Shutterless ion mobility spectrometer with fast pulsed electron source.

Ion mobility spectrometers (IMS) are devices for fast and very sensitive trace gas analysis. The measuring principle is based on an initial ionization process of the target analyte. Most IMS employ radioactive electron sources, such as 63Ni or 3H. These radioactive materials have the disadvantage of legal restrictions and the electron emission has a predetermined intensity and cannot be controlled or disabled. In this work, we replaced the 3H source of our IMS with 100 mm drift tube length with our nonradioactive electron source, which generates comparable spectra to the 3H source. An advantage of our emission current controlled nonradioactive electron source is that it can operate in a fast pulsed mode with high electron intensities. By optimizing the geometric parameters and developing fast control electronics, we can achieve very short electron emission pulses for ionization with high intensities and an adjustable pulse width of down to a few nanoseconds. This results in small ion packets at simultaneously high ion densities, which are subsequently separated in the drift tube. Normally, the required small ion packet is generated by a complex ion shutter mechanism. By omitting the additional reaction chamber, the ion packet can be generated directly at the beginning of the drift tube by our pulsed nonradioactive electron source with only slight reduction in resolving power. Thus, the complex and costly shutter mechanism and its electronics can also be omitted, which leads to a simple low-cost IMS-system with a pulsed nonradioactive electron source and a resolving power of 90.

A compact high-resolution X-ray ion mobility spectrometer.

For the ionization of gaseous samples, most ion mobility spectrometers employ radioactive ionization sources, e.g., containing (63)Ni or (3)H. Besides legal restrictions, radioactive materials have the disadvantage of a constant radiation with predetermined intensity. In this work, we replaced the (3)H source of our previously described high-resolution ion mobility spectrometer with 75 mm drift tube length with a commercially available X-ray source. It is shown that the current configuration maintains the resolving power of R = 100 which was reported for the original setup containing a (3)H source. The main advantage of an X-ray source is that the intensity of the radiation can be adjusted by varying its operating parameters, i.e., filament current and acceleration voltage. At the expense of reduced resolving power, the sensitivity of the setup can be increased by increasing the activity of the source. Therefore, the performance of the setup can be adjusted to the specific requirements of any application. To investigate the relation between operating parameters of the X-Ray source and the performance of the ion mobility spectrometer, parametric studies of filament current and acceleration voltage are performed and the influence on resolving power, peak height, and noise is analyzed.

A compact high resolution electrospray ionization ion mobility spectrometer.

Electrospray is a commonly used ionization method for the analysis of liquids. An electrospray is a dispersed nebular of charged droplets produced under the influence of a strong electrical field. Subsequently, ions are produced in a complex process initiated by evaporation of neutral solvent molecules from these droplets. We coupled an electrospray ionization source to our previously described high resolution ion mobility spectrometer with 75 mm drift tube length and a drift voltage of 5 kV. When using a tritium source for chemical gas phase ionization, a resolving power of R=100 was reported for this setup. We replaced the tritium source and the field switching shutter by an electrospray needle, a desolvation region with variable length and a three-grid shutter for injecting ions into the drift region. Preliminary measurements with tetraalkylammonium halides show that the current configuration with the electrospray ionization source maintains the resolving power of R=100. In this work, we present the characterization of our setup. One major advantage of our setup is that the desolvation region can be heated separately from the drift region so that the temperature in the drift region stays at room temperature even up to desolvation region temperatures of 100 °C. We perform parametric studies for the investigation of the influence of temperature on solvent evaporation with different ratios of water and methanol in the solvent for different analyte substances. Furthermore, the setup is operated in negative mode and spectra of bentazon with different solvents are presented.

First-principles determination of ultrahigh thermal conductivity of boron arsenide: a competitor for diamond?

We have calculated the thermal conductivities (κ) of cubic III-V boron compounds using a predictive first principles approach. Boron arsenide is found to have a remarkable room temperature κ over 2000 W m(-1) K(-1); this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in boron arsenide to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high κ materials, and to relatively weak phonon-isotope scattering. We also find that cubic boron nitride and boron antimonide will have high κ with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultrahigh κ material of potential interest for passive cooling applications.

Thermal conductivity and large isotope effect in GaN from first principles.

We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb. In GaN we find a large increase to the thermal conductivity with isotopic enrichment, ~65% at room temperature. We show that both the high thermal conductivity and its enhancement with isotopic enrichment in GaN arise from the weak coupling of heat-carrying acoustic phonons with optic phonons. This weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap between acoustic and optic phonons compared to other materials. Rigorous understanding of these features in GaN gives important insights into the interplay between intrinsic phonon-phonon scattering and isotopic scattering in a range of materials.

Optical control of coherent interactions between electron spins in InGaAs quantum dots.

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with µeV strength.

Directing nuclear spin flips in InAs quantum dots using detuned optical pulse trains.

We find that detuning an optical pulse train from electronic transitions in quantum dots controls the direction of nuclear spin flips. The optical pulse train generates electron spins that precess about an applied magnetic field, with a spin component parallel to the field only for detuned pulses. This component leads to asymmetry in the nuclear spin flips, providing a way to stabilize and control the nuclear spin polarization. This effect is observed using two-color, time-resolved Faraday rotation and ellipticity.

Photoluminescence spectroscopy of the molecular biexciton in vertically stacked InAs-GaAs quantum dot pairs.

We present photoluminescence studies of the molecular neutral biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum dot pairs. We tune either the hole or the electron levels of the two dots into tunneling resonances. The spectra are described well within a few-level, few-particle molecular model. Their properties can be modified broadly by an electric field and by structural design, which makes them highly attractive for controlling nonlinear optical properties.

Theory of fast optical spin rotation in a quantum dot based on geometric phases and trapped states.

A method is proposed for the optical rotation of the spin of an electron in a quantum dot using excited trion states to implement operations significantly faster than those of most existing proposals. Key ingredients are the geometric phase induced by 2pi hyperbolic secant pulses, use of coherently trapped states and use of naturally dark states. Our proposal covers a variety of quantum dots by addressing different parameter regimes. Numerical simulations with typical parameters for InAs self-assembled quantum dots, including their dissipative dynamics, give fidelities of the operations in excess of 99%.

Electrically tunable g factors in quantum dot molecular spin states.

We present a magnetophotoluminescence study of individual vertically stacked InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied electric field tunes the relative energies of the two dots, we observe a strong resonant increase or decrease in the g factors of different spin states that have molecular wave functions distributed over both quantum dots. We propose a phenomenological model for the change in g factor based on resonant changes in the amplitude of the wave function in the barrier due to the formation of bonding and antibonding orbitals.

Coherent photonic coupling of semiconductor quantum dots.

We report a new type of coupling between quantum dot excitons mediated by the strong single-photon field in a high-finesse micropillar cavity. Coherent exciton coupling is observed for two dots with energy differences of the order of the exciton-photon coupling. The coherent coupling mode is characterized by an anticrossing with a particularly large line splitting of 250 microeV. Because of the different dispersion relations with temperature, the simultaneous photonic coupling of quantum dot excitons can be easily distinguished from cases of sequential strong coupling of two quantum dots.

Optical signatures of coupled quantum dots.

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots. This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing.

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

Control of vertically coupled InGaAs/GaAs quantum dots with electric fields.

Controllable interactions that couple quantum dots are a key requirement in the search for scalable solid state implementations for quantum information technology. From optical studies of excitons and corresponding calculations, we demonstrate that an electric field on vertically coupled pairs of In(0.6)Ga(0.4)As/GaAs quantum dots controls the mixing of the exciton states on the two dots and also provides controllable coupling between carriers in the dots.

Chemical detection with a single-walled carbon nanotube capacitor.

We show that the capacitance of single-walled carbon nanotubes (SWNTs) is highly sensitive to a broad class of chemical vapors and that this transduction mechanism can form the basis for a fast, low-power sorption-based chemical sensor. In the presence of a dilute chemical vapor, molecular adsorbates are polarized by the fringing electric fields radiating from the surface of a SWNT electrode, which causes an increase in its capacitance. We use this effect to construct a high-performance chemical sensor by thinly coating the SWNTs with chemoselective materials that provide a large, class-specific gain to the capacitance response. Such SWNT chemicapacitors are fast, highly sensitive, and completely reversible.

Strong coupling in a single quantum dot-semiconductor microcavity system.

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

Fine structure of excitons in InAs/GaAs coupled auantum dots: a sensitive test of electronic coupling.

Exciton fine structure in InAs/GaAs coupled quantum dots has been studied by photoluminescence spectroscopy in magnetic fields up to 8 T. Pronounced anticrossings and mixings of optically bright and dark states as functions of magnetic field are seen. A theoretical treatment of the mixing of the excitonic states has been developed, and it traces observed features to structural asymmetries. These results provide direct evidence for coherent coupling of excitons in quantum dot molecules.

[Chronic myoepithelial sialadenitis - symptomatology, clinical signs, differential diagnostics]. / Die chronische myoepitheliale Sialadenitis - Symptomatologie, Klinik und Differenzialdiagnostik -

UNLABELLED: In the differential diagnosis of mass lesions of the salivary glands, myoepithelial sialadenitis (MESA), i. e. benign lymphoepithelial lesion, carries particular importance because of its association with Sjoegren's syndrome and development of malignant lymphoma. In the present study, epidemiology and clinical findings were analysed in relation to presence of MESA, Sjoegren's syndrome and lymphoma development. MATERIAL AND METHOD: 67 patients, histopathologically classified by the salivary gland registry, were analysed retrospectively in regard to their clinical presentation, especially in regard to the chronical process of inflammation as present in MESA. RESULTS: MESA primarily affects women in the 5th and 6th decade and regularly the parotid gland; in 44.8 % of the cases, there is multiple organ presentation. Xerostomy (38.5 %) is usually present (in 88,9 % of all cases) before or at clinical onset of gland inflammation, whereas xerophthalmy (28.4 %) did not show such a correlation. In general, rheumatic diseases (23.9 %) precede the gland-symptoms in 77.8 % of the patients. In 31.3 % of the cases a Sjoegren's syndrome was present. 26.9 % of the patients developed a malignant Non Hodgkin's Lymphoma (88.9 % of the MALT-type). CONCLUSION: The most important clinical relevance of MESA lies in the higher probability to develop malignant lymphoma; this requires adequate staging-procedures and proper histopathological examination of sialogenic and nodal masses, especially over the course of the disease.

Isomeric photonic molecules formed from coupled microresonators.

Isomeric photonic molecules were formed by connecting four identical cavities in different geometries: a chain, a square, and a T shape. The optical mode spectrum in these structures exhibits three-dimensionally confined photonic states, which have been studied by photoluminescence spectroscopy. The energies of the optical modes depend strongly on the molecule geometry. The experimental data are in good agreement with detailed calculations of the fields in the cavities.

Inhibition and enhancement of the spontaneous emission of quantum dots in structured microresonators.

Spontaneous emission of quantum dot systems in laterally structured microcavities that exhibit photon confinement in all three directions has been studied by time-resolved photoluminescence spectroscopy. For on-resonance conditions, we find that the dot emission rate is increased substantially over that of the unstructured planar cavity. For off-resonance conditions, we are able to suppress the emission rate by an order of magnitude by using cavities with metal coatings, which we attribute to the suppression of leaky optical modes in these structures.