Femtosecond Optical Breakdown in Dielectrics

Abstract

We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For t , 100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 10 14 W͞cm 2 slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials. [S0031-9007(98)05969-9] PACS numbers: 79.20.Ds, 42.65.Re, 78.47. + pLaser-induced breakdown resulting in damage to dielectrics has been the subject of extensive experimental and theoretical investigations since powerful lasers became available [1-3]. It has been described in terms of three major processes: (i) the excitation of electrons in the conduction band by impact and multiphoton ionization (MPI), (ii) heating of the conduction-band (henceforth free) electrons by the radiation, and (iii) transfer of the plasma energy to the lattice. For pulses of a few picoseconds or shorter, heat diffusion is "frozen" during the interaction [4] and the shocklike energy deposition leads to ablation. This new regime of laser-matter interactions holds promise for a number of intriguing applications in science and technology.Although breakdown experiments were recently extended to the subpicosecond regime [5-9], both the nature of the avalanche and the role of multiphoton ionization have remained controversial up to now. Du et al. [5] were the first to observe a deviation from the p t scaling of breakdown threshold fluence F th and an increasingly deterministic character of breakdown for t , 10 ps as opposed to longer pulses. These observations were explained in terms of an avalanche scaling with the square root of the laser intensity, and MPI was found to serve only for the production of seed electrons for the avalanche. By contrast, Stuart et al. 's model [6] yields an avalanche that scales linearly with the laser intensity. Combining this model with the Keldysh MPI rate [10], these investigators found that MPI is likely to become the dominant channel for free electron generation for t , 100 fs, which was predicted to result in F th as low as ,0.1 J͞cm 2 for t ഠ 10 fs in fused silica. In this Letter, we report F th ഠ 1.5 J͞cm 2 in fused silica for t # 10 fs. Our investigations confirm a linear scaling of the avalanche with intensity [6] and yield MPI rates which are orders of magnitude lower than predicted by Keldysh's theory [11].This work significantly extends previous studies of ultrashort-pulse-induced breakdown in several respects.The pulses for femtosecond breakdown experiments are delivered by a spatially filtered beam for the first time. The high-quality beam and the absence of heat diffusion a...

Ultrashort laser pulse ablation of copper, silicon and gelatin: effect of the pulse duration on the ablation thresholds and the incubation coefficients

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Time-dependent density functional theory for strong electromagnetic fields in crystalline solids

“…Experimentally, electron-hole plasmas are generated by irradiating solids with strong laser pulses, and the threshold for dielectric breakdown has been measured [18][19][20][21][22][23][24][25][26][27] . To describe the phenomena, model approaches such as a rate equation for electronic excitations have been developed [28][29][30][31][32][33] .…”

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