  We present theoretical studies on real-time probing the electron density
evolution of an atom in a strong infrared (IR) laser field with few-cycle
near-infrared (NIR) and attosecond extreme-ultraviolet (XUV) pulses. Our
results indicate that the electron density near the tunneling barrier is
reflected in the additional tunneling ionization yield with a delayed NIR pulse
and the electron density near the nucleus can be probed by the single
photoionization yield with a delayed XUV pulse. It turns out the NIR-probing
scheme can be used to study the polarization of the system in an external IR
field and the XUV-probing can be additionally applied to explore excitation
dynamics during and after the IR field interaction.
