Abstract
One critical goal of ultra-intense laser applications is the achievement of an ultra-high fi eld, which has
never been obtained in laboratories; this would open a new fi eld of science. To realize a high fi eld with
high power lasers that are as strong as possible, extremely small f-number or fast focusing optics have
been recently developed using a plasma mirror (PM) technique and introduced to high power laser
systems. To optimize PM utilization, we experimentally studied the focus abilities of ultra-intense laser
light with PM by measuring the focal spot and the laser light refl ectivity. Such PM surface dynamics as
thermal expansion depend on the laser intensity and could become one critical factor to determine the
PM performance by comparing the experimental result of focus spot with calculations of the laser light
foci by a PM.
