Phase and Absorption Metrology for Thick Photopolymer Devices

Studies of development kinetics in volume photopolymers typically use transmission holography to quantify the index distribution. This method has advantages including simplicity, quantitative index data and natural mapping onto theories using harmonic expansion of the material response. A particular disadvantage is that the low spatialfrequency response corresponding to the intensity of the writing beams can never be Bragg matched and thus remains invisible. In configurations where the exposure is not primarily sinusoidal, the holographic method is not applicable. Important examples include bit-oriented data storage, direct-write lithography, and the object beam of page-based holography. In these cases the exposure intensity is essentially arbitrary and there is a need for metrology tools that can quantitatively measure the real and imaginary parts of the weak 3D index perturbation. Images produced by bright-field and phase-contrast microscopes are generally not quantitative and are corrupted by objects out of the focal plane. We have developed two methods, a form of optical diffraction tomography and a scanning transmission microscope, that are specifically designed to measure the 3D index response of holographic materials. Both are optimized to measure the extremely weak absorption and phase structures typical of photopolymers and have passbands that match the expected spatial frequencies.