Stokes Shift

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: the reduction in photon energy in certain processes

Category: article belongs to category nonlinear optics nonlinear optics

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DOI: 10.61835/q9d Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What is a Stokes Shift?

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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a Stokes shift?

A Stokes shift is the phenomenon where emitted radiation has a lower photon energy (and thus a longer wavelength) than the radiation that was absorbed to cause the emission. The energy difference is typically converted into microscopic vibrations (heat).

In which physical processes does a Stokes shift occur?

Stokes shifts are common in many optical processes, including fluorescence, the operation of optically pumped lasers, and in Raman scattering and Brillouin scattering.

What is the relation between Stokes shift and the quantum defect of a laser?

In a laser, the quantum defect is the fraction of pump photon energy which is lost and converted into heat. This energy loss corresponds to the Stokes shift between the pump and laser photons.

How is the magnitude of a Stokes shift specified?

It can be specified as the difference in photon energies, optical frequencies, or inverse wavelengths (wavenumbers), with the latter being common in spectroscopy and often expressed in units of cm⁻¹.

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