Terahertz Lasers Based on Intersubband Transitions

Semiconductor quantum wells are human-made quantum mechanical systems in which the energy levels can be designed and engineered to be of any value. Consequently, unipolar lasers based on intersubband transitions (electrons that make lasing transitions between subband levels within the conduction band) were proposed for long-wavelength sources as early as the 1970s. However, because of the great challenge in epitaxial material growth and the unfavorable fast nonradiative relaxation rate, unipolar intersubband-transition lasers (also called quantum-cascade lasers) at midinfrared wavelengths were developed only recently at Bell Laboratories. This achievement is remarkable, but the technique used in the original quantum-cascade lasers will not be directly applicable for the longerwavelength THz range because of two major obstacles. First, the energy levels corresponding to THz frequencies (1 THz = 4 meV) are quite narrow, so the requirements for the design and fabrication of suitable quantum wells are demanding. Because of the narrow separation between subband levels, heating and electronelectron scattering will have a much greater effect. Also, the small energy scales of THz photons make the detection and analysis of spontaneous emission (a crucial step toward developing lasers) quite difficult. Second, mode confinement, which is essential for any laser oscillation, is difficult at longer wavelengths.