Physics Colloquium, January 20, 2004

Interaction of intense laser pulses with nano-clusters

Howard Milchberg

University of Maryland

Atoms or molecules exhibit short-range attractions for one anther owing to Van der Waals forces. Under rapid cooling, such as in nozzle ejection of a gas puff into vacuum, hundreds to a few tens of millions of atoms can aggregate together to make nano-scale clusters, typically of diameter less than a few hundred angstroms. Recently, there has been great interest in the interaction of intense laser pulse with clusters. If an intense, ultrashort laser pulse is focused in a gas of clusters, the clusters are almost instantaneously heated to temperatures up to ~10**8 K -- many times hotter than the sun - and they explode violently. Such high temperatures means laser-heated clusters are a copious source of X-rays, and the speeds of the particles thrown off by the explosion are high enough that fusion has been demonstrated from the collisions of deuterium nuclei. The explosion scenario in the presence of intense laser fields has been a subject of much discussion, and the details affect most of the applications of the laser-cluster interaction. We have measured, with femtosecond time resolution, the cluster explosion dynamics. An interesting and practical implication is that a gas of exploding clusters causes strong self-focusing of intense laser pulses. Our results illustrate how high energy density femtosecond plasma physics at the nanoscale affects what at first appears to be a conventional laser- gas-phase interaction.

3.30 p.m., Smith Laboratory, Room 1005

Refreshments served in Smith 1094 at 3:00 p.m.