Photopolymerizable Glass Outperforms Traditional Holographic Materials
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observations also revealed that pores are still present in the center of many granules. No features were revealed when operating in visible light. These results demonstrated that the liquid immersion method using IR exposure leads to significant improvement in detecting defects, although the minimum size of detectable features is 1 µm. However, the researchers indicate that this is not a problem because the size of fabrication defects in this type of ceramic is usually larger. SIARI S. SOSA
Photopolymerizable Glass Outperforms Traditional Holographic Materials Studies of organically modified silica glass have produced a material with high refractive index modulation, high optical quality, and photosensitivity. Plus, the material can be cast as a monolith or a film. In the March 12 issue of Applied Physics Letters, Pavel Cheben of the National Research Council of Canada and Maria L. Calvo of the Complutense University of Madrid have reported that by combining a titanocene photoinitiator and an acrylic monomer with a sol-gel precursor, a material is produced which is ideal for volume holographic recording. Photoinitiation of ethylene glycol phenyl ether acrylate was accomplished with bis(µ5-2,4-cyclopentadien-1-yl)-bis-[2,6difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium. Both photoinitiator and monomer were added to the silica sol prior to the gelation point in concentrations of 0.4–0.8 wt% and 40 wt%, respectively. The solution was stirred for 20 min at 40°C until homogeneous, at which point it was cast into Teflon vials and allowed to gel and dry for 10 days at 60°C. Thin films of ~260 µm thickness were prepared by casting 0.1 mL of the solution onto a borosilicate glass plate and dried for 5 days at 60°C. Characterization of the holographic performance of the glass was achieved by writing a grating pattern into the monolith or thin film, followed by reading the pattern with another beam. The glass refractive index was measured as n = 1.47 by a prism-coupling technique. The diffraction efficiency dependence on the angle of incidence was measured by detuning the readout beam from the Bragg condition, yielding the grating angular selectivity. The refractive index modulation (∆n) amplitude and effective thickness of the grating were calculated using Kogelnik’s coupled-wave theory. The gratings in the monoliths produced ∆n = 1.35 E-3 for E ~ 1.5 J/cm2, and thus a sensitivity of S = 8.6 E-4 cm2/J (E is exposure). A dynamic sensitivity S* = d(∆n)/dE
= 1.8 E-3 cm2/J was achieved in the region of exposure 0.1–0.35 J/cm2. The gratings in the 260-µm-thick films produced ∆n = 4.5 E-3 for E = 0.72 J/cm2 and thus a sensitivity of S = 6.2 E-3 cm2/J. The researchers discussed the highly efficient formation of gratings in this material in terms of photopolymerization in the light regions. As the monomer is depleted in the light regions, mechanisms involving diffusion and capillary forces move monomers from the dark regions into the light regions where they participate in the photopolymerization process. The hybrid organic-inorganic g
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