(color online) The time-varying ionization degrees with the laser wavelengths (black line), 800 nm (red line), and 1600 nm (blue line) for the given laser intensity of and pulse duration of τ = 50 fs. The electron density reaches the same saturation after the laser pulse passage. Inset shows the time-varying ionization degrees in an optical period of 1600 nm laser for the three wavelength lasers. In the short time interval, the electron density jumps twice for the 1600 nm laser but four times for the 800 nm laser.