In this paper, a new approach is proposed to realize a lens-less and 90 degrees light-bending by a volume-holographic element, consisting of 3D sub-wavelength index-gratings throughout the sample. In this approach, a top incident plane-wave is diffracted by 90 degrees and guided into a planar volume hologram. Conversely, a side-incident light may be guided and diffracted out of the top sample surface. The diffraction-efficiency eta of light was studied in real-time during light-exposure and also in the dark after exposure to observe "dark-enhancement". It is shown that 90 degrees light-bending at normal incidence can be achieved with a high efficiency, eta similar to 70%, and a small angular-tolerance, Delta theta=0.02 degrees. It is further shown, as a proof-of-concept, that a 5 degrees cylindrical-wave top-incident beam may be used to improve angular tolerance to Delta theta=5 degrees, but with a reduced efficiency of eta similar to 0.80%. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.
- Holographic optical elements; Volume gratings; Diffractive optics