Volume holograms are tremendously versatile optical components, with uses as diverse as head-mounted displays, narrowband filters, and high density data storage. They are made by exposing a thick film of material to an interference pattern, which then changes the refractive index in the bright parts of the pattern. Photopolymers are a promising material to make these from, as they can be processed easily and respond well to high-frequency fringes. However, achieving the required refractive index contrast for wide-angle applications (requiring thin holograms) is difficult. Incorporating small dielectric nanoparticles is a trick that's been used in the past to improve contrast, but Tomita et al. have taken it a step further by using hyperbranched polymer particles with a much higher refractive index than has been available previously. Thanks to the incorporation of a triazine and multiple aromatic rings, their polymer particles have a refractive index of over 1.8, meaning the grating can attain a refractive index contrast better than 0.02. This is enough for nearly 100% efficiency at the design wavelength of 532nm. This is a step forward for volume holographic gratings with a wide acceptance angle, advantageous for many display applications.
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