The Meta-Substituted Isomer of TMPyP Enables More Effective Photodynamic Bacterial Inactivation than Para-TMPyP In Vitro.

Porphyrinoid-based photodynamic inactivation (PDI) provides a promising approach to treating multidrug-resistant infections. However, available agents for PDI still have optimization potential with regard to effectiveness, toxicology, chemical stability, and solubility. The currently available photosensitizer TMPyP is provided with a para substitution pattern (para-TMPyP) of the pyridinium groups and has been demonstrated to be effective for PDI of multidrug-resistant bacteria. To further improve its properties, we synthetized a structural variant of TMPyP with an isomeric substitution pattern in a meta configuration (meta-TMPyP), confirmed the correct structure by crystallographic analysis and performed a characterization with NMR-, UV/Vis-, and IR spectroscopy, photostability, and singlet oxygen generation assay. Meta-TMPyP had a hypochromic shift in absorbance (4 nm) with a 55% higher extinction coefficient and slightly improved photostability (+6.9%) compared to para-TMPyP. Despite these superior molecular properties, singlet oxygen generation was increased by only 5.4%. In contrast, PDI, based on meta-TMPyP, reduced the density of extended spectrum ß-lactamase-producing and fluoroquinolone-resistant Escherichia coli by several orders of magnitude, whereby a sterilizing effect was observed after 48 min of illumination, while para-TMPyP was less effective (p < 0.01). These findings demonstrate that structural modification with meta substitution increases antibacterial properties of TMPyP in PDI.

Gradients in Three-Dimensional Core-Shell GaN/InGaN Structures: Optimization and Physical Limitations.

Three-dimensional InGaN/GaN nano- and microstructures with high aspect ratios and large active sidewall areas are still of great interest in the field of optoelectronics. However, when grown by metalorganic chemical vapor deposition (MOCVD), their optical performance can be negatively affected by gradients in thickness and peak emission wavelength along their sidewalls, which is still a key obstacle for using such structures in commercial products. In this work, we present a detailed study on the different mechanisms causing this gradient, as well as means to alleviate it. Gas-phase mass transport and surface diffusion are found to be the two main processes governing the shell growth, and the predominance of one process over the other is varying with the geometry of the 3D structures as well as the spacing between them. Consequently, variations in temperature, which mainly affect surface diffusion, will have a stronger impact on structures with small separation between them rather than larger ones. On the other hand, variations in pressure modify gas-phase diffusion, and thus, structures with a large spacing will be more strongly affected. A proper design of the dimensions of 3D architectures as well as the separation between them may improve the gradient along the sidewalls, but a tradeoff with the active area per wafer footprint is inevitable.

Influence of common lighting conditions and time-of-day on the effort-related cardiac response.

Melanopic stimuli trigger diverse non-image-forming effects. However, evidence of a melanopic contribution to acute effects on alertness and performance is inconclusive, especially under common lighting situations. Effects on cognitive performance are likely mediated by effort-related physiological changes. We assessed the acute effects of lighting in three scenarios, at two times of day, on effort-related changes to cardiac contraction as indexed by the cardiac pre-ejection period (PEP). In a within-subject design, twenty-seven participants performed a cognitive task thrice during a morning and a late-afternoon session. We set the lighting at 500 lux in all three lighting scenarios, measured horizontally at the desk level, but with 54 lux, 128 lux, or 241 lux melanopic equivalent daylight illuminance at the eye level. Impedance cardiography and electrocardiography measurements were used to calculate PEP, for the baseline and task period. A shorter PEP during the task represents a sympathetic heart activation and therefore increased effort. Data were analysed with linear mixed-effect models. PEP changes depended on both the light scene and time of day (p = 0.01 and p = 0.002, respectively). The highest change (sympathetic activation) occurred for the medium one of the three stimuli (128 lux) during the late-afternoon session. However, effect sizes for the singular effects were small, and only for the combined effect of light and time of day middle-sized. Performance scores or self-reported scores on alertness and task demand did not change with the light scene. In conclusion, participants reached the same performance most efficiently at both the highest and lowest melanopic setting, and during the morning session. The resulting U-shaped relation between melanopic stimulus intensity and PEP is likely not dependent solely on intrinsic ipRGC stimuli, and might be moderated by extrinsic cone input. Since lighting situations were modelled according to current integrative lighting strategies and real-life indoor light intensities, the result has implications for artificial lighting in a work environment.

Spectral-temporal dynamics of (Al,In)GaN laser diodes.

(Al,In)GaN laser diodes have various relevant applications, especially in projection systems for virtual and augmented reality devices and in optical communication, all requiring fast modulation. This corresponds to pulses in the nanosecond to microsecond range, where a rich longitudinal mode dynamics occurs. We investigate this spectral-temporal dynamics experimentally with a streak camera system and simulate it using a longitudinal multi-mode rate equation model. We observe an interplay of effects, which have been observed selectively, such as relaxation oscillations, mode competition and inhomogeneous pumping of multiple quantum wells. A mechanism is included in the simulations to model the red-shift of the gain spectrum due to the carrier density in the quantum wells exceeding threshold density, which is amplified by inhomogeneous pumping. Mode competition leads to spectral cycles of the active mode with a noticeable jitter, which is observed in single pulse measurements in comparison to multi pulse averaged measurements where blurring occurs. Here, some statistical behavior as well as repeating patterns are investigated at the same effect. Also thermal effects as laser diode self-heating are discussed and have been measured over six orders of magnitude in time.

Phosphor-based light conversion for miniaturized optical tools.

This paper describes the application of phosphor-based light conversion for its use in optogenetic experiments to tailor the wavelength of light emitted from implantable miniaturized light sources. Gallium-nitride-based blue light-emitting diodes are used in combination with orthosilicate phosphor immersed in an epoxy matrix and emitting in the yellow wavelength range. The miniaturization of the phosphor-containing polymer droplets toward diameters as small as 300 µm provides the compatibility with implantable optical probes. The parameter study applied here varied the concentration of the phosphor material in the polymer matrix as well as the droplet height in order to tailor the characteristics of blue-to-yellow light conversion.

Quasi-Bessel beams from asymmetric and astigmatic illumination sources.

We study the spatial intensity distribution and the self-reconstruction of quasi-Bessel beams produced from refractive axicon lenses with edge emitting laser diodes as asymmetric and astigmatic illumination sources. Comparing these to a symmetric mono-mode fiber source, we find that the asymmetry results in a transition of a quasi-Bessel beam into a bow-tie shaped pattern and eventually to a line shaped profile at a larger distance along the optical axis. Furthermore, we analytically estimate and discuss the effects of astigmatism, substrate modes and non-perfect axicons. We find a good agreement between experiment, simulation and analytic considerations. Results include the derivation of a maximal axicon angle related to astigmatism of the illuminating beam, impact of laser diode beam profile imperfections like substrate modes and a longitudinal oscillation of the core intensity and radius caused by a rounded axicon tip.

Quasi-epitaxial growth of [Ru(bpy)3 ](2+) by confinement in clay nanoplatelets yields polarized emission.

A nano confinement strategy is presented to control the spatial orientation and emission polarization of phosphorescent metal complexes. Through nano-confinement of the phosphorescent metal complex [Ru(bpy)3 ](2+) by attaching it to anionic clay nanoplatelets, it is possible to simultaneously lock the spatial orientation of the complex and fix its emission polarization. This quasi-epitaxial approach may provide a future work strategy directed at light emitting diodes and lasers.

Longitudinal mode competition and mode clustering in (Al,In)GaN laser diodes.

Longitudinal mode competition in (Al,In)GaN laser diodes at λ = 445nm and 515 nm with mode competition frequencies from 10 MHz to 150 MHz is observed. Up to two dozen lasing modes oscillate with the lasing mode rolling from the short wavelength edge to the long wavelength edge of the gain profile. The experimental results can be described very well with a set of multi-mode rate equations including self-, symmetric and asymmetric cross gain saturation. By tuning essential parameters of the gain saturation terms, mode competition disappears and single mode operation as well as mode clustering is found. This proves that the mechanisms of gain saturation have not only a profound impact on the complex temporal-spectral behavior but also explains mode clustering in (Al,In)GaN laser diodes, both in pulsed and continuous wave (cw) operation as a natural nonlinear effect without the necessity to add noise.

Maximal power output by solar cells with angular confinement.

Angularly selective filters can increase the efficiency of radiatively limited solar cells. A restriction of the acceptance angle is linked to the kind of utilizable solar spectrum (global or direct radiation). This has to be considered when calculating the potential enhancement of both the efficiency and the power output. In this paper, different concepts to realize angularly selective filters are compared regarding their limits for efficiency and power output per unit area. First experimental results of a promising system based on a thin-film filter as the angularly selective element are given to demonstrate the practical relevance of such systems.

Superior functionality by design: selective ozone sensing realized by rationally constructed high-index ZnO surfaces.

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high-index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c-axis-oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO-ZnO core-shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high-index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO-ZnO core-shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core-shell nanowires are largely non-responsive to varying O(3) concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect-rich high-index polar surfaces.

Sub-surface channels in sapphire made by ultraviolet picosecond laser irradiation and selective etching.

We demonstrate the realization of sub-surface channels in sapphire prepared by ultraviolet picosecond laser irradiation and subsequent selective wet etching. By optimizing the pulse energy and the separation between individual laser pulses, an optimization of channel length can be achieved with an aspect ratio as high as 3200. Due to strong variation in channel length, further investigation was done to improve the reproducibility. By multiple irradiations the standard deviation of the channel length could be reduced to 2.2%. The achieved channel length together with the high reproducibility and the use of a commercial picosecond laser system makes the process attractive for industrial application.

Transient memory effect in the photoluminescence of InGaN single quantum wells.

The transition to maximum photoluminescence of InGaN single quantum wells is a phenomena that has time constants in the range of few seconds. Using a systematic illumination/darkening procedure we found that these characteristics are related to previous stimulations as if the sample has a memory of past illumination events. Choosing opportune time sequences, time constants were observed to vary more than 100%. These facts suggest the presence of carrier trapping/de-trapping processes that act beyond the single illumination event, accumulating over time in a complex effect.

Spectral dynamics of 405 nm (Al,In)GaN laser diodes grown on GaN and SiC substrate.

We investigate the spectral properties of violet 405 nm (Al,In)GaN laser diodes (LDs). Depending on the substrate the LDs are grown on, the lasing spectra show significant differences. LDs grown on low dislocation GaN substrate have a broad spectrum with several longitudinal modes, while LDs grown on SiC substrate are lasing on a single longitudinal mode.With increasing current, the laser emission of LDs grown on SiC substrate jumps from one longitudinal mode to another (mode hopping), whereas GaN substrate LDs show a smooth but asymmetric mode comb. The different envelopes of these spectra can be understood by assuming a variation of the gain for each individual longitudinal mode. With a high spectral resolution setup, we measure the gain of each longitudinal mode, employing the Hakki-Paoli method. Measurements show a slightly fluctuating gain for the modes of GaN substrate LDs, but much larger fluctuations for LDs on SiC substrate. We carry out simulations of the longitudinal mode spectrum of (Al,In)GaN laser diodes using a rate equation model with nonlinear gain (self saturation, symmetric and asymmetric cross saturation) and including gain fluctuations. With a set of parameters which is largely identical for LDs on either substrate, the simulated spectra truly resemble those typical for LDs on GaN or SiC substrate.

Measurement and simulation of filamentation in (Al,In)GaN laser diodes.

(Al,In)GaN-based laser diodes with ridge widths broader than a few micrometer tend to show filamentation effects in the lateral direction. By time-resolved scanning near-field optical microscopy, we find different kinds of filaments depending on ridge width and lateral position. We investigate these effects systematically and compare them to the results of corresponding simulations, which are based on a simple rate equation model including the lateral dimension. By this comparison we find a consistent and reasonable set of material parameters that can describe the laser diode. Furthermore, we discuss several reasons for filamentation dynamics like ridge asymmetry or spatial hole-burning, as well as critical temperatures that induce filamentation.

Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes.

Optical gain spectra presented for (Al,In)GaN laser diodes with lasing wavelength ranging from UV (375 nm) to aquamarine (470 nm) show a strong increase in inhomogeneous broadening, caused by Indium composition and quantum well width fluctuations which increase with Indium mole fraction. These gain spectra provides a standard data set for the calibration of microscopic many-body simulations. We demonstrate by comparison with basic simulations that the different assumptions of a global constant carrier density or of global constant quasi-Fermi levels for electrons and holes lead to a strikingly different dependency of optical gain on carrier density. For constant quasi-Fermi levels the threshold carrier density becomes insensitive to inhomogeneous broadening for realistic parameters. This is in agreement with the observation that the threshold current is nearly independent over the wavelength range from near UV to aquamarine.

Stokes parameters in the unfolding of an optical vortex through a birefringent crystal.

Following our earlier work [F. Flossmann et al., Phys. Rev. Lett. 95 253901 (2005)], we describe the fine polarization structure of a beam containing optical vortices propagating through a birefringent crystal, both experimentally and theoretically.We emphasize here the zero surfaces of the Stokes parameters in three-dimensional space, two transverse dimensions and the third corresponding to optical path length in the crystal. We find that the complicated network of polarization singularities reported earlier -lines of circular polarization (C lines) and surfaces of linear polarization (L surfaces) - can be understood naturally in terms of the zeros of the Stokes parameters.

Polarization singularities from unfolding an optical vortex through a birefringent crystal.

Optical vortices (nodal lines and phase singularities) are the generic singularities of scalar optics but are unstable in vector optics. We investigate experimentally and theoretically the unfolding of a uniformly polarized optical vortex beam on propagation through a birefringent crystal and characterize the output field in terms of polarization singularities (C lines and points of circular polarization; L surfaces and lines of linear polarization). The field is described both in the 2-dimensional transverse plane, and in three dimensions, where the third is abstract, representing an optical path length propagated through the crystal. Many phenomena of singular optics, such as topological charge conservation and singularity reconnections, occur naturally in the description.

Observation of Ince-Gaussian modes in stable resonators.

We report what is to our knowledge the first observation of Ince-Gaussian modes directly generated in a stable resonator. By slightly breaking the symmetry of the cavity of a diode-pumped Nd:YVO4 laser and its pump beam configuration we were able to generate single high-order Ince-Gaussian modes of high quality. The observed transverse modes have an inherent elliptic structure and exhibit remarkable agreement with theoretical predictions.