The spaser is a nano plasmonic counterpart of the laser. The spasing modes are surface plasmons whose localization length is on the nanoscale. The resonator for spasing can be specially designed plasmonic metal nanostructures. This resonator should be surrounded by the gain medium that overlaps with the spasing SP eigenmode spatially and whose emission line overlaps with this eigenmode spectrally.
The gain medium can be semiconductor nanocrystals, dye molecules, rare-earth ions, or electron–hole excitations of a bulk semiconductor. The pump excites electron–hole pairs in the gain medium which relax to form excitons. The excitons constitute the two-level systems that are the donors of energy for the SP emission into the spacing mode.
In vacuum, the excitons would recombine emitting photons. The plasmons in the spaser mode create the high local fields that excite the gain medium and stimulate more emission to this mode, which is the feedback mechanism. The SP eigenmodes are described by a wave equation with homogeneous boundary conditions. Electrons and photons play a central role in the operation of quantum devices. The behaviour of electrons in metals and semiconductors at a finite temperature depends on the Fermi–Dirac distribution. Similarly, the properties of photons are governed by the Bose–Einstein distribution. The gain medium was treated quantum mechanically using density matrix equations (optical Bloch equations) but the electric field operator of the quantized SPs were defined quasi-classically using the wave equation.