Fig. 6

Fabrication process for holographic colour pixels. a In the direct laser writing exposure process, a 780 nm femtosecond pulsed IR laser beam is focused into a liquid puddle of negative-tone photoresist. At the focal point of the laser spot, the UV-sensitive photoresist is cross-linked by two-photon polymerisation and becomes solid. (Unexposed photoresist remains as a liquid and is later washed away during development.) The laser spot is translated in three dimensions (x, y, and z) according to the sequence in b to create complex structures. Resolution in the z-direction is not limited by the axially elongated shape of the point spread function as it is determined by the positioning accuracy of the laser spot. b Process flow for fabricating a holographic colour pixel using direct laser writing: (1) Blocks are created by rastering the laser spot to fill a square in the xy-plane with a continuous line exposure (hatching); (2) Step 1 is optionally repeated at higher z-positions to give thicker blocks (slicing); (3) Pillar arrays are created by point exposures, where the diameter is controlled by the exposure dose; (4) Step 3 is optionally repeated at higher z-positions to give taller pillars. Block thickness and pillar height are controlled in Steps 2 and 4 by overlapping multiple layers of exposures along the z-direction. A full holographic colour print is made by patterning an array of many pixels with different block thicknesses and pillar dimensions