ResearchResearch Areas
Task Group M3

Surface polishing, coatings and optical elements: Research on integrated multilayer 2D surface optics

The research of the Task Group M3

© Alexander Wolf/IPeG

Our Research Mission

The Task Group M3 - Surface Polishing, Coatings and Optical Elements research group focuses on the optical functionalisation of surfaces and their integration into photonic components. Thin coatings enhance or suppress the reflection of light from surfaces. Transmission and reflection can be adjusted independently for different wavelengths by complex layer stacks. On the other hand, structuring surfaces creates diffractive optical elements such as gratings, holograms, CDs, or optical waveguides.

OUR MOTIVATION

The Task Group's research contributes to PhoenixD's vision in particular through miniaturisation and integration. By combining different technologies, complex integrated optical systems are created.

Thin layer stacks of different materials are first produced two-dimensionally by advanced optical coating processes. They are then precisely cut into sub-millimetre sized platelets and used as microfilters. They can reflect one wavelength, for example, while another passes through the stack undisturbed. In this way, optical signals are separated or combined. Scientists also use this concept to realise complex functions such as distributing, filtering or switching light signals.

Embossing technology enables cost-efficient mass production of optics with spatially structured surfaces.

Embossing technology enables cost-efficient mass production of optics with spatially structured surfaces. Diffractive optical elements such as holograms or optical gratings require a sub-micrometre resolution. An alternative process for producing volume bodies with almost arbitrarily shaped but equally micro-structured surfaces is injection moulding.

Embossing or injection moulding will be used in the Task Group M3 to create optical carrier substrates into which optical microcomponents will be inserted. The optical interconnection technology is based on waveguides in the carrier substrate and the electrical contacting on printed or laser-written conductor paths.

Our Research

The basis for optical integration in the M3 working group is an optical platform (see Figure 1), which is manufactured using precision injection moulding. The standard for fine structuring of the mould inserts for injection moulding is photolithography and electroplating. The resolutions in the range of micrometres, that can be achieved with these techniques, are sufficient for passive waveguide structures.

Thermoplastics are generally suitable as materials for injection moulding. Beyond their suitability for precision injection moulding, they must fulfil many other boundary conditions. In particular, chemical, thermal and mechanical compatibility with subsequent process steps is therefore essential.

For mounting and electrical contacting of active optical components such as light sources and detectors, the substrate is provided with electrical conductor paths. One possibility is metallic coating followed by lithographic patterning. Here, however, additive processes such as direct laser writing of conductor paths are to be used preferably.

© Florian Carstens/LZH
Figure 1: Schematic structure of the optical platform

Planar substrates, in particular, can also be produced very cost-effectively by precision embossing. A combination of both processes is also conceivable. For example, the researchers first use injection moulding to create substrates with optical waveguides and then embossing technology to supplement them with delicate diffractive structures.

Optical gratings (see Figures 2 and 3) are used, for example, in waveguides for coupling light in and out or in optical sensors.

© Sebastian Bengsch/IMPT)
Figure 2: Optical gratings in mould inserts for injection moulding

The structure sizes of such diffractive optical elements are well below one micrometre. Therefore, it is necessary to use electron-beam lithography, for example, to structure the mould inserts or the embossing stamps.

© Sebastian Bengsch/IMPT
Figure 3: Optical gratings in plastic polyetheretherketone

Other examples of optical elements with very fine structures are holograms, photonic crystals or plasmonic structures. As with injection moulding, the process technology for precision embossing of structures in the sub-micrometre range presents a significant challenge. To achieve consistent quality, one-off optimisation of the process parameters is not enough. Instead, manufacturing with adaptive process control in real-time is required.

© Marc Jupé/LZH
Figure 4: Facility for the production of optical coatings with ion beam sputtering

Spectral filters and mirrors with dimensions in the micrometre range are essential for optical integration on carrier substrates with waveguides. The layer stacks receive their coating within a vacuum (see Figure 4). However, the researchers cannot directly integrate this kind of fabrication into a process chain. Instead, the coatings are first produced over a large area and then divided into small platelets with high precision using lasers (see figure 5).

© Florian Carstens/LZH
Figure 5: Filters (250µm x 250µm) singled out with laser processes on a carrier substrate

In addition to these passive layer systems, the working group also researches the development of active layers. The goal is optical filter layers whose spectral characteristic can be influenced by applying an electrical voltage during operation. For example, various optical signals can be decoupled from or fed into an optical data stream with such filters.

With a lift-off process, the thin filter or mirror platelets can be detached from the silicon wafer. In a unique pick-and-place system, which will be installed soon, the platelets are then inserted into the corresponding groves on the optical carrier. Before fixing them, an individual active adjustment is implemented. For this purpose, the unique production facility can couple optical signals into the waveguides and monitor the signals.

The Task Group M3 collaborates in many ways with the other PhoenixD working groups. For example, there is intensive cooperation with the researchers of the IHF - Institute for High-Frequency Technology at TU Braunschweig and the Institute of Inorganic Chemistry at Leibniz University Hannover to develop layered materials switching electro-optically. In this context, the Task Group M3 jointly researches with Task Group S2 - Optical Materials on modelling the corresponding layer structures. Scientists at the ITA - Institute of Transport and Automation Technology and the HOT - Hannover Centre for Optical Technologies develop specific filter concepts to select optical signals in integrated optical systems. Besides, the Task Group investigates the coating of ultra-thin glass as an alternative process for the production of miniaturised thin-film filters for the work in Task Group M2 - Additive/Subtractive Manufacturing.

Contact

Prof. Dr. Detlev Ristau
Executive Board
Address
Hollerithallee 8
30419 Hannover
Prof. Dr. Detlev Ristau
Executive Board
Address
Hollerithallee 8
30419 Hannover