Intersubband transitions (also known as intraband transitions) are dipolar allowed optical excitations between the quantized electronic energy levels within the conduction band of semiconductor heterostructures. Intersubband transitions when coupled with an optical resonator form new, mixed-state photons. This mixing is referred to as an intersubband cavity-polariton. These transitions exhibit an anticrossing in energy with a separation known as vacuum-Rabi splitting, similar to level repulsion in atomic physics.

A cascading of intersubband transitions is the mechanism behind a quantum cascade laser which produces a monochromatic coherent light-source at infrared wavelengths.

Most metals reflect almost all visible light, due to the presence of free charges, and are therefore silvery in color or mirror-like. However, some metals like gold and copper are more reddish, and this is due to absorption from intersubband transitions that occur at blue wavelengths.

• Fluorescence (interband transitions)

• Fox, Mark (2010). Optical Properties of Solids (2 ed.). Oxford University Press. p. 166-167. ISBN 978-0199573370.
• Hofmann, Philip (26 May 2015). Solid State Physics (2 ed.). Wiley-VCH. p. 84-85. ISBN 978-3527412822. Archived from the original on 18 March 2020. Retrieved 21 December 2019.