Abstract

The ability of pulsed lasers to cause breakdown in gaseous media has been observed and studied nearly as long as lasers have been available. A breakdown microplasma is initiated at the focus of a laser when atomic and molecular species are ionized following the resonant and/or nonresonant absorption of laser radiation. Free electrons that are initially produced absorb laser radiation through a process known as inverse bremsstrahlung. After absorbing enough energy, the electrons collide with other species causing them to ionize and the process is repeated until a cascade ionization occurs. When the electron density is sufficiently high, the microplasma becomes opaque to the laser radiation which results in a very rapid heating of the plasma for the duration of the laser pulse. Temperatures in laser produced microplasmas have been estimated as high as 10⁴ to 10⁵ K with electron densities on the order of 10¹⁵ to 10¹⁹ per cm³. Studies of microplasmas produced using laser radiation from the ultraviolet to the far infra-red have shown that the intensity required for optical breakdown to occur (the breakdown threshold) generally decreases as the laser wavelength decreases (1-5). Similarly, the "rise time" of the microplasma, or the time it takes for the plasma emission intensity to reach a maximum value, has been found to decrease as the laser wavelength decreases.

© 1992 Optical Society of America