Abstract
Quantum cascade lasers (QCLs) are one of the best arenas for wavefunction engineering using ultrathin
layers. Such basic properties of QCLs as transition strength, the lifetime of each energy level, and the
coupling strength among energy levels are uniquely determined by wavefunction designs in quantum
cascade structures. QCLs with identical material systems, such as InGaAs/InAlAs/InP, GaAs/AlGaAs,
and InAs/AlSb, cover a broad wavelength: mid-infrared or the terahertz (THz) region. For practical
applications in the mid-infrared region, QC lasers based on anti-crossed dual-upper-state (DAU) designs
are a promising candidate because of its broad bandwidth and high device performances. Furthermore,
for room-temperature THz generation, THz QCL sources based on intra-cavity difference frequency
generation (DFG) in dual-wavelength mid-IR QCLs, so-called THz DFG-QCLs, have made dramatic
progress in recent years. We also report the performance of THz DFG-QCLs based on the DAU active
region.
