where n e is the electron plasma density, n c r = m e ω ω 0 2 / 4 π π e 2 is the critical plasma density and a 0 = e A / m e c 2 is the normalized vector potential. Here m e is the electron mass, e is the electron charge, c is the speed of light and ω ω 0 is the laser frequency. Note that CGS units were used above.