Quantum cascade laser
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The quantum cascade laser or QC laser is a unipolar solid-state laser. Unlike conventional solid-state lasers, the optical transitions occur between electric subbands rather than between the conduction band and valence bands. The "cascade" is a series of equal energy steps built into the material matrix while the crystal is being grown. When the electrons are transmitted through the laser crystal, they emit one photon at each of these cascade steps, unlike diode lasers which only emit one photon per electron transmitted.
The laser was invented and demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, Claire Gmachl, and Alfred Cho at Bell Laboratories in 1994.
Fabrication
It consists of alternating layers of two different semiconductors like for example GaAs/AlGaAs or AlInAs/InGaAs, forming a quantum heterostructure. The layers are grown on a substrate using a process called molecular beam epitaxy. The emitted wavelength of the laser is determined primarily by the thickness of the crystal layers and the layer materials itself. This is a great advantage over diode lasers, whose wavelengths depend primarily on the band gap of the given material and is therefore restricted.Often a structure called distributed feedback (DFB) is built on top of the laser crystal to prevent it from emitting at other than the desired wavelength.
Intersubband Transitions
Within a bulk semiconductor crystal, electrons may occupy states in one of two continuous energy bands - the valence band, which is heavily populated with low energy electrons and the conduction band, which is sparsely populated with high energy electrons. The two energy bands are separated by an energy bandgap in which there are no permitted states available for electrons to occupy.Conventional semiconductor lasers rely on a single photon of light being emitted when a high energy electron from the conduction band "falls" into an unoccupied hole in the valence band. The wavelength of light emitted is therefore strongly dependent upon the energy bandgap of the laser material.
The quantum cascade laser however does not use bulk semiconductor materials in its optically active region. Instead, it consists of a periodic series of thin layers of varying material composition. The varying composition introduces a varying electric potential across the length of the device, meaning that there is a varying probability of electrons occupying different positions over the length of the device. This is referred to as one-dimensional multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of electric subbands. For any given energy, an electron may occupy a number of different subbands, each with a different momentum.
By careful selection of material composition and thickness of each layer in the device, and the applied external electric field, an electric subband minimum in a given period of the device may be aligned with a higher energy subband minimum in the adjacent period. Therefore, an electron may take part in an optical transition between electric subbands in a given period before tunneling into the next period of the structure and performing another optical transition. This process may occur dozens of times for each electron moving through the device, giving high optical power output.
Advantages
The laser's high optical power output, tuning range and room temperature operation make it useful for spectroscopic applications like the remote sensing of environmental gases and pollutants in the atmosphere. It may eventually be used for vehicular cruise control in conditions of poor visibility, collision avoidance radar, industrial process control, and medical diagnostics such as breath analyzers. The independence of laser operation from the conduction and valence band edge characteristics allows much greater flexibility of emission wavelengths from conventional semiconductor materials such as the GaAs/AlGaAs material system. Furthermore, laser operation from indirect bandgap materials such as the Si/SiGe material system is made possible.References
- [Bell Labs summary]
- * [Bell Labs technical information]
- [PhysicsWeb: Quantum Cascade Lasers]
- [Walter Schottky Institute explanation, with diagrams]
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