Optimization of holographic diffractive optical element used for fabricating photonic crystals

A technique for fabricating optimized holographic diffractive optical element (HDOE) used for fabricating photonic crystals is proposed in this paper. The HDOE consists of three identical transmission gratings symmetrically placed around the center with 120° between each other. Usually the polarization of the diffracted beam of a grating will be changed from that of the incident beam depending on the orientation of the grating. As the orientations of the three gratings in a HDOE are different, the polarizations of the diffracted beams and their diffraction efficiencies are different, resulting in low contrast of the interference pattern. This problem can be solved by controlling the parameters in grating fabrication. Rigorous coupled-wave analysis is used to calculate the influence of grating depth and polarization angle on diffraction efficiency and polarization of diffracted beams. Calculated results demonstrate that under 458 nm illumination, if the three sinusoidal gratings in a HDOE have 0.729 μm period and 0.33 μm depth, the polarization and diffraction efficiencies of the three first-order diffracted beams will be equal. The experimental results are presented to prove the practicability of the technique.