Alois Renn

A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency

Single-photon sources have been discussed as the building blocks of quantum cryptography, optical quantum computation, spectroscopy, and metrology. However, when using sources based on single emitters, the success of these proposals depends on the ability to achieve near-unity collection efficiency into well-defined modes. Some of the current state-of-the-art efforts aimed at achieving these criteria have been demonstrated, but despite an impressive progress the results still fall short. In particular, a collection efficiency of 38% were reported using microresonators [1], while a nanowire device reached an efficiency of 72% at cryogenic temperatures [2]. Here we report on a broad-band room-temperature scheme, which uses a layered dielectric antenna for realizing ultra-bright single photon sources with near-unity collection efficiency.

High-speed nanoscopic tracking of the position and orientation of a single virus.

Optical studies have revealed that, after binding, virions move laterally on the plasma membrane, but the complexity of the cellular environment and the drawbacks of fluorescence microscopy have prevented access to the molecular dynamics of early virus-host couplings, which are important for cell infection. Here we present a colocalization methodology that combines scattering interferometry and single-molecule fluorescence microscopy to visualize both position and orientation of single quantum dot–labeled Simian virus 40 (SV40) particles. By achieving nanometer spatial and 8 ms temporal resolution, we observed sliding and tumbling motions during rapid lateral diffusion on supported lipid bilayers, and repeated back and forth rocking between nanoscopic regions separated by 9 nm. Our findings suggest recurrent swap of receptors and viral pentamers as well as receptor aggregation in nanodomains. We discuss the prospects of our technique for studying virus-membrane interactions and for resolving nanoscopic dynamics of individual biological nano-objects.

A single-molecule optical transistor

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities or waveguides. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second ‘gating’ beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Synthesis of Free‐Standing, Monolayered Organometallic Sheets at the Air/Water Interface

The recent discovery of how to obtain and utilize individual layers of graphite, that is, graphene, and other inorganic monolayered sheets has turned the spotlight brighter than ever on a basically ignored field of chemistry, the rational synthesis of two-dimensional polymers. While there have been numerous reports on the synthesis of monolayered polymer films with irregularly networked internal structures since the pioneering work by Gee in 1935, little is known to date about the synthesis of a free-standing, 2D network with an ordered internal structure. This paucity is contrasted by the richness of fragments of such networks that were obtained by approaches such as iterative organic synthesis, selfassembly, or on-surface polymerization. As of now, the lateral dimensions of these fragments are too small to expect sheet-like properties; furthermore, they cannot yet be isolated and manipulated. Considering the huge application potential for structurally well-defined, free-standing 2D networks, which ranges from ultrasensitive membranes, molecular sieves, and devices based on high charge carrier mobility to materials with outstanding mechanical strength and the like, we felt the need to establish a synthesis program aiming at filling the gap between the one-dimensional (linear synthetic and biological polymers, carbon nanotubes, etc.) and the three-dimensional structures (hyperbranched and cross-linked bulk polymers, laminar crystals such as graphite, diamond, etc.) by providing access to structurally defined 2D polymers. Herein we report the synthesis of a free-standing monolayer sheet consisting of the hexafunctional terpyridine (tpy)-based D6h-symmetric monomer 1 (Figure 1) which was designed for the present purpose 12] and is to be held together by metal ion complexes between ideally all six tpy units of one monomer with one of the tpy units of each of the six neighboring monomers. This mode of polymerization is in principle reversible and could allow for dynamic bond formation. It has often been used for construction of complex but well-defined compounds as well as supramolecular assemblies. 15, 16] Figure 1 shows a targeted network. To avoid any three-dimensional growth of the coordination network during polymerization, monomer 1 was confined to two dimensions prior to polymerization by spreading it at the air/water interface on a Langmuir–Blodgett (LB) trough. The main advantages of using the air/water interface for the present synthesis instead of solid substrates include 1) the flat and uniform surface on a large length scale, 2) the availability of the water subphase as a pool of reagents and catalysts, 3) the straightforward preparation and facile isolation of single sheets by transfer onto solid substrates and supports of all sorts, 20] 4) the possibility to preset the lateral surface pressure and lateral concentration of monomers prior to the polymerization, and 5) the ease in performing polymerization under ambient conditions. A sub-monolayer of monomer 1 was spread at the air/ water interface from chloroform solution and compressed to a pressure of 30 mNm . The compression process was monitored by Brewster angle microscopy up to 10 mNm 1 and found to be fully reversible (Figure S1 in the Supporting Information) and to provide thin layers that are homogenous at the resolution of micrometers (Figure S2 in the Supporting Information). The point of inflection of the corresponding surface pressure–area isotherm (Figure 2a) was observed at a pressure of approximately 10 mNm , from which a mean molecular area of approximately 520 2 is estimated. This preliminary value is in good agreement with the formation of a dense monolayer in which the monomers lie flat on the interface. This arrangement was supported by AFM contactmode scratching and tapping-mode imaging experiments after vertical transfer of the compressed monolayer (at 10 mNm ) onto a mica substrate. Figure 2 b shows the scratched area and the corresponding height profile, providing the apparent height happ 0.8 nm. While happ values obtained by ambient-condition AFM are known to not accurately reflect real heights, the value of approximately 0.8 nm nevertheless suggests a monolayer. Not only is it in a reasonable range for a conjugated structure, parts of which may significantly deviate from coplanarity, but also a [*] T. Bauer, Dr. Z. Zheng, Dr. J. Sakamoto, Prof. A. D. Schl ter Department of Materials, Institute of Polymers Swiss Federal Institute of Technology, ETH Z rich HCI J 541, 8093 Z rich (Switzerland) E-mail: schlueter@mat.ethz.ch sakamoto@mat.ethz.ch

Single molecule spectroscopy: Stark effect of pentacene in p-terphenyl

Abstract Stark shifts of the zero phonon line of the vibrationless S 1 ←S 0 transition of single pentacene molecules embedded in p -terphenyl crystals have been investigated at cryogenic temperatures. The energy shifts are mainly determined by the quadratic Stark effect. The contribution of the linear Stark effect is rather small and varies strongly from molecule to molecule. The polarizability differences for the ground and first excited state have been calculated from the experimental data obtained from a detailed study of eight single molecules.

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Fluorescence microscopy of single molecules

Abstract Using fluorescence excitation spectroscopy single pentacence molecules were studied under a microscope at a temperature of 1.8 K. The microscope has a resolution of 3 μm and allows the location of a single emitting molecule with an accuracy of 0.5 μm, in a total viewing field of 235 μm×175 μm. 133 images have been recorded by increasing the excitation frequency of the light at 592.344 nm in steps of 2 MHz. Each single molecule has a Lorentzian absorption profile with a width of about 14 MHz and can be observed in 10 consecutive images. The “position” of the single molecule is characterized by the x - and y -coordinates in the spatial domain and by its absorption frequency in the spectral domain.

Coupled rotation within single F0F1 enzyme complexes during ATP synthesis or hydrolysis

F0F1 ATP synthases are the smallest rotary motors in nature and work as ATP factories in bacteria, plants and animals. Here we report on the first observation of intersubunit rotation in fully coupled single F0F1 molecules during ATP synthesis or hydrolysis. We investigate the Na+‐translocating ATP synthase of Propionigenium modestum specifically labeled by a single fluorophore at one c subunit using polarization‐resolved confocal microscopy. Rotation during ATP synthesis was observed with the immobilized enzyme reconstituted into proteoliposomes after applying a diffusion potential, but not with a Na+ concentration gradient alone. During ATP hydrolysis, stepwise rotation of the labeled c subunit was found in the presence of 2 mM NaCl, but not without the addition of Na+ ions. Moreover, upon the incubation with the F0‐specific inhibitor dicyclohexylcarbodiimide the rotation was severely inhibited.

Quantum interference of tunably indistinguishable photons from remote organic molecules

We demonstrate two-photon interference using two remote organic molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we explore the effect of photon distinguishability.

Direct observation of the triplet lifetime quenching of single dye molecules by molecular oxygen

The influence of oxygen on the photophysical properties of individual DiI18 molecules has been investigated by means of both wide field and confocal scanning optical fluorescence microscopy. Excited close to saturation intensity, single-molecule fluorescence detected in wide field by a charge-coupled device camera showed a dramatic increase at exposure to air compared to the fluorescence when the sample was protected from oxygen by nitrogen flush. The change of the triplet lifetime of individual dye molecules due to oxygen quenching was measured in real time by means of time resolved single photon counting. In the presence of oxygen, the triplet state lifetime decreases from several tens of milliseconds down to fractions of a millisecond, whereas no changes of the intersystem crossing quantum yield and the fluorescence lifetime are observed. The triplet lifetimes in the presence and absence of oxygen, respectively, are anti-correlated indicative of heterogeneity of the polymer surrounding the dye molecules.