A quantitative theory of attosecond pulse generation in relativistically driven overdense plasma slabs is presented based on an explicit analysis of synchrotron-type electron trajectories. The subcycle, field-controlled release, and subsequent nanometer-scale acceleration of relativistic electron bunches under the combined action of the laser and ionic potentials give rise to coherent radiation with a high-frequency cutoff, intensity, and radiation pattern explained in terms of the basic laws of synchrotron radiation. The emerging radiation is confined to time intervals much shorter than the half-cycle of the driver field. This intuitive approach will be instrumental in analyzing and optimizing few-cycle-laser-driven relativistic sources of intense isolated extreme ultraviolet and x-ray pulses.

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We study XUV generation with several-cycle laser pulses of intensity up to 1015 W cm−2 using numerical solution of the 3D Schrödinger equation for a hydrogen atom. Ionization of the atom mainly takes place in a barrier-suppression regime for such driving intensities. We find that in this regime XUV yield stops growing with the laser intensity and then even essentially decreases. The calculated yield dependence on the laser intensity agrees well with predictions of our theory. The latter shows several factors that lead to the decrease of the XUV yield in the barrier-suppression regime. The calculated cut-off in the XUV spectrum is displaced to slightly lower energies than those predicted by the Ip + 3.17Up law. The change in the cut-off can be due to the non-zero initial velocity of the electron detached under the barrier-suppression ionization. We find that essential population of both the free wave packet returning to the origin and the ground state at the instant of the return is required for effective XUV generation. Rapid ground state depopulation leads to shortening of the attopulse train generated by the two-cycle laser pulse when laser intensity increases. In particular, we find that the cut-off XUV generated under essential barrier-suppression with sine-like laser pulse provides an isolated attopulse, while two attopulses are generated for lower laser intensity.

We study the generation of attosecond x-ray and ultraviolet pulses from relativistically driven overdense plasma targets with two-color incident light. Particle-in-cell simulations show that significant improvement in pulse intensity and isolation is achievable with appropriate laser and plasma parameters. Conversion of 5% of incident laser energy to its second harmonic can enhance the intensity of generated attosecond pulses by an order of magnitude. This approach allows the generation of higher attosecond pulse intensities with existing experimental laser technology and offers a powerful tool for the analysis of the dynamics of relativistic laser-plasma interaction.

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We report a bulk void-like micromodification of fused silica using two-color μJ-energy level tightly focused (NA = 0.5) co-propagating seeding (visible, 0.62 μm) and heating (near-IR, 1.24 μm) femtosecond laser pulses with online third harmonic diagnostics of created microplasmas as well as subsequent laser-induced void-like defects. It has been shown experimentally and theoretically that production of seeding electrons through multiphoton ionization by visible laser pulses paves the way for controllability of the energy deposition and laser-induced micromodification via carrier heating by delayed infrared laser pulses inside the material. Experimental results demonstrate wide possibilities to increase the density of energy deposited up to 6 kJ cm−3 inside the dielectric by tight focusing of two color fs-laser pulses and elliptical polarization for infrared heating fs-laser pulses. The developed theoretical approach predicts the enhancement of deposited energy density up to 9 kJ cm−3 using longer (mid-IR) wavelengths for heating laser pulses.

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