A p-type semiconductor barrier layer is provided in the vicinity of
undoped quantum dots, and holes in the p-type semiconductor barrier layer
are injected in advance in the ground level of the valence band of the
quantum dots. Lowering the threshold electron density of conduction
electrons in the ground level of the conduction band of quantum dots in
this way accelerates the relaxation process of electrons from an excited
level to the ground level in the conduction band.