Inversionless terahertz lasing in a four-level Raman-type scheme within mid-infrared quantum cascade structures

This paper applies the density matrix formalism to investigate a feasibility of inversionless terahertz (THz) lasing within a mid-infrared (mid-IR) quantum cascade laser (QCL). The proposed structure aims to use a mid-IR QCL as an efficient pump source that integrated with a four-level Raman-type scheme to generate THz radiation through strong coherence excited in the system. A key aspect of the design is that the THz generation is achieved by interplay between inversion-based lasing and coherent effects due to quantum interference between the microscopic polarizations of the dipole-allowed transitions in the four-level system. THz intensity gain arising from both the inversion-based lasing and coherent effects is derived from the off-diagonal density matrix elements in the four-level system. The simulation results indicate that the proposed design demonstrates the potential to achieve THz lasing with good temperature characteristics and without the need for population inversion.