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(Acronym: QCL)
Definition: semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region
The quantum cascade laser is a special kind of semiconductor laser, usually emitting mid-infrared light. Such a laser is operating on laser transitions not between different electronic bands but on intersubband transitions of a semiconductor structure. Figure 1 shows what happens to an electron injected into the gain region: in each period of the structure, it undergoes a first transition (blue arrow) between two sublevels of a quantum well (which is the laser transition on which stimulated emission occurs), then a non-radiative transition (red arrow) to the lowest sublevel, before tunneling (green arrow) into the upper level of the next quantum well. By using a multitude of quantum wells in a series (i.e., a cascade), a higher optical gain and multiple photons per electron are obtained at the expense of a higher required electrical voltage.
Figure 1: Principle of the gain region of a quantum cascade laser. The diagram shows the electron energy versus position in the structure, which contains three quantum wells. The overall downward trend of energy towards the right-hand side is caused by an applied electric field.
As the transition energies are defined not by fixed material properties but rather by design parameters (particularly by layer thickness values of quantum wells), quantum cascade lasers can be designed for operation wavelengths ranging from a few microns to well above 10 μm, or even in the terahertz region. High efficiencies are possible by using a cascade of laser transitions, where a single electron can generate dozens of mid-infrared photons.
In a quantum cascade laser, the mentioned quantum well structure is embedded in a waveguide, and the laser resonator is mostly of DBR or DFB type.
While continuously operating room-temperature devices are normally limited to moderate output power levels in the lower milliwatt region (although hundreds of milliwatts can be generated in exceptional cases), hundreds of milliwatts are easily possible with liquid-nitrogen cooling. Even at room temperature, watt-level peak powers are possible when using short pump pulses.
Most quantum cascade lasers emit mid-infrared light. However, quantum cascade lasers can also be made for generating terahertz waves. Such devices constitute very compact and simple sources of terahertz radiation.
Perhaps the most important applications for quantum cascade lasers will be in the area of spectroscopy of trace gases, e.g. to detect very small concentrations of pollutants in air. Besides the suitable wavelength range, QCLs usually feature a relatively narrow linewidth and good wavelength tunability, making them very suitable for such applications.
Bibliography
| [1] | R. F. Kazarinov et al., "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice", Fizika i Tekhnika Poluprovodnikov 5 (4), 797 (1971) |
| [2] | N. Uehara and K. I. Ueda, "193-mHz beat linewidth of frequency-stabilized laser-diode-pumped Nd:YAG ring lasers", Opt. Lett. 18 (7), 505 (1993) |
| [3] | J. Faist et al., "Quantum cascade lasers", Science 264, 553 (1994) |
| [4] | R. M. Williams et al., "Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers", Opt. Lett. 24 (24), 1844 (1999) |
| [5] | M. Beck et al., "Continuous wave operation of a mid-infrared semiconductor laser at room temperature", Science 295, 301 (2002) |
| [6] | R. Köhler et al., "Terahertz semiconductor-heterostructure laser", Nature 417, 156 (2002) |
| [7] | B. S. Williams et al., "Operation of terahertz quantum-cascade lasers at 164 K in pulsed mode and at 117 K in continuous-wave mode", Opt. Express 13 (9), 3331 (2005) |
| [8] | B. S. Williams, "Terahertz quantum-cascade lasers", Nat. Photonics 1, 517 (2007) |
| [9] | A. Kosterev et al., "Application of quantum cascade lasers to trace gas analysis", Appl. Phys. B. 90, 165 (2008) |
See also: semiconductor lasers
