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Volume diffractive optical elements promise widespread application in laser beam shaping, imaging and optical data storage by structuring refractive index modulation in the third axial dimension. Femtosecond laser direct writing has been exploited inside fused silica to assemble multiple 1D grating layers on Talbot planes and overcome the inherent weak diffraction efficiency otherwise found in low-contrast volume gratings. Here, we extend laser structuring from linear (1D) to orthogonally crossed (2D) gratings with the aim to write 3D photonic crystal templates in photoresist by 3D interference lithography. The formation of crossed grating structures present challenges in balancing the efficiency of diffraction orders owing to blazing and index overwriting effects requiring compensation by tuning the grating design and laser power exposure. In this way, six-layer grating designs have been fabricated and applied to exposure of thick photoresist, enabling the formation of 3D photonic crystal templates with bicontinuous structure. A systematic offsetting of orthogonal grating layers to establish phase offsets over 0 to π/2 range presents a precise means for controlling the photonic crystal structure symmetry between body centered tetragonal (BCT) and woodpile-like tetragonal (wTTR).

Femtosecond laser writing of phase-tuned volume gratings for symmetry control in 3D photonic crystal holographic lithography