Laser Diffraction Zones and Spots from Three-Dimensional Graded Photonic Super-Crystals and Moire Photonic Crystals

The laser diffraction from periodic structures typically shows isolated and sharp point patterns at zeroth and +/- nth orders. Diffraction from 2D graded photonic super-crystals (GPSCs) has demonstrated over 1000 spots due to the fractional diffractions. Here, we report the holographic fabrication of three types of 3D GPSCs through nine beam interferences and their characteristic diffraction patterns. The diffraction spots due to the fractional orders are merged into large-area diffraction zones for these three types of GPSCs. Three distinguishable diffraction patterns have been observed: (a) 3 x 3 Diffraction zones for GPSCs with a weak gradient in unit super-cell, (b) 5 x 5 non-uniform diffraction zones for GPSCs with a strong modulation in long period and a strong gradient in unit super-cell, (c) more than 5 x 5 uniform diffraction zones for GPSCs with a medium gradient in unit super-cell and a medium modulation in long period. The GPSCs with a strong modulation appear as moire photonic crystals. The diffraction zone pattern not only demonstrates a characterization method for the fabricated 3D GPSCs, but also proves their unique optical properties of the coupling of light from zones with 360 degrees azimuthal angles and broad zenith angles.