QCL - ACRONYM

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.

Electron emits a cascade of photons as it tunnels through a series of quantum wells in this simplified view of a QCL.

PMT - ACRONYM

PMT - Photomultiplier Tube, a vacuum tube light sensor in which input photons cause a cathode to emit electrons that are amplified through a chain of electron amplifiers called a multiplier. Invented in 1934, PMTs are still in use. They offer high gain, low noise, reasonably fast response, and a large collecting area, all important for detecting faint signals.

First observed in 1887 by Heinrich Hertz, the light-induced emission of electrons became an important puzzle because classical physics could not explain why electron emission occurred only for wavelengths shorter than a threshold value rather than depending on light intensity. Albert Einstein won the Nobel prize in 1921 for showing that the photoelectric effect was due to photons needing to have a threshold energy to free electrons from atoms.

The first photoemissive detectors were vacuum photodiodes, in which light illuminated a metal cathode, freeing electrons collected by an anode when a voltage was applied across the tube. Alkali metal cathodes were used to detect visible light. In the days before semiconductor electronics, these devices were called photodiodes or photocells. They were too insensitive for use in early electronic television cameras, so engineers added amplification stages, called dynodes, inside the tube; electrons collided with the dynodes, producing additional or secondary electrons. A series of acceleration stages and dynodes multiplied the photocurrent, thus earning the name photomultiplier. Developed in the 1930s, PMTs became the detectors of choice for applications demanding high sensitivity, low noise, and high speed.

This high performance has made the PMT a remarkably durable technology, one of the last vacuum tubes that is still a standard product in the age of solid-state photonics. Continuing refinements in design and packaging have adapted PMTs for modern applications such as single-photon counting. PMTs continue to new challenges, such as silicon photomultipliers, containing a hundred to several thousand tiny avalanche photodiodes (APDs) connected in parallel for single-photon detection, also called multichannel APDs. But PMTs just keep plugging along.

Modern metal-channel dynode PMT, shown in cutaway. (Image courtesy of Hamamatsu)