Magnetic field in-plane quantization and tuning of population inversion in a THz superlattice quantum cascade laser

The operation of a 4.4 THz, GaAs-based quantum cascade laser is investigated in a 0–7.5 T magnetic field applied parallel to the growth axis. Pronounced oscillations are found in the threshold current and output power as the magnetic field is increased. At 4.3 T the threshold current density is drastically reduced to $160{\mathrm{A}/\mathrm{c}\mathrm{m}}^2$ from the $290{\mathrm{A}/\mathrm{c}\mathrm{m}}^2$ measured at 0 T. We attribute these results to a modulation of the lifetime of the upper state of the laser transition owing to the combined effects of inter-Landau level resonances and a progressive quenching of nonradiative relaxation channels due to the additional in-plane quantization. The latter is also responsible for a dramatic decrease in the conductivity, with a reduction in the number of carriers injected into the upper state of the laser transition at fixed voltage. This process limits the maximum output power and eventually prevents lasing action at high magnetic fields.