Toward Near-Perfect Diffractive Optical Elements via Nanoscale 3D Printing

Diffractive optical elements (DOEs) are widely applied as compact solutions to generate desired optical patterns in the far field by wavefront shaping. They consist of microscopic structures of varying heights to control the phase of either reflected or transmitted light. However, traditional methods to achieve varying thicknesses of structures for DOEs are tedious, requiring multiple aligned lithographic steps each followed by an etching process. Additionally, the reliance on photomasks precludes rapid prototyping and customization in manufacturing complex and multifunctional surface profiles. To achieve this, we turn to nanoscale 3D printing based on two-photon polymerization lithography (TPL). However, TPL systems lack the precision to pattern diffractive components where subwavelength variations in height and position could lead to observable loss in diffraction efficiency. Here, we employed a lumped TPL parametric model and a workaround patterning strategy to achieve precise 3D printing of DOEs using optimized parameters for laser power, beam scan speed, hatching distance, and slicing distance. In our case study, millimeter scale near-perfect Dammann gratings were fabricated with measured diffraction efficiencies near theoretical limits, laser spot array nonuniformity as low as 1.4%, and power ratio of the zero-order spot as low as 0.4%. Leveraging on the advantages of additive manufacturing inherent to TPL, the 3D-printed optical devices can be applied for precise wavefront shaping, with great potential in all-optical machine learning, virtual reality, motion sensing, and medical imaging.