There are various kinds of optical processes where the resulting radiation has a decreased photon energy (or an increased wavelength) compared with the incoming radiation. Such a loss of photon energy is often called Stokes shift, referring to the Irish physicist George G. Stokes. Stokes shifts occur e.g. in the following cases:
- When some material is excited to emit fluorescence, the average photon energy of the fluorescent light is usually smaller than that of the incoming light, and the energy difference results in microscopic vibrations (heating). However, this is not always the case, see the article on optical refrigeration. The Stokes shift is always positive if it is defined via the absorption and emission band maxima for a given transition.
- The energy of photons emitted by an optically pumped laser is usually smaller than that of the pump radiation. The smallest possible Stokes shifts (→ quantum defect) for lasers (e.g. with ytterbium-doped laser gain media) correspond to less than 1% of the pump photon energy, but more usual values are between several percent and 40%.
- In Raman amplifiers and Raman lasers and also in devices based on Brillouin scattering, the photon energy of amplified light is also smaller than that of the pump light.
- In Raman spectroscopy, most of the scattered light has a reduced photon energy. Some anti-Stokes lines with increased photon energies can also often be detected.
A Stokes shift may be quantified in different ways:
- One can specify the difference in photon energies (i.e., energy per involved atom).
- Alternatively, one may take the difference of optical frequencies.
- Particularly in spectroscopy, it is common to specify the difference in inverse wavelengths in units of cm−1.
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