Bessel beam

Last updated
Evolution of a Bessel beam. Bessel beam.gif
Evolution of a Bessel beam.
Diagram of axicon and resulting Bessel beam Bessel beam.svg
Diagram of axicon and resulting Bessel beam
Cross-section of the Bessel beam and graph of intensity Bessek beam intensity.svg
Cross-section of the Bessel beam and graph of intensity
Bessel beam re-forming central bright area after obstruction Bessel beam reform.svg
Bessel beam re-forming central bright area after obstruction

A Bessel beam is a wave whose amplitude is described by a Bessel function of the first kind. [1] [2] [3] Electromagnetic, acoustic, gravitational, and matter waves can all be in the form of Bessel beams. A true Bessel beam is non-diffractive. This means that as it propagates, it does not diffract and spread out; this is in contrast to the usual behavior of light (or sound), which spreads out after being focused down to a small spot. Bessel beams are also self-healing, meaning that the beam can be partially obstructed at one point, but will re-form at a point further down the beam axis.

Contents

As with a plane wave, a true Bessel beam cannot be created, as it is unbounded and would require an infinite amount of energy. Reasonably good approximations can be made, [4] however, and these are important in many optical applications because they exhibit little or no diffraction over a limited distance. Approximations to Bessel beams are made in practice either by focusing a Gaussian beam with an axicon lens to generate a Bessel–Gauss beam, by using axisymmetric diffraction gratings, [5] or by placing a narrow annular aperture in the far field. [3] High order Bessel beams can be generated by spiral diffraction gratings. [6]

Properties

The properties of Bessel beams [7] [8] make them extremely useful for optical tweezing, as a narrow Bessel beam will maintain its required property of tight focus over a relatively long section of beam and even when partially occluded by the dielectric particles being tweezed. Similarly, particle manipulation with acoustical tweezers was achieved [9] [10] with a Bessel beam that scatters [11] [12] [13] [14] and produces a radiation force resulting from the exchange of acoustic momentum between the wave-field and a particle placed along its path. [15] [16] [17] [18] [19] [20] [21] [22] [23]

The mathematical function which describes a Bessel beam is a solution of Bessel's differential equation, which itself arises from separable solutions to Laplace's equation and the Helmholtz equation in cylindrical coordinates. The fundamental zero-order Bessel beam has an amplitude maximum at the origin, while a high-order Bessel beam (HOBB) has an axial phase singularity along the beam axis; the amplitude is zero there. HOBBs can be of vortex (helicoidal) or non-vortex types. [24]

X-waves are special superpositions of Bessel beams which travel at constant velocity, and can exceed the speed of light. [25]

Mathieu beams and parabolic (Weber) beams [26] are other types of non-diffractive beams that have the same non-diffractive and self-healing properties of Bessel beams but different transverse structures.

Acceleration

In 2012 it was theoretically proven [27] and experimentally demonstrated [28] that, with a special manipulation of their initial phase, Bessel beams can be made to accelerate along arbitrary trajectories in free space. These beams can be considered as hybrids that combine the symmetric profile of a standard Bessel beam with the self-acceleration property of the Airy beam and its counterparts. Previous efforts to produce accelerating Bessel beams included beams with helical [29] and sinusoidal [30] trajectories as well as the early effort for beams with piecewise straight trajectories. [31]

Attenuation-compensation

Beams may encounter losses as they travel through materials which will cause attenuation of the beam intensity. A property common to non-diffracting (or propagation-invariant) beams, such as the Airy beam and Bessel beam, is the ability to control the longitudinal intensity envelope of the beam without significantly altering the other characteristics of the beam. This can be used to create Bessel beams which grow in intensity as they travel and can be used to counteract losses, therefore maintaining a beam of constant intensity as it propagates. [32] [33]

Applications

Imaging and microscopy

In light-sheet fluorescence microscopy, non-diffracting (or propagation-invariant) beams have been utilised to produce very long and uniform light-sheets which do not change size significantly across their length. The self-healing property of Bessel beams has also shown to give improved image quality at depth as the beam shape is less distorted after travelling through scattering tissue than a Gaussian beam. Bessel beam based light-sheet microscopy was first demonstrated in 2010 [34] but many variations have followed since. In 2018, it was shown that the use of attenuation-compensation could be applied to Bessel beam based light-sheet microscopy and could enable imaging at greater depths within biological specimens. [35]

Acoustofluidics

Bessel beams are a good candidate for the selectively trapping because of the concentric circles of pressure maximum and minimum in the transverse planes.

Related Research Articles

Optical tweezers are scientific instruments that use a highly focused laser beam to hold and move microscopic and sub-microscopic objects like atoms, nanoparticles and droplets, in a manner similar to tweezers. If the object is held in air or vacuum without additional support, it can be called optical levitation.

The Wolf effect is a frequency shift in the electromagnetic spectrum. The phenomenon occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light. It was first predicted by Emil Wolf in 1987 and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox, and a year later in optic sources by Dean Faklis and George Morris in 1988.

<span class="mw-page-title-main">Optical vortex</span> Optical phenomenon

An optical vortex is a zero of an optical field; a point of zero intensity. The term is also used to describe a beam of light that has such a zero in it. The study of these phenomena is known as singular optics.

A superlens, or super lens, is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is a feature of conventional lenses and microscopes that limits the fineness of their resolution depending on the illumination wavelength and the numerical aperture NA of the objective lens. Many lens designs have been proposed that go beyond the diffraction limit in some way, but constraints and obstacles face each of them.

Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision. These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing and radiation dosimetry. It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed. It can also be used to generate contours representing the form of the surface.

Digital holography refers to the acquisition and processing of holograms with a digital sensor array, typically a CCD camera or a similar device. Image rendering, or reconstruction of object data is performed numerically from digitized interferograms. Digital holography offers a means of measuring optical phase data and typically delivers three-dimensional surface or optical thickness images. Several recording and processing schemes have been developed to assess optical wave characteristics such as amplitude, phase, and polarization state, which make digital holography a very powerful method for metrology applications .

<span class="mw-page-title-main">Point diffraction interferometer</span>

A point diffraction interferometer (PDI) is a type of common-path interferometer. Unlike an amplitude-splitting interferometer, such as a Michelson interferometer, which separates out an unaberrated beam and interferes this with the test beam, a common-path interferometer generates its own reference beam. In PDI systems, the test and reference beams travel the same or almost the same path. This design makes the PDI extremely useful when environmental isolation is not possible or a reduction in the number of precision optics is required. The reference beam is created from a portion of the test beam by diffraction from a small pinhole in a semitransparent coating. The principle of a PDI is shown in Figure 1.

<span class="mw-page-title-main">Talbot effect</span>

The Talbot effect is a diffraction effect first observed in 1836 by Henry Fox Talbot. When a plane wave is incident upon a periodic diffraction grating, the image of the grating is repeated at regular distances away from the grating plane. The regular distance is called the Talbot length, and the repeated images are called self images or Talbot images. Furthermore, at half the Talbot length, a self-image also occurs, but phase-shifted by half a period. At smaller regular fractions of the Talbot length, sub-images can also be observed. At one quarter of the Talbot length, the self-image is halved in size, and appears with half the period of the grating. At one eighth of the Talbot length, the period and size of the images is halved again, and so forth creating a fractal pattern of sub images with ever-decreasing size, often referred to as a Talbot carpet. Talbot cavities are used for coherent beam combination of laser sets.

<span class="mw-page-title-main">Ptychography</span>

Ptychography is a computational method of microscopic imaging. It generates images by processing many coherent interference patterns that have been scattered from an object of interest. Its defining characteristic is translational invariance, which means that the interference patterns are generated by one constant function moving laterally by a known amount with respect to another constant function. The interference patterns occur some distance away from these two components, so that the scattered waves spread out and "fold" into one another as shown in the figure.

<span class="mw-page-title-main">Airy beam</span>

An Airy beam, is a propagation invariant wave whose main intensity lobe propagates along a curved parabolic trajectory while being resilient to perturbations (self-healing).

Speckle, speckle pattern, or speckle noise is a granular noise texture degrading the quality as a consequence of interference among wavefronts in coherent imaging systems, such as radar, synthetic aperture radar (SAR), medical ultrasound and optical coherence tomography. Speckle is not external noise; rather, it is an inherent fluctuation in diffuse reflections, because the scatterers are not identical for each cell, and the coherent illumination wave is highly sensitive to small variations in phase changes.

Codes for electromagnetic scattering by spheres - this article list codes for electromagnetic scattering by a homogeneous sphere, layered sphere, and cluster of spheres.

<span class="mw-page-title-main">Digital holographic microscopy</span>

Digital holographic microscopy (DHM) is digital holography applied to microscopy. Digital holographic microscopy distinguishes itself from other microscopy methods by not recording the projected image of the object. Instead, the light wave front information originating from the object is digitally recorded as a hologram, from which a computer calculates the object image by using a numerical reconstruction algorithm. The image forming lens in traditional microscopy is thus replaced by a computer algorithm. Other closely related microscopy methods to digital holographic microscopy are interferometric microscopy, optical coherence tomography and diffraction phase microscopy. Common to all methods is the use of a reference wave front to obtain amplitude (intensity) and phase information. The information is recorded on a digital image sensor or by a photodetector from which an image of the object is created (reconstructed) by a computer. In traditional microscopy, which do not use a reference wave front, only intensity information is recorded and essential information about the object is lost.

The technique of vibrational analysis with scanning probe microscopy allows probing vibrational properties of materials at the submicrometer scale, and even of individual molecules. This is accomplished by integrating scanning probe microscopy (SPM) and vibrational spectroscopy. This combination allows for much higher spatial resolution than can be achieved with conventional Raman/FTIR instrumentation. The technique is also nondestructive, requires non-extensive sample preparation, and provides more contrast such as intensity contrast, polarization contrast and wavelength contrast, as well as providing specific chemical information and topography images simultaneously.

<span class="mw-page-title-main">Whispering-gallery wave</span> Wave that can travel around a concave surface

Whispering-gallery waves, or whispering-gallery modes, are a type of wave that can travel around a concave surface. Originally discovered for sound waves in the whispering gallery of St Paul's Cathedral, they can exist for light and for other waves, with important applications in nondestructive testing, lasing, cooling and sensing, as well as in astronomy.

A common-path interferometer is a class of interferometers in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, Zernike phase-contrast interferometer, and the point diffraction interferometer. A common-path interferometer is generally more robust to environmental vibrations than a "double-path interferometer" such as the Michelson interferometer or the Mach–Zehnder interferometer. Although travelling along the same path, the reference and sample beams may travel along opposite directions, or they may travel along the same direction but with the same or different polarization.

<span class="mw-page-title-main">Light sheet fluorescence microscopy</span> Fluorescence microscopy technique

Light sheet fluorescence microscopy (LSFM) is a fluorescence microscopy technique with an intermediate-to-high optical resolution, but good optical sectioning capabilities and high speed. In contrast to epifluorescence microscopy only a thin slice of the sample is illuminated perpendicularly to the direction of observation. For illumination, a laser light-sheet is used, i.e. a laser beam which is focused only in one direction. A second method uses a circular beam scanned in one direction to create the lightsheet. As only the actually observed section is illuminated, this method reduces the photodamage and stress induced on a living sample. Also the good optical sectioning capability reduces the background signal and thus creates images with higher contrast, comparable to confocal microscopy. Because light sheet fluorescence microscopy scans samples by using a plane of light instead of a point, it can acquire images at speeds 100 to 1,000 times faster than those offered by point-scanning methods.

Wide-field multiphoton microscopy refers to an optical non-linear imaging technique tailored for ultrafast imaging in which a large area of the object is illuminated and imaged without the need for scanning. High intensities are required to induce non-linear optical processes such as two-photon fluorescence or second harmonic generation. In scanning multiphoton microscopes the high intensities are achieved by tightly focusing the light, and the image is obtained by beam scanning. In wide-field multiphoton microscopy the high intensities are best achieved using an optically amplified pulsed laser source to attain a large field of view (~100 µm). The image in this case is obtained as a single frame with a CCD without the need of scanning, making the technique particularly useful to visualize dynamic processes simultaneously across the object of interest. With wide-field multiphoton microscopy the frame rate can be increased up to a 1000-fold compared to multiphoton scanning microscopy. Wide-field multiphoton microscopes are not yet commercially available, but working prototypes exist in several optics laboratories.

Three-photon microscopy (3PEF) is a high-resolution fluorescence microscopy based on nonlinear excitation effect. Different from two photon excitation microscopy, it uses three exciting photons. It typically uses 1300 nm or longer wavelength lasers to excite the fluorescent dyes with three simultaneously absorbed photons. The fluorescent dyes then emit one photon whose energy is three times the energy of each incident photon. Compared to two-photon microscopy, three-photon microscopy reduces the fluorescence away from the focal plane by , which is much faster than that of two-photon microscopy by . In addition, three-photon microscopy employs near-infrared light with less tissue scattering effect. This causes three photon microscopy to have higher resolution than conventional microscopy.

<span class="mw-page-title-main">Virtually imaged phased array</span> Dispersive optical device

A virtually imaged phased array (VIPA) is an angular dispersive device that, like a prism or a diffraction grating, splits light into its spectral components. It works almost independently of polarization. In contrast to prisms or regular diffraction gratings, it has a much higher angular dispersion but has a smaller free spectral range. This aspect is similar to that of an Echelle grating which is usually used in reflection, since high diffraction orders are also used there. The VIPA can be a compact optical component with high wavelength resolving power.

References

  1. ↑ Garcés-Chávez, V.; McGloin, D.; Melville, H.; Sibbett, W.; Dholakia, K. (2002). "Simultaneous micromanipulation in multiple planes using a self-reconstructing light beam". Nature . 419 (6903): 145–7. Bibcode:2002Natur.419..145G. doi:10.1038/nature01007. PMID   12226659. S2CID   4426776.
  2. ↑ McGloin, D.; Dholakia, K. (2005). "Bessel beams: diffraction in a new light". Contemporary Physics . 46 (1): 15–28. Bibcode:2005ConPh..46...15M. doi:10.1080/0010751042000275259. S2CID   31363603.
  3. 1 2 Durnin, J. (1987). "Diffraction-free beams". Physical Review Letters . 58 (15): 1499–1501. Bibcode:1987PhRvL..58.1499D. doi:10.1103/PhysRevLett.58.1499. PMID   10034453.
  4. ↑ Cox, A.J.; D'Anna, Joseph (1992). "Constant-axial-intensity nondiffracting beam". Optics Letters. 17 (4): 232–234. Bibcode:1992OptL...17..232C. doi:10.1364/OL.17.000232. PMID   19784285.
  5. ↑ Jiménez, N.; et al. (2014). "Acoustic Bessel-like beam formation by an axisymmetric grating". Europhysics Letters . 106 (2): 24005. arXiv: 1401.6769 . Bibcode:2014EL....10624005J. doi:10.1209/0295-5075/106/24005. S2CID   55703345.
  6. ↑ Jiménez, N.; et al. (2016). "Formation of high-order acoustic Bessel beams by spiral diffraction gratings". Physical Review E . 94 (5): 053004. arXiv: 1604.08353 . Bibcode:2016PhRvE..94e3004J. doi:10.1103/PhysRevE.94.053004. PMID   27967159. S2CID   27190492.
  7. ↑ Fahrbach, F. O.; Simon, P.; Rohrbach, A. (2010). "Microscopy with self-reconstructing beams". Nature Photonics . 4 (11): 780–785. Bibcode:2010NaPho...4..780F. doi:10.1038/nphoton.2010.204.
  8. ↑ Mitri, F. G. (2011). "Arbitrary scattering of an electromagnetic zero-order Bessel beam by a dielectric sphere". Optics Letters . 36 (5): 766–8. Bibcode:2011OptL...36..766M. doi:10.1364/OL.36.000766. PMID   21368976.
  9. ↑ Hill, M. (2016). "Viewpoint: A One-Sided View of Acoustic Traps". Physics . 9 (3): 3. doi: 10.1103/Physics.9.3 .
  10. ↑ D. Baresch, J.L. Thomas, and R. Marchiano, Physical review letters, 2016, 116(2), 024301.
  11. ↑ Marston, P. L. (2007). "Scattering of a Bessel beam by a sphere". The Journal of the Acoustical Society of America . 121 (2): 753–758. Bibcode:2007ASAJ..121..753M. doi:10.1121/1.2404931. PMID   17348499.
  12. ↑ Silva, G. T. (2011). "Off-axis scattering of an ultrasound bessel beam by a sphere". IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control . 58 (2): 298–304. doi:10.1109/TUFFC.2011.1807. PMID   21342815. S2CID   38969143.
  13. ↑ Mitri, F. G.; Silva, G. T. (2011). "Off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid sphere". Wave Motion . 48 (5): 392–400. doi:10.1016/j.wavemoti.2011.02.001.
  14. ↑ Gong, Z.; Marston, P. L.; Li, W.; Chai, Y. (2017). "Multipole expansion of acoustical Bessel beams with arbitrary order and location". The Journal of the Acoustical Society of America . 141 (6): EL574–EL578. Bibcode:2017ASAJ..141L.574G. doi: 10.1121/1.4985586 . PMID   28679251.
  15. ↑ Mitri, F. G. (2008). "Acoustic radiation force on a sphere in standing and quasi-standing zero-order Bessel beam tweezers". Annals of Physics . 323 (7): 1604–1620. Bibcode:2008AnPhy.323.1604M. doi:10.1016/j.aop.2008.01.011.
  16. ↑ Mitri, F. G.; Fellah, Z. E. A. (2008). "Theory of the acoustic radiation force exerted on a sphere by standing and quasistanding zero-order Bessel beam tweezers of variable half-cone angles". IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control . 55 (11): 2469–2478. doi:10.1109/TUFFC.954. PMID   19049926. S2CID   33064887.
  17. ↑ Mitri, F. G. (2009). "Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere". IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control . 56 (5): 1059–1064. doi:10.1109/TUFFC.2009.1139. PMID   19473924. S2CID   33955993.
  18. ↑ Mitri, F. G. (2009). "Acoustic radiation force on an air bubble and soft fluid spheres in ideal liquids: Example of a high-order Bessel beam of quasi-standing waves". The European Physical Journal E . 28 (4): 469–478. Bibcode:2009EPJE...28..469M. doi:10.1140/epje/i2009-10449-y. PMID   19408023. S2CID   12972708.
  19. ↑ Mitri, F. G. (2009). "Negative axial radiation force on a fluid and elastic spheres illuminated by a high-order Bessel beam of progressive waves". Journal of Physics A . 42 (24): 245202. Bibcode:2009JPhA...42x5202M. doi:10.1088/1751-8113/42/24/245202. S2CID   122118984.
  20. ↑ Mitri, F. G. (2008). "Acoustic scattering of a high-order Bessel beam by an elastic sphere". Annals of Physics . 323 (11): 2840–2850. Bibcode:2008AnPhy.323.2840M. doi:10.1016/j.aop.2008.06.008.
  21. ↑ Mitri, F. G. (2009). "Equivalence of expressions for the acoustic scattering of a progressive high-order bessel beam by an elastic sphere". IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control . 56 (5): 1100–1103. doi:10.1109/TUFFC.2009.1143. PMID   19473927. S2CID   22404158.
  22. ↑ Marston, P. L. (2006). "Axial radiation force of a Bessel beam on a sphere and direction reversal of the force". The Journal of the Acoustical Society of America . 120 (6): 3518–3524. Bibcode:2006ASAJ..120.3518M. doi:10.1121/1.2361185. PMID   17225382.
  23. ↑ Marston, P. L. (2009). "Radiation force of a helicoidal Bessel beam on a sphere". The Journal of the Acoustical Society of America . 125 (6): 3539–3547. Bibcode:2009ASAJ..125.3539M. doi:10.1121/1.3119625. PMID   19507935.
  24. ↑ Mitri, F. G. (2011). "Linear axial scattering of an acoustical high-order Bessel trigonometric beam by compressible soft fluid spheres". Journal of Applied Physics . 109 (1): 014916–014916–5. Bibcode:2011JAP...109a4916M. doi:10.1063/1.3518496.
  25. ↑ Bowlan, P.; et al. (2009). "Measurement of the Spatiotemporal Electric Field of Ultrashort Superluminal Bessel-X Pulses". Optics and Photonics News . 20 (12): 42. Bibcode:2009OptPN..20...42M. doi:10.1364/OPN.20.12.000042. S2CID   122056218.
  26. ↑ Bandres, M. A.; Gutiérrez-Vega, J. C.; Chávez-Cerda, S. (2004). "Parabolic nondiffracting optical wave fields". Optics Letters . 29 (1): 44–6. Bibcode:2004OptL...29...44B. doi:10.1364/OL.29.000044. PMID   14719655.
  27. ↑ Chremmos, I. D.; Chen, Z; Christodoulides, D. N.; Efremidis, N. K. (2012). "Bessel-like optical beams with arbitrary trajectories" (PDF). Optics Letters . 37 (23): 5003–5. Bibcode:2012OptL...37.5003C. doi:10.1364/OL.37.005003. PMID   23202118.
  28. ↑ Juanying, Z.; et al. (2013). "Observation of self-accelerating Bessel-like optical beams along arbitrary trajectories" (PDF). Optics Letters . 38 (4): 498–500. Bibcode:2013OptL...38..498Z. doi:10.1364/OL.38.000498. PMID   23455115.
  29. ↑ Jarutis, V.; Matijošius, A.; DiTrapani, P.; Piskarskas, A. (2009). "Spiraling zero-order Bessel beam". Optics Letters . 34 (14): 2129–31. Bibcode:2009OptL...34.2129J. doi:10.1364/OL.34.002129. PMID   19823524.
  30. ↑ Morris, J. E.; Čižmár, T.; Dalgarno, H. I. C.; Marchington, R. F.; Gunn-Moore, F. J.; Dholakia, K. (2010). "Realization of curved Bessel beams: propagation around obstructions". Journal of Optics . 12 (12): 124002. Bibcode:2010JOpt...12l4002M. doi:10.1088/2040-8978/12/12/124002. S2CID   120332951.
  31. ↑ Rosen, J.; Yariv, A. (1995). "Snake beam: a paraxial arbitrary focal line". Optics Letters . 20 (20): 2042–4. Bibcode:1995OptL...20.2042R. CiteSeerX   10.1.1.9.3156 . doi:10.1364/OL.20.002042. PMID   19862244.
  32. ↑ Zamboni-Rached, Michel (2004-08-23). "Stationary optical wave fields with arbitrary longitudinal shape by superposing equal frequency Bessel beams: Frozen Waves". Optics Express. 12 (17): 4001–4006. arXiv: physics/0407128 . Bibcode:2004OExpr..12.4001Z. doi:10.1364/opex.12.004001. PMID   19483938. S2CID   14469395.
  33. ↑ Čižmár, Tomáš; Dholakia, Kishan (2009-08-31). "Tunable Bessel light modes: engineering the axial propagation". Optics Express. 17 (18): 15558–15570. Bibcode:2009OExpr..1715558C. doi: 10.1364/oe.17.015558 . PMID   19724554.
  34. ↑ Fahrbach, Florian O.; Simon, Philipp; Rohrbach, Alexander (2010). "Microscopy with self-reconstructing beams". Nature Photonics. 4 (11): 780–785. Bibcode:2010NaPho...4..780F. doi:10.1038/nphoton.2010.204.
  35. ↑ Nylk, Jonathan; McCluskey, Kaley; Preciado, Miguel A.; Mazilu, Michael; Yang, Zhengyi; Gunn-Moore, Frank J.; Aggarwal, Sanya; Tello, Javier A.; Ferrier, David E. K. (2018-04-01). "Light-sheet microscopy with attenuation-compensated propagation-invariant beams". Science Advances. 4 (4): eaar4817. arXiv: 1708.02612 . Bibcode:2018SciA....4R4817N. doi:10.1126/sciadv.aar4817. PMC   5938225 . PMID   29740614.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.