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A quantum well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. Laser diodes are formed in compound semiconductor materials that (quite unlike silicon) are able to emit light efficiently. The wavelength of the light emitted by a quantum well laser is determined by the width of the active region rather than just the bandgap of the material from which it is constructed.This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function.
In 1972, Charles H. Henry, a physicist and newly appointed Head of the Semiconductor Electronics Research Department at Bell Laboratories, had a keen interest in the subject of integrated optics, the fabrication of optical circuits in which the light travels in waveguides.
Later that year while pondering the physics of waveguides, Henry had a profound insight. He realized that a double heterostructure is not only a waveguide for light waves, but simultaneously for electron waves. Henry was drawing upon the principles of quantum mechanics, according to which electrons behave both as particles and as waves. He perceived a complete analogy between the confinement of light by a waveguide and the confinement of electrons by the potential well that is formed from the difference in bandgaps in a double heterostructure.
C.H. Henry realized that, just as there are discrete modes in which light travels within a waveguide, there should be discrete electron wavefunction modes in the potential well, each having a unique energy level. His estimate showed that if the active layer of the heterostructure is as thin as several tens of nanometers, the electron energy levels would be split apart by tens of milli-electron volts. This amount of energy level splitting is observable. The structure Henry analyzed is today called a "quantum well."
Henry proceeded to calculate how this "quantization" (i.e., the existence of discrete electron wavefunctions and discrete electron energy levels) would alter the optical absorption properties (the absorption "edge") of these semiconductors. He realized that, instead of the optical absorption increasing smoothly as it does in ordinary semiconductors, the absorption of a thin heterostructure (when plotted versus photon energy) would appear as a series of steps.
In addition to Henry's contributions, the quantum well (which is a type of double-heterostructure laser) was actually first proposed in 1963 by Herbert Kroemer in Proceedings of the IEEEand simultaneously (in 1963) in the U.S.S.R by Zh. I. Alferov and R.F. Kazarinov. Alferov and Kroemer shared a Nobel Prize in 2000 for their work in semiconductor heterostructures.
In early 1973, Henry proposed to R. Dingle, a physicist in his department, that he look for these predicted steps. The very thin heterostructures were made by W. Wiegmann using molecular beam epitaxy. The dramatic effect of the steps was observed in the ensuing experiment, published in 1974.
After this experiment showed the reality of the predicted quantum well energy levels, Henry tried to think of an application. He realized that the quantum well structure would alter the density of states of the semiconductor, and result in an improved semiconductor laser requiring fewer electrons and electron holes to reach laser threshold. Also, he realized that the laser wavelength could be changed merely by changing the thickness of the thin quantum well layers, whereas in the conventional laser a change in wavelength requires a change in layer composition. Such a laser, he reasoned, would have superior performance characteristics compared to the standard double heterostructure lasers being made at that time.
Dingle and Henry received a patent on this new type of semiconductor laser comprising a pair of wide bandgap layers having an active region sandwiched between them, in which "the active layers are thin enough (e.g., about 1 to 50 nanometres) to separate the quantum levels of electrons confined therein. These lasers exhibit wavelength tunability by changing the thickness of the active layers. Also described is the possibility of threshold reductions resulting from modification of the density of electron states." The patent was issued on September 21, 1976, entitled "Quantum Effects in Heterostructure Lasers," U.S. Patent No. 3,982,207.
Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructure lasers. A well-designed quantum well laser can have an exceedingly low threshold current.
Moreover, since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by the electrons and holes, very high quantum efficiencies can be achieved with the quantum well laser.
To compensate for the reduction in active layer thickness, a small number of identical quantum wells are often used. This is called a multi-quantum well laser.
While the term "quantum well laser" was coined in the late 1970s by Nick Holonyak and his students at the University of Illinois at Urbana Champaign, the first observation of quantum well laser operation was madein 1975 at Bell Laboratories. The first electrically pumped "injection" quantum well laser was observed by P. Daniel Dapkus and Russell D. Dupuis of Rockwell International, in collaboration with the University of Illinois at Urbana Champaign (Holonyak) group in 1977. Dapkus and Dupuis had, by then, pioneered the metalorganic vapour phase epitaxy MOVPE (also known as OMCVD, OMVPE, and MOCVD) technique for fabricating semiconductor layers. The MOVPE technique, at the time, provided superior radiative efficiency as compared to the molecular beam epitaxy (MBE) used by Bell Labs. Later, however, Won T. Tsang at Bell Laboratories succeeded in using MBE techniques in the late 1970s and early 1980s to demonstrate dramatic improvements in performance of quantum well lasers. Tsang showed that, when quantum wells are optimized, they have exceedingly low threshold current and very high efficiency in converting current-in to light-out, making them ideal for widespread use.
The original 1975 demonstration of optically pumped quantum well lasers had threshold power density of 35 kW/cm2. Ultimately, it was found that the lowest practical threshold current density in any quantum well laser is 40 Amperes/cm2, a reduction of approximately 1,000x. [ full citation needed ]
Extensive work has been performed on quantum well lasers based on gallium arsenide and indium phosphide wafers. Today, however, lasers utilizing quantum wells and the discrete electron modes researched by C.H. Henry during the early 1970s, fabricated by both MOVPE and MBE techniques, are produced at a variety of wavelengths from the ultraviolet to the THz regime. The shortest wavelength lasers rely on gallium nitride-based materials. The longest wavelength lasers rely on the quantum cascade laser design.
The story of the origin of the quantum well concept, its experimental verification, and the invention of the quantum well laser is told by Henry in more detail in the foreword to "Quantum Well Lasers," ed. by Peter S. Zory, Jr.
Quantum well lasers are important because they are the basic active element (laser light source) of the Internet fiber optic communication. Early work on these lasers focused on GaAs gallium arsenide based wells bounded by Al-GaAs walls, but wavelengths transmitted by optical fibers are best achieved with indium phosphide walls with indium gallium arsenide phosphide based wells. The central practical issue of light sources buried in cables is their lifetimes to burn-out. The average burn-out time of early quantum well lasers was less than one second, so that many early scientific successes were achieved using rare lasers with burn-out times of days or weeks. Commercial success was achieved by Lucent (a spin-off from Bell Laboratories) in the early 1990s with quality control of quantum well laser production by MOVPE Metalorganic vapour phase epitaxy, as done using high-resolution X rays by Joanna (Joka) Maria Vandenberg. Her quality control produced Internet lasers with median burn-out times longer than 25 years.
Aluminium gallium arsenide (AlxGa1−xAs) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. The x in the formula above is a number between 0 and 1 - this indicates an arbitrary alloy between GaAs and AlAs.
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move in the solid; however, if some electrons transfer from the valence to the conduction band, then current can flow. Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap.
A laser diode, (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction. Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the development of nanotechnologies. MBE is used to fabricate diodes and MOSFETs at microwave frequencies, and to manufacture the lasers used to read optical discs.
Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. For example, GaN is the substrate which makes violet (405 nm) laser diodes possible, without requiring nonlinear optical frequency-doubling.
A quantum well is a potential well with only discrete energy values.
Indium gallium arsenide (InGaAs) is a ternary alloy of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are elements of the periodic table while arsenic is a element. Alloys made of these chemical groups are referred to as "III-V" compounds. InGaAs has properties intermediate between those of GaAs and InAs. InGaAs is a room-temperature semiconductor with applications in electronics and photonics.
Quantum cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.
Indium gallium nitride is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN). It is a ternary group III/group V direct bandgap semiconductor. Its bandgap can be tuned by varying the amount of indium in the alloy. InxGa1−xN has a direct bandgap span from the infrared for InN to the ultraviolet of GaN. The ratio of In/Ga is usually between 0.02/0.98 and 0.3/0.7.
Aluminium gallium indium phosphide is a semiconductor material that provides a platform for the development of novel multi-junction photovoltaics and optoelectronic devices, as it spans a direct bandgap from deep ultraviolet to infrared.
Aluminium gallium nitride (AlGaN) is a semiconductor material. It is any alloy of aluminium nitride and gallium nitride.
Catastrophic optical damage (COD), or catastrophic optical mirror damage (COMD), is a failure mode of high-power semiconductor lasers. It occurs when the semiconductor junction is overloaded by exceeding its power density and absorbs too much of the produced light energy, leading to melting and recrystallization of the semiconductor material at the facets of the laser. This is often colloquially referred to as "blowing the diode." The affected area contains a large number of lattice defects, negatively affecting its performance. If the affected area is sufficiently large, it can be observable under optical microscope as darkening of the laser facet, and/or as presence of cracks and grooves. The damage can occur within a single laser pulse, in less than a millisecond. The time to COD is inversely proportional to the power density.
Dmitri Z. Garbuzov was one of the pioneers and inventors of room temperature continuous-wave-operating diode lasers and high-power diode lasers.
Charles H. Henry was an American physicist. He was born in Chicago, Illinois. He received an M.S. degree in physics in 1959 from the University of Chicago, and a Ph.D. degree in physics in 1965 from the University of Illinois, under the direction of Charlie Slichter. In March 2008, he was featured in an article in the Physics Illinois News, a publication of the Physics Department of the University of Illinois.
The Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) is a research institute, which is a member of the Gottfried Wilhelm Leibniz Scientific Community. The institute is located in Berlin at the Wissenschafts- und Wirtschaftsstandort Adlershof (WISTA), its research activity is applied science in the fields of III-V electronics, photonics, integrated quantum technology and III-V technology
Interband cascade lasers (ICLs) are a type of laser diode that can produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum. They are fabricated from epitaxially-grown semiconductor heterostructures composed of layers of indium arsenide (InAs), gallium antimonide (GaSb), aluminum antimonide (AlSb), and related alloys. These lasers are similar to quantum cascade lasers (QCLs) in several ways. Like QCLs, ICLs employ the concept of bandstructure engineering to achieve an optimized laser design and reuse injected electrons to emit multiple photons. However, in ICLs, photons are generated with interband transitions, rather than the intersubband transitions used in QCLs. Consequently, the rate at which the carriers injected into the upper laser subband thermally relax to the lower subband is determined by interband Auger, radiative, and Shockley-Read carrier recombination. These processes typically occur on a much slower time scale than the longitudinal optical phonon interactions that mediates the intersubband relaxation of injected electrons in mid-IR QCLs. The use of interband transitions allows laser action in ICLs to be achieved at lower electrical input powers than is possible with QCLs.
Morton B. Panish is an American physical chemist who, with Izuo Hayashi, developed a room-temperature continuous wave semiconductor laser in 1970. For this achievement he shared the Kyoto Prize in Advanced Technology in 2001.
James J. Coleman is an electrical engineer who worked at Bell Labs, Rockwell International, and the University of Illinois, Urbana. He is best known for his work on semiconductor lasers, materials and devices including strained-layer indium gallium arsenide lasers and selective area epitaxy. Coleman is a Fellow of the IEEE and a member of the US National Academy of Engineering.