Achieving a high-quality beam—one whose profile is near Gaussian and as narrow as diffraction allows—can be a challenge in semiconductor laser design. That’s especially true in the few-terahertz frequency regime, an elusive part of the electromagnetic spectrum just below the reach of most optical technologies and just above the reach of electronics.
No natural material has an open bandgap small enough to emit coherent photons below tens of terahertz. The quantum cascade laser (QCL), one of the few devices that can operate at a few terahertz, relies on stacked, two-dimensional layers of semiconductors, which create quantum wells that act as gain material. The spatial confinement of electrons in the wells splits the conduction band into discrete subbands, separated by energies as small as a few meV, depending on the semiconductor-layer thickness (1 THz corresponds to about 4 meV).
Transitions between the subbands are at the heart of the QCL (see...
