Editing Birefringence (section)

== Applications ==

[[Image:LCD layers.svg|left|thumb|250px| Reflective twisted-nematic [[liquid-crystal display]]. Light reflected by the surface (6) (or coming from a [[backlight]]) is horizontally polarized (5) and passes through the liquid-crystal modulator (3) sandwiched in between transparent layers (2, 4) containing electrodes. Horizontally polarized light is blocked by the vertically oriented polarizer (1), except where its polarization has been rotated by the liquid crystal (3), appearing bright to the viewer.]]

===Optical devices===
Birefringence is used in many optical devices. [[Liquid-crystal display]]s, the most common sort of [[flat-panel display]], cause their pixels to become lighter or darker through rotation of the polarization (circular birefringence) of linearly polarized light as viewed through a sheet polarizer at the screen's surface. Similarly, [[electro-optic modulator|light modulators]] modulate the intensity of light through [[Pockels effect|electrically induced birefringence]] of polarized light followed by a polarizer. The [[Lyot filter]] is a specialized narrowband spectral filter employing the wavelength dependence of birefringence. [[Waveplate]]s are thin birefringent sheets widely used in certain optical equipment for modifying the polarization state of light passing through it.

To manufacture polarizers with high transmittance, birefringent crystals are used in devices such as the [[Glan–Thompson prism]], [[Glan–Taylor prism]] and other variants.<ref name="Home f364">{{cite web | title=Birefringent_ Polarizers | website=Home | url=https://www.pmoptics.com/birefringent_polarizers.html | access-date=2024-03-15}}</ref> Layered birefringent polymer sheets can also be used for this purpose.<ref name="Gilbert Weber Strharsky Stover 2001 p. FA2">{{cite conference | last1=Gilbert | first1=Larry | last2=Weber | first2=M.F. | last3=Strharsky | first3=R.J. | last4=Stover | first4=C.A. | last5=Nevitt | first5=T.J. | last6=Ouderkirk | first6=A.J. | title=Optical Interference Coatings | chapter=Giant birefringent optics in multilayer polymer filters | publisher=OSA | date=2001 | isbn=978-1-55752-682-3 | doi=10.1364/OIC.2001.FA2 | page=FA2}}</ref>

Birefringence also plays an important role in [[second-harmonic generation]] and other [[Nonlinear optics|nonlinear optical processes]]. The crystals used for these purposes are almost always birefringent. By adjusting the angle of incidence, the effective refractive index of the extraordinary ray can be tuned in order to achieve [[Nonlinear optics#Phase matching|phase matching]], which is required for the efficient operation of these devices.

===Medicine===
Birefringence is utilized in medical diagnostics. One powerful accessory used with optical microscopes is a pair of crossed [[polarizer|polarizing]] filters. Light from the source is polarized in the {{mvar|x}} direction after passing through the first polarizer, but above the specimen is a polarizer (a so-called ''analyzer'') oriented in the {{mvar|y}} direction. Therefore, no light from the source will be accepted by the analyzer, and the field will appear dark. Areas of the sample possessing birefringence will generally couple some of the {{mvar|x}}-polarized light into the {{mvar|y}} polarization; these areas will then appear bright against the dark background. Modifications to this basic principle can differentiate between positive and negative birefringence.

{{multiple image
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| footer = Gout and pseudogout crystals viewed under a microscope with a red compensator, which slows red light in one orientation (labeled "polarized light axis").<ref name="Frances1983">{{cite journal | last=Frances Lixey | first=Mary | title=Inexpensive Compensator for a Polarizing Microscope | journal=Laboratory Medicine | publisher=Oxford University Press (OUP) | volume=14 | issue=6 | date=1983-06-01 | issn=0007-5027 | doi=10.1093/labmed/14.6.382 | pages=382}}</ref> Urate crystals ('''left''' image) in [[gout]] appear yellow when their long axis is parallel to the slow transmission axis of the red compensator and appear blue when perpendicular.  The opposite colors are seen in [[calcium pyrophosphate dihydrate crystal deposition disease]] (pseudogout, '''right''' image): blue when parallel and yellow when perpendicular.
| image1 = Birefringence microscopy of gout, annotated.jpg
| image2 = Birefringence microscopy of pseudogout, annotated.jpg
| caption2 = 
}}

For instance, needle aspiration of fluid from a [[gout]]y joint will reveal negatively birefringent [[monosodium urate crystals]]. [[Calcium pyrophosphate]] crystals, in contrast, show weak positive birefringence.<ref>{{cite journal |vauthors=Hardy RH, Nation B |title=Acute gout and the accident and emergency department |journal=Arch Emerg Med |volume=1 |issue=2 |pages=89–95 |date=June 1984 |pmid=6536274 |pmc=1285204 |doi= 10.1136/emj.1.2.89}}</ref> Urate crystals appear yellow, and calcium pyrophosphate crystals appear blue when their long axes are aligned parallel to that of a red compensator filter,<ref>[http://emedicine.medscape.com/article/336054-workup#a0721 The Approach to the Painful Joint Workup] Author: Alan N. Baer; Chief Editor: Herbert S. Diamond. Updated: Nov 22, 2010.</ref> or a crystal of known birefringence is added to the sample for comparison.

The birefringence of tissue inside a living human thigh was measured using polarization-sensitive optical coherence tomography at 1310&nbsp;nm and a single mode fiber in a needle. Skeletal muscle birefringence was Δn = 1.79 × 10<sup>−3</sup> ± 0.18×10<sup>−3</sup>, adipose Δn = 0.07 × 10<sup>−3</sup> ± 0.50 × 10<sup>−3</sup>, superficial aponeurosis Δn = 5.08 × 10<sup>−3</sup> ± 0.73 × 10<sup>−3</sup> and interstitial tissue Δn = 0.65 × 10<sup>−3</sup> ±0.39 × 10<sup>−3</sup>.<ref>{{Cite journal |last1=McBride |first1=Jeffrey M. |last2=Hackmann |first2=Michael J. |last3=Nimphius |first3=Sophia |last4=Cense |first4=Barry |date=2022 |title=In vivo PS-OCT needle probe scan of human skeletal muscle |url=https://doi.org/10.1364/BOE.446169 |journal=[[Biomedical Optics Express]] |volume=13 |issue=3 |pages=1386–1397 |doi=10.1364/BOE.446169 |pmid=35414965 |pmc=8973164 |via=Optica}}</ref> These measurements may be important for the development of a less invasive method to diagnose [[Duchenne muscular dystrophy]].

Birefringence can be observed in [[amyloid]] plaques such as are found in the brains of [[Alzheimer's disease|Alzheimer's]] patients when stained with a dye such as Congo Red. Modified proteins such as [[immunoglobulin]] light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet [[Chemical structure|conformation]]. [[Congo red]] dye [[intercalation (biochemistry)|intercalates]] between the folds and, when observed under polarized light, causes birefringence.

In [[ophthalmology]], binocular [[retinal birefringence screening]] of the ''Henle fibers'' (photoreceptor axons that go radially outward from the fovea) provides a reliable detection of [[strabismus]] and possibly also of [[Amblyopia#Refractive|anisometropic amblyopia]].<ref>{{cite journal|author1=Reed M. Jost|author2=Joost Felius|author3=Eileen E. Birch|title=High sensitivity of binocular retinal birefringence screening for anisometropic amblyopia without strabismus|journal=Journal of American Association for Pediatric Ophthalmology and Strabismus|volume=18|number=4|pages=e5–e6|date=August 2014|doi=10.1016/j.jaapos.2014.07.017}}</ref> In healthy subjects, the maximum retardation induced by the Henle fiber layer is approximately 22 degrees at 840&nbsp;nm.<ref>{{Cite journal|last1=Cense|first1=Barry|last2=Wang|first2=Qiang|last3=Lee|first3=Sangyeol|last4=Zhao|first4=Liang|last5=Elsner|first5=Ann E.|last6=Hitzenberger|first6=Christoph K.|last7=Miller|first7=Donald T.|date=2013-11-01|title=Henle fiber layer phase retardation measured with polarization-sensitive optical coherence tomography|url=https://www.osapublishing.org/boe/abstract.cfm?uri=boe-4-11-2296|journal=Biomedical Optics Express|language=EN|volume=4|issue=11|pages=2296–2306|doi=10.1364/BOE.4.002296 |pmid=24298395| pmc=3829392 |issn=2156-7085}}</ref> Furthermore, [[scanning laser polarimetry]] uses the birefringence of the [[optic nerve]] fiber layer to indirectly quantify its thickness, which is of use in the assessment and monitoring of [[glaucoma]]. Polarization-sensitive optical coherence tomography measurements obtained from healthy human subjects have demonstrated a change in birefringence of the retinal nerve fiber layer as a function of location around the optic nerve head.<ref>{{Cite journal|last1=Cense|first1=Barry|last2=Chen|first2=Teresa C.|last3=Park|first3=B. Hyle|last4=Pierce|first4=Mark C.|last5=Boer|first5=Johannes F. de|date=2004-08-01|title=Thickness and Birefringence of Healthy Retinal Nerve Fiber Layer Tissue Measured with Polarization-Sensitive Optical Coherence Tomography|url=https://iovs.arvojournals.org/article.aspx?articleid=2124143|journal=Investigative Ophthalmology & Visual Science|language=en|volume=45|issue=8|pages=2606–2612|doi=10.1167/iovs.03-1160|pmid=15277483|issn=1552-5783}}</ref> The same technology was recently applied in the living human retina to quantify the polarization properties of vessel walls near the optic nerve.<ref>{{Cite journal |last1=Afsharan |first1=Hadi |last2=Hackmann |first2=Michael J. |last3=Wang |first3=Qiang |last4=Navaeipour |first4=Farzaneh |last5=Jayasree |first5=Stephy Vijaya Kumar |last6=Zawadzki |first6=Robert J. |last7=Silva |first7=Dilusha |last8=Joo |first8=Chulmin |last9=Cense |first9=Barry |date=2021-07-01 |title=Polarization properties of retinal blood vessel walls measured with polarization sensitive optical coherence tomography |url=https://www.osapublishing.org/boe/abstract.cfm?uri=boe-12-7-4340 |journal=[[Biomedical Optics Express]] |language=EN |volume=12 |issue=7 |pages=4340–4362 |doi=10.1364/BOE.426079 |issn=2156-7085 |pmc=8367251 |pmid=34457418}}</ref> While retinal vessel walls become thicker and less birefringent in patients who suffer from hypertension,<ref>{{Cite journal |last1=Afsharan |first1=Hadi |last2=Anilkumar |first2=Vidyalakshmi |last3=Silva |first3=Dilusha |last4=Dwivedi |first4=Girish |last5=Joo |first5=Chulmin |last6=Cense |first6=Barry |date=2024-01-01 |title=Hypertension-associated changes in retinal blood vessel walls measured in vivo with polarization-sensitive optical coherence tomography |journal=Optics and Lasers in Engineering |volume=172 |pages=107838 |doi=10.1016/j.optlaseng.2023.107838 |issn=0143-8166|doi-access=free |bibcode=2024OptLE.17207838A }}</ref> hinting at a decrease in vessel wall condition, the vessel walls of diabetic patients do not experience a change in thickness, but do see an increase in birefringence,<ref>{{Cite journal |last1=Afsharan |first1=Hadi |last2=Silva |first2=Dilusha |last3=Joo |first3=Chulmin |last4=Cense |first4=Barry |date=August 2023 |title=Non-Invasive Retinal Blood Vessel Wall Measurements with Polarization-Sensitive Optical Coherence Tomography for Diabetes Assessment: A Quantitative Study |journal=Biomolecules |language=en |volume=13 |issue=8 |pages=1230 |doi=10.3390/biom13081230 |issn=2218-273X |pmc=10452597 |pmid=37627295 |doi-access=free }}</ref> presumably due to fibrosis or inflammation.

Birefringence characteristics in [[sperm head]]s allow the selection of spermatozoa for [[intracytoplasmic sperm injection]].<ref>{{cite journal |author=Gianaroli L. |author2=Magli M. C. |author3=Ferraretti A. P. |title=Birefringence characteristics in sperm heads allow for the selection of reacted spermatozoa for intracytoplasmic sperm injection |journal=Fertil. Steril. |volume= 93|issue= 3|pages= 807–813|date=December 2008 |pmid=19064263 |doi=10.1016/j.fertnstert.2008.10.024 |display-authors=etal|doi-access=free }}</ref> Likewise, ''[[zona imaging]]'' uses birefringence on [[oocyte]]s to select the ones with highest chances of successful pregnancy.<ref>{{cite journal |author=Ebner T. |author2=Balaban B. |author3=Moser M. |title=Automatic user-independent zona pellucida imaging at the oocyte stage allows for the prediction of preimplantation development |journal=Fertil. Steril. |volume= 94|issue= 3|pages= 913–920|date=May 2009 |pmid=19439291 |doi=10.1016/j.fertnstert.2009.03.106 |display-authors=etal|doi-access=free }}</ref> Birefringence of particles biopsied from pulmonary nodules indicates [[silicosis]].

Dermatologists use dermatoscopes to view skin lesions. Dermoscopes use polarized light, allowing the user to view crystalline structures corresponding to dermal collagen in the skin. These structures may appear as shiny white lines or rosette shapes and are only visible under polarized [[dermoscopy]].

===Stress-induced birefringence===
[[Image:Birefringence Stress Plastic.JPG|left|thumb|250px|Color pattern of a plastic box with "frozen in" [[mechanical stress]] placed between two crossed [[polarizer]]s]]
[[Isotropic]] solids do not exhibit birefringence. When they are under [[mechanical stress]], birefringence results. The stress can be applied externally or is "frozen in" after a birefringent plastic ware is cooled after it is manufactured using [[injection molding]]. When such a sample is placed between two crossed polarizers, colour patterns can be observed, because polarization of a light ray is rotated after passing through a birefringent material and the amount of rotation is dependent on wavelength. The experimental method called [[photoelasticity]] used for analyzing stress distribution in solids is based on the same principle. There has been recent research on using stress-induced birefringence in a glass plate to generate an [[optical vortex]] and full Poincare beams (optical beams that have every possible polarization state across a cross-section).<ref>{{Cite journal|last1=Beckley|first1=Amber M.|last2=Brown|first2=Thomas G.|last3=Alonso|first3=Miguel A.|date=2010-05-10|title=Full Poincaré beams|url=https://www.osapublishing.org/abstract.cfm?uri=oe-18-10-10777|journal=Optics Express|language=EN|volume=18|issue=10|pages=10777–10785|doi=10.1364/OE.18.010777|pmid=20588931|issn=1094-4087|bibcode=2010OExpr..1810777B|doi-access=free}}</ref>
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===Other cases of birefringence===

[[File:Rutile birefringence.gif|right|thumb|Birefringent [[rutile]] observed in different polarizations using a rotating polarizer (or ''analyzer'')]]
Birefringence is observed in anisotropic [[elastic deformation|elastic]] materials. In these materials, the two polarizations split according to their effective refractive indices, which are also sensitive to stress.

The study of birefringence in [[shear wave]]s traveling through the solid Earth (the Earth's liquid core does not support shear waves) is widely used in [[seismology]]. {{citation needed|date=July 2020}}

Birefringence is widely used in mineralogy to identify rocks, minerals, and gemstones.{{citation needed|date=July 2020}}
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