Model for ultrashort laser pulse-induced ionization dynamics in transparent solids

Abstract

A comprehensive model of ultrafast laser-induced plasma generation intended for coupling with pulse propagation simulations in transparent solids is introduced. It simultaneously accounts for the changing spectrum of a propagating ultrashort laser pulse while coupling to the evolution of the energy-resolved nonequilibrium free-carrier distribution. The presented results indicate that strong pulse chirps lead to ionization dynamics that are not captured by the standard monochromatic treatment of laser-induced plasma formation. These results have strong implications for ultrafast laser-solid applications that depend on ionization in a strong nonlinear focus.

Ultrafast modification of band structure of wide-band-gap solids by ultrashort pulses of laser-driven electron oscillations

“…Ultrafast laser interaction with electron sub-system of non-metal solids has been under intensive studies for several decades . The strong interest to that field results from the fundamental fact: the laser-electron interaction and energy deposition into the electron sub-system substantially contribute to all high-intensity ultrafast laser interactions with wide-band-gap crystals, e.g., laser ablation [1][2][3][4][5][6][7][8], laser-induced damage [7][8][9][10][11][12][13][14][15], excitation of transient optical response [16][17][18], direct writing of nanostructures [19,20], and ultrafast nonlinear propagation [21][22][23][24]. A recent development of petawatt (PW) laser systems launched another wave of research activity in this field [25].…”

“…Correspondingly, those approaches consider slow time variations driven by pulse envelope rather than the sub-cycle dynamics driven by instant electric field of a laser pulse. They employ the approximation of decoupled inter-and intra-band laser-driven excitations [1,2,4,5,[7][8][9][10][11][12][18][19][20][21][22][23][24]. In particular, the approaches of this group use the Keldysh formula [54] to evaluate the rate of the field-driven inter-band transitions that is not coupled to electron-particle collision and intraband absorption rates of conduction-band electrons.…”

“…They provide transparent interpretation of the experimental data and significantly assist in identifying specific contributions to the overall interactions. They deliver analytical scaling of the laserinduced effects with laser and material parameters due to the use of analytical formulae for the rates of the electron excitations [1,2,4,5,[7][8][9][10][11][12][18][19][20][21][22][23][24]. The analytical models are a highly supporting complement to the numerical simulations and are indispensable for qualitative analysis of the physics of ultrafast high-intensity laser interaction with quantum systems [55].…”

“…It implies singlefrequency cosine variations of electric field at constant amplitude that is in obvious contradiction with realistic time variations of electric field of a laser pulse. The usual way to overcome this issue is to replace the constant electric-field amplitude with the slowly varying amplitude of a pulse envelope [1,2,4,5,[7][8][9][10][11][12][14][15][16][17][18][19][20][21][22][23][24]. Although this approach raises significant concerns, e.g., when the monochromatic Keldysh formula is applied to laser pulses containing 2-25 cycles [22,[60][61][62], no analysis has been done so far to determine a range of laser parameters where such modification of the monochromatic approximation is valid.…”

“…In this pulse-width range, the validity of the simple analytical models is highly questionable due to the failure of the monochromatic approximation while the numerical approaches are not applied because of various reasons. Recently reported attempts to build analytical non-monochromatic models either assume low irradiance and the perturbative regime of the laser-electron interaction [60,61] or are obtained by improper modifications of the Keldysh formula [22,62]. However, exactly this range of pulse widths is of special significance for various applications, e.g., to support the development of the PW laser systems.…”