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The Diffraction Grating
Published in Abdul Al-Azzawi, Photonics, 2017
Holographic gratings are formed by the interference fringes of two laser beams when the standing wave pattern is exposed to a polished substrate coated with photo resist. Processing of the exposed medium results in a pattern of straight lines with a sinusoidal cross section, as shown in Figure 19.3. Holographic gratings produce less stray light than ruled gratings. They also can be produced with different numbers of grooves per millimetre for greater resolving power. Due to their sinusoidal cross section, holographic gratings cannot be easily blazed, and their efficiency is usually considerably less than a comparable ruled grating. There are, however, special exceptions, which should be noted. When the ratio of groove spacing to wavelength is near one, a holographic grating has virtually the same efficiency as the ruled version. A holographic grating with 1800 grooves per millimetre has the same efficiency at 0.500 µm as a blazed ruled grating.
Optical Components
Published in Rajpal S. Sirohi, Mahendra P. Kothiyal, Optical Components, Systems, and Measurement Techniques, 2017
Rajpal S. Sirohi, Mahendra P. Kothiyal
Holographic gratings are realized by recording interference patterns on an optically good surface. The surface can be plane, spherical or aspherical. The fringe spacing may also be variable. Such gratings can be aluminized to give higher irradiance; they can also be blazed to give higher irradiance in the order of interest. These gratings are free from ghost images and have very low scattering. They can also be produced in large sizes. These are available in the following three types:
Optical Tamm states in a hybrid structure with a holographic polymer-liquid crystal grating
Published in Liquid Crystals, 2023
V. Yu. Reshetnyak, I. P. Pinkevych, N. P. Godman, T. J. Bunning, D. R. Evans
Liquid crystals (LCs) easily change their state under the action of external fields and therefore are used to control surface and Tamm plasmons [24–31]. Moreover, since light in cholesteric LC undergoes a Bragg reflection if the circular polarisation of the light coincides with the cholesteric helix, cholesteric LC can be used as an analogue of the Bragg mirror in designs for OTS excitation [32,33]. However, holographic polymer-liquid crystal gratings (HPLCG) having a form of periodically alternating polymer-rich and LC-rich layers can also be used for this purpose, with even greater possibilities, since the polarisation of the incident light can be arbitrary. There are various methods that ensure the recording of such holographic gratings by the interference of two intersecting coherent laser beams in a photosensitive mixture of a monomer and LC [34–45].
Liquid crystal light valves as optically addressed liquid crystal spatial light modulators: optical wave mixing and sensing applications
Published in Liquid Crystals Reviews, 2018
S. Residori, U. Bortolozzo, J. P. Huignard
Dynamical holographic gratings [13] can be realized by sending two or more interfering beams onto the LCLV. In such a case optical wave mixing occurs, leading to the dynamical exchange of phase and amplitude between the interfering beams. The setup for optical wave mixing is schematically depicted in Figure 2(a). A reference beam, , is sent onto the LCLV together with a signal beam, . The total electric field at the input of the LCLV can be written as where are the amplitudes of the reference and the signal waves, respectively, their respective propagation vectors and their frequencies. The two beams produce an intensity fringe pattern where is the total input intensity, is the grating wave vector and the frequency detuning between the two waves. The ratio between the two intensities, , is a parameter of the wave-mixing process and is usually kept much larger than one. In such a case, and a large gain coefficient can be achieved for the signal beam while the reference beam can be considered undepleted.