D. Du

Self-channeling of high-peak-power femtosecond laser pulses in air.

The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.

LASER-INDUCED BREAKDOWN BY IMPACT IONIZATION IN SIO2 WITH PULSE WIDTHS FROM 7 NS TO 150 FS

Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

Machining of sub-micron holes using a femtosecond laser at 800 nm

Abstract The ability to machine very small features in materials has a number of technological applications. We have ablated holes, by laser ablation, into a metal film. Using 200 fs, 800 nm pulses from a Ti:sapphire laser, focused to a spot size of 3000 nm, we have produced holes with a diameter of 300 nm and a depth of 52 nm. The production of these small features is possible because the effects of thermal diffusion are minimized with the short pulses.

THERMOPHYSICAL EFFECTS IN LASER PROCESSING OF MATERIALS WITH PICOSECOND AND FEMTOSECOND PULSES

Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin‐film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseco...

Reduction of multi-photon ionization in dielectrics due to collisions

The collisional effect due to the multi-photon ionization process in dielectric material has been studied. We found that the breakdown threshold of fused silica is the same for both linearly and circularly polarized light at 55 fs and 100 fs, which we believe is an indication of the suppression of multi-photon ionization in solids. By numerically solving the time-dependent Schrödinger equation with scattering, for the first time, we have observed substantial reduction of the multi-photon ionization rate in dielectrics due to collisions.

Photodisruption in the human cornea as a function of laser pulse width

BACKGROUND We investigated the role of laser pulse width in determining fluence thresholds and efficiency for corneal photodisruption. METHODS A laser system that delivers a wide range of pulse energies and pulse widths was used to produce ablations at pulse widths from 100 femtoseconds (fs) to 7 nanoseconds (ns). The laser-induced breakdown fluence threshold at each pulse width was determined by monitoring individual plasma emissions. Using multiple shots, the photodisruption threshold and cutting depth at each pulse width were determined histologically. RESULTS Corneal breakdown thresholds decreased at a faster rate from 7 ns to approximately 10 picoseconds (ps), compared to further reductions in pulse width below 10 ps, where little variation was seen. Breakdown for pulse widths below 10 ps showed little intershot variability, resulting in highly reproducible fluence thresholds. Corneal tissue examined histologically showed similar fluence dependency. CONCLUSIONS Corneal tissue photodisruption thresholds demonstrate pulse width dependence. At pulse widths less than 10 ps and with fluences near the breakdown threshold, ablations are maximally precise and efficient. These findings suggest optimal laser parameters for corneal surgery.

Injection of ultrafast regenerative amplifiers with low energy femtosecond pulses from an Er-doped fiber laser

Abstract A very compact, low-noise and stable frequency-doubled Er-fiber laser system has been used to injection-seed a 10 Hz and a 250 kHz Ti:sapphire regenerative amplifier systems.

Laser induced breakdown as a function of pulse duration: From 7 ns to 150 fs

Single-shot laser induced breakdown, in wide band gap materials such as SiO2 and MgF2, has been studied over almost 5 orders of magnitude in duration from 150 fs to 7 ns. A Ti:sapphire chirped pulse amplification system was used in this experiement, so the pulse duration could be continuously adjusted without changing any other parameters. The damage threshold was detected by looking at the plasma formation and the change of material transmission coefficient. The avalanche mechanism was found to dominate over the entire pulse-width range even for 150 fs pulses where we would expect multi-photon processes to take over. A strong departure from the conventional fluence threshold scaling law is observed for pulses shorter than 10 ps, where beyond this point the fluence threshold increases. Also, it is observed for the first time that for short pulses the damage threshold becomes very accurate and less statistical than that for longer pulses.

Electron-Lattice Heating from Avalanche Ionization in Silicon with Near Infrared Ultrafast Laser Pulses

Absorption and redistribution of ultrafast pulsed laser energy in silicon is examined with 800 nm radiation for pulse durations ranging from 80 fs to 7 ns. The results are successfully interpreted in terms of an avalanche ionization process. Thresholds for vaporization are measured and compared to a theoretical model that incorporates a two temperature electron-lattice heating process. Ionization layer thickness and time-resolved melting and vaporization events are described in terms of a thermodynamic process involving the absorption of ionized electron energy by the lattice and its subsequent phase transformation to the liquid and vapor state. An average solid density plasma heating layer of 78 nm is extracted from the model and used to fit threshold fluence data. More detailed experimental results are found to provide close agreement with, and continuation of, the avalanche ionization coefficient curve of silicon [1] for electric field strengths extending to 4 × 107 volts/cm.