  Advances in light sources and time resolved spectroscopy have made it
possible to excite specific atomic vibrations in solids and to observe the
resulting changes in electronic properties but the mechanism by which phonon
excitation causes qualitative changes in electronic properties, has remained
unclear. Here we show that the dominant symmetry-allowed coupling between
electron density and dipole active modes implies an electron density-dependent
squeezing of the phonon state which provides an attractive contribution to the
electron-electron interaction, independent of the sign of the bare
electron-phonon coupling and with a magnitude proportional to the degree of
laser-induced phonon excitation. Reasonable excitation amplitudes lead to
non-negligible attractive interactions that may cause significant transient
changes in electronic properties including superconductivity. The mechanism is
generically applicable to a wide range of systems, offering a promising route
to manipulating and controlling electronic phase behavior in novel materials.
