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.
These transitions exhibit an anticrossing in energy with a separation known as vacuum-Rabi splitting, similar to level repulsion in atomic physics.
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.