Intersubband polariton

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An intersubband transition is a dipole-allowed optical jump between quantized energy levels inside the conduction band of a semiconductor structure. When these transitions interact with an optical cavity, they mix with light to form hybrid light–matter states called intersubband cavity-polaritons. The energy levels show anticrossing with a gap called the vacuum-Rabi splitting, similar to level repulsion seen in atoms. A cascade of such transitions powers quantum cascade lasers, which emit coherent infrared light. In metals, most visible light is reflected because of free electrons, giving a silver-like appearance, while metals like gold and copper look reddish because intersubband absorption occurs at blue wavelengths.