Optical axis gratings (OAGs) are
gratings of
optical axis
An optical axis is an imaginary line that passes through the geometrical center of an optical system such as a camera lens, microscope or telescopic sight. Lens elements often have rotational symmetry about the axis.
The optical axis defines ...
of a
birefringent
Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefring ...
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
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
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
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
.
Cycloidal OAGs have already been proven to be very efficient in
beam steering and
optical switching.
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 nm).
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.
Applications
Optical axis gratings can be implemented in various materials, including
liquid crystals
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...
,
polymers
A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...
, birefringent crystals,
magnetic
Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
crystals and sub
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
gratings. This new type of grating has broad potential in imaging,
liquid crystal display
A liquid-crystal display (LCD) is a flat-panel display or other Electro-optic modulator, electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liq ...
, communication, and numerous military applications.
References
[{{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 ]
See also
*
Diffraction grating
In optics, a diffraction grating is an optical grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...
*
Liquid crystal
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...
Optical devices