A Simple Self - Triggered Plasma Shutter

Kecently it has been shown that a laser induced gas breakdown can be triggered by injecting the starting electrons into the focal volume [ 11. This “plasma shutter” has been applied to the generation of ultrashort CO, puises by optical free induction decay and has potential application as an optical isolator in high power laser systems. It has also been reported that the delay and the jitter are both in the subnanosecond range. This is consistent with the avalanche ionization concept of gas breakdown [e.g. 21, the validity of which depends on the presence of a starting electron. When a starting electron is not available, the avalanche theory of the breakdown threshold will not work and in fact, the threshold can be made very high. It was found that in ultraclean gases with subnanosecond CO, puises, the breakdown threshold can be made to be as high as 1013 W/cm2 [3]. The idea of the plasma shutter is then to inject electrons into these regions of ultrahigh field strengths to initiate the avalanche. Due to the ultrahigh laser intensity, the shutter speed is very fast and was measured to be -10 ps [4]. Previously, the injection was done with a d.c. spark source placed close to the focal volume. The d.c. surface spark photoionized the atoms and molecules in the focal region by its uv radiation. The electronic circuitry was rather complicated, however. In this communication, we wish to describe a very simple plasma shutter which requires no external electronics. Fig. 1 shows the experimental setup. The idea