Rainer Bornemann

High Photocurrent in Gated Graphene–Silicon Hybrid Photodiodes

Graphene/silicon (G/Si) heterojunction based devices have been demonstrated as high responsivity photodetectors that are potentially compatible with semiconductor technology. Such G/Si Schottky junction diodes are typically in parallel with gated G/silicon dioxide (SiO2)/Si areas, where the graphene is contacted. Here, we utilize scanning photocurrent measurements to investigate the spatial distribution and explain the physical origin of photocurrent generation in these devices. We observe distinctly higher photocurrents underneath the isolating region of graphene on SiO2 adjacent to the Schottky junction of G/Si. A certain threshold voltage (VT) is required before this can be observed, and its origins are similar to that of the threshold voltage in metal oxide semiconductor field effect transistors. A physical model serves to explain the large photocurrents underneath SiO2 by the formation of an inversion layer in Si. Our findings contribute to a basic understanding of graphene/semiconductor hybrid devices which, in turn, can help in designing efficient optoelectronic devices and systems based on such 2D/3D heterojunctions.

Optimizing the optical and electrical properties of graphene ink thin films by laser-annealing

We demonstrate a facile fabrication technique for graphene-based transparent conductive films. Highly flat and uniform graphene films are obtained through the incorporation of an efficient laser an ...

High-power solid-state cw dye laser

In the present paper we describe a high-power tunable solid-state dye laser setup that offers peak output power up to 800 mW around 575 nm with excellent long-time power stability and low noise level. The spectral width of the laser emission is less than 3 GHz and can be tuned over more than 30 nm. A nearly circular mode profile is achieved with an M(2) better than 1.4. The device can be integrated in a compact housing (dimensions are 60 × 40 × 20 cm(3)). The limitation of long-time power stability is mainly given by photo decomposition of organic dye molecules. These processes are analyzed in detail via spatially resolved micro-imaging and spectroscopic studies.

Efficient, robust, and scale-invariant decomposition of Raman spectra

Raman spectroscopy is used to identify unknown constituent minerals and their abundances since Raman spectra convey characteristic information about the samples chemical structure. We present a novel method to identify constituting pure minerals in a mixture by comparing the measured Raman spectra with a reference database. Our method comprises of two major components: A novel scale-invariant spectral matching technique, that allows to compare measured spectra with the reference spectra from the database even when the band intensities are not directly comparable and an iterative unmixing scheme to decompose a measured spectrum into its constituent minerals and compute their abundances.

High-power CW tunable solid state dye lasers: from the visible to UV

We describe a high power CW solid-state dye laser setup. With perylene orange in PMMA as gain medium an output power up to 800 mW at 576 nm and a tuning range between 565 and 595 nm is reached. The laser output shows good long time power stability. The durability can be adjusted by variation of the pump power. A feedback loop controls the laser output. At a setpoint of e.g. 100 mW, the laser output can be provided for more than eight hours with a low noise level (RMS < 10%). The spectral width of the laser emission is less than 3 GHz and can be tuned over more than 30 nm. A circular mode-profile is achieved with M2 < 1.4 [1]. Via intra-cavity second harmonic generation more than 1 mW of 290 nm UV-radiation is achieved. As nonlinear element a 7 mm BBO (Beta-Barium Borate) crystal is used. The UV laser radiation can be tuned over 10 nm. The theoretical limit of UV output is estimated to 3.5 mW. To our knowledge we present the first tunable CW polymer UV laser. While the output stability at the fundamental wavelength is reasonably good, in the UV region a significant enhancement of the noise level is observed. In addition to this the long time stability is reduced to few minutes. The limitation is mainly given by the photo-decomposition of the organic dye molecules.

Visual Analysis of Confocal Raman Spectroscopy Data using Cascaded Transfer Function Design

2D Confocal Raman Microscopy (CRM) data consist of high dimensional per‐pixel spectral data of 1000 bands and allows for complex spectral and spatial‐spectral analysis tasks, i.e., in material discrimination, material thickness, and spatial material distributions. Currently, simple integral methods are commonly applied as visual analysis solutions to CRM data which exhibit restricted discrimination power in various regards.

Experimental evidence for cm propagation lengths of long-range guided terahertz radiation by thin layers of water

Terahertz waves can be guided with low losses along thin layers of a strongly absorbing medium embedded in a dielectric. By means of end-fire coupling experiments, we demonstrate the polarization dependent excitation, propagation, and detection of this loss-guided mode, demonstrating long propagation lengths for terahertz radiation along thin water films. Broadband frequency and time resolved analysis using transmission terahertz time domain spectroscopy are performed. A theoretical model is used to isolate the coupled mode from co-propagating spurious modes. Hereby, experimental propagation lengths in the cm range are demonstrated, 2–3 orders of magnitude above the intrinsic propagation length in bulk water.