S. Sartania

Compression of high-energy laser pulses below 5 fs.

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.

Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate

A compact all-solid-state femtosecond Ti:sapphire oscillator¿amplifier system using no grating-based pulse stretcher produces 20-fs, 1.5-mJ pulses at a 1-kHz repetition rate. The pulses are subsequently compressed in a hollow-fiber chirped-mirror compressor. The system delivers bandwidth-limited 5-fs, 0.5-mJ pulses at 780 nm in a diffraction-limited beam.

Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps

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.

Toward a terawatt-scale sub-10-fs laser technology

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.

Laser micromachining of barium aluminium borosilicate glass with pulse durations between 20 fs and 3 ps

Laser-micromachining of high-tech glass has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. A systematic electronmicroscopic study shows that, below 100 fs, an extreme precision and a substantial decrease of the ablation threshold fluence with respect to pulse laser processing with pulses in the picosecond and nanosecond range could be achieved. The technical relevance of this novel microtechnology is discussed. The morphology of the ablated areas is not determined by thermal processes, i.e. the heat affected zone. It is controlled by non-linear optical coupling effects. Multi-photon absorption becomes highly efficient below laser pulse durations of 100 fs so that light penetration is minimized and ablation cavities become smooth. At longer pulse durations, a higher light penetration due to a lower number of non-linearly absorbed photons allows mechanical relaxation processes in the glass material leading to roughening.

Amplitude and chirp characterization of high-power laser pulses in the 5-fs regime.

Frequency-resolved optical gating (FROG) based on second-harmonic generation has been demonstrated to be capable of high-fidelity measurement of the electric-field envelope and of the temporal evolution of the instantaneous carrier frequency of 0.1-TW 5-fs pulses without the need for any correction for systematic experimental errors. At a 1-kHz repetition rate, pulse energies of a few microjoules are sufficient for reliable FROG characterization of pulses with durations down to the single-cycle regime. The results obtained reveal that carefully designed hollow-fiber chirped-mirror compressors are able to deliver high-power sub-10-fs pulses with a smooth Gaussianlike leading edge that has an intensity contrast of approximately 10(-2) .

Few-optical-cycle laser pulses: from high peak power to frequency tunability

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of /spl sim/5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared.

Structuring of dielectric and metallic materials with ultrashort laser pulses between 20 fs and 3 ps

Laser-micromachining of barium aluminum borosilicate glass, fused silica and stainless steel has been extended down to a pulse duration of 20 fs generated by a Ti:sapphire laser system at a wavelength of 0.8 micrometer. A systematic study shows that, below 100 fs, an enhanced precision and a substantial decrease of the ablation threshold fluence in comparison to pulse laser processing with pulses in the picosecond and nanosecond range could be achieved. The physical mechanism and the technical relevance of this novel microtechnology is discussed.

Compression of high energy femtosecond pulses by the hollow fiber technique: Generation of sub-5-fs multigigawatt pulses

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high energy femtosecond pulses to duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow fiber filled with noble gases is studied under two excitation regimes with high energy input pulses of 140-fs and 20-fs duration respectively. With 20-fs input pulses and under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high throughput prism-chirped mirror delay line.

Nanometer precision material ablation and optical breakdown with sub-10 fs pulses

We report measurements of the optical breakdown threshold and ablation depth in dielectrics for laser pulse durations between 5 ps and 5 fs at the Ti:sapphire wavelength. The application of the shortest pulses leads to sub-μm ablation rates, smooth morpholgies of the generated structures and reduces the statistical behaviour of material removal. With 5-fs pulses a light intensity of 1014 W/cm2 can be applied to fused silica without causing damage