  We consider theoretically a closed (zero-dimensional) semiconductor
microcavity where confined vacuum photonic mode is coupled to electrons in
valence band of the semiconductor. It is shown that vacuum-induced virtual
electron transitions between valence and conduction bands result in
renormalization of electron energy spectrum. As a consequence, vacuum-induced
band gaps appear within the valence band. Calculated values of the band gaps
are of sub-meV scale, that makes this QED effect to be measurable in
state-of-the-art experiments.
