Markus Guttmann

Hot Embossing of Microoptical Components Prototyped by Deep Proton Writing

In this letter, we present the replication of out-of-plane coupling microcomponents using hot embossing, through the fabrication of a metal mould by electroforming a polymer template patterned by means of deep proton writing (DPW). We compare the surface roughness and the optical performance of the hot embossed replicas with the DPW prototypes and can conclude that the replicated components exhibit only a small increase in surface roughness and a very small decrease in coupling performance. This paves the way towards low-cost mass replication of DPW-fabricated prototypes in a variety of high-tech plastics.

Tailored Surface-Enhanced Raman Nanopillar Arrays Fabricated by Laser-Assisted Replication for Biomolecular Detection Using Organic Semiconductor Lasers

Organic semiconductor distributed feedback (DFB) lasers are of interest as external or chip-integrated excitation sources in the visible spectral range for miniaturized Raman-on-chip biomolecular detection systems. However, the inherently limited excitation power of such lasers as well as oftentimes low analyte concentrations requires efficient Raman detection schemes. We present an approach using surface-enhanced Raman scattering (SERS) substrates, which has the potential to significantly improve the sensitivity of on-chip Raman detection systems. Instead of lithographically fabricated Au/Ag-coated periodic nanostructures on Si/SiO2 wafers, which can provide large SERS enhancements but are expensive and time-consuming to fabricate, we use low-cost and large-area SERS substrates made via laser-assisted nanoreplication. These substrates comprise gold-coated cyclic olefin copolymer (COC) nanopillar arrays, which show an estimated SERS enhancement factor of up to ∼ 10(7). The effect of the nanopillar diameter (60-260 nm) and interpillar spacing (10-190 nm) on the local electromagnetic field enhancement is studied by finite-difference-time-domain (FDTD) modeling. The favorable SERS detection capability of this setup is verified by using rhodamine 6G and adenosine as analytes and an organic semiconductor DFB laser with an emission wavelength of 631.4 nm as the external fiber-coupled excitation source.

Sensitivity optimization of injection-molded photonic crystal slabs for biosensing applications

For label-free assays employing photonic crystal slabs (PCSs), the sensitivity is one of the most important properties influencing the detection limit. We investigate the bulk sensitivity and the surface sensitivity of 24 different PCSs fabricated by injection molding of PMMA and subsequent sputtering of a Ta2O5 high-index layer. The duty cycle of the linear grating is varied in steps of 0.1 between 0.2 and 0.7. Four different Ta2O5 layer thicknesses (89 nm, 99 nm, 189 nm, 301 nm) are deposited. Both bulk and surface sensitivity are optimal for a Ta2O5 layer thickness of 99 nm. The maximum bulk sensitivity of 138 nm/RIU is achieved for a duty cycle of 0.7, while the maximum surface sensitivity of 47 nm/RIU is obtained for a duty cycle of 0.5. Good agreement between experimental results and finite-difference time-domain (FDTD) simulations is observed. The PCSs sensitivity is linked to the mode intensity distribution.

Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog

Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix-fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

Replication of micro-optical components and nano-structures for mass production

Microstructured metallic moulding tools or mould inserts are needed for mass production of micro-optical components. These tools are used for hot embossing or injection moulding of micro components in plastic. Because of the extremely tight specifications like small sidewall roughness and high aspect ratios these tools are usually fabricated by lithographic procedures followed by electroforming. In this case the structural geometry is limited to Manhattan-like structures and only a limited number of technologies can be used to fabricate the master structures. Applicable techniques are e.g. X-ray lithography (LIGA technology) or Deep Proton Writing (DPW). However these processes are not suitable for low-cost mass production. They are limited by the exposure area and the design of the microstructures. To overcome these limitations a new process has been developed which allows the transfer of micro-optical structures fabricated by other technologies as well as assembled structures or structures with varying geometries into a moulding tool. The master structures, either plastic, glass, metal or a combination of these materials, serve as sacrificial parts. With electroforming technology, a negative copy of the microstructured master is built up in the metal subsequently used as a moulding tool. Low-cost mass production is possible with these moulding tools. We present the process chain in this paper and demonstrate its feasibility by producing reliable moulding tools from three challenging and different components. The possibility of mass fabrication of the components by replication was demonstrated.

Multi-periodic nanostructures for photon control

We propose multi-periodic nanostructures yielded by superposition of multiple binary gratings for wide control over photon emission in thin-film devices. We present wavelength- and angle-resolved photoluminescence measurements of multi-periodically nanostructured organic light-emitting layers. The spectral resonances are determined by the periodicities of the individual gratings. By varying component duty cycles we tune the relative intensity of the main resonance from 12% to 82%. Thus, we achieve simultaneous control over the spectral resonance positions and relative intensities.

Extraction of guided modes from organic emission layers by compound binary gratings

The extraction of guided modes from a 100 nm organic emission layer by compound binary gratings with multiple superimposed periods at different ratios is investigated. We measure angle-dependent photoluminescence from samples with double-period (350 and 450 nm), triple-period (350, 400, and 450 nm), and multiperiod (350, 400, 450, and 500 nm) gratings and show that each period component produces two outcoupling features due to first-order Bragg scattering of the TE(0) guided mode. The averaged angular color change is reduced by up to a factor of 11 compared to a single-period grating structuring.

Replication of deep micro-optical components prototyped by Deep Proton Writing

Using our rapid prototyping technology called Deep Proton Writing (DPW), we have in recent years made a wide range of micro-optical components with a large depth (500-μm) for a variety of applications. One of these components is a pluggable out-of-plane coupler for printed circuit board-level optical interconnections. Whereas DPW is capable of rapidly fabricating high-quality master components, the technology is not suitable for low-cost mass fabrication. Therefore, we investigate the replication of out-of-plane coupling components using hot embossing, through the fabrication of a metal mould of the DPW master by applying electroplating. We compare these hot embossed replicas with components replicated using the elastomeric mould vacuum casting technology.

Mould insert fabrication of a single- mode fibre connector alignment structure optimized by justified partial metallization

For mass production of multiscale-optical components, microstructured moulding tools are needed. Metal tools are used for hot embossing or injection moulding of microcomponents made of a thermoplastic polymer. Microstructures with extremely tight specifications, e.g. low side wall roughness and high aspect ratios are generally made by lithographic procedures such as x-ray lithography or deep proton writing. However, these processes are unsuitable for low-cost mass production. An alternative manufacturing method of moulding tools has been developed at the Karlsruhe Institute of Technology (KIT). This article describes a mould insert fabrication and a new replication process for self-centring fibre alignment structures for low loss field installable single-mode fibre connectors, developed and fabricated by the Vrije Universiteit Brussel (VUB) in collaboration with TE Connectivity. These components are to be used in fibre-to-the-home networks and support the deployment and maintenance of fibre optic links. The special feature of this particular fibre connector is a self-centring fibre alignment, achieved by means of a through hole with deflectable cantilevers acting as micro-springs. The particular challenge is the electroforming of through holes with a centre hole diameter smaller than 125 µm. The fibre connector structure is prototyped by deep proton writing in polymethylmethacrylate and used as a sacrificial part. Using joining, physical vapour deposition and electroforming technology, a negative copy of the prototyped connector is transferred into nickel to be used as a moulding tool. The benefits of this replication technique are a rapid and economical fabrication of moulding tools with high-precision microstructures and a long tool life. With these moulding tools low-cost mass production is possible. We present the manufacturing chain we have established. Each individual manufacturing step of the mould insert fabrication will be shown in this report. The process reliability and suitability for mass production was tested by hot embossing.

Alternative technology for fabrication of nano- or microstructured mould inserts used for optical components

For mass production of multiscale-optical components, micro- and nanostructured moulding tools are needed. Metal tools are used for hot embossing or injection moulding of microcomponents in plastics. Tools are typically produced by classical forming processes such as mechanical manufacturing e.g. turning or milling, laser manufacturing or electrical discharge machining (EDM). Microstructures with extremely tight specifications, e.g. low side wall roughness and high aspect ratios are generally made by lithographic procedures such as LIGA or DPW technology. However, these processes are unsuitable for low-cost mass production. They are limited by the exposure area and structure design. In cooperation with international partners alternative manufacturing methods of moulding tools have been developed at the Institute of Microstructure Technology (IMT). In a new replication procedure, mould inserts are fabricated using micro- and nanoscale optics. The multiscale structured prototypes, either in plastics, glass, metal or material combinations are used as sacrificial parts. Using joining technology, electroforming and EDM technology, a negative copy of a prototype is transferred into metal to be used as a moulding tool. The benefits of this replication technique are rapid and economical production of moulding tools with extremely precise micro- and nanostructures, large structured area and long tool life. Low-cost mass replication is possible with these moulding tools. In this paper, an established manufacturing chain will be presented. Multiscale and multimaterial optical prototypes e.g. out-of-plane coupler or microinterferometer were made by DPW or laser technology. The mould insert fabrication of each individual manufacturing step will be shown. The process reliability and suitability for mass production was tested by hot embossing.