3D diffractive elements through fs-laser direct writing

Diffractive optical elements (DOEs) are powerful tools for shaping light into almost arbitrary patterns. They find increasing use in advanced laser material processing and imaging applications, for instance to parallelize and thus accelerate fabrication steps. DOEs are commonly made by etching micro-meter structures into the surface of a glass blank. However, it is known that they could be even more powerful and versatile if they were three- dimensional, i.e., if the structures were directly written into the bulk of a small glass slab. Such 3D DOEs could exhibit exquisite sensitivity to color and beam incidence angle, thus enabling a new class of optical elements. For instance, they could be made small enough to fit onto a microscopy glass slide and at the same time sufficiently sensitive to differentiate closely resembling cell types, solely from the light they reflect. Current limitations of realizing 3D DOEs of higher complexity are given by the required large computational costs as well as the lack of fabrication strategies that are sufficiently fast and capable of writing sub-micron sized voxels in millimetre depths. Our research aims to take significant steps towards the realization of 3D DOEs. We plan to develop new algorithms for their design as well as a novel fabrication approach which is apt to fulfil the high demands imposed by the fabrication task. Our approach will be based on parallel femtosecond-laser direct writing (FLDW), where many voxels are simultaneously produced by irradiating the material with short-pulsed laser foci. In the course of our project, we will address the design and production of DOEs with increasing difficulty and complexity. In three subsequent stages we will design and produce 2D, multilayer and finally 3D DOEs.

This project is an international research collaboration between the Institute of Biomedical Physics at the Medical University of Innsbruck (MUI) in Austria (project leader: Alexander Jesacher) and the Institute of Photonic Technologies at the University of Erlangen-Nuremberg in Germany (project leader: Michael Schmidt). The main goal of our three-year research was to make significant steps towards the realization of laser-fabricated volume holograms in glass. Such elements are sometimes called aperiodic photonic volume elements (APVEs) and consist of hundreds of thousands of microscopic 3D pixels ("voxels") inscribed directly into the glass with a focused laser. APVEs are expected to become millimeter-sized, functional optical elements that can perform important tasks, such as splitting or combining light according to specific properties (e.g., its color characteristics or spatial distribution). APVEs could find future use as building blocks of optical computers or in fiber-optic-based telecommunications. The research was divided into two work packages: The Erlangen team focused on fabricating the elements using laser direct writing in glass. The Innsbruck team developed methods for optical characterization of the three-dimensional test voxels inside glass fabricated in Erlangen, as well as computer methods to design APVEs based on these measurements. The main questions were the following: 1. how should an APVE be structured to work as well as possible? How should individual voxels be shaped or how should they be arranged in 3D? 2) How can the optical properties of individual voxels be determined with sufficient accuracy? 3) Can hundreds of thousands of voxels be produced with sufficient reproducibility? 4) How well do APVEs work in practice? Which tasks can already be realistically performed today? With reference to these questions, we were able to find many important answers in the course of the research project: 1. Using computer simulations, we identified a particular voxel shape and arrangement that proved promising in terms of practical implementation. 2. We developed an optical tomography method that can characterize individual voxels with the high accuracy needed to realize APVEs. 3. Our partners were able to show that laser fabrication of an APVE can be done in a short time (~10 minutes per element) and with sufficient accuracy. 4. In the final phase of the project, we were able to design and fabricate several APVEs. The measured light efficiencies are in the range of 30% - 80% and are many times higher than previously achieved (~15%).