Christoph Wächter

Optimization of angularly resolved Bloch surface wave biosensors.

Bloch surface wave (BSW) sensors to be used in biochemical analytics are discussed in angularly resolved detection mode and are compared to surface plasmon resonance (SPR) sensors. BSW supported at the surface of a dielectric thin film stack feature many degrees of design freedom that enable tuning of resonance properties. In order to obtain a figure of merit for such optimization, the measurement uncertainty depending on resonance width and depth is deduced from different numerical models. This yields a limit of detection which depends on the sensors free measurement range and which is compared to a figure of merit derived previously. Stack design is illustrated for a BSW supporting thin film stack and is compared to the performance of a gold thin film for SPR sensing. Maximum sensitivity is obtained for a variety of stacks with the resonance positioned slightly above the TIR critical angle. Very narrow resonance widths of BSW sensors require sufficient sampling but are also associated with long surface wave propagation lengths as the limiting parameter for the performance of this kind of sensors.

Intrinsic OLED emitter properties and their effect on device performance

The optical properties of OLEDs can be tailored by optimization of the layer stack and by implementation of micro- and/or nano-optical structures. But the effect of changing the OLED configuration on the device performance crucially depends on the optical properties of the emissive layer. The aim of this contribution is twofold. First, we will propose and demonstrate a general and reliable method to determine the intrinsic optical emitter properties via simple optical characterization and corresponding reverse simulations. Second, the influence of the emitter properties on homogeneous OLEDs as well as on OLEDs with micro/nano-optical structures is discussed.

Micro-optically assisted high-index waveguide coupling

Adapting the concept of solid immersion lenses, we numerically study a micro-optical scheme for conventional high-index and photonic-crystal waveguide coupling by using a combination of different numerical methods such as ray tracing, angular-spectrum propagation, finite-difference time-domain simulations, and finite-element-method simulations. The numerical findings are discussed by means of impedance, group- or energy-velocity, spot-size, and phase-matching criteria. When fabrication constraints for high-index immersion lenses made of silicon are taken into account, a coupling efficiency of -80% can be reached for monomode silicon-on-insulator waveguides with a quadratic cross section of the core and rectangular cross sections of moderate aspect ratio. Similar coupling efficiencies of -80% can be obtained for silicon-on-insulator photonic-crystal waveguides. Tolerances that are due to misalignments and variations of the substrate thickness of the silicon lens are discussed.

Measuring the internal luminescence quantum efficiency of OLED emitter materials in electrical operation

One major performance parameter for organic light-emitting diodes (OLEDs) in display and illumination applications is the overall efficiency of the device, which is directly affected by the emitters internal luminescence quantum efficiency q. It is very desirable to determine q of the emitter in-situ, i.e. in electrically driven operation, since photoluminescence measurements of q provide a rather rough (over-)estimation. In a layered system (LS), the value q of the emitting material (EM) is associated with the coupling probability of emitted radiation into the different modes provided by the LS (air, substrate, guided, surface plasmon). We show the in-situ determination of q by optical means from a relative comparison of current efficiencies of OLEDs with varying emitter-cathode distance. As a prerequisite, we outline procedures for a complete characterization of the passive and active optical properties of the LS and the EM, respectively. Then, precise optical simulation allows determining q without additional assumptions.

Micro optical pattern shaping for tailored light emission from Organic LEDs

The application of large area OLEDs for lighting and signage purposes potentially requires essential changes of the common Lambert-like emission pattern. We demonstrate an array based micro optical approach for pattern shaping of area light sources based on distorted Fourier imaging of an aperture array with a micro lens array. Narrow angular emission patterns of ± 35° and ± 18° FWHM obtained experimentally demonstrate the pattern shaping with low stray light levels. The internal recycling of initially rejected photons yields intensity enhancements exceeding a factor two in forward direction that is still well below the theoretical limits due to limited reflectivity.

Micro-optical beam-shaper for tailoring light emission from OLEDs

In this contribution, we show that micro-optical elements are well suited to exploit the potential of organic light-emitting diode (OLED) based light sources. They may not only increase the OLED efficiency significantly but also enable for tailoring the common Lambertian-like emission pattern of OLEDs in order to reach desired light distributions corresponding to application demands. An OLED beam-shaping scheme is demonstrated utilizing thin micro-optical arrays where each channel consists of a half-ball lens and an adapted reflective/absorptive aperture. The combination of (a) light recycling, (b) distorted and arrayed imaging of the apertures, and (c) potential substrate-mode-outcoupling allows for efficient tailoring the light emission pattern of large area OLEDs. By means of such a beam-shaping concept, several different illumination patterns (e.g. circular, triangular beams or even more complex light distributions like letters) with various divergence angles below ±40° are demonstrated. Furthermore, a reduction of the divergence angle down to about ±10° accompanied by a stray light level minimization to <1% at larger angles is presented. In either case, intensity enhancements by a factor of >2 can be realized while the thickness of the optics remains below 2 mm.

Design rules for combined label-free and fluorescence Bloch surface wave biosensors

We report on the fabrication and physical characterization of optical biosensors implementing simultaneous label-free and fluorescence detection and taking advantage of the excitation of Bloch surface waves at a photonic crystals truncation interface. Two types of purposely designed one-dimensional photonic crystals on molded organic substrates with micro-optics were fabricated. These crystals feature either high or low finesse of the Bloch surface wave resonances and were tested on the same optical readout system. The experimental results show that designing biochips with a large resonance quality factor does not necessarily lead in the real case to an improvement of the biosensor performance. The conditions for optimal biochip design and operation of the complete bio-sensing platform are established.

Effects of metallic nanoparticle arrays in Si solar cell structures

Metallic inclusions in layered structures can have noticeable effects onto scattering and absorption due to the coupling of the external electromagnetic field and local charge oscillations. These effects are strongly related to both the geometry of the individual particle as well as to the array structure. Having in mind the efficiency improvement of silicon solar cells due to plasmonic effects, we report on the modeling and the fabrication of periodic arrays of metallic nanoparticles on planar substrates. Different characterization techniques as atomic force microscopy (AFM), scanning electron microscope (SEM) and optical measurements are applied which provide particular information with respect to the fabricated structures, each. Special emphasis is placed on the clarification of the dominant features of the optical characterization by detailed numerical analysis. This allows identifying significant modes of the planar geometry which is complemented by the nanostructures, whose interplay with the radiation field does establish changes of the absorption in the silicon layer, finally. These findings may be helpful for optimization and clarification of specific details of technology, later on.

Simulation of Metallic Nanoparticles in Layered Structures

Metallic thin films and metallic nanoparticles give rise to peculiar optical effects in frequency regions where an external electromagnetic field can couple to charge oscillations resonantly. Plasmons localized at interfaces and closed surfaces give rise to huge local field enhancements which can be used for a great variety of applications based on linear and non‐linear light‐matter‐interaction. Except for a few geometry‐types an accurate modelling of plasmons and plasmon‐related effects is feasible only by sophisticated numerics which combines reliability and efficiency, preferably. We report on FEM‐based calculations for 3D metallic nanoparticles whith special emphasis on both test configurations as well as on effects in layered structures as they are typical for silicon solar cells.

Micro- and Nano-Optical Modeling of Organic LED

The optical performance of organic LED can be optimized by using diffractive and/or refractive structures. Simulation of such complex systems requires mixed modeling of the emission from thin film stacks, diffractive, and refractive elements.