Abstract

When intense laser pulses interact with matter, ionisation is initiated by multiphoton effects. In some cases the electrons, gaining energy from the laser field, can collide with other atoms and ionise them, triggering an avalanche ionisation. It is generally believed that the laser pulse has to be sufficiently long (> 100 fs) for the electrons to gain energy required for an avalanche. However, this is still a well-debated issue as some experimental observations suggest that avalanche effects can prevail even for shorter pulses [1]. Recent theoretical works suggest that in the presence of intense fields, even electrons with lower-than-the-bandgap energy can cause ionisation [2]. This field-assisted collisional ionisation or cold avalanche could be present even for few cycle pulses. In solids for example, several collisions are possible during the presence of intense fields due to the high atomic density. However, it is difficult to distinguish this from the normal avalanche process and the presence of field assisted collisional ionisation has not yet been experimentally verified.

© 2009 IEEE