HOME

TheInfoList



OR:

The Hanle effect, also known as zero-field level crossing, is a reduction in the polarization of light when the atoms emitting the light are subject to a magnetic field in a particular direction, and when they have themselves been excited by polarized light. Experiments which utilize the Hanle effect include measuring the lifetime of
excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...
s, and detecting the presence of magnetic fields.


History

The first experimental evidence for the effect came from
Robert W. Wood Robert Williams Wood (May 2, 1868 – August 11, 1955) was an American physicist and inventor who made pivotal contributions to the field of optics. He pioneered infrared and ultraviolet photography. Wood's patents and theoretical work inform m ...
, and
Lord Rayleigh John William Strutt, 3rd Baron Rayleigh ( ; 12 November 1842 – 30 June 1919), was an English physicist who received the Nobel Prize in Physics in 1904 "for his investigations of the densities of the most important gases and for his discovery ...
. The effect is named after
Wilhelm Hanle Wilhelm Hanle (13 January 1901 – 29 April 1993) was a German experimental physicist. He is known for the Hanle effect. During World War II, he made contributions to the German nuclear energy project, also known as the Uranium Club. From 1941 ...
, who was the first to explain the effect, in terms of
classical physics Classical physics refers to physics theories that are non-quantum or both non-quantum and non-relativistic, depending on the context. In historical discussions, ''classical physics'' refers to pre-1900 physics, while '' modern physics'' refers to ...
, in ''
Zeitschrift für Physik ''Zeitschrift für Physik'' (English: ''Journal for Physics'') is a defunct series of German peer-reviewed physics journals established in 1920 by Springer Berlin Heidelberg. The series ended publication in 1997, when it merged with other journal ...
'' in 1924. Initially, the causes of the effect were controversial, and many theorists mistakenly thought it was a version of the
Faraday effect The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...
. Attempts to understand the phenomenon were important in the subsequent development of
quantum physics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...
. An early theoretical treatment of level crossing effect was given by
Gregory Breit Gregory Breit (, ; July 14, 1899 – September 13, 1981) was an American physicist born in Mykolaiv, Russian Empire (now Mykolaiv, Ukraine). He was a professor at New York University (1929–1934), University of Wisconsin–Madison (1934–194 ...
.


Theory


Classical model

The classical explanation for this effect involves the
Lorentz oscillator model The Lorentz oscillator model (classical electron oscillator or CEO model) describes the optical response of Polarization density#Definition, bound charges. The model is named after the Dutch physicist Hendrik Antoon Lorentz. It is a Classical ph ...
, which treats the electron bound to the nucleus as a classical oscillator. When light interacts with this oscillator, it sets the electron in motion in the direction of its polarization. Consequently, the radiation emitted by this moving electron is polarized in the same direction as the incident light, as explained by classical electrodynamics. Consider light propagating along the y-axis, linearly polarized with the electric field along the x-axis, incident on a single atom in a vapor cell. The vapor cell is placed in a uniform magnetic field along the z-axis. A detector is placed to monitor the fluorescent light emitted along the z-axis from the vapor cell and measure its polarization in the x-y plane. The oscillating electric field of the incident light induces oscillations in the electron along the x axis. The electric field generated by the induced dipole along the z-axis is given by \mathbf E(z, t) = \frac \mathbf'(t - z/c)\frac where \omega_0 is the angular frequency of the incident light and t_0 is the time at which the electron is excited. The component of the fluorescent light polarized at angle \alpha with respect to the x axis at the detector has intensity given by I(\alpha, t) = \frac e^ where \omega_L = g_J eB/(2m_0) is the Larmor frequency and 1/\tau is the damping rate of the fluorescence radiation. Consider N atoms in the vapor cell, excited at a constant rate R. The steady-state intensity at the detector can be obtained by integrating over the excitation time t_0 from -\infty to t, which gives I(\alpha) = NR \frac \left frac + \frac + \frac\right The polarization degree is P = \frac = \frac where I_x and I_y are the intensities of the x-polarized component (\alpha = 0) and the y-polarized component (\alpha = \pi/2). This has a Lorentzian shape as a function of the magnetic field strength B.


Applications

Observation of the Hanle effect on the light emitted by the Sun is used to indirectly measure the magnetic fields within the Sun, see: *
Polarization in astronomy Polarization of electromagnetic radiation is a useful tool for detecting various astronomical phenomenon. For example, energy can become polarized by passing through interstellar dust or by magnetic fields. Microwave energy from the primordial uni ...
*
Imaging spectroscopy Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
The effect was initially considered in the context of gasses, followed by applications to
solid state physics Solid-state physics is the study of rigid matter, or solids, through methods such as solid-state chemistry, quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state p ...
. It has been used to measure both the states of localized electrons and free electrons. For spin-polarized electrical currents, the Hanle effect provides a way to measure the effective spin lifetime in a particular device.


Related effects

The zero-field Hanle level crossings involve magnetic fields, in which the states which are degenerate at zero magnetic field are split due to the
Zeeman effect The Zeeman effect () is the splitting of a spectral line into several components in the presence of a static magnetic field. It is caused by the interaction of the magnetic field with the magnetic moment of the atomic electron associated with ...
. There is also the closely analogous zero-field Stark level crossings with electric fields, in which the states which are degenerate at zero electric field are split due to the
Stark effect The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to the presence of an external electric field. It is the electric-field analogue of the Zeeman effect, where a spectral line is split into several compon ...
. Tests of zero field Stark level crossings came after the Hanle-type measurements, and are generally less common, due to the increased complexity of the experiments.


See also

*
Larmor precession Sir Joseph Larmor (; 11 July 1857 – 19 May 1942) was an Irish mathematician and physicist who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influential work was ...
*
Resonance fluorescence Resonance fluorescence is the process in which a two-level atom system interacts with the quantum electromagnetic field if the field is driven at a frequency near to the natural frequency Natural frequency, measured in terms of '' eigenfreque ...
*
Optical pumping Optical pumping is a process in which light is used to raise (or "pump") electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction to pump the active laser medium so as to achieve popu ...


References

{{DEFAULTSORT:Hanle Effect Atomic physics Magnetism Foundational quantum physics Physical phenomena