intraband transitions, these later resulting from parametric oscillation. We also analyze the temporal characteristics of the harmonic emission.
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 (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.
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.
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
In the non-relativistic limit, the dynamics of the interaction of light with matter is described via a Hamiltonian
that does not include spin operators. However, the actual spin configuration of the interacting particles still
plays a fundamental role, via the Pauli's exclusion principle, by forcing a particular symmetry of the spatial
part of the wavefunction. In this paper we analyze the role of symmetry in the process of ionization of two and