Birefringent Materials for Polarization Optics - A Technical Guide
Table of Contents
Polarization
By polarized light we mean optical radiation whose electric field oscillates in a specific, regular manner. Every ordinarily polarized electric field can be decomposed into two orthogonally polarized components. If the light is linearly polarized, the electric field oscillates in a single plane — the plane of polarization — and the two components are in phase with one another. If the light is elliptically polarized, the components have a constant phase difference, and the tip of the electric field vector follows a three-dimensional ellipse as the beam propagates.
Circularly polarized light is a special case of elliptically polarized light in which the two components have a phase difference of 90° relative to one another, and the electric field vector traces a spiral with a circular cross-section.
If one looks at a right circularly polarized beam in the direction of the light source, it has a circular vector rotating clockwise, whereas a left circularly polarized beam describes a circle counterclockwise.
Linear Birefringence
Linearly birefringent uniaxial crystalline materials can be characterized by having a single axis of symmetry, called the optic axis, which determines the propagation of the light beam within the crystal.
This can happen in two ways: either as an ordinary ray, which is polarized in a plane perpendicular to the optic axis, or as an extraordinary ray, whose plane of polarization contains the optic axis. Each of these two rays has a different refractive index, so that both the phase velocities (wave normals) and the angles of refraction of the two rays differ from one another. It is this latter property that makes it possible to cut and align prisms made of birefringent material in such a way that they are suitable as polarizers or polarizing beam splitters.
Circular Birefringence
If a linearly polarized beam propagates along the optic axis of a material exhibiting circular birefringence, it is split into two collinear, circularly polarized beams whose propagation velocities differ slightly from one another. Upon exiting the material, these two components recombine into a single linearly polarized beam whose plane of polarization has been rotated relative to that of the incident beam.
This effect of producing a progressive rotation of the plane of polarization with path length is called optical activity and is used in the production of optical rotators.
Birefringent Materials
For the production of our components, we use a variety of materials from a fairly wide range of birefringent crystals — calcite (calcspar), quartz, magnesium fluoride (MgF₂), YVO₄, and α-BBO.
Calcite (CaCO₃, calcspar)
Calcite is often the preferred material due to its very high birefringence, broad spectral transmission, and the availability of crystals of suitable size. Unfortunately, calcite cannot be produced synthetically. Only natural crystals exist, which limits the maximum size of crystals with good optical quality. It has the advantage of not being hygroscopic, so protection from the atmosphere is not required, although it is a fairly soft crystal and is easily scratched.
Through the perfection of several proprietary cutting, grinding, and polishing processes, we achieve calcite prisms with perfect optical surfaces, typically with flatness down to λ/8 over several cm with a 10/5 scratch-dig rating. This capability is reflected in the high quality of the finished components and allows our polarizers to be used with high-peak-power lasers.
Quartz (SiO₂)
Quartz is an extremely useful birefringent material and is available both as natural crystals and as synthetic single-crystal bodies. Natural crystals occur as rough lumps that require orientation to determine the optic axis before the machining process, while synthetic crystals have reference surfaces that are aligned to the crystallographic axis. When deciding which type of quartz to use for a component, two parameters should be considered, in which the two types differ significantly from one another: size and spectral transmission.
In general, components made from synthetic quartz are limited to a maximum size of about 70mm in the direction of the optic axis, while natural quartz single crystals are available large enough to achieve a usable diameter of 100mm. The other significant distinguishing feature between natural and synthetic quartz is the short-wavelength cutoff. Natural quartz has usable transmission (about 80%) starting at 220nm, while synthetic quartz similarly transmits starting at 190nm. Both transmit up to 2.6µm.
Quartz is very hard (Mohs scale 7) and very stable. It is therefore suitable for producing very thin low-order retardation plates. Unlike calcite or magnesium fluoride, quartz exhibits circular birefringence, and there is no uniform direction (optic axis) along which the ordinary and extraordinary rays propagate with the same refractive index and the same velocity. Instead, the optic axis is the direction in which the two refractive indices are closest: a beam propagating along this axis does so as two polarized beams of opposite handedness (chirality). This produces a progressive optical rotation of the plane-polarized incident beam. This effect is of great use in the production of rotators.
Magnesium Fluoride (MgF₂)
Single-crystal magnesium fluoride is another very useful material for the production of polarizers due to its very broad spectral transmission, extending from 120nm in the vacuum UV to beyond 7.0µm in the IR region. Because suitable refractive-index-matching cements do not transmit below 220nm, polarizers made from magnesium fluoride are produced by optically contacting the two prisms.
Thin plates can also be made from this material for use in achromatic retardation plates. In these components, the spectral dispersion of magnesium fluoride is used to compensate for that of quartz, so that a nearly constant retardation is achieved over a defined spectral range.
Yttrium Orthovanadate (YVO₄)
Yttrium orthovanadate is very useful due to its extended wavelength range. We offer polarizers that are functional up to a wavelength of 4000nm (4µm). Magnesium fluoride is also a very good candidate, but the separation angle when using YVO₄ for Wollaston and Rochon polarizers is much larger.
α-Barium Borate (α-BBO)
The α form of BBO (α-BaB₂O₄) at high temperatures is a negative, uniaxial crystal. It has high birefringence over a broad transparency range from 190nm to 3500nm. Due to its exceptional transparency in the UV, good mechanical properties, and high damage threshold, α-BBO is an excellent crystal for replacing calcite, TiO₂, LiNbO₃, etc. in Glan-Taylor and Glan-Thompson polarizers, as well as in beam-displacer prisms — particularly for high-power and UV polarizers. Since the crystal is centrosymmetric, it cannot be used for nonlinear optical applications.
We hope this brief tutorial has been useful to you. Please contact us if you require further information on the available birefringent materials or possible components.
About the Author:
Heike Schleusener
Optics Specialist & Sales
Heike Schleusener is Artifex Engineerings’s optics specialist, with over 10 years of hands-on experience in precision optics. Having personally handled thousands of components, testing and checking them along the way, she has developed a deep understanding of the challenges involved in working with optics. Close contact with customers taught her the questions and problems they run into and time spent visiting companies to help and learn, has kept her closely connected to the industry.
