Laser frequency upshift, self-defocusing, and ring formation in tunnel ionizing gases and plasmas

The combined effects of tunnel ionization of gases on laser frequency upshi ft, defocusing, and ring formation are considered self-consistently. A high -intensity short pulse laser causes rapid tunnel ionization of a gas. The i ncreasing plasma density leads to a decreasing refractive index, modulating the phase of the laser as it propagates and causing frequency upshift and supercontinuum generation. For laser intensity profile peaking on axis, the tunnel ionization produces a minimum of refractive index on axis, thus def ocusing the laser. The defocusing reduces the ionization rate and frequency upshift. As the laser propagates over a Rayleigh length its trailing porti on develops a ring shape distribution due to stronger defocusing of rays on axis than the off-axis rays, as seen in a recent experiment. (C) 2000 Amer ican Institute of Physics. [S1070-664X(00)02911-6].