The Keldysh model of the photoionization [L. V. Keldysh, Sov. Phys. JETP 20, 1307 (1965)] is extended by deriving a formula for the photoionization rate of crystals based on the cosine energy-momentum relation. The relation is characteristic of tight-binding approximation and directly takes into account the influence of Bragg-type reflections of oscillating electrons at the edges of the first Brillouin zone. Due to the reflections and oscillations, the dependence of the photoionization rate on laser and material parameters takes form of a multibranch function with the branches separated by singularity points with unlimited increasing of the rate. Each of the singularities is coupled to the flattening of the effective-band structure. The laser intensity corresponding to the first singularity is found to be about $10\mathrm{TW}∕{\mathrm{cm}}^2$ for most wide band-gap crystals. We also show that the lowest-order branch of the photoionization rate completely corresponds to the multiphoton regime, and the first singularity takes place before the tunneling regime starts to dominate. Analysis of the ionization-rate asymptotic in the vicinity of the first-singularity point suggests possibility of ionization suppression by high-intensity radiation for certain ranges of laser wavelength.

Silicon is one of the most abundant materials which is used in many areas of modern research and technology. A variety of those applications require surface nanopatterning with minimum structure defects. However, the high-quality nanostructuring of large areas of silicon surface at industrially acceptable speed is still a challenge. Here, we report a rapid formation of highly regular laser-induced periodic surface structures (HR-LIPSS) in the regime of strong ablation by infrared femtosecond laser pulses at sub-MHz repetition rate. Parameters of the laser-surface interactions and obtained experimental results suggest an important role of electrostatically assisted bond softening in initiating the HR-LIPSS formation.

Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MIR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ² scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MIR pulses (λ = 2.7–4.2 μm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and μ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.

The chirped pulse amplification technique has enabled the generation of pulses of a few femtosecond duration with peak powers multi-Tera and Peta–Watt in the near infrared. Its implementation to realize even shorter pulse duration, higher energy, and higher repetition rate laser systems relies on overcoming the limitations imposed by laser damage of critical components. In particular, the laser damage of coatings in the amplifiers and in post-compression optics have become a bottleneck. The robustness of optical coatings is typically evaluated numerically through steady-state simulations of electric field enhancement in multilayer stacks. However, this approach cannot capture crucial characteristics of femtosecond laser induced damage (LID), as it only considers the geometry of the multilayer stack and the optical properties of the materials composing the stack. This approach neglects that in the interaction of an ultrashort pulse and the materials there is plasma generation and associated material modifications. Here, we present a numerical approach to estimate the LID threshold of dielectric multilayer coatings based on strong field electronic dynamics. In this dynamic scheme, the electric field propagation, photoionization, impact ionization, and electron heating are incorporated through a finite-difference time-domain algorithm. We applied our method to simulate the LID threshold of bulk fused silica, and of multilayer dielectric mirrors and gratings. The results are then compared with experimental measurements. The salient aspects of our model, such as the implementation of the Keldysh photoionization model, the impact ionization model, the electron collision model for ‘low’-temperature, dense plasma, and the LID threshold criterion for few-cycle pulses are discussed.

The electric field of high-intensity ultrashort laser pulses substantially perturbs an electron subsystem of a crystal and affects its band structure. Laser-driven oscillations of electrons and holes are frequently referred to as a major mechanism of the perturbation in dielectrics and semiconductors. New physical effects arise when the band structure is modified by an ultrashort pulse of the oscillations driven by a few-cycle laser pulse. Assuming the laser-pulse envelope varies slowly compared to the carrier frequency, we derive analytical relations for the laser-modified band structure by utilizing the Keldysh cycle-averaged nonperturbative approach under the approximation of constant effective mass. Formation of indirect-gap transient bands, suppression of the nonlinear absorption on the leading edge of a laser pulse, and cycle-averaged photocurrent generation driven by the pulse envelope are predicted. Analytical scaling with six laser and material parameters is obtained. The reported results establish the limits of validity of the Keldysh photoionization model and advance understanding of the fundamental effects involved in high-intensity ultrafast laser-solid interactions.

A constant magnetic field can significantly improve the quality and speed of ablation by nanosecond laser pulses. These improvements are usually attributed to the confinement of laser-produced plasma by the magnetic field and specific propagation effects in the magnetized plasma. Here we report a strong influence of constant axial magnetic field on the ablation of silicon by 20-ns laser pulses at wavelength 355 nm, which results in an increase of ablation depth by a factor of 1.3 to 69 depending on laser parameters and magnitude of the magnetic field. The traditional plasma effects do not explain this result, and magneto-absorption of silicon is proposed as one of the major mechanisms of the significant enhancement of ablation.