Population inversion by resonant magnetic confinement in terahertz quantum-cascade lasers

Ultralow-threshold terahertz laser emission exploiting in-plane confinement arising from perpendicular magnetic field applied on a quantum-cascade structure is reported. A special design strategy has been adopted that takes advantage of the selective opening and closing of relaxation channels by elastic scattering between Landau levels. The key effect is a reduction of the lower state lifetime of the lasing transition that produces population inversion. The structure shows laser action only with applied magnetic field and yields threshold current densities as low as $\text{19\hspace{0.17em}}{\mathrm{A/cm}}^2$ at 4.2 K and $\text{32\hspace{0.17em}}{\mathrm{A/cm}}^2$ at 60 K at a frequency of 3.6 THz.