Tunable enhancement of high harmonic emission from laser solid interactions

Selective generation of narrow-band harmonics by a relativistic laser pulse interaction with a detuned plasma grating

The spectral characteristics of high-order harmonics generated by the interaction of a linearly polarized relativistic laser pulse with a plasma grating target are investigated. Through particle-in-cell simulations and an analytical model, it is shown that a plasma grating target with periodic structure can select special harmonics with integer multiples of the grating frequency, and that low-order harmonics with frequencies being integer times of the laser frequency are generated nearly parallel to the target surface from a Fresnel zone plate target with an aperiodic structure. Spectral control of the harmonics can be achieved by introducing a correction factor β to the radius formula of the Fresnel zone plate, which can create a slightly detuned plasma grating, and then only the narrow-band harmonics can be selected nearly parallel to the target surface. The center order of the narrow-band harmonics can be tuned by adjusting the correction factor β, while the bandwidth of the harmonics can be selected by adjusting the other parameter λ_{f} of the detuned plasma grating.

High-quality high-order harmonic generation through preplasma truncation

By introducing preplasma truncation to cases with an initial preplasma scale length larger than 0.2λ, the efficiency of high-order harmonics generated from relativistic laser-solid interactions can be enhanced by more than one order of magnitude and the angular spread can be confined into near-diffraction-limited divergence. Numerical simulations show that density truncation results in more compact oscillation of the surface electron sheet and the curvature of the reflection surface for the driving laser is greatly reduced. This leads to an overall improvement in the harmonic beam quality. More importantly, density truncation makes the harmonic generation weakly dependent on the preplasma scale length, which provides a way to relax the extremely high requirement on the temporal contrast of the driving laser pulse. A feasible scheme to realize the required preplasma truncation is also proposed and demonstrated by numerical simulations.

Spectral control of high harmonics from relativistic plasmas using bicircular fields

We introduce two-color counterrotating circularly polarized laser fields as a way to spectrally control high harmonic generation (HHG) from relativistic plasma mirrors. Through particle-in-cell simulations, we show that only a selected group of harmonic orders can appear owing to the symmetry of the laser fields and the related conservation laws. By adjusting the intensity ratio of the two driving field components, we demonstrate the overall HHG efficiency, the relative intensity of allowed neighboring harmonic orders, and that the polarization state of the harmonic source can be tuned. The HHG efficiency of this scheme can be as high as that driven by a linearly polarized laser field.

Spatial Properties of High-Order Harmonic Beams from Plasma Mirrors: A Ptychographic Study

Spatial properties of high-order harmonic beams produced by high-intensity laser-matter interactions carry rich information on the physics of the generation process, and their detailed understanding is essential for applications of these light beams. We present a thorough study of these properties in the case of harmonic generation from plasma mirrors, up to the relativistic interaction regime. In situ ptychographic measurements of the amplitude and phase spatial profiles of the different harmonic orders in the target plane are presented, as a function of the key interaction parameters. These measurements are used to validate analytical models of the harmonic spatial phase in different generation regimes, and to benchmark ultrahigh-order Maxwell solvers of particle-in-cell simulation codes.

Ultrasharp-front laser pulses generated by energetic-electron flux triggering of laser propagation in overdense plasmas

This paper reports that an initially opaque plasma foil, irradiated by a laser pulse with intensity below the self-induced transparency (SIT) threshold, will become transparent, if a flux of energetic electrons is present. Based on this phenomenon, named flux-induced transparency (FIT), an approach to obtaining ultrasharp-front laser pulses is proposed. With the presence of an energetic-electron flux generated by a p-polarized laser irradiating an overdense plasma foil from the rear side, the propagation of an s-polarized laser irradiating the front surface of the foil can be manipulated. The transmitted s-polarized laser pulse has an ultrasharp front which rises by three orders of magnitude within a few laser cycles. The profile of the transmitted pulse is tunable by controlling the time at which the energetic-electron flux arrives at the front surface.

Beaming of high-order harmonics generated from laser-plasma interactions

Beam divergences of high-order extreme ultraviolet harmonics from intense laser interactions with steep plasma density gradients are studied through experiment and Fourier analysis of the harmonic spatial phase. We show that while emission due to the relativistically oscillating mirror mechanism can be explained by ponderomotive surface denting, in agreement with previous results, the divergence of the emission due to the coherent wake emission mechanism requires a combination of the dent phase and an intrinsic emission phase. The temporal dependence of the divergences for both mechanisms is highlighted while it is also shown that the coherent wake emission divergence can be small in circumstances where the phase terms compensate each other.