Powerful flashes of ultraviolet, visible and near-infrared laser light can cause breakdown and plasma formation in gases which are normally transparent at these wavelengths. The ionization processes involved are multiphoton absorption and inverse bremsstrahlung or cascade collisional ionization, in competition with deionization by diffusion, recombination and radiative energy losses. The relative importance of these mechanisms and their roles in leading to the initiation of breakdown and the earliest stages of plasma formation is examined. Theoretical treatments are summarized, and experimental results obtained with a variety of gases and metal vapours at pressures from millitorrs to hundreds of atmospheres and with flashes from picosecond duration to quasi-continuous illumination are reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/38/5/002/pdf

Laser-induced breakdown of gases

Fundamentals of high-energy spark ignition with lasers

We investigated plasma formation in distilled water by 30-ps and 6-ns Nd:YAG laser pulses of 1064-nm and 532-nm wavelength for focusing angles between 1.7° and 32°. We determined the optical breakdown thresholds and analyzed the plasma length achieved at superthreshold irradiance, The parameter range investigated covers the parameters used for intraocular laser surgery. The experimental results are compared to theoretical models for the calculation of breakdown thresholds and the description of plasma growth for superthreshold breakdown. We found that at λ=1064 nm the measured thresholds for both pulse durations coincide with the calculated thresholds for the generation of seed electrons by multiphoton ionization. The breakdown process is completed by avalanche ionization. The seed electron density required for breakdown is about 4×10/sup 9/ cm for the 6-ns pulses, and 1.4×10/sup 11/ cm/sup -3/ for the 30-ps pulses. No spot size dependence of the irradiance threshold for breakdown was observed. The average threshold is by a factor of 5.9 higher for 30-ps pulses (I/sub th/=4.5×1011 W/cm2) than for 6-ns pulses (Ith=0.76×10/sup 11/ W/cm/sup 2/). At angles below approximately 2°, the threshold is influenced by self-focusing effects. The breakdown thresholds at 532 nm are slightly lower than at 1064 nm. Here, multiphoton ionization contributes considerably to the generation of free electrons throughout the whole process of plasma formation. Our results for plasma formation at superthreshold energies support a "breakdown wave" mechanism of plasma growth. For picosecond pulses, the breakdown threshold can be considered to be time-invariant, but for nanosecond pulses there is probably a decrease of the threshold during the laser pulse which may be due to UV-radiation emitted from plasma created at the beginning of the pulse.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. I. Optical breakdown at threshold and superthreshold irradiance

This paper provides a review of papers published on laser induced gas breakdown. The material is divided into four parts: 1) breakdown mechanism, 2) plasma expansion under the influence of the laser pulse, 3) plasma decay after the end of the laser pulse, and 4) bibliography of papers published on the subject up to the end of May, 1968.

Laser induced gas breakdown: A bibliographical review

We report on a theoretical and experimental study of the temporal and spatial dynamics of plasmas produced in liquids by single Nd: YAG laser pulses of nanosecond duration This study was motivated by the increasing attention paid to the phenomenon of optical breakdown and to its related effects on tissues and media in connection with microsurgical techniques developed for ophthalmology and urology Streak camera recordings of the emission from laser-induced plasmas were taken in distilled and tap water in controlled irradiation conditions From streak recordings, plasma starting times as a function of the axial distance from focus, the overall length of the plasma column, plasma lifetimes, and plasma absorption were derived and analyzed In this first paper we analyze the curves of plasma starting time, as a function of the irradiation parameters and of the properties of the medium We show that a model obtained by upgrading the theory of the moving breakdown allows accurate interpretation of the experimental observations

Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 1: Analysis of the plasma starting times.

The plasma created by the output of a 2 to 10 J Q‐switched ruby laser focused into air, argon, helium and hydrogen at pressures from 15 to 1000 psi has been photographically recorded during and after the laser heating pulse. The growth of the luminous plasma front during laser irradiation was in accordance with a prediction based on a radiatively supported detonation‐wave model in which a Gaussian pulse shape was used for the instantaneous energy addition. With hydrogen at atmospheric pressure an appreciable fraction of the laser energy was transmitted. Consequently, the detonation‐wave model had to include only partial absorption of the incident laser beam. This was done by using the inverse bremsstrahlung absorption coefficient and a constant depth of energy addition. The shock wave created by this sudden energy addition was observed by using a Schlieren system. The growth of the shock wave was not in accordance with that predicted by spherical‐blast‐wave theory. The shock‐wave position from the initial point of breakdown varied as time from breakdown to the (0.20 ± 20%) power independent of gas species or operating condition.

J. W. Daiber

Papers

Laser‐Driven Detonation Waves in Gases