Norbert Danz

Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter’s radiative quantum efficiency

The efficiency of organic light-emitting diodes is limited as only a fraction of the consumed electrical power is converted into light that is finally extracted to air. Especially, the radiative quantum efficiency of the guest-host system is of interest and should be close to unity to achieve highly efficient devices. We show that the red phosphorescent emitter Ir(MDQ)2(acac) doped in an α-NPD matrix exhibits a profound non-isotropic dipole orientation. Ignoring this feature leads to a significant overestimation of the emitter efficiency. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel emitter orientation in phosphorescent organic light-emitting diodes.

Dipole lifetime in stratified media

The field of an electric dipole inside an arbitrary system of parallel slabs is evaluated with a Green’s function approach. Application of boundary conditions yields a matrix formalism that allows compact formulation of the problem. The method is extended to the general case of parallel stratified medialike cavities containing a dipole. Effects on spontaneous-emission rate of dipole emitters are evaluated and discussed for different types of planar microcavities.

A full ellipsometric approach to optical sensing with Bloch surface waves on photonic crystals

We report on the investigation on the resolution of optical sensors exploiting Bloch surface waves sustained by one dimensional photonic crystals. A figure of merit is introduced to quantitatively assess the performance of such sensors and its dependency on the geometry and materials of the photonic crystal. We show that the figure of merit and the resolution can be improved by adopting a full ellipsometric phase-sensitive approach. The theoretical predictions are confirmed by experiments in which, for the first time, such type of sensors are operated in the full ellipsometric scheme.

Surface plasmon resonance platform technology for multi-parameter analyses on polymer chips

The development of a surface plasmon resonance (SPR) spectrometer comprising angular‐resolved analysis for quasi‐monochromatic illumination is reported. The optical system utilizes disposable, injection‐molded chips combined with a lateral imaging optical system for parallel analysis of one‐dimensional spot arrays. Further parallelization is achieved by introducing a segmented light source. This source sequentially illuminates three neighbored one‐dimensional arrays in order to keep angular‐resolved analysis without introducing any mechanically moving parts. This system is applied to detect genetic variations among different DNA samples obtained from polymerase chain reaction (PCR). For this purpose, 135 spots on the chip surface have been prepared by spotting and analyzed separately.

A Fluorescent One-Dimensional Photonic Crystal for Label-Free Biosensing Based on Bloch Surface Waves

A one-dimensional photonic crystal (1DPC) based on a planar stack of dielectric layers is used as an optical transducer for biosensing, upon the coupling of TE-polarized Bloch Surface Waves (BSW). The structure is tailored with a polymeric layer providing a chemical functionality facilitating the covalent binding of orienting proteins needed for a subsequent grafting of antibodies in an immunoassay detection scheme. The polymeric layer is impregnated with Cy3 dye, in such a way that the photonic structure can exhibit an emissive behavior. The BSW-coupled fluorescence shift is used as a means for detecting refractive index variations occurring at the 1DPC surface, according to a label-free concept. The proposed working principle is successfully demonstrated in real-time tracking of protein G covalent binding on the 1DPC surface within a fluidic cell.

Measuring the profile of the emission zone in polymeric organic light-emitting diodes

The profile of the emission zone (PEZ) in the emissive layer (EML) of polymeric organic light-emitting diodes (OLEDs) is determined by fitting the measured electroluminescence spectrum of the device with a series of simulated spectra that correspond to different emitter locations. The studied OLEDs are based on two different blue-emitting conjugated polymers. In one case, the PEZ is confined to a 10 nm thin sheet at the anode indicating electron dominated current in the EML. In OLEDs based on the other emitter, the PEZ spreads over the entire EML.

Combining label-free and fluorescence operation of Bloch surface wave optical sensors

We report on the design, fabrication, and characterization of optical sensors based on Bloch surface waves propagating at the truncation edge of one-dimensional photonic crystals. The sensors can be simultaneously operated in both a label-free mode, where small refractive index changes at the surface are detected, and a fluorescence mode, where the fluorescence from a novel heptamethyne dye label in the proximity of the surface is collected. The two modes operate in the near-infrared spectral range with the same configuration of the optical reading apparatus. The limit of detection is shown to be smaller than that of equivalent surface plasmon sensors and the fluorescence collection efficiency is such that it can be efficiently analyzed by the same camera sensor used for label-free operation.

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.

DNA microarrays for hybridization detection by surface plasmon resonance spectroscopy

We report on the development of a new platform technology for the detection of genetic variations by means of surface plasmon resonance (SPR) spectroscopy. TOPAS chips with integrated optics were exploited in combination with microfluidics. Within minutes, the detection of hybridization kinetics was achieved simultaneously at all spots of the DNA microarray. A nanoliter dispenser is used to deposit thiol-modified single-stranded probe DNA on the gold surface of the chips. We investigated the influence of different parameters on hybridization using model polymerase chain reaction (PCR) products. These PCR products comprised a single-stranded tag sequence being complementary to an anti-tag sequence of probes immobilized on the gold surface. The signals increased with increasing length of PCR products (60, 100 or 300 base pairs) as well as with their concentration. We investigated hybridizations on DNA microarrays comprising 90 spots of probe DNA with three different sequences. Furthermore, we demonstrate that sequences with possible hairpin structures significantly lower the binding rate, and thus, the SPR signals during hybridization.

Probing losses of dielectric multilayers by means of Bloch surface waves

We exploit the excitation of electromagnetic surface waves on high-quality dielectric multilayers to measure the very low extinction coefficient of the structures, with a resolution down to 4·10(-7) and in a simple optical configuration. The effect of exposition to a rhodamine 6G solution in water and ethanol is also reported, including dye adsorption in the layers and bleaching upon resonant excitation.