Eric Diebold's Quantum Mechanics Page
The Quantum Cascade Laser, or QCL, was invented in 1994 by scientists at Bell Labs in Murray Hill, New Jersey. Using a radically new laser concept, they created a semiconductor laser unlike any other that previously existed. Through the use of Molecular Beam Epitaxy (MBE), nanometer-thick layers of alternating semiconductors were deposited on a substrate, forming a superlattice potential structure. Similar to ordinary bulk semiconductor materials, superlattice structures have energy mini-bands in which electrons are allowed to occupy certain quantum energy states. Because of MBE, engineers are able to design superlattice structures with almost any energy mini-band scheme they desire. It is through electron transitions in the semiconductor conduction band between these superlattice mini-bands that the optical emission of photons occurs. When a population inversion between the correct mini-bands exists, lasing in the structure becomes possible.
The "cascade" effect is termed from repeated optical transitions of the same electron as it "cascades" down the laser structure. Passing through injectors and active transition regions repeatedly, a single electron can emit 25-75 photons as it travels through the structure, depending on the number of active regions (~25-75 per laser). This feature is a great advantage over the standard double heterostructure semiconductor laser, in which one electron combining with one hole emits a single photon.
There are many other details that describe the QCL's operation, but the primary benefits of this new laser are its large, continuous range of wavelength possibilities (4-24µm), high power (up to 300mW in pulsed mode - QCL's can carry large currents due to their large material band gap structure), and low cost. These great benefits make the QCL an ideal laser source in applications such as infrared spectroscopy (QCL's emit light in the "molecular fingerprint" region of the spectrum making it a convenient choice in gas trace detection), optical wireless communications, and medical diagnostics. One of the fundamentally new developments of the last decade, the great benefits of the Quantum Cascade Laser will surely propel it to become one of the most widely used semiconductor lasers in the world.
Here are links to the papers I am using as sources for my presentation on the operation of the Quantum Cascade Laser:
Physics Today article: "Quantum Cascade Lasers," May, 2002
This article describes, in somewhat popular form, the operation of the QCL. Starting from a somewhat historical perspective, Federico Capasso (the author of the article, and one of the primary inventors of the QCL) motivates the invention of the QCL by examining the shortcomings of the modern double heterostructure semiconductor laser, and proceeds to highlight the QCL's distinct comparative advantages. The author then discusses the energy band structure engineering required to develop a QCL. Through several diagrams, the quantum transitions of the active electrons in the laser are presented, and the author is able to pictorially describe the general process by which the QCL emits light. Several different designs of QCLs are included, such as the Multiwavelength QCL, and the QCL without injectors. Finally, Capasso describes some of the many relevant applications of the QCL, and gives data from several trace gas detection experiments recently completed by various other research groups.
Nature Article: "Ultra-broadband semiconductor laser," February 2002
This article discusses the operation of a broadband QCL. Through band-structure engineering, the authors were able to create a QCL that emits a continuum of wavelengths in the 5-8µm range. Because mini-bands within the superlattice active region structures can be designed at arbitrary energies, multiple active regions can be combined in a Quantum Cascade structure, with each active region designed to emit a different wavelength of light. To achieve a threshold current density in the laser that is independent of output wavelength, a procedure must be taken to correctly order the stack of active regions. This ordering of the stack is important, such that each wavelength's active region may be traversed by the correct wavelength mode, allowing stimulated emission and lasing to occur. The authors derive a relation in order to create a flat wavelength response at the current threshold, and provide a suitable stacking scheme such that broadband lasing is possible. The authors also give examples of several applications for a broadband laser, such as custom-tailored light sources for spectroscopy or microscopy of specimens with complex optical responses, a wavelength-tunable laser source, or a source of ultra-short laser pulses, via mode-locking.