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.

VCSEL - ACRONYM

VCSEL - Vertical Cavity Surface-Emitting Laser, a type of semiconductor laser in which the resonant cavity is perpendicular to the junction layer and light is emitted from the surface of the chip.

All early diode lasers oscillated in the plane of the junction or active layer and emitted from the edge of the semiconductor chip. This design is logical because keeps laser oscillation in the plane of the active layer where recombination of current carriers produces a population inversion, so the round-trip gain in the cavity is high. However, because the active layer is very thin, the beams from edge-emitting diode lasers diverge rapidly, particularly in the direction perpendicular to the active layer.

VCSELs oscillate vertically, in a cavity formed by reflective layers on the top and bottom of the chip. Single-pass gain much lower than in edge emitters because only a very thin layer of gain material is between the cavity mirrors but the emitting aperture typically is much wider than the active layer is thick, producing a higher-quality, circular beam. First demonstrated in 1979, VCSELs went through a series of structural refinements to improve their performance and fabrication processes. In current designs, one or often both of the reflectors are multilayer Bragg reflectors containing many the tens of pairs of layers needed to produce the very high reflectivity needed to sustain oscillation with only a thin gain medium.

The short length of VCSEL cavities brings some advantages, including allowing direct current modulation at speeds to 40 gigabits per second (Gbit/s) and without the mode hopping possible in edge emitters. Yet ironically, a commercial attraction of the more complex VCSEL design is that it makes them more economical. All the hard parts of VCSEL production are done by highly automated semiconductor manufacturing techniques. The resulting VCSELs can be tested on the wafer, unlike edge emitters, which can't be tested until the wafer is diced into chips. That combined with their larger emitting area greatly reduces packaging expenses, which account for more of finished product costs than the laser chips. So more complex winds up being cheaper, as well as better for many applications.

And VCSEL types and applications keep growing. Recently, a VCSEL-type cavity was used in optically pumped colloidal quantum dot lasers emitting red, green and blue light.

Complexity in a VCSEL:  Beam Express (Lausanne, Switzerland) makes 1310 nm VCSELs by bonding AlGaAs/GaAs distributed Bragg reflectors on top and bottom of an InAlGaAs/InP gain layer containing strained quantum wells and a tunnel junction because high-contrast Bragg reflectors are not practical in InP-based materials.

EUV/XUV - ACRONYM

EUV or XUV:  Extreme Ultraviolet, the short-wavelength or high-energy end of the ultraviolet spectrum, from 120 (or 200) nanometers to about 10 nanometers (nm).

Atmospheric transmission in the ultraviolet decreases sharply with wavelength, and air absorption is so strong that wavelengths shorter than 150 to 200 nm must be studied in a vacuum. The first explorers of the EUV spectrum were astronomers using satellite instruments. The quest to squeeze more and smaller transistors onto semiconductor chips has changed that by shrinking chip features to dimensions on the scale of EUV waves. The semiconductor industry is now testing the first wave of EUV photolithography systems operating at 13.5 nm.

Such a sudden technological interest in a long-neglected part of the spectrum is a great recipe for muddied definitions of spectral bands. Astronomers consider the 121 nm Lyman alpha line of hydrogen to be the major landmark in the EUV spectrum, so they picked 120 nm as the long-wave end of the EUV band. However, the major technological landmark for the semiconductor industry is the 13.5 nm lithography wavelength, so they often define EUV as having a wavelength of 13.5 nm. The laser community uses a broader definition of 10 to 120 or 200 nm as it explores a broader range of applications, enabled by new techniques such as high-harmonic generation.

The EUV largely overlaps the older vacuum-ultraviolet (VUV) band, usually defined as 10 to 200 nm. A draft document from the International Standardization Organization (developed for space observations) also defines two other overlapping bands, the far-ultraviolet (FUV) at 122 to 200 nm and the germicidal Ultraviolet C (UVC) band at 100 to 280 nm.

XUV often is an alternative abbreviation for extreme ultraviolet, substituting the fashionable X for the relatively drab E, but the ISO lists it as an abbreviation for soft X rays at 0.1 to 10 nm. A quick Google search gives the impression XUV is the more popular form, with 5.6 million hits compared to 3.2 million for EUV and 1.8 million for VUV. But that's misleading because XUV also is shorthand for "crossover utility vehicle," a sport-utility vehicle based on a car rather than a truck. That usage may be popular on the Internet, but it was new to me--and for years I've been driving a Toyota RAV4, which is classed as an XUV.

APD - ACRONYM

An avalanche photodiode (APD) is a semiconductor photodetector in which incident light generates a photocurrent, which is then multiplied by an avalanche process to give a stronger signal.

An avalanche photodiode is a single device that incorporates two distinct semiconductor stages. The first is a photodiode detector, in which light with energy above the bandgap of a semiconductor delivers enough energy to valence electrons for them to enter the conduction band. The electron leaves behind a hole in the valence band, which also functions as a current carrier. Application of a voltage across the device pulls the electrons and holes in opposite directions.

The second stage applies a strong reverse bias across the semiconductor to accelerate electrons. When electrons reach high enough velocities, they can ionize other atoms in the semiconductor, producing an avalanche of electrons. This multiples the original photocurrent and produces a much stronger response than a simple photodiode. Typically, silicon APDs are biased with about 100 V to multiply photocurrent by around a factor of 100. Further increasing the reverse voltage increases the dark current as it approaches the ionization threshold of the semiconductor. Breakdown occurs at a reverse bias of about 150 to 200 V for silicon, depending on device design, and at different voltages in other semiconductors.

You can think of APDs as solid-state counterparts of photomultiplier tubes (PMTs), but they are far from plug-in replacements. As you expect from solid-state devices, APDs are much smaller and their bias voltages are much lower--about a tenth of those in PMTs. However, PMTs are less subject to noise, their gain does not depend as strongly on bias voltage, and they can be engineered to respond to different wavelengths than APDs, so they are among the few survivors of the vacuum-tube era.

New designs and new materials are extending the range of APDs for applications ranging from single-element fiber-optic detectors to focal-plane arrays for imaging. Germanium/silicon ADPs have reached a gain-bandwidth product of 105 GHz, attractive for high-speed optical interconnects. APD elements designed for single-photon counting can be assembled in arrays to make a single-photon counting camera.

Gain or multiplication factor of an APD increases sharply as voltage approaches breakdown, but so does dark current, limiting usable gain. (From Jeff Hecht, Understanding Fiber Optics: 5th edition [Pearson Prentice-Hall, 2006])