Properties of Holographic Gratings Photoinduced in Polymethyl Methacrylate

Abstract

Polymethyl methacrylate, PMMA, sensitized for λ = 0.325 μm, is shown to exhibit a peak refractive index change of 2.3 × 10⁻³. The index change has been characterized in relation to its sensitivity, temperature dependence, and development time. The sensitivity of the material is shown to be 1.7 × 10⁻⁴α, where α is the intensity absorption coefficient. Laser light scattered by an exposed region is found to produce a double ring pattern due to the graininess of the index variation. Three-dimensional holographic diffraction gratings were made in the PMMA, and its diffraction efficiency was measured as a function of thickness, refractive index change, and reconstruction angle. The efficiencies measured agree fairly well with the theoretical sin² curve; however, higher peak diffraction efficiencies were obtained further out on this oscillatory curve. A maximum diffraction efficiency of 96% was obtained. Angular sensitivity measurements indicated that the effective thickness of the grating was less than its actual thickness due to the nonuniformity of the index variation with thickness. Potential applications as a dielectric waveguide, diffraction grating, and wavelength selector are discussed. Scattering, the relatively small maximum index change, and poor reproducibility are the chief limiting factors.