Intersubbandübergänge

Intersubband transitions refer to electronic excitations that occur between quantized energy states, or subbands, in semiconductor nanostructures such as quantum wells, wires, or dots. When a charge carrier, typically an electron, moves from one subband to another, energy is absorbed or emitted in the range of a few millielectronvolts to a few hundred millielectronvolts, corresponding to infrared or terahertz wavelengths. The energy separation between subbands depends on the width of the quantum well, the material bandgap, and the effective mass of the carriers. These transitions are only allowed for light polarized perpendicular to the growth direction of the structure because of the selection rules derived from the symmetry of the envelope functions.

The phenomenon is exploited in a range of optoelectronic devices. Quantum cascade lasers use a cascade of intersubband transitions to produce coherent light in the mid‑infrared and terahertz regimes. Intersubband absorption provides the working principle for photodetectors such as quantum well infrared photodetectors and quantum cascade detectors. Devices engineered for terahertz emission also rely on phonon‑assisted or electric‑dipole intersubband transitions.

The design of intersubband devices requires precise control over layer thickness, doping concentrations, and interface quality. Modern growth techniques like molecular beam epitaxy and metal–organic chemical vapor deposition enable the fabrication of structures with sub‑nanometer precision, reducing linewidths and enhancing transition dipole moments. Recent research explores hybrid structures combining two‑dimensional materials with conventional quantum wells, aiming to tailor intersubband spectra for sensing, imaging, and communication applications.