Using acrylamide-based photopolymers for fabrication of holographic optical elements in solar energy applications.

A holographic device is under development that aims to improve light collection in solar cells. The aim is to explore the potential of using photopolymer holographic optical elements (HOEs) to collect light from a moving source, such as the sun, and redirect it for concentration by a holographic lens. A working range of 45° is targeted for such a device to be useful in solar applications without tracking. A photopolymer HOE is capable of efficiently redirecting light, but the angular selectivity of a single grating is usually of the order of one degree at the thicknesses required for high efficiency. The challenge here is to increase the angular and wavelength range of the gratings so that a reasonable number may be multiplexed and/or combined to create a device that can concentrate light incident from a large range of angles. In this paper, low spatial frequency holographic recording is explored to increase the angular and wavelength range of an individual grating. Ultimately, a combination of gratings will be used so that a broad range of angles of incidence are accepted. A design is proposed for the combination of such elements into a holographic solar collector. The first step in achieving this is optimization of recording at low spatial frequency. This requires a photopolymer material with unique properties, such as a fast monomer diffusion rate. This paper reports results on the efficiency of holograms recorded in an acrylamide-based photopolymer at low spatial frequencies (100, 200, and 300  l/mm). The diffraction efficiency and angular selectivity of recorded holograms have been studied for various photopolymer layer thicknesses and different intensities of the recording beams. A diffraction efficiency of over 80% was achieved at a spatial frequency of 200  l/mm. The optimum intensity of recording at this spatial frequency was found to be 1  mW/cm2. Individual gratings and focusing elements with high efficiency and FWHM angles of 3° are experimentally demonstrated.