A lot of the work of our group is concerned with what happens when an intense laser pulse interacts with matter. This basic issue is incredibly rich and complex, leading a wide range of interesting phenomena and useful processes.
One of the most important parameters is the laser intensity, i.e. the energy density. Although generally the high power lasers we use don’t have a lot of energy (about enough to heat a gram of water to 1 degree above room temperature), we first compress the laser pulse to an incredibly short duration (ranging from 10 femtoseconds to 10 nanoseconds) and then focus the pulse to a few micron size spot. This gives us an impressively high intensity, up to >1021 W/cm2 , although only for quite a short time! At such high-intensities matter is rapidly ionised and the electrons are free to move in the electro-magnetic fields of the laser pulse. Ions, owing to their higher mass, move much more sluggishly and don’t respond so much to the laser directly, as the laser fields oscillate too quickly (we typically use near infrared laser pulses which have a frequency of about 300 THz). However, they do respond the electrostatic force set up by the displacement of the electrons – a phenomena which is employed to great effect in laser-driven ion acceleration.
With increasing laser intensity, electrons in the laser field oscillate more rapidly, and for the intensities above 1018 W/cm2 becomes highly relativistic. A simplified calculation of the electron momentum in a laser field gives rise the cryptically named ‘normalised vector potential’ a0 = eE/(ωmec), which is the maximum electron velocity (eE/ω) divided by the electron rest mass (me) and the speed of light (c). When > 1, the laser pulse is said to be relativistic.