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...

High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.

Coherent extreme-ultraviolet radiation extending to wavelengths below the carbon K edge at 4.37 nanometers (nm) has been generated at a repetition rate of 1 kilohertz by focusing 5-femtosecond near-infrared (780 nm) laser pulses into a helium gas jet. The incident light field performs just a few oscillations, which results in the emission of an x-ray supercontinuum rather than discrete harmonics. Owing to the extremely short rise time of the driving pulses, neutral atoms can be exposed to high fields before they are depleted by ionization. As a result, the observed x-ray radiation extends well into the water window and is delivered in a well-collimated beam (divergence less than 1 milliradian). The high repetition rate and spatial coherence result in a brightness of about 5 x 1 O8 photons per square millimeter per square milliradian per second in a I -percent bandwidth at 4.37 nm, the carbon edge of the water window. The compact laboratory system holds promise as a source for biological holography and nonlinear optics in the x-ray regime.

Laser-induced ablation has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. Barium aluminum borosilicate glass with an extremely high glass transformation temperature (∼600 °C) served as target material. The most significant observation was a substantial decrease of the ablation threshold fluence at pulse durations below 100 fs. All results indicate a dominant role of multiphoton absorption in addition to collisional ionization in this time domain.