Direct pumping into the emitting level
Direct pumping is also called "in-band pumping" or resonnant pumping.
These are the same idea realizing high slope efficiency in laser
oscillator. The efficiency of a laser oscillation is defined by mainly 3
conditions: the first is the setup of its resonnator, the second is the
effeciecy of gain media, and the third is the stokes shift between pump
and output. This stokes shift reduce the photon energy and this is
expressed by "Atomic quantum efficiency". The direct pumping technplogy
can improve this atomic quantum efficiency by the appropriate selection of
the frequency domain of pumping source.