**5. Experimental**

**Figure 4.** (a) Sketch of the experimental set-up used by Hung et al. [64] for the doubly slanted layer structure. Diffrac-

Castro et al. [12] reported a detailed study on the design and characterization of a holographic grating used to address the direct sunlight on PV cell with the maximized energy efficiency possible. In particular, they analyzed the effects of incident spectra that vary hourly, daily and seasonally. To maximize the energy collection efficiency during the course of a year, the authors

**Figure 5.** (a) Holographic solar concentrator structure proposed by Castro et al. [12]. Dashed box: unit cell. (b) Holo-

The unit cell includes two cascaded holographic grating on each side of the PV cell (holograms A and B). The holograms on each side of the PV cell are conjugated (i.e. A and A' or B and B') to provide peak energy collection at different seasons. In order to reduce the optical crosstalk of the V-HOEs, the two cascaded holograms are designed to diffract light in opposite directions with the incident angles in different quadrants of the Bragg circle (**Figure 5(b)**). Moreover, the geometrical parameters of the system (such as the hologram width and the distance hologram PV cell) are optimized to assure that maximum of the diffracted rays of the sunlight within the solar responsivity spectrum of PV cell can reach the surface of the cell independently of the incident angle. An energy increase due to the concentrator averaged over a particular day of 147% can be obtained, and nearly 50% of the available energy illuminating hologram areas can

Hsieh et al. presented a solar concentrator based on a so-called 90° hologram that allows obtaining a compact and wide-angle structure [60]. The conceptual recording set-up is

proposed the system based on the structure illustrated in **Figure 5(a)**.

be collected by photovoltaic cells without the need of tracking.

tion patterns evaluated taken from behind (b) and above (c).

38 Holographic Materials and Optical Systems

graphic design to reduce the optical crosstalk.

#### **5.1. Photopolymer**

The recording material was a prototype of photopolymer sensitive to light at wavelength of 532 nm. It was obtained by sol-gel reaction of functionalized alkoxysilanes in acidic conditions and by adding a mixture of acrylic monomers and bis[2,6-difluoro-3-(1-hydropyrrol-1-yl) phenyl]titanocene photoinitiator before thin-film deposition. The mixture was made of halogenated high refractive index species dissolved in phenoxyethyl acrylate and methacrylic acid. Photopolymer was deposited through bar-coating method to obtain 30 μm thickness films. The films were exposed to green light pattern for hologram recording and subsequently to halogen lamp to bleach the unreacted photoinitiator. The final modulation of refractive index showed by this photopolymer is of about 0.02 [65].

In our previous paper, we experimentally demonstrated that this new photosensitive material allows to record volume, holographic diffraction gratings with a very good diffraction efficiency of about 94% [66].

#### **5.2. Recording set-up**

The dimensions of an individual HOE range from 1 cm × 1 cm to 10 cm × 10 cm. In a step-bystep exposure process by coherent and monochromatic light (laser), the holograms are produced in patterns on a film, which can have a maximum size of 1 m × 2 m at the present state of technology.

The experimental set-up used to record holographic in-line spherical lenses was a typical Michelson interferometer with a concave mirror with a focal length of 5 cm placed on the object beam. A recording light source emitting at 532 nm (green) with a maximum power of 750 mW in CW and a coherence length up to 100 m was used. The diameter of the hologram was about 4 cm. To record an off-axis holographic cylindrical lens, the experimental set-up was modified, and two beams of equal intensity interfere with an angle α at the surface of the recording medium. A commercial cylindrical lens, with a focal length of 5.08 cm, is placed on the object beam. Finally, to record a volume holographic grating (VHG), two collimated beams interfere with an angle α at the surface of the recording medium.
