homogeneous broadening (original) (raw)

Definition: the increase in the linewidth of an atomic transition caused by effects which affect different radiating or absorbing atoms in the same way

Category: physical foundations

Related: optical transitionslinewidthinhomogeneous broadeninggain bandwidthgain saturation

Opposite term: inhomogeneous broadening

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DOI: 10.61835/x7b Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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What is Homogeneous Broadening of Transitions?

Homogeneous broadening, as opposed to inhomogeneous broadening, refers to effects which increase the optical linewidth of an electronic transition by equally affecting different radiating or absorbing atoms, ions or molecules. The spectral shapes of the transition cross-sections of all involved atoms are then equal, and each atom or ion exposed to a certain optical intensity will exhibit the same rates of optical transitions.

Some common cases with homogeneous broadening are as follows:

• Isolated non-moving atoms or ions exhibit transitions with a natural linewidth, resulting from the finite lifetimes of the energy levels involved.
• In a gas (e.g. of a gas laser), all atoms or molecules of some species experience the same average rate of collisions, which lead to collisional broadening.
• In many laser crystals, laser-active ions can occupy only one type of site in the crystal lattice. (This applies e.g. to Nd3+ ions in Nd:YAG, replacing yttrium (Y3+) ions of the YAG host.) The interaction of such ions with the crystal lattice via phonons then affects all these ions equally. Rapid transitions between the sublevels of the involved Stark level manifolds lead to strongly reduced sublevel lifetimes, and thus to linewidth values which are orders of magnitude larger than the natural linewidth as expected from the lifetimes of the whole Stark level manifolds.

For homogeneous broadening, one often observes a Lorentzian spectral line shape. This naturally results from the exponential excitation decay. For inhomogeneous broadening, different line shapes are often observed, e.g. Gaussian ones.

Homogeneous broadening is strongly related to homogeneous saturation in laser gain media. In comparison with inhomogeneous broadening, it makes it easier to achieve single-frequency operation.

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 homogeneous broadening?

Homogeneous broadening describes effects that increase the optical linewidth of a transition by affecting all involved atoms or ions equally. As a result, every atom has the same spectral line shape and transition rates.

What are common causes of homogeneous broadening?

The main causes are the finite lifetime of energy levels (natural linewidth), collisions in a gas (collisional broadening), and the interaction with lattice vibrations (phonons) in a solid-state gain medium.

What spectral line shape is characteristic for homogeneous broadening?

Homogeneous broadening typically results in a Lorentzian spectral line shape. This shape is a natural consequence of the exponential decay of the excitation in the involved atoms or ions.

How does homogeneous broadening influence laser operation?

It is associated with homogeneous saturation, where a laser beam depletes the gain over the entire line profile. This behavior makes it easier to achieve single-frequency operation compared to inhomogeneous broadening.

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