Quantum cascade lasers: The semiconductor solution for lasers in the mid‐ and far‐infrared spectral regions

The quantum cascade laser is a semiconductor light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these devices the principles of operation are not re lated to the physical properties of the constituent materials, but arise from an artificial potential designed using a sequence of very thin layers of different semiconductor materials. The quantum design implemented by highly sophisticated epitaxial growth, allows one to ascribe in semiconductor crystals this artificial potential with the desired electronic energy levels and wavefunctions. In recent years the performance of these devices has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid‐far infrared lasers in a very wide spectral range (3.5–160 µm). At present, the best performances are obtained at wavelengths between 5–12 µm, where continuous‐wave room temperature operation is routinely achieved and record average power in the order of 1 W has been demonstrated. The long wavelength region has been only recently explored and lasers operating temperatures do not currently exceed 150 K. Interesting nonlinear mixing of diode lasers with a THz laser has been recently demonstrated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)