A poloidal high-k scattering system for NSTX-U

A previous 5-channel tangential high-k scattering system is being replaced by an 8-channel, poloidal high-k scattering system on the National Spherical Torus eXperiment Upgrade (NSTX-U) device located in Princeton, NJ, USA. The 693 GHz poloidal scattering system replaces a 280 GHz tangential scattering system to study high-k electron density fluctuations on NSTX-U, thereby considerably enhancing planned turbulence physics studies by providing a measurement of the k θ -spectrum of both electron temperature gradient (ETG) and ion temperature gradient (ITG) modes. Two approaches to generating the 693 GHz probe beam are under development: an optically-pumped far-infrared (FIR) laser that generates ∼50 mW, and a compact gyrotron that can potentially generate in excess of 5 W. Large aperture optics collect radiation scattered from density fluctuations in the plasma core at 8 simultaneous scattering angles ranging from 2 to 15° corresponding to poloidal wavenumbers that extend to >40 cm−1. Steerable launch optics coupled with receiver optics mounted on a 5-axis receiver carriage allow the scattering volume to be placed radially from r/a = 0.3 out to the pedestal region (r/a ∼ 0.99) and translated horizontally as needed to satisfy wavenumber matching.

Quantification of the Enhancement Factor in Surface-Enhanced Raman Scattering

<p>This thesis presents a rigorous stepwise methodology towards the accurate measurement and quantification of the SERS enhancement factor (EF), the key parameter in describing the SERS effect. The work represents, we believe, a successful attempt to resolve some of the inconsistencies in the literature and to refocus the field by emphasizing the importance of consistent definitions and rigorous quantification to elucidate matters of fundamental importance in SERS. The success in our approach is that it combines careful experimental measurements upon a sound theoretical framework, and utilizes a 'toolbox' of techniques developed in recent years, such as bi-analyte SERS (BiASERS) techniques for single-molecule (SM) detection, and isotopic editing. In experimental work, we measure the bare Raman cross-sections of five common probes used in SERS as a first step in measuring the analytical enhancement factor (AEF) and single-molecule enhancement factor (SMEF). The methodology in measuring these EFs involved the use of a reference standard of known cross-section along with a careful characterization of the scattering volume through beam profiling experiments. As a guide to validating the reference cross-section we make extensive use of density functional theory (DFT) calculations to obtain estimates for the intrinsic Raman cross-sections of small, non-resonant probes. The results of this work showed that previous upper limits for the EF reported in the literature of 1014 were based on a faulty normalization of the EF. In fact, EFs of 108 were sufficient to see single molecules, which is much lower than previously expected; under optimum conditions, even lower EFs, possibly down to 105 could be sufficient for the SM detection of resonant probes. As a valuable extension of BiASERS, we elaborate on the synthesis of isotopic analogues of a rhodamine dye as ideal partners for SM experiments. The synthesis and definitive characterization of these probes enable their use in an experiment to determine the SM regime in a liquid colloidal sample. Isotopically edited dyes such as these, in combination with the methodologies of EF quantification outlined herein, set the standard for those interested in accurate quantification of the SERS effect. This approach is useful in terms of both basic theoretical questions and applications such as the effective comparison of SERS substrates. Finally, we extend the techniques developed over the thesis to a long-standing and largely unresolved question in SERS: What is the minimum intrinsic Raman cross-section that can be measured as a single molecule in standard SERS conditions. In this work, we explore the SM detection non-resonant probes, which are the molecules of interest for many practical applications such as forensics and biological assays. Specifically, we demonstrate the successful SM detection of isotopically edited adenine probes.</p>

Quantification of the Enhancement Factor in Surface-Enhanced Raman Scattering

<p>This thesis presents a rigorous stepwise methodology towards the accurate measurement and quantification of the SERS enhancement factor (EF), the key parameter in describing the SERS effect. The work represents, we believe, a successful attempt to resolve some of the inconsistencies in the literature and to refocus the field by emphasizing the importance of consistent definitions and rigorous quantification to elucidate matters of fundamental importance in SERS. The success in our approach is that it combines careful experimental measurements upon a sound theoretical framework, and utilizes a 'toolbox' of techniques developed in recent years, such as bi-analyte SERS (BiASERS) techniques for single-molecule (SM) detection, and isotopic editing. In experimental work, we measure the bare Raman cross-sections of five common probes used in SERS as a first step in measuring the analytical enhancement factor (AEF) and single-molecule enhancement factor (SMEF). The methodology in measuring these EFs involved the use of a reference standard of known cross-section along with a careful characterization of the scattering volume through beam profiling experiments. As a guide to validating the reference cross-section we make extensive use of density functional theory (DFT) calculations to obtain estimates for the intrinsic Raman cross-sections of small, non-resonant probes. The results of this work showed that previous upper limits for the EF reported in the literature of 1014 were based on a faulty normalization of the EF. In fact, EFs of 108 were sufficient to see single molecules, which is much lower than previously expected; under optimum conditions, even lower EFs, possibly down to 105 could be sufficient for the SM detection of resonant probes. As a valuable extension of BiASERS, we elaborate on the synthesis of isotopic analogues of a rhodamine dye as ideal partners for SM experiments. The synthesis and definitive characterization of these probes enable their use in an experiment to determine the SM regime in a liquid colloidal sample. Isotopically edited dyes such as these, in combination with the methodologies of EF quantification outlined herein, set the standard for those interested in accurate quantification of the SERS effect. This approach is useful in terms of both basic theoretical questions and applications such as the effective comparison of SERS substrates. Finally, we extend the techniques developed over the thesis to a long-standing and largely unresolved question in SERS: What is the minimum intrinsic Raman cross-section that can be measured as a single molecule in standard SERS conditions. In this work, we explore the SM detection non-resonant probes, which are the molecules of interest for many practical applications such as forensics and biological assays. Specifically, we demonstrate the successful SM detection of isotopically edited adenine probes.</p>

On incoherent diffractive imaging

Incoherent diffractive imaging (IDI) promises structural analysis with atomic resolution based on intensity interferometry of pulsed X-ray fluorescence emission. However, its experimental realization is still pending and a comprehensive theory of contrast formation has not been established to date. Explicit expressions are derived for the equal-pulse two-point intensity correlations, as the principal measured quantity of IDI, with full control of the prefactors, based on a simple model of stochastic fluorescence emission. The model considers the photon detection statistics, the finite temporal coherence of the individual emissions, as well as the geometry of the scattering volume. The implications are interpreted in view of the most relevant quantities, including the fluorescence lifetime, the excitation pulse, as well as the extent of the scattering volume and pixel size. Importantly, the spatiotemporal overlap between any two emissions in the sample can be identified as a crucial factor limiting the contrast and its dependency on the sample size can be derived. The paper gives rigorous estimates for the optimum sample size, the maximum photon yield and the expected signal-to-noise ratio under optimal conditions. Based on these estimates, the feasibility of IDI experiments for plausible experimental parameters is discussed. It is shown in particular that the mean number of photons per detector pixel which can be achieved with X-ray fluorescence is severely limited and as a consequence imposes restrictive constraints on possible applications.

Microsecond hydrodynamic interactions in dense colloidal dispersions probed at the European XFEL

Many soft-matter systems are composed of macromolecules or nanoparticles suspended in water. The characteristic times at intrinsic length scales of a few nanometres fall therefore in the microsecond and sub-microsecond time regimes. With the development of free-electron lasers (FELs) and fourth-generation synchrotron light-sources, time-resolved experiments in such time and length ranges will become routinely accessible in the near future. In the present work we report our findings on prototypical soft-matter systems, composed of charge-stabilized silica nanoparticles dispersed in water, with radii between 12 and 15 nm and volume fractions between 0.005 and 0.2. The sample dynamics were probed by means of X-ray photon correlation spectroscopy, employing the megahertz pulse repetition rate of the European XFEL and the Adaptive Gain Integrating Pixel Detector. We show that it is possible to correctly identify the dynamical properties that determine the diffusion constant, both for stationary samples and for systems driven by XFEL pulses. Remarkably, despite the high photon density the only observable induced effect is the heating of the scattering volume, meaning that all other X-ray induced effects do not influence the structure and the dynamics on the probed timescales. This work also illustrates the potential to control such induced heating and it can be predicted with thermodynamic models.

Optimisation of a Polarisation Nephelometer

An analysis of the influence caused by polarization nephelometer parameters on the scattering matrix measurement accuracy in a non-isotropic medium is presented. The approximation errors in the actual scattering volume and radiation beam by an elementary scattering volume and an elementary radiation beam are considered. A formula for calculating the nephelometer base is proposed. It is shown that requirements to an irradiation source of a polarizing nephelometer, i.e. mono-chromaticity and high radiation intensity and directivity in a wide spectral range can be satisfied by a set of high brightness LEDs with a radiating (self-luminous) small size body. A 5-wavelength monochromatic irradiation source, with an emission flux of (0.15–0.6) W required for a polarization nephelometer, is described. The design of small-sized polarizing phase control units is shown. An electronic circuit of a radiator control unit based on an AVR-Atmega 8-bit microcontroller with feedback and drive control realized by means of an incremental angular motion sensor and a software PID controller is presented. Precise and smooth motion of the radiator is ensured by standard servo-driven numerical control mathematics and the use of precision gears. The system allows both autonomous adjustment of the radiator’s reference positions and adjustment by means of commands from a personal computer. Both the computer and microcontroller programs were developed with the use of free software, making it possible to transfer the programs to Windows‑7(10), Linux and embedded Linux operating systems. Communication between the radiator’s position control system and the personal computer is realised by means of a standard noise immune USB-RS485 interface.

On Compton scattering as a source of background in coherent diffraction imaging experiments

Compton scattering is generally neglected in diffraction experiments because the incoherent radiation it generates does not give rise to interference effects and therefore is negligible at Bragg peaks. However, as the scattering volume is reduced, the difference between the Rayleigh (coherent) and Compton (incoherent) contributions at Bragg peaks diminishes and the incoherent part may become substantial. The consequences can be significant for coherent diffraction imaging at high scattering angles: the incoherent radiation produces background that smears out the secondary interference fringes, affecting thus the achievable resolution of the technique. Here, a criterion that relates the object shape and the resolution is introduced. The Compton contribution for several object shapes is quantified, and it is shown that the maximum achievable resolution along different directions has a strong dependence on the crystal shape and size.

Optimization of a Polarization Nephelometer

The effect of parameters of a polarization nephelometer on its accuracy characteristic is analyzed. Errors in approximation of the actual scattering volume and actual optical beam by the elementary scattering volume and elementary beam are considered. A five-wave monochromatic source of radiation with the high spectral intensity of 0.15÷0.6 W is described. The design of polarization units is demonstrated.

The Interpretation of a Polarimetric Coherency Matrix with Four Scattering Models Considering Polarimetric Symmetry

&lt;p&gt;For a multi-look polarimetric synthetic aperture radar (POLSAR) image, each pixel corresponds to a polarimetric coherency matrix. Model-based incoherent polarimetric decomposition is a technique which is widely used to analyze multi-look POLSAR data. Traditional model-based incoherent polarimetric decomposition algorithms have some inherent drawbacks such as negative power components, polarimetric information loss, and non-model-based decomposition results. This study tries to completely interpret a polarimetric coherency matrix by the incoherent sum of four scattering mechanisms. Therefore, the proposed algorithm can be regarded as a new type of model-based incoherent polarimetric decomposition. All the four scattering models are firstly derived with polarimetric symmetry. The four scattering models correspond to surface scattering, double-bounce scattering, volume scattering and helix scattering, respectively. Then a new four-component model-based incoherent decomposition algorithm is found. After extracting the helix scattering component and the maximum possible volume scattering component, the remaining coherency matrix is decomposed into two components with an orientation angle difference of 45&amp;#176;. With the new algorithm, most pixels of a real POLSAR image can be completely decomposed into four components which are exactly consistent with helix scattering, volume scattering, surface scattering, and double-bounce scattering, respectively. Moreover, the proposed decomposition algorithm fully utilizes the polarimetric information, and all scattering component powers are nonnegative. Experiments with E-SAR, RADARSAT-2, and GF-3 data are presented to illustrate the effectiveness of analyzing the scattering mechanisms of real terrain targets with the proposed decomposition algorithm. The proposed decomposition algorithm is also compared with classic four-component model-based incoherent polarimetric decomposition algorithms.&lt;/p&gt;