Power efficiency and quantum efficiencies of electron-beam pumped lasers

The object is to identify and assess factors and mechanisms that control the conversion of electron-beam power into coherent light through excitation of a semiconductor laser cavity. First, we examine the question of pump power losses associated with electron backscattering and pair creation. It is shown that power retention and ionization yield reflect target characteristics (atomic number and bandgap energy) only. The external quantum efficiency, which is best expressed as a product of quantum yield, coherence ratio, and escape probability, involves two parameters: pumping ratio and output coupling. This leads to a straightforward optimization procedure. Heating effects are analyzed in terms of a differential quantum efficiency and are shown to degrade the saturation value of the efficiency by a factor roughly proportional to the pulse rise time, if adiabatic conditions hold. These considerations are illustrated using power-efficiency figures reported for CdS, CdTe, and GaAs lasers; it is demonstrated that the photon-loss coefficient of excited "perfect" CdS must be less than 1.5 cm^-1, at 4.2°K.