Some possibilities for coherent optical amplification of a normally incident and weak radiation field are reviewed based on various physical mechanisms, such as electronic quantum interference induced by a coupling laser field in a three-level system, field enhancement through the cavity confinement of a radiation field in a photonic crystal and field concentration seen in a transmitted near field through a metallic surface grating due to excitation of surface-plasmon-polariton modes. Numerical results are presented and discussed to demonstrate these interesting effects. The modification to the spontaneous emission inside a photonic crystal is also studied. The important role played by a longitudinal field resulting from the absorption by an induced three-dimensional plasma wave inside a doped semiconductor is analyzed using a nonlocal and nonadiabatic model. Moreover, the coupling between two-dimensional plasmons and surface plasmon modes in the nonretardation limit is explored.