## 3 Results
### Two Lenses in Stacked in Dual-Sensitive Photopolymer Layer  
For the final device, the dual-sensitized layer was prepared as described above. Two lenses were then recorded sequentially using the same optical set-up (described above) but switching an Ar-Ion lasers lasing wavelength from 514 nm and 488 nm between the recording, using the lasers coupling prism. For each exposure, the intensity and duration followed the optimized conditions found in the previous sections. In the object beam, a conventional cylindrical lens was introduced to create a diverging beam in one axis, which interfered with a collimated reference beam. The resulting interference pattern was recorded in each photopolymer layer. Firstly, a laser (\(\lambda\) = 514 nm) records the lens pattern in the erythrosine-B-sensitized photopolymer layer, which is the second layer in the stack. Secondly, the laser at 488 nm records the lens in the acriflavine-sensitized photopolymer material, which is the first layer in the stack. After recording, the stacked lenses were probed at three positions (center, +3 mm and \(-\)3 mm), over a wide range of angles using a 633 nm wavelength laser to obtain the zero-order transmission angular selectivity curve. A set of four stacked lenses were recorded with the same optimized recording conditions (from Section 3.3) and their results are shown in Figure 11.  
In Table 2, the angular separation of the incident angles at which maximum diffraction occurs for 633 nm light is tabulated for the three positions in all four samples. A theoretical angular separation is also shown, based on the diffraction angle expected for the recorded spatial frequency at that location. Neither lens is designed for 633 nm light, which is why some secondary peaks were observed (not noted in the table). The peak positions are closer to the positions for the lens designed for blue light, except in the case of sample 2, possibly because this is the lens that the incident beam meets first in the stack.  
Figure 11: Zero-order transmission angular selectivity curves when the stacked lens is probed by a He-Ne laser at three positions. (**a**), (**b**), (**c**) and (**d**) show the results for samples (1), (2), (3) and (4) respectively.  
In order to examine the diffraction angles at the target wavelengths, sample 1 was then tested at the design/recording wavelengths. Using a manual rotation stage, the angular position of the diffraction peaks was obtained, and the angular separation or full diffraction angle was calculated. Table 3 shows the measured diffraction angle (angular separation of +1 and \(-1\) diffraction orders) in comparison to the theoretical diffraction angles for both gratings in the stack while probing at the recording/design wavelengths. In most cases (+3 mm G1 is the exception) there is better than 0.5\({}^{\circ}\) agreement between the measured diffraction angle for the stack with the expected diffraction angle for that wavelength.  
The stack was then illuminated with laser light. The input beam was a horizontally diverging beam produced by the cylindrical lens (the same lens that was used in recording). It was arranged that this be incident on the stack at the appropriate angle and the laser was then switched between wavelengths of 488 nm and 514 nm. In both cases, an intense diffracted beam was clearly observed (to the right in Figure 12). While the transmitted portion of the input beam (seen on the left in each figure) was highly diverging, the diffracted output beam was observed to have much lower divergence and to have a circular cross-section. This demonstrates that the stack functions well for both wavelengths. As shown in Figure 12c, the process was repeated with a white light which was collimated and passed through the same cylindrical lens. Again, the stack produced a strong diffracted beam with a circular cross section (to the right in the image). Of course, some dispersion of the colors in the white light source is also evident, especially for the red and violet colors at the extreme ends of the visible spectrum.