Wang and co-workers have created switchable holographic diffraction gratings with a line spacing that changes from one side of the grating to the other, using polymer-dispersed liquid crystals. Such gratings (known as Variable Line Spacing or VLS gratings) are often used to correct aberrations resulting from their use at grazing incidence in UV or X-ray spectrometers, and can be used in other contexts to provide easy tuning of wavelength (for example, in a monochromator or tunable laser). In this Applied Optics article, they describe a fabrication method for these gratings using the interference pattern between two laser beams, one of which passes through a cylindrical lens. The fringes so created have a variable fringe pattern that is well described by diffraction theory, which is then transferred to the polymer-dispersed liquid crystal. The pattern creates some regions which are high in polymer and low in liquid crystal (LC), and other regions which are predominantly LC. The polymer is chosen to have the same refractive index as the LC when the LC molecules are aligned by an electric field (around 2.2 V/μm), so that the grating disappears on the application of a voltage (around 120V). This allows it to be switched on and off with a switching time below 1ms. When the electric field is removed, the liquid crystal reorients and (because the refractive index depends on the orientation of the molecules) the grating reappears. With optimised exposure times, the grating could be over 80% efficient, and the authors suggest it will find use in position sensing, modulation, and potentially the next generation of 3D displays.
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