  In systems having an anisotropic electronic structure, such as the layered
materials graphite, graphene and cuprates, impulsive light excitation can
coherently stimulate specific bosonic modes, with exotic consequences for the
emergent electronic properties. Here we show that the population of E$_{2g}$
phonons in the multiband superconductor MgB$_2$ can be selectively enhanced by
femtosecond laser pulses, leading to a transient control of the number of
carriers in the {\sigma}-electronic subsystem. The nonequilibrium evolution of
the material optical constants is followed in the spectral region sensitive to
both the a- and c-axis plasma frequencies and modeled theoretically, revealing
the details of the $\sigma$-$\pi$ interband scattering mechanism in MgB$_2$.
