Intersubband Lasers and Detectors

The term “intersubband transitions” has been coined to describe optical transitions between confined electronic states in semiconductor quantum structures. In this lecture I will discuss the basic underlying physics of intersubband transitions in quantum wells and discuss their recent applications for real optoelectronic devices, as there are the quantum cascade laser (QCL) and the quantum well infrared photodetector (QWIP).

Concerning the physics, the basic concept of intersubband transitions appears extremely simple; yet a detailed understanding of all aspects, such as absorption strength and frequency, polarization dependence and linewidth, requires a careful consideration of band structure, many-body physics and electromagnetism.

The first observation of intersubband absorption was reported in 1974 in Si accumulation layers and 1985 in GaAs quantum wells. Since then, intense research has driven the development of high-sensitivity intersubband infrared detectors and focal-plane arrays, which are now commercially available. Intersubband lasers have been realized in 1994 and are now covering a spectral range from 4 to 17 m. I will discuss the physics and performance of these optoelectronic devices, also in comparison to conventional band-gap detectors and lasers.