Editing Liquid crystal (section)

==Applications of liquid crystals==
{{see also|Liquid-crystal display}}
[[File:LCD layers.svg|thumb|Structure of liquid crystal display: 1 – vertical polarization filter, 2, 4 – glass with electrodes, 3 – liquid crystals, 5 – horizontal polarization filter, 6 – reflector]]
[[File:Wikipedia Liquid Crystal Display Arduino.jpg|thumb|left|"Wikipedia" displayed on an LCD]]

Liquid crystals find wide use in liquid crystal displays, which rely on the [[optics|optical]] properties of certain liquid crystalline substances in the presence or absence of an [[electric field]]. In a typical device, a liquid crystal layer (typically 4 μm thick) sits between two [[polarizer]]s that are crossed (oriented at 90° to one another). The liquid crystal alignment is chosen so that its relaxed phase is a twisted one (see [[Twisted nematic field effect]]).<ref name=castellano/> This twisted phase reorients light that has passed through the first polarizer, allowing its transmission through the second polarizer (and reflected back to the observer if a reflector is provided). The device thus appears transparent. When an electric field is applied to the LC layer, the long molecular axes tend to align parallel to the electric field thus gradually untwisting in the center of the liquid crystal layer. In this state, the LC molecules do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the device loses transparency with increasing voltage. In this way, the electric field can be used to make a pixel switch between transparent or opaque on command. Color LCD systems use the same technique, with color filters used to generate red, green, and blue pixels.<ref name=castellano/> Chiral smectic liquid crystals are used in ferroelectric LCDs which are fast-switching binary light modulators. Similar principles can be used to make other liquid crystal based optical devices.<ref>{{cite journal|title = Integrating liquid crystal based optical devices in photonic crystal| doi = 10.1007/s11082-007-9139-8|journal=Optical and Quantum Electronics|volume = 39| issue =12–13|date =2007| page = 1009|last1 = Alkeskjold|first1 = Thomas Tanggaard|last2 = Scolari|first2 = Lara|last3 = Noordegraaf|first3 = Danny|last4 = Lægsgaard|first4 = Jesper|last5 = Weirich|first5 = Johannes|last6 = Wei|first6 = Lei|last7 = Tartarini|first7 = Giovanni|last8 = Bassi|first8 = Paolo|last9 = Gauza|first9 = Sebastian| last10 = Wu| first10 = Shin-Tson| last11 = Bjarklev| first11 = Anders | name-list-style = vanc | s2cid = 54208691}}</ref>

[[Liquid crystal tunable filter]]s are used as [[electrooptical|electro-optical]] devices,<ref>{{cite book|url=https://books.google.com/books?id=PKwlrW-CrxUC&pg=PA16|title=Piezoelectric Nanomaterials for Biomedical Applications|last1=Ciofani|first1=Gianni|last2=Menciassi|first2=Arianna | name-list-style = vanc |date=2012|publisher=Springer Science & Business Media|isbn=9783642280443}}</ref><ref>{{cite journal |author=A.D. Chandra & A. Banerjee |url=https://www.tandfonline.com/doi/abs/10.1080/09500340.2020.1760954 |title=Rapid phase calibration of a spatial light modulator using novel phase masks and optimization of its efficiency using an iterative algorithm |journal=Journal of Modern Optics |year=2020 |volume=67 |issue=7 |pages=628–637 |publisher=Journal of Modern Optics, Volume 67, Issue 7, 18 May 2020 |doi=10.1080/09500340.2020.1760954 |arxiv=1811.03297 |bibcode=2020JMOp...67..628C |s2cid=219646821 |access-date=January 7, 2021 |archive-date=April 10, 2022 |archive-url=https://web.archive.org/web/20220410015422/https://www.tandfonline.com/doi/abs/10.1080/09500340.2020.1760954 |url-status=live }}</ref> e.g., in [[hyperspectral imaging]].

[[thermochromism|Thermotropic]] chiral LCs whose pitch varies strongly with temperature can be used as crude [[liquid crystal thermometer]]s, since the color of the material will change as the pitch is changed. Liquid crystal color transitions are used on many aquarium and pool thermometers as well as on thermometers for infants or baths.<ref>{{cite patent | inventor = Plimpton RG | title = Pool thermometer | country = US | number = 4738549 | gdate = 1988-04-19}}</ref> Other liquid crystal materials change color when stretched or stressed. Thus, liquid crystal sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, and so on. Liquid crystal in fluid form is used to detect electrically generated hot spots for [[failure analysis]] in the [[semiconductor]] industry.<ref>{{cite web|url=http://www.acceleratedanalysis.com/LC_hotspotdetection_procedure.html |title=Hot-spot detection techniques for ICs |access-date=May 5, 2009 |work=acceleratedanalysis.com |url-status=dead |archive-url=https://web.archive.org/web/20090211050934/http://www.acceleratedanalysis.com/LC_hotspotdetection_procedure.html |archive-date=February 11, 2009 }}</ref>

Liquid crystal lenses converge or diverge the incident light by adjusting the refractive index of liquid crystal layer with applied voltage or temperature. Generally, the liquid crystal lenses generate a parabolic refractive index distribution by arranging molecular orientations. Therefore, a plane wave is reshaped into a parabolic wavefront by a liquid crystal lens. The [[focal length]] of liquid crystal lenses could be continuously tunable when the external electric field can be properly tuned. Liquid crystal lenses are a kind of [[adaptive optics]]. Imaging systems can benefit from focusing correction, image plane adjustment, or changing the range of [[depth-of-field]] or [[depth of focus]]. The liquid crystal lense is one of the candidates to develop vision correction devices for [[myopia]] and [[presbyopia]] (e.g., tunable eyeglass and smart contact lenses).<ref>{{cite journal| vauthors = Sato S |title=Liquid-Crystal Lens-Cells with Variable Focal Length|journal=Japanese Journal of Applied Physics|volume=18|issue=9|pages=1679–1684|doi=10.1143/JJAP.18.1679|year=1979|bibcode=1979JaJAP..18.1679S|s2cid=119784753 }}</ref><ref>{{cite journal| vauthors = Lin YH, Wang YJ, Reshetnyak V |s2cid=139938136|title=Liquid crystal lenses with tunable focal length|journal=Liquid Crystals Reviews|volume=5|issue=2|pages=111–143|doi=10.1080/21680396.2018.1440256|year=2017}}</ref> Being an optical [[phase modulator]], a liquid crystal lens feature space-variant [[optical path length]] (i.e., optical path length as the function of its pupil coordinate). In different imaging system, the required function of [[optical path length]] varies from one to another. For example, to converge a plane wave into a diffraction limited spot, for a physically-planar liquid crystal structure, the refractive index of liquid crystal layer should be spherical or paraboloidal under [[paraxial approximation]]. As for projecting images or sensing objects, it may be expected to have the liquid crystal lens with aspheric distribution of optical path length across its aperture of interest. Liquid crystal lenses with electrically tunable refractive index (by addressing the different magnitude of electric field on liquid crystal layer) have potentials to achieve arbitrary function of [[optical path length]] for modulating incoming wavefront; current liquid crystal [[Freeform surface machining|freeform]] optical elements were extended from liquid crystal lens with same optical mechanisms.<ref>{{cite journal |doi=10.1364/OE.389647 |doi-access=free |title=Phase modulators with tunability in wavefronts and optical axes originating from anisotropic molecular tilts under symmetric electric field II: Experiments |year=2020 |last1=Wang |first1=Yu-Jen |last2=Lin |first2=Yi-Hsin |last3=Cakmakci |first3=Ozan |last4=Reshetnyak |first4=Victor |journal=Optics Express |volume=28 |issue=6 |pages=8985–9001 |pmid=32225513 |bibcode=2020OExpr..28.8985W |s2cid=214734642 }}</ref> The applications of liquid crystals lenses includes pico-projectors, prescriptions lenses (eyeglasses or contact lenses), smart phone camera, augmented reality, virtual reality etc.

[[Liquid crystal laser]]s use a liquid crystal in the [[Active laser medium|lasing medium]] as a distributed feedback mechanism instead of external mirrors. Emission at a [[Photonic crystal|photonic bandgap]] created by the periodic dielectric structure of the liquid crystal gives a low-threshold high-output device with stable monochromatic emission.<ref name="Kopp1998">{{cite journal | vauthors = Kopp VI, Fan B, Vithana HK, Genack AZ | title = Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals | journal = Optics Letters | volume = 23 | issue = 21 | pages = 1707–9 | date = November 1998 | pmid = 18091891 | doi = 10.1364/OL.23.001707 | bibcode = 1998OptL...23.1707K }}</ref><ref name="Dolgaleva2008">{{cite journal | vauthors = Dolgaleva K, Wei SK, Lukishova SG, Chen SH, Schwertz K, Boyd RW | title = Enhanced laser performance of cholesteric liquid crystals doped with oligofluorene dye | journal=Journal of the Optical Society of America | date = 2008 | volume = 25 | issue = 9 | pages = 1496–1504| doi=10.1364/JOSAB.25.001496 | bibcode=2008JOSAB..25.1496D}}</ref>

[[Smart glass#Polymer-dispersed liquid-crystal devices|Polymer dispersed liquid crystal]] (PDLC) sheets and rolls are available as adhesive backed [[Smart film]] which can be applied to windows and electrically switched between transparent and opaque to provide privacy.

Many common fluids, such as [[soap|soapy water]], are in fact liquid crystals. Soap forms a variety of LC phases depending on its concentration in water.<ref>{{cite journal| title = Structure of the Liquid-Crystal Phases of the Soap–water System: Middle Soap and Neat Soap| journal=Nature|volume = 180|date = 1957| issue =4586| page = 600| doi =10.1038/180600a0|bibcode = 1957Natur.180..600L| vauthors = Luzzati V, Mustacchi H, Skoulios A | s2cid=4163714}}</ref>

Liquid crystal films have revolutionized the world of technology. Currently they are used in the most diverse devices, such as digital clocks, mobile phones, calculating machines and televisions. The use of liquid crystal films in optical memory devices, with a process similar to the recording and reading of [[Compact disc|CDs]] and [[DVD]]s may be possible.<ref>{{cite journal | vauthors = Silva MC, Sotomayor J, Figueirinhas J | title = Effect of an additive on the permanent memory effect of polymer dispersed liquid crystal films. | journal = Journal of Chemical Technology & Biotechnology | date = September 2015 | volume = 90 | issue = 9 | pages = 1565–9 | doi =  10.1002/jctb.4677| bibcode = 2015JCTB...90.1565S }}</ref><ref>{{cite journal | vauthors = da Silva MC, Figueirinhas JL, Sotomayor JC | title = Improvement of permanent memory effect in PDLC films using TX-100 as an additive. | journal = Liquid Crystals | date = January 2016 | volume = 43 | issue = 1 | pages = 124–30 | doi = 10.1080/02678292.2015.1061713 | s2cid = 101996816 }}</ref>

Liquid crystals are also used as basic technology to imitate [[quantum computing|quantum computers]], using [[electric field|electric fields]] to manipulate the orientation of the liquid crystal [[molecule|molecules]], to store data and to encode a different value for every different degree of misalignment with other molecules.<ref>{{cite journal|first1=Karmela|last1=Padavic-Callaghan|url=https://www.newscientist.com/article/2334577-computer-made-from-liquid-crystals-would-ripple-with-calculations/|title=Computer made from liquid crystals would ripple with calculations|journal=Science Advances|publisher=[[New Scientist]]|doi=10.1126/sciadv.abp8371|date=August 19, 2022|volume=8|issue=33|pages=eabp8371|pmid=35984880|pmc=9390992|hdl=1721.1/145669|access-date=August 24, 2022|archive-date=August 24, 2022|archive-url=https://web.archive.org/web/20220824200410/https://www.newscientist.com/article/2334577-computer-made-from-liquid-crystals-would-ripple-with-calculations/|url-status=live}}</ref><ref>{{cite web|url=https://eandt.theiet.org/content/articles/2022/08/liquid-crystals-could-be-used-to-make-computers-mit-study-says|title=Researchers claim that ripples and imperfections in liquid crystals like those found in LCD TVs could be used to build a new type of computer|publisher=[[ Engineering and Technology]]|date=August 22, 2022|access-date=August 24, 2022|archive-date=August 26, 2022|archive-url=https://web.archive.org/web/20220826104728/https://eandt.theiet.org/content/articles/2022/08/liquid-crystals-could-be-used-to-make-computers-mit-study-says/|url-status=live}}</ref>