FOCUS I PHOTONIC TECHNIQUES AND TECHNOLOGIES

THE FRESNEL TRIPRISM AND the circular of light

Oriol ARTEAGA 1,2 In 1822 Augustin Fresnel discovered the of Enric GARCIA-CAUREL 2 light with an experiment in which a plane polarized beam was Razvigor OSSIKOVSKI 2 1 Departament de Física Aplicada, resolved into its left- and right- circularly polarized components IN2UB, Universitat de Barcelona, after refraction at slightly different angles at the interface between 08028 Barcelona, Spain two different species of quartz that formed a composite , 2 LPICM, CNRS, École polytechnique, called the Fresnel triprism. Fresnel’s landmark experiment, once 91128 Palaiseau, France popular, remains today a very little known method for producing [email protected] circularly polarized light.

n a rapid sequence of discoveries Indeed this was the earliest descrip- 1811, could be explained by a special made in France, starting in 1811 tion of a Fresnel rhomb, but it would form of double refraction and made an Iwith the observation of optical take Fresnel a few years more to realize experiment to demonstrate it. rotation in quartz by François Arago that the light was actually circularly Fresnel took a very acute isosce- [1], and roughly spanning over 10 polarized instead of being depolarized. les prism of right-handed quartz and years, most of our modern unders- The concepts of linear polarization, cemented it in between two half tanding about the polarization of light circular polarization and elliptical of left-handed quartz, forming a com- was established. The leading figure was polarization in the wave picture of posite prism, named “Fresnel triprism” the brilliant mathematical physicist light were established by Fresnel in the and schematically shown in Figure 1. In Agustin Fresnel, often stimulated by conclusion of his memoir from 1822 the experiment, light always propagated preliminary experimental discoveries [4]. The main subject of this memoir substantially parallel to the optic axes of Arago. By 1820 Fresnel had already was the circular double refraction of of the quartz prism components, so realized that light could be understood light propagating along the optic axis it could not be affected by standard as a transverse wave and the Fresnel- of a quartz crystal (cristal de roche) in double refraction but, at the exit facet, Arago laws of interference and pola- an experiment that is little known Fresnel still observed two beams emer- rization had been just published [2]. today, but that was crucial for Fresnel ging with a small angular separation. At that time however, circular polari- to understand circular polarization Fresnel used double refracting calcite to zation was still not fully understood and its relation to . check that these emerging beams were and the property of “polarization of Unlike the well-known double refrac- not linearly polarized as in standard light” was, by default, associated only tion phenomenon in calcite, already double refraction, yet he checked that with linear polarization. Nonetheless, well understood by Fresnel in terms when the beams were totally internally circular polarization was already a of polarization, light does not split reflected in a glass prism, linear pola- main ingredient of the experimen- by polarization in two different paths rizations were recovered, each beam tal research as, for example, in 1817 when propagating along the optic axis being orthogonal to the other. With Fresnel had already reported that after of a quartz crystal. However, Fresnel this experiment Fresnel postulated two total internal reflections in a glass thought that the phenomenon of opti- the circular polarization of light and prism light seemed “depolarized” [3]. cal rotation, first observed by Arago in understood that the optical rotation of quartz and other materials was due to the different indices of refraction for left- and right-circular polarizations. To generate the angular divergence (i.e. the circular double refraction) interfaces between quartz prisms of different Figure 1. The Fresnel triprism. A prism made of quartz cemented in between two other half species were needed, since according prisms made of quartz of opposite handedness splits the incident light into its right (R) and to Snell-Descartes’s law left- and right- left (L) circularly polarized components. The double headed arrows indicate the directions handed waves refract with slightly of the optic axes of the crystals. different angles at such interfaces.

44 Photoniques Special EOS Issue 3 ❚ www.photoniques.com This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, https://doi.org/10.1051/photon/2019S444 distribution, and reproduction in any medium, provided the original work is properly cited. FOCUS

Figure 2. Photo of the historical Fresnel polyprism composed of nine prisms. The inset shows the beam splitting of a laser beam. More details in Ref. [5].

Fresnel triprisms became popular angular separation between the emer- optical devices in the XIX and the first ging light beams, so with this device it part of the XX century as they were often is easy to achieve a large macroscopic used in teaching demonstrations that separation between the right and left plane polarized light can be resolved circularly polarized beams, something into right-and left-circularly polarized that we could test in situ in a series of components. However, Fresnel triprisms experiments we performed with this his- remain nowadays very little known opti- torical polyprism during the summer cal devices and they are not available as of 2018 [5]. commercial optical components. In our The Fresnel triprism, or more quest to find a Fresnel triprism we were generally, polyprism can be regarded lucky to discover that in the recently as the experimental culmination of inaugurated museum of the École what perhaps was the most fruitful polytechnique (Palaiseau, France), ins- decade in the history of polarization titution where both Arago and Fresnel optics, started with Arago’s observa- were trained, there is one of such com- tion of optical rotation also found in posite prisms (see Fig. 2) manufactured quartz. Beyond this historical signifi- by Henri Soleil some time before 1872. cance, we think that this optical device, This unit held a surprise: instead of a tri- today almost forgotten, can be again prism, it contained nine different prisms of scientific relevance in the XXI cen- of alternating handedness, so it is better tury as methods to produce and detect referred to as a polyprism, a feature that circularly polarized light are becoming to our knowledge is unique of this unit. more and more important in today’s Essentially, each interface doubles the research and technology. n

FURTHER READING [1] D.F. Arago, “Mémoire sur une modification remarquable qu’éprouvent les rayons lumineux dans leur passage à travers certains corps diaphanes, et sur quelques autres nouveaux phéno- mènes d’optique”. Mm. Inst. vol. 1, 93-134 (1811) [2] F. Arago and A. Fresnel, “Mémoire sur l’action que les rayons de lumière polarisée exercent les uns sur les autres”. Ann. Chim. Phys. vol. 10, 288-305 (1819) [3] A. Fresnel, “Mémoire sur les modifications que la réflexion imprime à la lumière polarisée”. Œuvres vol. 1, 441-485 (1817) [4] A. Fresnel, “Mémoire sur la double réfraction que les rayons lumineux éprouvent en traversant les aiguilles de cristal de roche suivant les directions parallèles à l'axe”. Œuvres vol. 1, 731-751 (1822) [5] O. Arteaga, E. Garcia-Caurel, and R. Ossikovski, “Stern-Gerlach experiment with light: separating photons by spin with the method of A. Fresnel”. Opt. Express vol. 27, 4758-4768 (2019)

Photoniques Special EOS Issue 3 www.photoniques.com ❚ Photoniques Special EOS Issue 3 45 https://doi.org/10.1051/photon/2019S444