Buried volume holograms fabricated by two-photon polymerization

Area of Science:

• Optics and Photonics
• Materials Science
• Nanofabrication

Background:

• Computer-generated volume holograms (CGVHs) are crucial optical elements.
• Two-photon polymerization (2PP) is a common fabrication method for CGVHs.
• Surface irregularities in 2PP-fabricated CGVHs cause optical path differences (OPDs) and degrade performance.

Purpose of the Study:

• To develop a method to mitigate OPDs caused by surface variations in 2PP-fabricated CGVHs.
• To improve the optical performance and stability of CGVHs.
• To enable the fabrication of complex CGVHs with isolated optical functionality.

Main Methods:

• Fabrication of CGVHs using 2PP lithography.
• Application of a uniform cover layer to the fabricated CGVHs.
• Characterization of surface topography using confocal microscopy and focused ion beam-scanning electron microscopy (FIB-SEM).
• Evaluation of optical performance using computational shear interferometry (CoSI).

Main Results:

• A cover layer effectively concealed surface height variations (≲400nm) at the grating-cover interface.
• Undesired OPDs were minimized to a negligible level.
• Dynamic refractive index modulation of 2×10^-3 was achieved in volume gratings.
• The optical functionality of the CGVHs was successfully isolated from the environment.

Conclusions:

• The developed cover layer technique significantly improves the optical performance of 2PP-fabricated CGVHs.
• This approach provides a simple and effective way to fabricate complex CGVHs.
• The method enhances the stability and reliability of CGVH devices.