Specular reflectivity of plasma mirrors as a function of intensity, pulse duration, and angle of incidence

The specular reflectivity of plasma mirrors formed by subpicosecond pulses from a titanium:sapphire laser has been measured for different angles of incidence and for two different pulse lengths as a function of the laser intensity. Laser pulses with energies up to 250 mJ and pulse durations of 90 and 500 fs were focused onto a fused silica substrate. For angles of incidence between 6° and 45° the specular reflectivity increases to values of about 80% for intensities above a certain threshold intensity. The threshold intensity varies with the pulse length but is nearly independent of the angle of incidence. For very high intensities the specular reflectivity drops again to values of only a few percent.