  The average effective mass of charge carriers produced by an intense
ultrashort laser pulse in a transparent solid increases significantly as the
excitation mechanism changes from multiphoton transitions to interband
tunneling. We theoretically investigate this phenomenon for several dielectrics
and semiconductors. For diamond as a representative dielectric, we present a
detailed analysis of the laser-induced change of optical properties. When the
concentration of free carriers is high, we find that the average effective mass
controls not only the intraband charge-carrier transport but also the interband
contributions to the optical response. We observe that the excitation-induced
birefringence is particularly large for parameters where the plasma response
compensates for the linear response of an unperturbed solid.
