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The emission characteristics of top-emitting organic light-emitting devices ͑OLEDs͒ have been studied experimentally and theoretically to derive a quantitative understanding of the effect of a dielectric capping layer. We demonstrated that the angular intensity distribution and the spectral characteristics can be tuned and the light outcoupling enhanced simply by varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. With the capping-layer concept, the outcoupled light intensity in forward direction was increased by a factor of 1.7, and concomitantly a high color purity achieved. An optical model based on a classical approach was used to calculate the emission characteristics. The excellent agreement between measured and simulated data shows that the capping layer controls the interplay between different interference effects such as wide-angle and multiple-beam interference occurring in top-emitting OLEDs. The strength of the capping layer concept is in particular that the optical and the electrical device performance can be optimized separately.

The Poisson-Nernst-Planck equations describe the dynamics of charge transport in an electric field. Although they are relevant in many applications, a general solution is not known and several aspects are not well understood. In many situations nonlinear effects arise for which no analytical description is available. In this work, we investigate charge transport in a planar device on application of a voltage step. We derive analytical expressions for the dynamical behavior in four extreme cases. In the "geometry limited" regime, applicable at high voltages and low charge contents, we neglect diffusion and the electric field induced by the charges. This leads to a uniform movement of all charges until the bulk is completely depleted. In the "space charge limited" regime, for high voltages and high charge contents, diffusion is still neglected but the electric field is almost completely screened over transient space charge layers. Eventually, however, the bulk becomes depleted of charges and the field becomes homogeneous again. This regime is solved under the assumption of a homogeneous current density, and is characterized by a typical t -3/4 behavior. In the "diffusion limited" regime, valid for low voltages and low charge contents, diffusion is the dominant transport mechanism and prevents the charges from separating. This results in only very small deviations from a homogeneous charge distribution throughout the device. In the "double layer limited" regime, for low voltages and high charge contents, the combination of dominant diffusion and screening of the electric field results in large variations occurring only in thin double layers near the electrodes. Numerical simulations confirm the validity of the derived analytical expressions for each of the four regimes, and allow us to investigate the parameter values for which they are applicable. We present transient current measurements on a nonpolar liquid with surfactant and compare them with the external current predicted by the theoretical description. The agreement of the analytical expressions with the experiments allows us to obtain values for a number of properties of the charges in the liquid, which are consistent with results in other works. The confirmation by simulations and measurements of the derived theoretical expressions gives confidence about their usefulness to understand various aspects of the Poisson-Nernst-Planck equations and the effects they represent in the dynamics of charge transport.

Silicon nitride (SiN) is emerging as a competitive platform for CMOS-compatible integrated photonics. However, active devices such as modulators are scarce and still lack in performance. Ideally, such a modulator should have a high bandwidth, good modulation efficiency, low loss, and cover a wide wavelength range. Here, we demonstrate the first electro-optic modulators based on ferroelectric lead zirconate titanate (PZT) films on SiN, in both the O-band and C-band. Bias-free operation, bandwidths beyond 33 GHz and data rates of 40 Gbps are shown, as well as low propagation losses (α ≈ 1 dB cm−1). A half-wave voltage-length product of 3.2 V cm is measured. Simulations indicate that further improvement is possible. This approach offers a much-anticipated route towards high-performance phase modulators on SiN.

We propose a direct method to determine absorption anisotropy of colloidal quantum rods. In this method, the rods are aligned in solution by using an alternating electric field and we measure simultaneously the resulting average change in absorption. We show that a frequency window for the electric field exists in which the change in absorbance as a function of field strength can be analyzed in terms of the quantum-rod dipole moment and the absorption coefficient for light that is polarized parallel or perpendicular to the long axis of the rod. The approach is verified by measuring the absorbance change of CdSe rods at 400 nm as a function of field strength, where we demonstrate excellent agreement between experiment and theory. This enables us to propose improved values for the CdSe quantum-rod extinction coefficient. Next, we analyze CdSe/CdS dot-in-rods and find that the absorption of the first exciton transition is fully anisotropic, with a vanishing absorption coefficient for light that is polarized perpendicularly to the long axis of the rods.

Obtaining sub‐resolution particle positions in fluorescence microscopy images is essential for single particle tracking and high‐resolution localization microscopy. While the localization precision of stationary single molecules or particles is well understood, the influence of particle motion during image acquisition has been largely neglected. Here, we address this issue and provide a theoretical description on how particle motion influences the centroid localization precision, both in case of 2‐D and 3‐D diffusion. In addition, a novel method is proposed, based on dual‐channel imaging, for the experimental determination of the localization precision of moving particles. For typical single particle tracking experiments, we show that the localization precision is approximately two‐fold worse than expected from the stationary theory. Strikingly, we find that the most popular localization method, based on the fitting of a Gaussian distribution, breaks down for lateral diffusion. Instead, the centroid localization method is found to perform well under all conditions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract. Liquid crystals are nowadays widely used in all types of display applications. However their unique electro-optic properties also make them a suitable material for nondisplay applications. We will focus on the use of liquid crystals in different photonic components: optical filters and switches, beam-steering devices, spatial light modulators, integrated devices based on optical waveguiding, lasers, and optical nonlinear components. Both the basic operating principles as well as the recent state-of-the art are discussed. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).[ DOI: 10.1117/1.3565046] Subject terms: liquid crystals; photonic applications; review; liquid-crystal lasers; spatial light modulators; tunable lenses; nematicons; optical nonlinearity.Paper 100913SSR received Nov. 5, 2010; revised manuscript received Jan. 11, 2011; accepted for publication Jan. 13, 2011; published online Jun. 14, 2011. IntroductionLiquid crystals (LCs) are organic materials that are liquid but that show a certain degree of ordering (positional and/or orientational). With this definition, many materials can be classified as liquid crystals, but the majority of liquid crystals that are used in photonic applications are of the thermotropic type. Thermotropic means that the liquid-crystal phase exists within a certain temperature interval (in contrast to lyotropic materials for which the material is liquid-crystal within a certain concentration range). Various types of thermotropic liquid-crystal materials exist, and many different mesophases have been discovered in the last decades: nematic, smectic A, smectic C, columnar, blue phases, and many many more. The diversity of liquid-crystal materials is huge, but this diversity is even overshadowed by the number of applications in which liquid crystals are used nowadays. The majority of applications are related to informationdisplay applications. Liquid crystals have conquered the major market share in different display application areas: television screens, laptop screens, screens in mobile phones, etc. Only in projection displays does a tough competitor exist, namely microelectromechanical systems or licenced by Texas Instruments: Digital light processing. Organic light emitting diodes (OLEDs) are the obvious next generation technology that could overtake the LC domination, but today OLED displays have not penetrated the market and only the future will tell if they will. In this review article, we will focus on nondisplay applications, but because we cannot go too broad, we restrict ourselves to photonic applications in which the light is actively manipulated by the LC. In this review article, a certain external influence (surface anchoring, electric fields, optical fields) is used to (re)orient the LC in a certain way. In turn, the LC then changes the light that is propagating through it. Of course, as in every review article, it is impossible to list all the fascinating new scientific results or breakthroughs. Therefore, the authors apologize for not i...