Electronic transport in a series of multiple arbitrary tunnel junctions

Monte Carlo simulations and an analytical approach within the framework of a semiclassical model are presented which permit the determination of Coulomb blockade and single electron charging effects for multiple tunnel junctions coupled in series. The Coulomb gap in the I(V) curves can be expressed as a simple function of the capacitances in the series. Furthermore, the magnitude of the differential conductivity at current onset is calculated in terms of the model. The results are discussed with respect to the number of junctions.Comment: 3 figures, revte

Optoelectronic printed circuit board: 3D structures written by two-photon absorption

ABSTRACT The integration of optical interconnects in printed circuit boards (PCB) is a rapidly growing field due to a continuously increasing demand for high data rates, along with a progressive miniaturization of devices and components. For high-speed data transfer, materials and integration concepts are searched for which enable high-speed short-range connections, accounting also for miniaturization, and costs. Many concepts are discussed so far for the integration of optics in PCB: the use of optical fibers, or the generation of waveguides by UV lithography, embossing, or direct laser writing. Most of the concepts require many different materials and process steps. In addition, they also need highly-sophisticated assembly steps in order to couple the optoelectronic elements to the optical waveguides. An innovative approach is presented which only makes use of only one individual inorganic-organic hybrid polymer material to fabricate optical waveguides by two-photon absorption (TPA) processes. Particularly, the waveguides can be directly integrated on pre-configured PCB by in situ positioning the optical waveguides with respect to the mounted optoelectronic components by the TPA process. Thus, no complex packaging or assembly is necessary, and the number of process steps is significantly reduced, where the process fits ideally into the PCB fabrication process. The material properties, the TPA processing of waveguides, and the integration concept will be discussed. Recent experiments employing vertical-cavity surface-emitting lasers demonstrated data rates exceeding 6 Gbit/s

Two-photon polymerization as method for the fabrication of large scale biomedical scaffold applications

Two-photon polymerization (2PP) using ultra-short laser pulses is a well-known methodology for the 3D free-form fabrica-tion of optical devices with resolutions down to 100 nm. However, the structure dimensions have been restricted to quite small sizes, mainly due to limitations of the focussing optics and due to very long fabrication times. Therefore, the large-scale fabrication of biomedical scaffold structures with dimensions in the mm-range still remains challenging. Using a diode-pumped Ytterbium laser system emitting 325 fs laser pulses at 515 nm after second harmonic generation we are able to write arbitrary 3D structures in inorganic-organic hybrid polymers (ORMOCER®s). Our setup is able to produce structures with mm extension normal to the substrate at a structural resolution of a few microns. In particular, a 3D porous inner structure can be provided, which is required for three-dimensional cell growth to support cell adhesion and prolifera-tion. Scaffold stru ctures were produced with different parameters, and they were characterized in order to demonstrate their potential concerning resolution and scaffold quality. It is found that not only the experimental setup, but also the substrate material plays an important role for the scaffold fabrication process and structural quality

Compensation of spherical aberration influences for two-photon polymerization patterning of large 3D scaffolds

Two-photon polymerization using femtosecond laser pulses at a wavelength of 515 nm is used for three-dimensional patterning of photosensitive, biocompatible inorganic-organic hybrid polymers (ORMOCER(A (R))s). In order to fabricate millimeter-sized biomedical scaffold structures with interconnected pores, medium numerical aperture air objectives with long working distances are applied which allow voxel lengths of several micrometers and thus the solidification of large scaffolds in an adequate time. It is demonstrated that during processing the refraction of the focused laser beam at the air/material interface leads to strong spherical aberration which decreases the peak intensity of the focal point spread function along with shifting and severely extending the focal region in the direction of the beam propagation. These effects clearly decrease the structure integrity, homogeneity and the structure details and therefore are minimized by applying a positioning and laser power adaptation throughout the fabrication process. The results will be discussed with respect to the resulting structural homogeneity and its application as biomedical scaffold