March 05, 2020

Off-axis holography by additive microfabrication

Schema of the off-axis holographic projection
Schema of the off-axis holographic projection with blazed top facet DOE. Image: Hao Wang, Singapore University of Technology and Design

A new type of diffractive optical elements (DOE) was developed by an international research team using Nanoscribe’s microfabrication technology. The DOEs enable holographic far-field projections free of both zero diffraction order and shadowing effects, while keeping a high diffraction efficiency. The novel modification of DOEs can be applicable for precise wavefront shaping, e.g. in facial recognition hardware, mixed-reality technology and 3D displays.

If you remember the Holodeck from the Star Trek science fiction series, you might immediately associate holography with 3D projections. There is no doubt that holography continues to captivate us with its visual effects. But holography also inspires numerous applications in augmented reality, medical imaging, anticounterfeiting, biometric facial recognition, 3D optical tweezing and more.

Diffractive optical elements with sub-pixel manipulation

The generation of far-field projections is, however, still a challenging endeavor. To this end, diffractive optical elements provide a compact solution to shape light by diffraction, manipulating the wavefront that travels through a computer-generated nanostructured pattern. Unfortunately, DOEs suffer from the zero order spot, which is a result of undiffracted light. The light of a laser beam travels along its optical axis and through the DOE without being altered, causing a disturbing bright spot.

To solve this issue, an international research team developed a novel DOE by means of Nanoscribe’s Photonic Professional GT 3D printer. In this project the Singapore University of Technology and Design collaborates with the National University of Singapore, the International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology at the Shenzhen University and the Institute of Materials Research and Engineering at A*STAR.

Thanks to the design freedom and high resolution of 3D Microfabrication, an ingenious DOE design was additively manufactured with sub-pixel manipulation. The researchers combined diffractive optical elements with blazed gratings to achieve blazed facets DOE structures that are capable to shift the projection. The novel DOE structures produce projections free of zero order spot, shadowing effects and with a high diffraction efficiency.

Nanostructures by Two-Photon Grayscale Lithography

In June 2019, Nanoscribe presented Quantum X maskless lithography system with a new technology that offers advanced capabilities for DOE fabrication: Nanoscribe’s patent-pending technology of Two-Photon Grayscale Lithography (2GL®). Compared to one-photon grayscale lithography, 2GL allows a truly three-dimensional control of the voxel position and size. To raise throughput in DOE fabrication, Quantum X systems employ 2GL at high laser scanning speed. The laser beam thus rapidly scans through one plane and adjusts the right exposure dose at each exact position. Multilevel DOE patterns are generated in one continuous printing process. This is how 2GL offers a maskless fabrication approach that reduces the fabrication time needed for multilevel DOEs from days to a few hours.

Read the publication in Advanced Optical Materials: Off‐Axis Holography with Uniform Illumination via 3D Printed Diffractive Optical Elements

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