Quantum-cascade laser
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 Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.
The energy of the photon and hence the emission wavelength of laser diodes is therefore determined by the band gap of the material system used.
Instead, it consists of a periodic series of thin layers of varying material composition forming a superlattice.
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 discrete electronic subbands.
By suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system which is required in order to achieve laser emission.
Additionally, in semiconductor laser diodes, electrons and holes are annihilated after recombining across the band gap and can play no further part in photon generation.
This process of a single electron causing the emission of multiple photons as it traverses through the QCL structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than semiconductor laser diodes.
Assuming that no other subbands are populated, the rate equations for the three level lasers are given by: In the steady state, the time derivatives are equal to zero and
The general rate equation for electrons in subband i of an N level system is therefore: Under the assumption that absorption processes can be ignored, (i.e.
The scattering rates are tailored by suitable design of the layer thicknesses in the superlattice which determine the electron wave functions of the subbands.
The figure shows the wave functions in a three quantum well (3QW) QCL active region and injector.
A vertical transition is one in which the upper laser level is localised in mainly the central and right-hand wells.
[4] In 1998 GaAs/AlGaAs QCLs were demonstrated by Sirtori et al. proving that the QC concept is not restricted to one material system.
[7] InAs/AlSb QCLs have quantum wells 2.1 eV deep and electroluminescence at wavelengths as short as 2.5 μm has been observed.
[8] The couple InAs/AlSb is the most recent QCL material family compared to alloys grown on InP and GaAs substrates.
The main advantage of the InAs/AlSb material system is the small effective electron mass in quantum wells, which favors a high intersubband gain.
[9] This benefit can be better exploited in long-wavelength QCLs where the lasing transition levels are close to the bottom of the conduction band, and the effect of nonparabolicity is weak.
InAs-based QCLs have demonstrated room temperature (RT) continuous wave (CW) operation at wavelengths up to
[14] QCLs may also allow laser operation in materials traditionally considered to have poor optical emission properties.
Indirect bandgap materials such as silicon have minimum electron and hole energies at different momentum values.
[15] Intersubband electroluminescence from non-polar SiGe heterostructures has been observed for mid-infrared and far-infrared wavelengths, first in the valence band.
[16][17][18] Higher gain can be achieved by using strain to push parasitic light-hole states above the heavy-hole to heavy hold transitions.
Light is emitted from the cleaved ends of the waveguide, with an active area that is typically only a few micrometers in dimension.
Buried heterostructure waveguides are efficient at removing heat from the QC active area when light is being produced.
Although the quantum cascade gain medium can be used to produce incoherent light in a superluminescent configuration,[23] it is most commonly used in combination with an optical cavity to form a laser.
Fabry–Pérot quantum cascade lasers are capable of producing high powers,[24] but are typically multi-mode at higher operating currents.
If a frequency-selective element is included in the external cavity, it is possible to reduce the laser emission to a single wavelength, and even tune the radiation.
There exists several methods to extend the tuning range of quantum cascade lasers using only monolithically integrated elements.
[32] The high optical power output, tuning range and room temperature operation make QCLs useful for spectroscopic applications such as remote sensing of environmental gases and pollutants in the atmosphere[33] and security.
[35] When used in multiple-laser systems, intrapulse QCL spectroscopy offers broadband spectral coverage that can potentially be used to identify and quantify complex heavy molecules such as those in toxic chemicals, explosives, and drugs.
