Ulf Geyer

Large-scale patterning of indium tin oxide electrodes for guided mode extraction from organic light-emitting diodes

We describe a cost-efficient and large area scalable production process of organic light-emitting diodes (OLEDs) with photonic crystals (PCs) as extraction elements for guided modes. Using laser interference lithography and physical plasma etching, we texture the indium tin oxide (ITO) electrode layer of an OLED with one- and two-dimensional PC gratings. By optical transmission measurements, the resonant mode of the grating is shown to have a drift of only 0.4% over the 5mm length of the ITO grating. By changing the lattice constant between 300 and 600nm, the OLED emission angle of enhanced light outcoupling is tailored from −24.25° to 37°. At these angles, the TE emission is enhanced up to a factor of 2.14.

Enhancing outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via nanostructured high index layers

We fabricated organic light-emitting diodes with one-dimensional Bragg gratings as light extraction elements for substrate and waveguide modes. A Ta2O5 layer was introduced to obtain a high refractive index contrast to the subsequent anode layer. As anode we employed a highly conductive polymer. Laser interference lithography and physical plasma etching were used to pattern gratings into the Ta2O5 layer with a lattice constant of 370 nm and various grating depths. Mainly attributed to the outcoupling of the substrate modes, the structured devices exhibit a luminous flux which is up to four times higher compared to the unstructured reference devices.

Optical characterization of photonic crystal slabs using orthogonally oriented polarization filters

We present an experimental method for direct analysis of guided-mode resonances in photonic crystal slab structures using transmission measurements. By positioning the photonic crystal slab between orthogonally oriented polarization filters light transmission is suppressed except for the guided-mode resonances. Angle resolved transmission measurements with crossed polarizers are performed to obtain the band structure around the Gamma-point. Results are compared to mode simulations. Spatially resolved measurements in a confocal microscope setup are used for homogeneity characterizations. Stitching errors and inhomogeneities in exposure dose down to 1.3% in photonic crystal slabs fabricated by electron beam lithography are observed using this method.

Metallic Bragg-gratings for light management in organic light-emitting devices

We used laser interference lithography to fabricate organic light-emitting diodes (OLEDs) with gold Bragg gratings on top of the indium-tin-oxide layer for efficient light management. We built polymer OLEDs with 15 and 30 nm gold grating thickness as well as reference devices with continuous gold layers and without gold layer. Electrical and optical device characterization was performed and compared to optical simulations. The gratings extract waveguide and substrate modes and change the angular and spectral emission characteristics by cavity effects. The combination of light extraction and cavity effects leads to 25–30% enhancement in power efficiency compared to reference devices.

Organic semiconductor lasers as integrated light sources for optical sensor systems

We demonstrate the feasibility of organic semiconductor lasers as light sources for lab-on-a-chip systems. These lasers are based on a 1D- or 2D-photonic crystal resonator structure providing optical feedback in the active laser material that is deposited on top, e.g. aluminum tris(8-hydroxyquinoline) (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). We investigated different fabrication methods for the resonator structures, like thermal nanoimprint, UV nanoimprint, and laser interference lithography. Different substrate materials commonly used in lab-on-a-chip systems, e.g. PMMA, Topas, and Ormocer were deployed. By changing the distributed feedback grating periodicity, we demonstrate a tuning range for a single material system of more than 120 nm. The investigated organic semiconductor lasers are optically pumped. External optical pumping provides a feasible way for one-time-use chips. Our recent success of pumping organic lasers with a low-cost laser diode also renders hand-held systems possible. As a further step towards the integration of organic lasers in sensor systems, we demonstrate the coupling of an organic laser into polymeric waveguides which can be combined with microfluidic channels. The integrated organic lasers and the waveguides are both fabricated on the same polished PMMA substrate using thermal nanoimprint lithography and deep-UV modification, respectively. We could demonstrate the guiding of the laser light in single-mode waveguides.

Optical spectroscopy with organic semiconductor lasers

We report on the fabrication of large-scale surface gratings by laser interference lithography and reactive ion etching on which we evaporated a thin film of the organic semiconductor tris(8-hydroxyquinoline) aluminum doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane. We created a thickness gradient by using a rotating shadow mask evaporation technique. This allowed us to continuously tune the emission wavelength from 606 nm to 661 nm on a single substrate. After encapsulation, we demonstrated the usefulness of such low-cost and tunable organic semiconductor lasers by conducting simple fluorescence excitation and transmission spectroscopy measurements using a minimal amount of additional optical components.

Material-based three-dimensional imaging with nanostructured surfaces

Visualizing three-dimensional (3D) structures at the micrometer and nanometer scale is essential not only for characterizing materials and corrosion but also biological samples. Here, we present a material-based nano-optical method using the near-field properties of periodically nanostructured surfaces (photonic crystal slabs) to obtain 3D images. The wavelength and the quality factor of resonances in the transmission spectrum provide optical thickness information of objects on the surface, which we use for rapid topography determination of cells.

Periodic nanostructuring for guided mode extraction in organic light-emitting diodes

We investigated guided mode extraction in organic light-emitting diodes and compare the experimental findings to transfer matrix (T-matrix) and finite difference time domain (FDTD) simulations. To this end, we patterned the indium tin oxide anode with Bragg gratings with lattice constants from 300 to 600 nm and varied the depth of the grating structures. The structuring was done by laser interference lithography and plasma etching. Both techniques allow for a rapid large area processing. We measured angle resolved electroluminescent spectra of the nanostructured devices and reference devices. To obtain the mode distribution in the devices we made use of T-matrix simulations. In addition we performed FDTD simulations of the emission characteristics of the patterned devices. The simulations are in agreement with our experimental findings and give insight into the outcoupling mechanisms.

Nano-structured metallic electrodes for plasmonic optimized light- emitting diodes

Metallic nanostructures have attracted large interest recently due to new optical properties caused by plasmonic effects. The exceptionally high transmission of light through periodically structured metals is originated by interactions between light and plasmonic resonances. These resonances are controllable by varying periodicity and geometrical dimensions of the metal gratings. Our aim is the utilization of these effects to improve the efficiency of conventional light-emitting diodes (LED). The application of one-dimensional periodic metallic gratings as top electrodes of LEDs offers advantages such as efficient and homogeneous current injection, enhanced light output, modified angular light emission characteristics and linear polarization of the emission. Based on finite-difference time-domain simulations, we optimized the parameters for gold and silver gratings on top of InGaAs/GaAs/AlAs heterostructures. Fabrication of these structures was carried out using laser interference lithography (LIL) and a lift-off process. We measured the optical transmission of these structures and were able to demonstrate a polarization- and wavelength-dependence in good consistency with our calculations.

Direct observation of photonic modes in photonic crystal slabs

Guided-mode resonances at the gamma-point are characterized via normal-incidence transmission measurements through photonic crystal slabs. By placing the photonic crystal slab between two orthogonally oriented polarization filters, direct light transmission is suppressed. Only light coupling to the guided-mode resonances is observed in transmission. Transmission properties of linear, quadratic, and hexagonal photonic crystal slabs are investigated. Experimental results are compared to finite-difference time-domain simulations.