Buried volume holograms fabricated by two-photon polymerization

Computer generated volume holograms (CGVHs) are optical elements designed to manipulate light for desired optical functionalities. Two-photon polymerization (2PP) lithography offers a straightforward fabrication method for such CGVHs. However, non-uniform height profiles at the air-polymer interface introduce unwanted optical path differences (OPDs) for 2PP-fabricated CGVHs that disturb their optical performance. We address this effect by printing a several  µm-thick cover layer with a uniform surface on top of the fabricated structures to minimize the undesired OPD caused by height profile variations at the grating-cover interface to a negligible level. The approach is successfully tested on simple volume gratings, where refractive index changes are achieved by varying the laser power used to print the volume grating. Confocal microscopy and focused ion beam-scanning electron microscopy determine the top surface height variations of ≲400nm, which are concealed beneath the uniform top cover layer. The optical performance of the buried diffractive structures is confirmed by computational shear interferometry (CoSI), demonstrating a dynamic refractive index modulation of 2 × 10^-3 as volume gratings are polymerized with different powers within the structure. This approach offers a new and simple way to design and fabricate complex CGVHs and isolate their optical functionality from the surrounding environment.