Benjamin Ewers

The optics and performance of dual-focus fluorescence correlation spectroscopy.

Fluorescence correlation spectroscopy (FCS) is an important spectroscopic technique which can be used for measuring the diffusion and thus size of fluorescing molecules at pico- to nanomolar concentrations. Recently, we introduced an extension of conventional FCS, which is called dual-focus FCS (2fFCS) and allows absolute diffusion measurements with high precision and repeatability. It was shown experimentally that the method is robust against most optical and sample artefacts which are troubling conventional FCS measurements, and is furthermore able to yield absolute values of diffusion coefficients without referencing against known standards. However, a thorough theoretical treatment of the performance of 2fFCS is still missing. The present paper aims at filling this gap. Here, we have systematically studied the performance of 2fFCS with respect to the most important optical and photophysical factors such as cover slide thick-ness, refractive index of the sample, laser beam geometry, and optical satu-ration. We show that 2fFCS has indeed a superior performance when com-pared with conventional FCS, being mostly insensitive to most potential ab-errations when working under optimized conditions.

Changes in the structural dimensionality of selenidostannates in ionic liquids: formation, structures, stability, and photoconductivity.

In situ transformations of selenidostannate frameworks in ionic liquids (ILs) were initiated by treatment of the starting phase K2[Sn2Se5] and the consecutive reaction products by means of temperature increase and/or amine addition. Along the reaction pathway, the framework dimensionalities of the five involved selenidostannate anions develop from 3D to 1D and back, both in top-down and bottom-up style. Addition of ethane-1,2-diamine (en) led to the reversion of the 2D→1D step from 2D-{[Sn24Se56](16-)} to 1D-{[Sn6Se14](4-)}. As rationalized by DFT investigations, the 2D anion is thermodynamically favored. Photoconductivity measurements reveal that all samples show Schottky contact behavior with absolute thresholds below 10 V. One of the samples exhibits conductive states within the energy range of visible photons.

Observation of Forbidden Exciton Transitions Mediated by Coulomb Interactions in Photoexcited Semiconductor Quantum Wells

W. D. Rice, J. Kono,1,2,∗ S. Zybell, S. Winnerl, J. Bhattacharyya, H. Schneider, M. Helm, B. Ewers, A. Chernikov, M. Koch, S. Chatterjee, G. Khitrova, H. M. Gibbs,†,6 L. Schneebeli, B. Breddermann, M. Kira, and S. W. Koch, Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany Technische Universität Dresden, 01062 Dresden, Germany Department of Physics, Philipps University Marburg, Renthof 5, D-35032 Marburg, Germany College of Optical Science, University of Arizona, Tucson, Arizona 85721-0094, USA ∗corresponding author: kono@rice.edu

Recent advances in time-correlated single-photon counting

We report about the time-resolved confocal fluorescence microscope MicroTime 200, which is completely based on TTTR format data acquisition and enables to perform very advanced FCS, FRET and FLIM analysis such as Fluorescence Lifetime Correlation Spectroscopy (FLCS) or Two Focus FCS (2fFCS). FLCS is a fundamental improvement of standard FCS overcoming many of its inherent limitations. The basic idea of FLCS is a weighting of the detected photons based on the additional picosecond timing information (TCSPC start-stop time) when using pulsed laser excitation. 2fFCS goes even further, combining Pulsed Interleaved Excitation (PIE) with a time-gated FCS analysis. The basic implementation of 2fFCS uses two synchronized but interleaved pulsed lasers of the same wavelength but of different polarisation to generate two close by excitation foci in a pre-determined distance acting as a submicron ruler. In this case it it no longer necessary to have prior knowledge about the size and shape of the confocal volume. Maintaining the information about the photon´s origin, the dual focus data allows a precise calculation of absolute diffusion coefficients.

Terahertz spectral computed tomography

The novel technique of spectral THz computed tomography is demonstrated at an example of a small plastic figure. The data are reconstructed by conventional filtered back-projection. The reconstruction are discussed.

Controlling the polarization dynamics by strong THz fields in photoexcited germanium quantum wells

The interaction of strong single-cycle THz pulses with the optically induced polarization in germanium quantum wells is studied. With increasing THz field strength, it is observed that the excitonic resonances shift toward higher energy and broaden before weak signatures of a splitting of the exciton line occur. In comparison with high-quality GaAs-based quantum wells, where a much clearer Autler–Townes splitting is observed, the germanium system response is significantly more broadened and shows signatures of a quasi-steady-state behavior due to the intrinsic fast dephasing times dominated by intervalley scattering.

Spectral terahertz topography

THz-time domain spectroscopy (TDS) is utilised for spectrally resolved topography. Beyond pure imaging we employ a reflection set-up in order to demonstrate capabilities of computing 3D volume data for components of everyday communication electronics. The techniques potentials in Non-Destructive Testing (NDT) are explored with respect to established methods.