Fano resonance

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Plot of scattering cross-section versus normalized energy for various values of the parameter q illustrating the asymmetric Fano line-shape. Fano-resonance-scs.png
Plot of scattering cross-section versus normalized energy for various values of the parameter q illustrating the asymmetric Fano line-shape.

In physics, a Fano resonance is a type of resonant scattering phenomenon that gives rise to an asymmetric line-shape. Interference between a background and a resonant scattering process produces the asymmetric line-shape. It is named after Italian-American physicist Ugo Fano, who in 1961 gave a theoretical explanation for the scattering line-shape of inelastic scattering of electrons from helium; [1] [2] however, Ettore Majorana was the first to discover this phenomenon. [3] Fano resonance is a weak coupling effect meaning that the decay rate is so high, that no hybridization occurs. [4] The coupling modifies the resonance properties such as spectral position and width and its line-shape takes on the distinctive asymmetric Fano profile. Because it is a general wave phenomenon, examples can be found across many areas of physics and engineering.

Contents

History

The explanation of the Fano line-shape first appeared in the context of inelastic electron scattering by helium and autoionization. The incident electron doubly excites the atom to the state, a sort of shape resonance. The doubly excited atom spontaneously decays by ejecting one of the excited electrons. Fano showed that interference between the amplitude to simply scatter the incident electron and the amplitude to scatter via autoionization creates an asymmetric scattering line-shape around the autoionization energy with a line-width very close to the inverse of the autoionization lifetime.

Explanation

The Fano resonance line-shape is due to interference between two scattering amplitudes, one due to scattering within a continuum of states (the background process) and the second due to an excitation of a discrete state (the resonant process). The energy of the resonant state must lie in the energy range of the continuum (background) states for the effect to occur. Near the resonant energy, the background scattering amplitude typically varies slowly with energy while the resonant scattering amplitude changes both in magnitude and phase quickly. It is this variation that creates the asymmetric profile.

For energies far from the resonant energy the background scattering process dominates. Within of the resonant energy, the phase of the resonant scattering amplitude changes by . It is this rapid variation in phase that creates the asymmetric line-shape.

Fano showed that the total scattering cross-section assumes the following form,

where describes the line width of the resonant energy and q, the Fano parameter, measures the ratio of resonant scattering to the direct (background) scattering amplitude. This is consistent with the interpretation within the Feshbach–Fano partitioning theory. In the case the direct scattering amplitude vanishes, the q parameter becomes zero and the Fano formula becomes :

Looking at transmission shows that this last expression boils down to the expected Breit–Wigner (Lorentzian) formula, as , the three parameters Lorentzian function (note that it is not a density function and does not integrate to 1, as its amplitude is 1 and not ).

Examples

Examples of Fano resonances can be found in atomic physics, nuclear physics, condensed matter physics, electrical circuits, microwave engineering, nonlinear optics, nanophotonics, magnetic metamaterials, [5] and in mechanical waves. [6]

Fano can be observed with photoelectron spectroscopy [7] and Raman spectroscopy. [5] The phenomenon can be also observed at visible frequencies using simple glass microspheres, which may allow enhancing the magnetic field of light (which is typically small) by a few orders of magnitude. [8]

See also

Related Research Articles

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<span class="mw-page-title-main">Lorentz force</span> Force acting on charged particles in electric and magnetic fields

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<span class="mw-page-title-main">Resonance</span> Tendency to oscillate at certain frequencies

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<span class="mw-page-title-main">Ionization</span> Process by which atoms or molecules acquire charge by gaining or losing electrons

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<i>Q</i> factor Parameter describing the longevity of energy in a resonator relative to its resonant frequency

In physics and engineering, the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lost in one radian of the cycle of oscillation. Q factor is alternatively defined as the ratio of a resonator's centre frequency to its bandwidth when subject to an oscillating driving force. These two definitions give numerically similar, but not identical, results. Higher Q indicates a lower rate of energy loss and the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Resonators with high quality factors have low damping, so that they ring or vibrate longer.

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<span class="mw-page-title-main">Møller scattering</span> Electron-electron scattering

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<span class="mw-page-title-main">Bhabha scattering</span> Electron-positron scattering

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In spectroscopy, the Autler–Townes effect, is a dynamical Stark effect corresponding to the case when an oscillating electric field is tuned in resonance to the transition frequency of a given spectral line, and resulting in a change of the shape of the absorption/emission spectra of that spectral line. The AC Stark effect was discovered in 1955 by American physicists Stanley Autler and Charles Townes.

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The Elliott formula describes analytically, or with few adjustable parameters such as the dephasing constant, the light absorption or emission spectra of solids. It was originally derived by Roger James Elliott to describe linear absorption based on properties of a single electron–hole pair. The analysis can be extended to a many-body investigation with full predictive powers when all parameters are computed microscopically using, e.g., the semiconductor Bloch equations or the semiconductor luminescence equations.

References

  1. " A. Bianconi Ugo Fano and shape resonances in X-ray and Inner Shell Processes" AIP Conference Proceedings (2002): (19th Int. Conference Roma June 24–28, 2002) A. Bianconi arXiv: cond-mat/0211452 21 November 2002
  2. Fano, U. (15 December 1961). "Effects of Configuration Interaction on Intensities and Phase Shifts". Physical Review. 124 (6). American Physical Society (APS): 1866–1878. doi:10.1103/physrev.124.1866. ISSN   0031-899X.
  3. Vittorini-Orgeas, Alessandra; Bianconi, Antonio (7 January 2009). "From Majorana Theory of Atomic Autoionization to Feshbach Resonances in High Temperature Superconductors". Journal of Superconductivity and Novel Magnetism. 22 (3): 215–221. arXiv: 0812.1551 . doi:10.1007/s10948-008-0433-x. ISSN   1557-1939. S2CID   118439516.
  4. Limonov, Mikhail F.; Rybin, Mikhail V.; Poddubny, Alexander N.; Kivshar, Yuri S. (2017). "Fano resonances in photonics". Nature Photonics. 11: 543–554. doi:10.1038/nphoton.2017.142.
  5. 1 2 Luk'yanchuk, Boris; Zheludev, Nikolay I.; Maier, Stefan A.; Halas, Naomi J.; Nordlander, Peter; Giessen, Harald; Chong, Chong Tow (23 August 2010). "The Fano resonance in plasmonic nanostructures and metamaterials". Nature Materials. 9 (9). Springer Nature: 707–715. doi:10.1038/nmat2810. ISSN   1476-1122. PMID   20733610.
  6. Martínez-Argüello, A. M.; Martínez-Mares, M.; Cobián-Suárez, M.; Báez, G.; Méndez-Sánchez, R. A. (1 May 2015). "A new Fano resonance in measurement processes". EPL (Europhysics Letters). 110 (5): 54003. arXiv: 1502.03488 . doi:10.1209/0295-5075/110/54003. ISSN   0295-5075. S2CID   124830448.
  7. Tjernberg, O.; Söderholm, S.; Karlsson, U. O.; Chiaia, G.; Qvarford, M.; Nylén, H.; Lindau, I. (1996-04-15). "Resonant photoelectron spectroscopy on NiO". Physical Review B. 53 (15): 10372–10376. doi:10.1103/PhysRevB.53.10372. ISSN   0163-1829. PMID   9982607.
  8. Wang, Z.B.; Luk'yanchuk, B.S.; Yue, L.; Yan, B.; Monks, J.; Dhama, R.; Minin, O.V.; Minin, I.V.; Huang, S.; Fedyanin, A. (30 Dec 2019). "High order Fano resonances and giant magnetic fields in dielectric microspheres". Scientific Reports. 9 (1). Springer Nature Limited: 20293. doi: 10.1038/s41598-019-56783-3 . ISSN   2045-2322. PMC   6937277 . PMID   31889112.