Self-consistent theory of high-order harmonic generation by relativistic plasma mirror

High-order harmonic generation in an electron-positron-ion plasma

The laser interaction with an electron-positron-ion mixed plasma is studied from the perspective of the associated high-order harmonic generation. For an idealized mixed plasma which is assumed with a sharp plasma-vacuum interface and uniform density distribution, when it is irradiated by a weakly relativistic laser pulse, well-defined signals at harmonics of the plasma frequency in the harmonic spectrum are observed. These characteristic signals are attributed to the inverse two-plasmon decay of the counterpropagating monochromatic plasma waves which are excited by the energetic electrons and the positron beam accelerated by the laser. Particle-in-cell simulations show the signal at twice the plasma frequency can be observed for a pair density as low as ∼10^{-5} of the plasma density. In the self-consistent scenario of pair production by an ultraintense laser striking a solid target, particle-in-cell simulations, which account for quantum electrodynamic effects (photon emission and pair production), show that dense (greater than the relativistically corrected critical density) and hot pair plasmas can be created. The harmonic spectrum shows weak low-order harmonics, indicating a high laser absorption due to quantum electrodynamic effects. The characteristic signals at harmonics of the plasma frequency are absent, because broadband plasma waves are excited due to the high plasma inhomogeneity introduced by the interaction. However, the high-frequency harmonics are enhanced due to the high-frequency modulations from the direct laser coupling with created pair plasmas.

Electron-Nanobunch-Width-Dominated Spectral Power Law for Relativistic Harmonic Generation from Ultrathin Foils

Relativistic high-order harmonic generation from solid-density plasma offers a compact source of coherent ultraviolet and x-ray light. For solid targets much thinner than the laser wavelength, the plasma thickness can be tuned to increase conversion efficiency; a reduction in total charge allows for balancing the laser and plasma driving forces, producing the most effective interaction. Unlike for semi-infinite plasma surfaces, we find that for ultrathin foil targets the dominant factor in the emission spectral shape is the finite width of the electron nanobunches, leading to a power-law exponent of approximately 10/3. Ultrathin foils produce higher-efficiency frequency conversion than solid targets for moderately relativistic (1<a_{0}<40) interactions and also provide unique insight into how the trajectories of individual electrons combine and interfere to generate reflected attosecond pulses.

Employing machine learning for theory validation and identification of experimental conditions in laser-plasma physics

The validation of a theory is commonly based on appealing to clearly distinguishable and describable features in properly reduced experimental data, while the use of ab-initio simulation for interpreting experimental data typically requires complete knowledge about initial conditions and parameters. We here apply the methodology of using machine learning for overcoming these natural limitations. We outline some basic universal ideas and show how we can use them to resolve long-standing theoretical and experimental difficulties in the problem of high-intensity laser-plasma interactions. In particular we show how an artificial neural network can “read” features imprinted in laser-plasma harmonic spectra that are currently analysed with spectral interferometry.

Frequency gating to isolate single attosecond pulses with overdense plasmas using particle-in-cell simulations

We present the isolation of single attosecond pulses for multi-cycle and few-cycle laser pulses from high harmonic generation in overdense plasmas, calculated with particle-in-cell simulations. By the combination of two laser pulses of equal amplitude and a small frequency shift between them, we demonstrate that it is possible to shorten the region in which the laser pulse is most intense, therefore restricting the generation of high harmonic orders in the form of attosecond pulses to a narrower time window. The creation of this window is achieved due to the combination of the laser pulse envelope and the slow oscillating wave obtained from the coherent sum of the two pulses. A parametric scan, performed with particle-in-cell simulations, reveals how the pulse isolation behaves for different input laser pulse lengths and which are the optimal frequency shifts between the two laser pulses in each case, giving the conditions for having a good isolation of an attosecond pulse when working with laser-plasma interaction in overdense targets.

Intense circularly polarized attosecond pulse generation from relativistic laser plasmas using few-cycle laser pulses

We have investigated the polarization of attosecond light pulses generated from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas using particle-in-cell simulation. Under suitable conditions, a desired polarization state of the generated attosecond pulse can be achieved by controlling the polarization of the incident laser. In particular, an elliptically polarized laser pulse of suitable ellipticity can generate an almost circularly polarized attosecond pulse without compromising the harmonic generation efficiency. The process is thus applicable as a new tabletop circularly-polarized XUV radiation source for probing attosecond phenomena with high temporal resolution.