Optical breakdown of solids by few-cycle laser pulses.

We show that a broadly accepted criterion of laser-induced breakdown in solids, defining the laser-breakdown threshold in terms of the laser fluence or laser intensity needed to generate a certain fraction of the critical electron density rc within the laser pulse, fails in the case of high-intensity few-cycle laser pulses. Such laser pulses can give rise to subcycle oscillations of electron density ρ with peak ρ values well above ρc even when the total energy of the laser pulse is too low to induce a laser damage of material. The central idea of our approach is that, instead of the ρ = ρ c ratio, the laser-breakdown threshold connects to the total laser energy coupled to the electron subsystem and subsequently transferred to the crystal lattice. With this approach, as we show in this work, predictions of the physical model start to converge to the available experimental data.

Amplitude concentration in a phase-modulated spectrum due to femtosecond filamentation.

We present a method by which the spectral intensity of an ultrafast laser pulse can be accumulated at selected frequencies by a controllable amount. Using a 4-f pulse shaper we modulate the phase of the frequency components of a femtosecond laser. By inducing femtosecond filamentation with the modulated pulse, we can concentrate the spectral amplitude of the pulse at various frequencies. The phase mask applied by the pulse shaper determines the frequencies for which accumulation occurs, as well as the intensity of the spectral concentration. This technique provides a way to obtain pulses with adjustable amplitude using only phase modulation and the nonlinear response of a medium. This provides a means whereby information which is encoded into spectral phase jumps may be decoded into measurable spectral intensity spikes.

Field-cycle-resolved photoionization in solids.

The Keldysh theory of photoionization in a solid dielectric is generalized to the case of arbitrarily short driving pulses of arbitrary pulse shape. We derive a closed-form solution for the nonadiabatic ionization rate in a transparent solid with a periodic dispersion relation, which reveals ultrafast ionization dynamics within the field cycle and recovers the key results of the Keldysh theory in the appropriate limiting regimes.

Attosecond shock waves.

Shock-wave formation is a generic scenario of wave dynamics known in nonlinear acoustics, fluid dynamics, astrophysics, seismology, and detonation physics. Here, we show that, in nonlinear optics, remarkably short, attosecond shock transients can be generated through a strongly coupled spatial and temporal dynamics of ultrashort light pulses, suggesting a pulse self-compression scenario whereby multigigawatt attosecond optical waveforms can be synthesized.