J. A. Pérez-Hernández

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.

Beyond carbon K-edge harmonic emission using a spatial and temporal synthesized laser field.

We present numerical simulations of high-order harmonic generation in helium using a temporally synthesized and spatially nonhomogeneous strong laser field. The combination of temporal and spatial laser field synthesis results in a dramatic cutoff extension far beyond the usual semiclassical limit. Our predictions are based on the convergence of three complementary approaches: resolution of the three dimensional time dependent Schrödinger equation, time-frequency analysis of the resulting dipole moment, and classical trajectory extraction. A laser field synthesized both spatially and temporally has been proven capable of generating coherent extreme ultraviolet photons beyond the carbon K edge, an energy region of high interest as it can be used to initiate inner-shell dynamics and study time-resolved intramolecular attosecond spectroscopy.

Harmonic emission beyond the Carbon K-edge using spatially and temporally synthesized laser field

We examine how the combination of temporal and spatial laser field synthesis results in a remarkable cutoff extension far beyond the conventional semi-classical limit. Our scheme allows coherent XUV photons generation beyond the carbon K-edge.

New Methods For Computing High-Order Harmonic Generation and Propagation

Due to its nonperturbative character, the theoretical modelization of strong field phenomena is a challenging aspiration. In this chapter, we shall consider the problem of high-order harmonic generation and propagation, and review some recent proposals that conform an alternative approach to the standard procedures. In particular, the semiclassical description of the single-atom response can be nowadays replaced to include the full quantum description. Also, the limits of the Strong-Field Approximation can be extended to include the influence of the strong field on the ground state. These two aspects allow for a new procedure, here referred to as SFA + , for calculating the high-order harmonic generation spectrum, which is demonstrated to improve the quantitative accuracy and to recover, for instance, the correct dependence of the harmonic yield with the laser wavelength. On the contrary, the problem of harmonic propagation has also been tackled recently from a new perspective: the combination of SFA + methods with a Discrete Dipole approach. This latter strategy is not based on the differential wave equation for the fields, but on its integral version, and finds some advantages with respect to the usual approximations (slowly varying envelopes, paraxial, etc).

Laser-matter phenomena driven by plasmonic near-fields

We study theoretically the high order harmonic generation and photoelectron emission in gases using plasmonic enhanced near-fields. We demonstrate that these fields have a great potential to generate high energetic electrons and coherent photons.

Ultrahigh-efficiency high harmonic generation driven by UV lasers

We demonstrate bright high harmonic generation driven by UV lasers with ultra-high conversion efficiency approaching 10-3 and ultra-narrow single-harmonic bandwidth of ~0.2%. The enhanced flux results from improved phase-matching combined with stronger single-atom yield.

Unified microscopic-macroscopic picture of high harmonic generation from the VUV to the keV X-ray region

We present a unified picture of phase matching of high harmonic upconversion spanning the electromagnetic spectrum from the VUV to keV, combining both microscopic and macroscopic physics. We validate this picture with experiment and theory.

Valley Structure in the Harmonic Efficiency at Ultra-high Laser Intensities

We demonstrate that Non Adiabatic Turn-on laser field allows one to avoid efficiency losses when the saturation level of atoms is rebased, providing new route for attosecond pulses production via high-order harmonic generation.

Sources of VUV radiation by high harmonic generation and their characteristics

We make a review of the high harmonic generation (HHG) process, its characteristics and properties, presenting our main results in the field. The HHG is a very interesting source on the deep ultraviolet region with high potential in applications such as time resolved measurements and high spatial resolution imaging