Optical axis gratings 5179603 215583777 2008-05-28T21:00:32Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. '''Optical axis gratings''' (OAGs) are [[diffraction grating | gratings]] of [[optical axis]] of a [[birefringence|birefringent]] material. In OAGs, the birefringence of the material is constant, while the direction of optical axis is periodically modulated in a fixed direction. In this way they are different from the regular phase gratings, in which the [[refractive index]] is modulated and the direction of the optical axis is constant. The optical axis in OAGs can be modulated in either transverse or the longitudinal direction, which causes it to act as a diffractive or a reflective component. Numerous modulation profiles allow variation in the optical properties of the OAGs. == Examples of OAGs == <!-- Unsourced image removed: [[Image:Cycloidal Optical Axis Grating.GIF|500px|right|thumb|Fig. 1. Schematic structure of the cycloidal optical axis grating. Left: top view; right: side view. The optical axis rotates in the transverse direction with a fixed rate, while the material birefringence stays the same. A grating only one micrometer thick can redirect 100% of the incident light, or switch it on and off with [[contrast ratio]] over 1000:1!]] --> An example of a transverse OAG, the so-called cycloidal OAG, is shown in Fig. 1. The optical axis in this grating is monotonously modulated in transverse direction. This grating is capable of diffracting ''all'' incident light into either +1st or −1st order in a [[micrometre|micrometer]]-thick layer <ref name="Sarkissian et al. 1">{{cite journal | author = H. Sarkissian, S.V. Serak, N.V. Tabiryan, L.B. Glebov, V. Rotar, B.Ya. Zeldovich | year = 2006 | month = August | title = Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals | journal = Optics Letters | volume = 31 | issue = 15 | pages = 2248–2250 | url = http://ol.osa.org/abstract.cfm?id=90803 | doi = 10.1364/OL.31.002248 }}</ref> <ref name="Sarkissian et al. 2">{{cite journal | author = H. Sarkissian, N. Tabiryan, B. Park, B. Zeldovich | year = 2006 | title = Periodically Aligned Liquid Crystal: Potential application for projection displays | journal = Molecular Crystals and Liquid Crystals | volume = 451 | pages = 1–19 | doi = 10.1080/154214090959957 }}</ref>. The cycloidal OAGs have already been proven to be very efficient in [[beam steering]] and [[optical switching]].<ref name="Sarkissian et al. 1"> </ref> <!-- Image with unknown copyright status removed: [[Image:Volume Optical Axis Grating with bandgap.GIF|300px|right|thumb|Fig. 2. The optical axis in this volume OAG is modulated with sub[[wavelength]] period. As a result the grating works similar to [[cholesteric liquid crystal]]s, reflecting light with frequencies within the bandgap. However, unlike cholesterics, reflection by OAG is insensitive to [[polarization]].]] --> In another type of OAG, the optical axis is modulated in the direction of light propagation (Fig. 2) with a modulation period equal to a fraction of the wavelength (200-3000 nm)<ref name="Sarkissian et al. 3">{{cite journal | author = H. Sarkissian, N. Tabiryan, B. Zeldovich | title = Polarization-universal bandgap in periodically twisted nematics | journal = Optics Letters | volume = 31 | issue = 11 | pages = 1678–1680 | url = http://ol.osa.org/abstract.cfm?id=89809 | doi = 10.1364/OL.31.001678 | year = 2006 }}</ref><ref name="Sarkissian et al. 4">{{cite journal | author = H. Sarkissian, B. Zeldovich, N. Tabiryan | title = Longitudinally modulated bandgap nematic structure | year = 2006 | uri = http://josab.osa.org/abstract.cfm?id=95673 | volume = 23 | pages = 1712–1717 }}</ref>. This modulation prevents these frequencies from propagating within the grating, acting as a [[band-stop filter]]. As a result, any light with frequency within the matching range will be reflected from the OAG. However, unlike [[Cholesterol#Cholesteric liquid crystals|cholesterics]] which reflect only one of two circular polarizations of incident light, this OAG reflects any polarization<ref name="Sarkissian et al. 3"> </ref><ref name="Sarkissian et al. 4"> </ref>. == Applications == Optical axis gratings can be implemented in various materials, including [[liquid crystals]], [[polymers]], birefringent crystals, [[magnetic]] crystals and sub[[wavelength]] gratings. This new type of grating has broad potential in imaging, [[liquid crystal display]], communication, and numerous military applications. == References == <div class="references-small"> <references/> </div> [[Category:Optics]] [[Category:Optical devices]]