62,870 research outputs found
Understanding the magnetic resonance spectrum of nitrogen vacancy centers in an ensemble of randomly-oriented nanodiamonds
Nanodiamonds containing nitrogen vacancy (NV-) centers show promise for a
number of emerging applications including targeted in vivo imaging and
generating nuclear spin hyperpolarization for enhanced NMR spectroscopy and
imaging. Here, we develop a detailed understanding of the magnetic resonance
behavior of NV- centers in an ensemble of nanodiamonds with random crystal
orientations. Two-dimensional optically detected magnetic resonance
spectroscopy reveals the distribution of energy levels, spin populations, and
transition probabilities that give rise to a complex spectrum. We identify
overtone transitions that are inherently insensitive to crystal orientation and
give well-defined transition frequencies that access the entire nanodiamond
ensemble. These transitions may be harnessed for high-resolution imaging and
generation of nuclear spin hyperpolarization. The data are well described by
numerical simulations from the zero- to high-field regimes, including the
intermediate regime of maximum complexity. We evaluate the prospects of
nanodiamond ensembles specifically for nuclear hyperpolarization and show that
frequency-swept dynamic nuclear polarization may transfer a large amount of the
NV- center's hyperpolarization to nuclear spins by sweeping over a small region
of its spectrum.Comment: 6 pages, 5 figure
A subradiant optical mirror formed by a single structured atomic layer
Efficient and versatile interfaces for the interaction of light with matter
are an essential cornerstone for quantum science. A fundamentally new avenue of
controlling light-matter interactions has been recently proposed based on the
rich interplay of photon-mediated dipole-dipole interactions in structured
subwavelength arrays of quantum emitters. Here we report on the direct
observation of the cooperative subradiant response of a two-dimensional (2d)
square array of atoms in an optical lattice. We observe a spectral narrowing of
the collective atomic response well below the quantum-limited decay of
individual atoms into free space. Through spatially resolved spectroscopic
measurements, we show that the array acts as an efficient mirror formed by only
a single monolayer of a few hundred atoms. By tuning the atom density in the
array and by changing the ordering of the particles, we are able to control the
cooperative response of the array and elucidate the interplay of spatial order
and dipolar interactions for the collective properties of the ensemble. Bloch
oscillations of the atoms out of the array enable us to dynamically control the
reflectivity of the atomic mirror. Our work demonstrates efficient optical
metamaterial engineering based on structured ensembles of atoms and paves the
way towards the controlled many-body physics with light and novel light-matter
interfaces at the single quantum level.Comment: 8 pages, 5 figures + 12 pages Supplementary Infomatio
meV resolution in laser-assisted energy-filtered transmission electron microscopy
The electronic, optical, and magnetic properties of quantum solids are
determined by their low-energy (< 100 meV) many-body excitations. Dynamical
characterization and manipulation of such excitations relies on tools that
combine nm-spatial, fs-temporal, and meV-spectral resolution. Currently,
phonons and collective plasmon resonances can be imaged in nanostructures with
sub-nm and 10s meV space/energy resolution using state-of-the-art
energy-filtered transmission electron microscopy (TEM), but only under static
conditions, while fs-resolved measurements are common but lack spatial or
energy resolution. Here, we demonstrate a new method of spectrally resolved
photon-induced near-field electron microscopy (SRPINEM) that allows us to
obtain nm-fs-resolved maps of nanoparticle plasmons with an energy resolution
determined by the laser linewidth (20 meV in this work), and not limited by
electron beam and spectrometer energy spreading. This technique can be extended
to any optically-accessible low-energy mode, thus pushing TEM to a previously
inaccessible spectral domain with an unprecedented combination of space, energy
and temporal resolution.Comment: 19 pages, 7 figure
Study of the properties of hyperpolarized xenon-129 for magnetic resonance imaging
Produkce hyperpolarizovaných plynů, především helia (3He) nebo xenonu (129Xe), nachází stále rostoucí rozsah aplikací v zobrazování magnetickou rezonancí (MRI). Helium ani xenon nejsou obyčejně obsaženy v těle a experimenty tedy nejsou ovlivněny nechtěným signálem z okolních tkání. Ukázalo se, že několika hyperpolarizačními technikami může být magnetická polarizace (magnetizace) jader vzácných plynů zvýšena na hladinu, se kterou jsou praktické aplikace proveditelné. Hyperpolarizované plyny mohou tedy být užitečným nástrojem pro neinvazivní zkoumání lidského dýchání, dovolující statické zobrazování během zadržení dechu nebo zkoumání dynamiky výdechu nebo nádechu, nebo funkčního zobrazování. V neživé přírodě, mohou být hyperpolarizovaný plyny využity jako kontrastní látka při studiu mikroporézních materiálů, jako jsou zeolity, stavební látky a hmoty, atd. V této doktorské práci je popsán vývoj a konstrukce aparatury pro hyperpolarizaci xenonu (izotopu 129Xe). Nákup hyperpolarizovaného xenonu od jiných výzkumných center v zahraničí a jeho dovážení by ovšem nebylo efektivní a to zejména z důvodu náročnosti zajištění potřebných fyzikálních podmínek pro přepravu hyperpolarizovaného plynu. Toto bylo hlavní motivací k vývoji vlastní technologie pro přípravu hyperpolarizovaného xenonu. Se zvládnutou technologií by bylo možné navázat spolupráci s medicínskými zařízeními, nebo s týmy zabývající se živou nebo neživou přírodou (např. při studiu mikroporézních materiálů, gelů, v zemědělských aplikacích nebo při výzkumu využívajících zvířat, atd.). Cílem této práce je studium teorie hyperpolarizovaných vzácných plynů se zaměřením na 129Xe a experimentální ověření a změření relaxačních časů pomocí jaderné magnetické rezonance. Vzhledem k tomu, že je možné hyperpolarizované vzácné plyny skladovat pro pozdější využití, se tato práce také zabývá možnostmi zásobníku hyperpolarizovaného vzácného plynu a jeho teoretickým a experimentálním řešením. V této práci jsou popsány především dva základní typy experimentů přípravy hyperpolarizovaného xenonu. V obou jsou využity zatavené válcové skleněné vzorky naplněné xenonem a doplňujícím plynem – dusíkem, heliem. První z experimentů se zabývá měřením vlastností termálně polarizovaného xenonu a druhý měřením vlastností hyperpolarizovaného xenonu. Pro hyperpolarizaci 129Xe bylo použito výkonového laseru a experimentálně byla zkoumána jednak míra polarizace na základě změny spektrální hustoty čerpacího laserového svazku a dále pak optimální doba optického čerpání 129Xe a relaxační časy xenonu.The production of hyperpolarized gases (HpG), predominantly helium (3He) or xenon (129Xe), have found a steadily increasing range of applications in magnetic resonance imaging (MRI). Neither helium nor xenon are normally present in the body, thus the magnetic resonance experiments do not suffer from unwanted background signals. It has been demonstrated by several techniques of hyperpolarization that the magnetic polarization (magnetization) of the noble gas nuclei can be increased to levels that make practical application feasible. Hence, hyperpolarized gases may become a useful tool for non-invasive investigation of human lung ventilation, permitting static imaging during breathhold or probing the dynamics of inhalation/exhalation, or functional imaging. In inanimate nature, hyperpolarized gas can be used as a contrast medium for microporous materials, such zeolites, constructive materials in civil engineering, etc. This thesis describes the development and construction of a xenon (129Xe) hyperpolarization (Hp) device. Buying hyperpolarized xenon from other research centres abroad is inefficient mainly because of a need of a fast transport of HpXe under specific conditions. That was the main motivation for developing of our own technology for production of HpXe. Well-handled technology could allow a medical cooperation or cooperation with teams dealing with in/animate nature (microporous material, gels, agriculture, animals, etc.). The aim of this work is to study the hyperpolarized noble gases theory with concern to 129Xe and to experimentally prove and measure xenon relaxation times by the NMR. Since it is possible to store hyperpolarized noble gases for later use, this doctoral thesis also explores the potentials of hyperpolarized noble gas storage system and its theoretical and experimental solution. Mainly two types of experiments are described in the thesis. In both experiments, sealed cylindrical Simax sample filled with xenon and supplement gas – nitrogen, helium were used. The first type of experiment is based on thermally polarized xenon and the second on hyperpolarized xenon. For hyperpolarization of 129Xe a high-power laser was used. In this experiment, the relation between power spectral density of optical pumping beam and efficiency of HpXe production process was investigated. The optimal duration of optical pumping and relaxation times of HpXe were investigated too.
Fast Fiber Orientation Estimation in Diffusion MRI from kq-Space Sampling and Anatomical Priors
High spatio-angular resolution diffusion MRI (dMRI) has been shown to provide
accurate identification of complex fiber configurations, albeit at the cost of
long acquisition times. We propose a method to recover intra-voxel fiber
configurations at high spatio-angular resolution relying on a kq-space
under-sampling scheme to enable accelerated acquisitions. The inverse problem
for reconstruction of the fiber orientation distribution (FOD) is regularized
by a structured sparsity prior promoting simultaneously voxelwise sparsity and
spatial smoothness of fiber orientation. Prior knowledge of the spatial
distribution of white matter, gray matter and cerebrospinal fluid is also
assumed. A minimization problem is formulated and solved via a forward-backward
convex optimization algorithmic structure. Simulations and real data analysis
suggest that accurate FOD mapping can be achieved from severe kq-space
under-sampling regimes, potentially enabling high spatio-angular dMRI in the
clinical setting.Comment: 10 pages, 5 figures, Supplementary Material
Comparison of imaging with sub-wavelength resolution in the canalization and resonant tunnelling regimes
We compare the properties of subwavelength imaging in the visible wavelength
range for metal-dielectric multilayers operating in the canalization and the
resonant tunnelling regimes. The analysis is based on the transfer matrix
method and time domain simulations. We show that Point Spread Functions for the
first two resonances in the canalization regime are approximately Gaussian in
shape. Material losses suppress transmission for higher resonances, regularise
the PSF but do not compromise the resolution. In the resonant tunnelling
regime, the MTF may dramatically vary in their phase dependence. Resulting PSF
may have a sub-wavelength thickness as well as may be broad with multiple
maxima and a rapid phase modulation. We show that the width of PSF may be
reduced by further propagation in free space, and we provide arguments to
explain this surprising observation.Comment: 17 pages,12 figure
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