  A theory of lasing in an optically thin layer of active centers disposed at a
metal surface is developed from first principles. The approach is based on a
rigorous account of the local field in a close vicinity of a reflective surface
which provides a feedback for dipole oscillations in active centers. It is
demonstrated that the gain medium thickness plays a crucial role in the lasing
condition and controls a switching from conventional lasing to lasing without
inversion. The numerical calculations are carried out for erbium doped glass
bordering a gold surface where radiation at telecom wavelength (1532 nm) can be
generated.
