Quantum cascade lasers

Quantum cascade lasers (QCLs) are semiconductor lasers that emit from the mid-infrared, at wavelengths around 3 μm, to terahertz waves in the far infrared, with wavelengths down to a few hundred micrometers. Their emission therefore covers two decades of the electromagnetic spectrum (in wavelength between 3 and 300 μm, or in frequency between 3 and 300 THz). A remarkable feature of these lasers is that this vast frequency range is basically covered by a single semiconductor material: AlInAs/GaInAs, grown on an indium phosphide (InP) substrate. Indeed, for this original laser concept, the emission wavelength is not linked to the semiconductor's band gap, but determined by the thickness and alternation of thin layers forming a quantum potential into which electrons are injected. What's more, these semiconductors are already widely used in telecommunications technology, which employs very similar alloys for laser diodes, detectors and other optoelectronic components. Quantum cascade lasers were therefore invented and produced without any real materials development. As a result, their performance has been able to progress rapidly thanks to conceptual improvements to the device, without having to wait for material refinement work. Finally, the nature of the materials making up the device is incidental, serving merely as a support for the implementation of the quantum concepts governing the operation of these lasers. Apart from the very important conceptual aspect, the fact that lasers can be realized over such a wide frequency range, always exploiting the same system of materials, greatly simplifies their manufacture. Once a manufacturing process has been perfected for one wavelength, it can be used identically for all other wavelengths.

There are two features unique to quantum cascade lasers that fundamentally distinguish them from other semiconductor lasers and, in general, from any conventional semiconductor-based light emitter. These are unipolarity (an electron-only device) and cascade (several photons are emitted by each electron passing through the structure). Unipolarity arises from the fact that the optical transitions of the quantum cascade laser occur between conduction-band electronic states (subbands). These transitions are commonly referred to as intersubband transitions, and result from the confinement of electrons in very thin semiconductor layers, known as quantum wells, and do not exist in bulk materials. In this sense, the quantum cascade laser is an intrinsically two-dimensional device. The other fundamental feature of QCLs is the multi-stage cascade scheme, in which electrons are recycled from one period to the next, each time contributing to photon gain and emission.

Quantum cascade lasers were first demonstrated in 1994 at the Bell Laboratories...