Origination of free-form micro-optical elements using one- and two-photon grayscale laser lithography.

We discuss and describe the development of an origination process for planar free-form micro-optical elements from a given optical design. The targeted masters serve as origination structures for a roll-to-roll mass fabrication process. Specifically targeted are complex, optically smooth, surface relief structures with variable structure heights in the range of 1-20 µm, with typical lateral sizes of more than 5 µm. The area of the targeted masters is in the range of 1cm2. The main part of the paper is devoted to the description of a self-developed grayscale laser direct-write platform enabling one- and two-photon absorption lithography, also in combination on one and the same sample. In the following, we describe both methods and show that both lead to excellent structural quality of surface micro-relief structures. As a showcase of what the system can do in principle, we designed and fabricated free-form micro-optical elements to project light from an LED as a defined light pattern onto a wall. The proper optical functionality of the fabricated element was shown within a demonstrator setup.

Multiscale optical simulation settings: challenging applications handled with an iterative ray-tracing FDTD interface method.

We show that with an appropriate combination of two optical simulation techniques-classical ray-tracing and the finite difference time domain method-an optical device containing multiple diffractive and refractive optical elements can be accurately simulated in an iterative simulation approach. We compare the simulation results with experimental measurements of the device to discuss the applicability and accuracy of our iterative simulation procedure.

Multiple interfacing between classical ray-tracing and wave-optical simulation approaches: a study on applicability and accuracy.

In this study the applicability of an interface procedure for combined ray-tracing and finite difference time domain (FDTD) simulations of optical systems which contain two diffractive gratings is discussed. The simulation of suchlike systems requires multiple FDTD↔RT steps. In order to minimize the error due to the loss of the phase information in an FDTD→RT step, we derive an equation for a maximal coherence correlation function (MCCF) which describes the maximum degree of impact of phase effects between these two different diffraction gratings and which depends on: the spatial distance between the gratings, the degree of spatial coherence of the light source and the diffraction angle of the first grating for the wavelength of light used. This MCCF builds an envelope of the oscillations caused by the distance dependent coupling effects between the two diffractive optical elements. Furthermore, by comparing the far field projections of pure FDTD simulations with the results of an RT→FDTD→RT→FDTD→RT interface procedure simulation we show that this function strongly correlates with the error caused by the interface procedure.