QCL: Quantum Cascade Laser, a semiconductor laser lacking a junction in which an electron passes through a series of quantum wells. In each quantum well, the electron emits a photon on an inter-subband transition before tunneling through to the next quantum well. QCLs are important sources in the mid- and far-infrared, including the terahertz band.
Semiconductor and diode lasers were long considered synonymous after demonstration of the first semiconductor diode laser in 1962, although other types had been proposed and lasing had been demonstrated in semiconductors without junctions that were pumped optically or with electron beams.
Russian physicists Rudolf Kazarinov and R. A. Suris took the first step toward the QCL in 1971 by suggesting electrons in a superlattice could tunnel between adjacent quantum wells, but the technology needed to make them was not yet available. The development of molecular beam epitaxy (MBE) revived interest in such complex semiconductor structures, and in 1986 Federico Capasso, Khalid Mohammed, and Alfred Cho of Bell Labs suggested that electrons tunneling through stacks of quantum wells might be used to make infrared lasers.
Their 1986 paper clearly shows the basic idea, but demonstrating QCLs took eight years, as long as it took to go from the first pulsed cryogenic diode laser to room-temperature operation. Not until 1994 did Jerome Faist, Capasso, Cho, and Deborah Sivco report the first QCL in a Science paper where they coined the evocative phrase "quantum cascade" to describe its operation; a Google Book search fails to find any earlier use of the phrase. Their device produced 8 milliwatt (mW) pulses at 4.2 micrometers (µm), but like the first diode lasers it required cryogenic cooling, with highest power at 10 degrees Kelvin (K), and operation at up to 90 K with a threshold of 14 kiloamperes (kA) per square centimeter (cm2), comparable to the threshold of the first diode lasers.
Today, QCLs are in the mainstream of laser technology, operating continuous-wave at room temperature with multiwatt output in the mid-infrared. Available commercially, QCLs operate through much of the infrared all the way to the terahertz band.