'''Optical axis gratings''' (OAGs) are gratings of optical axis of a 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 ==

<!-- 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!]] --> The optical axis in a transverse or ''cycloidal'' OAG is monotonously modulated in transverse direction. This grating is capable of diffracting ''all'' incident light into either +1st or −1st order in a micrometer-thick layer <ref name="Sarkissian et al. 1">{{cite journal |author1=H. Sarkissian |author2=S.V. Serak |author3=N.V. Tabiryan |author4=L.B. Glebov |author5=V. Rotar |author6=B.Ya. Zeldovich|author6-link=Yakov Zeldovich |date=August 2006 | title = Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals | journal = Optics Letters | volume = 31 | pmid = 16832448 | issue = 15 | pages = 2248–2250 | url = http://ol.osa.org/abstract.cfm?id=90803 | doi = 10.1364/OL.31.002248 |bibcode = 2006OptL...31.2248S |url-access = subscription }}</ref> .<ref name="Sarkissian et al. 2">{{cite journal |author1=H. Sarkissian |author2=N. Tabiryan |author3=B. Park |author4=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 |arxiv=cond-mat/0508555|s2cid=96404563 }}</ref> Cycloidal OAGs have already been proven to be very efficient in beam steering and optical switching.<ref name="Sarkissian et al. 1" />

<!-- 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 subwavelength period. As a result the grating works similar to cholesteric liquid crystals, 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 with a modulation period equal to a fraction of the wavelength (200–3000&nbsp;nm).<ref name="Sarkissian et al. 3">{{cite journal |author1=H. Sarkissian |author2=N. Tabiryan |author3=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 | pmid = 16688259 | year = 2006 |bibcode = 2006OptL...31.1678S | url-access = subscription }}</ref><ref name="Sarkissian et al. 4">{{cite journal | doi = 10.1364/JOSAB.23.001712 |author1=H. Sarkissian |author2=B. Zeldovich |author3=N. Tabiryan | title = Longitudinally modulated bandgap nematic structure | year = 2006 | url = http://josab.osa.org/abstract.cfm?id=95673 | journal = Journal of the Optical Society of America B | volume = 23 |issue=8 | pages = 1712–1717 |bibcode = 2006JOSAB..23.1712S | url-access = subscription }}</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 cholesterics which reflect only one of two circular polarizations of incident light, this OAG reflects any polarization.<ref name="Sarkissian et al. 3" /><ref name="Sarkissian et al. 4" />

== Applications == Optical axis gratings can be implemented in various materials, including liquid crystals, polymers, birefringent crystals, magnetic crystals and subwavelength gratings. This new type of grating has broad potential in imaging, liquid crystal display, communication, and numerous military applications.

== References == {{reflist}}<ref>{{Cite journal |last1=Nersisyan |first1=S. R. |last2=Tabiryan |first2=N. V. |last3=Steeves |first3=D. M. |last4=Kimball |first4=B. R. |date=March 2009 |title=Optical Axis Gratings in Liquid Crystals and their Use for Polarization Insensitive Optical Switching |url=https://www.worldscientific.com/doi/abs/10.1142/S0218863509004555 |journal=Journal of Nonlinear Optical Physics & Materials |language=en |volume=18 |issue=1 |pages=1–47 |doi=10.1142/S0218863509004555 |issn=0218-8635|url-access=subscription }}</ref>

==See also== *Diffraction grating *Liquid crystal

Category:Optical devices