140,586 research outputs found
Approximating Sparsest Cut in Low Rank Graphs via Embeddings from Approximately Low Dimensional Spaces
No full textWe consider the problem of embedding a finite set of points x_1, ... , x_n in R^d that satisfy l_2^2 triangle inequalities into l_1, when the points are approximately low-dimensional. Goemans (unpublished, appears in a work of Magen and Moharammi (2008) ) showed that such points residing in exactly d dimensions can be embedded into l_1 with distortion at most sqrt{d}. We prove the following robust analogue of this statement: if there exists a r-dimensional subspace Pi such that the projections onto this subspace satisfy sum_{i,j in [n]} norm{Pi x_i - Pi x_j}_2^2 >= Omega(1) * sum_{i,j \in [n]} norm{x_i - x_j}_2^2, then there is an embedding of the points into l_1 with O(sqrt{r}) average distortion. A consequence of this result is that the integrality gap of the well-known Goemans-Linial SDP relaxation for the Uniform Sparsest Cut problem is O(sqrt{r}) on graphs G whose r-th smallest normalized eigenvalue of the Laplacian satisfies lambda_r(G)/n >= Omega(1)*Phi_{SDP}(G). Our result improves upon the previously known bound of O(r) on the average distortion, and the integrality gap of the Goemans-Linial SDP under the same preconditions, proven in [Deshpande and Venkat, 2014], and [Deshpande, Harsha and Venkat 2016]
Advanced methods for enhanced sensing in biomedical Raman spectroscopy
No full textRaman spectroscopy is a powerful tool in the field of biomedicine for disease
diagnosis owing to its potential to provide the molecular fingerprint of biological
samples. However due to the inherent weak nature of the Raman process, there is a
constant quest for enhancing the sensitivity of this technique for enhanced diagnostic
efficiency. This thesis focuses on achieving this goal by integrating advanced methods
with Raman spectroscopy.
Firstly this thesis explores the applicability of a laser based fluorescence
suppression technique – Wavelength Modulated Raman Spectroscopy (WMRS) - for
suppressing the broad luminescence background which often obscure the Raman peaks.
The WMRS technique was optimized for its applications in single cell studies and tissue
studies for enhanced sensing without compromising the throughput. It has been
demonstrated that the optimized parameter would help to chemically profile single cell
within 6 s. A two fold enhancement in SNR of Raman bands was demonstrated when
WMRS was implemented in fiber Raman based systems for tissue analysis. The
suitability of WMRS on highly sensitive single molecule detection techniques such as
Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Resonance
Raman Spectroscopy (SERRS) was also explored. Further this optimized technique was
successfully used to address an important biological problem in the field of immunology.
This involved label-free identification of major immune cell subsets from human blood.
Later part of this thesis explores a multimodal approach where Raman
spectroscopy was combined with Optical Coherence Tomography (OCT) for enhanced
diagnostic sensitivity (>10%). This approach was used to successfully discriminate
between ex-vivo adenocarcinoma tissues and normal colon tissues.
Finally this thesis explores the design and implementation of a specialized fiber
Raman probe that is compatible with surgical environments. This probe was originally
developed to be compatible with Magnetic Resonance Imaging (MRI) environment. It
has the potential to be used for performing minimally invasive optical biopsy during
interventional MRI procedures
Chirped pulse Raman amplification in plasma
No full textRaman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage
Electrical conductivity and Raman imaging of double wall carbon nanotubes in a polymer matrix
No full textRaman spectroscopy is used to access the dispersion state of DWNTs in a PEEK polymer matrix. The interaction of the outer tube with the matrix can be determined from the line shape of the Raman G band. This allows us to distinguish regions where the nanotubes are well dispersed and regions where the nanotubes are agglomerated. The percolation threshold of the electrical conductivity of the double wall carbon nanotubes (DWNTs)/PEEK nanocomposites is found to be at 0.2–0.3 wt.%. We find a maximum electrical conductivity of 3 x 10-2 S/cm at 2 wt.% loading. We detect nanotube weight concentrations as low as 0.16 wt.% by Raman spectroscopy using a yellow excitation wavelength. We compare the Raman images with transmission electron microscopy images and electrical conductivity measurements. A statistical method is used to find a quantitative measure of the DWNTs dispersion in the polymer matrix from the Raman images
Stimulated terahertz emission due to electronic Raman scattering in silicon
No full textSilicon-based semiconductors are intensively investigated over the past years as promising candidates for optoelectronic devices at terahertz (THz) frequencies [1]. Optically pumped intracenter silicon lasers, realized in the past decade in the THz range, are based on direct optical transitions between shallow levels of different shallow donors [2]. Recently, terahertz Raman laser emission has been demonstrated in silicon doped by antimony [3] and phosphorus [4].
We report on realization of terahertz lasers based on intracenter electronic Raman scattering in silicon doped by arsenic (Si:As, frequency range 4.8 – 5.1 THz and 5.9 – 6.5 THz) and silicon doped by bismuth (Si:Bi, 4.6 – 5.9 THz) under optical excitation by infrared frequency-tunable free electron laser at low lattice temperatures. The Stokes shift of the observed laser emission is equal to the Raman-active donor electronic transition between the ground 1s(A1) and the excited 1s(E) donor states. Raman terahertz gain of the lasers is similar to those observed for the donor-type terahertz silicon donor lasers
Crystallography-assisted Raman spectroscopy: X-ray induced photodissociation monitored by Raman microscopy
No full textCurrently, few sites provide the possibility to perform simultaneous Raman/X-ray diffraction of single macromolecular crystals. In this recent Raman-assisted crystallography applications, Raman microscopy has been representing a fine servant of a dominant X-ray Crystallography, and its highest scope was the reinforcement of structural data [1, 2]. The strategy of our work is to overturn this paradigm, focusing Crystallography-assisted Raman on the rich spectroscopic information that are well transferable to many bio-analytical applications. A Synchrotron with available Raman microscope, SLS [3], has been used to collect high-resolution crystallographic data on unusual states of model biomolecules. Two cases of study are presented: a) NO photodissociation in hemoglobin crystal structure; b) ultra high resolution crystal structure (0.8 Å) of ribonuclease A at different X- ray doses.
[1] Katona, G., et al. (2007) Science, 316, 449. [2] Vergara, A et al. (2010) J. Biol. Chem. 285,, 32568. [3] Owen, R. L., et al. (2009). J. Synchrotron Rad. 16, 173-182
Advanced techniques in Raman tweezers microspectroscopy for applications in biomedicine
No full textThis thesis investigates the use of Raman tweezers microspectroscopy to interrogate the biochemistry of single biological cells. Raman tweezers microspectroscopy is a powerful technique, which combines traditional Raman microspectroscopy and optical trapping, allowing the manipulation and environmental isolation of a biological cell of interest whilst simultaneously probing its biochemistry gleaning a wealth of pertinent information.
The studies carried out in this thesis can be split into two broad categories: firstly, the exploitation of Raman tweezers microspectroscopy to study biological cells and secondly developments to the Raman tweezers microspectroscopy technique that extend its capabilities and the range of samples that can be studied. In the application of Raman tweezers, the stacking and interrogation of multiple cells is reported allowing a rapid representative Raman signal to be recorded from a small cell population with improved signal to noise. Also demonstrated is the ability of Raman spectroscopy to identify and grade the development of Human Papillomavirus induced cervical neoplasia with sensitivities of up to 96 %. These studies demonstrate the potential of Raman spectroscopy to study biological cells but it was noted that the traditional Raman tweezers system struggled to manipulate large cells thus a decoupled Raman tweezers microspectroscopy system is presented where a dual beam fibre optical trap is used to perform the trapping function and a separate Raman probe is introduced to probe the biochemical nature of the trapped cell. This development allowed the trapping and examination of very large cells whilst opening up the possibility of creating Raman maps of trapped objects. Raman tweezers microspectroscopy could potentially become an important clinical diagnostic and biological monitoring tool but is held back by the long signal integration times required due to the weak nature of Raman scattering. The final study presented in this thesis examines the potential of wavelength modulated Raman spectroscopy to improve signal to noise ratios and reduce integration times.
All these studies aim to demonstrate the potential and extend the performance of Raman tweezers microspectroscopy
Forensic applications of raman spectroscopy
No full textThe forensic applications of Raman spectroscopy have been explored and extended using the development of novel sampling techniques and task-specific instrumentation described in this thesis.
The phenomenon of Raman scattering, enhanced Raman scattering and their relevance in forensic investigations was reviewed. Particular emphasis was placed on current applications, experimental considerations relevant to in-situ Raman sampling and the deficiencies of instrumentation commercially available at the time. It was concluded that the development of novel, optimised instrumentation was
essential in the application of Raman spectroscopy to portable forensic applications.
The feasibility of achieving molecularly-specific and sensitive detection of TNT vapour using waveguide-enhanced, surface-enhanced resonance Raman spectroscopy was investigated using reference spectra measured using a calibrated optical system provided by a collaborator. Improvements in signal-to-noise ratio afforded by employing waveguide-enhanced sampling, higher excitation power,
long integration times and an improved spectrometer design were modelled, experimentally verified, and used to predict a detection limit of 10-16g for saturated vapour-phase TNT. The theoretical performance of the optical instrument is
described and verified using experimentally measured data.
The feasibility of conducting specific and sensitive long-range stand-off covert observation operations against unsuspecting targets in compliance with the UK Regulation of Investigative Powers act was established using a task-optimised laboratory simulation. Using a 5mW visible excitation, short integration times (under 20s) and multiplex detection it was possible to detect and identify a tagged object from a range of up to 50m. The feasibility study yielded a robust prototype handheld system comprising a modified telephoto camera with the integrated capability of sample discrimination using Raman spectroscopy. The instrument design is described
Raman microspectroscopy interrogating 19th and 20th century painted trades union banners
No full textWe have previously developed protocols for the application of Raman microspectroscopy to studies on painted textiles. We have further assessed the value of such microanalyses in the identification of both inorganic and organic constituents, including original components and consolidants used in conservation treatments. This paper presents the results of a recent study on a number of 19th- and 20th-century trades union banners directed at collating a spectral database of inorganic pigments used in the illustrations and at probing the preparative process prior to painting. Such information will contribute to an understanding of the manufacture of such banners and their current condition, leading to the development of optimum conservation procedures.While Raman spectroscopy has the potential to be used in situ and, with the appropriate protocol, is non-destructive, nonetheless we have found that the analysis of resin-embedded cross-sections is to be preferred with microtoming providing the cleanest sample surface. The optimum methodology for acquiring good quality Raman spectra is described including operation in the confocal mode, with consideration of fluorescence, interference from resin, laser-induced photochemistry, and so on
1.06 µm picosecond pulsed, normal dispersion pumping for generating efficient broadband infrared supercontinuum in meter-length single-mode tellurite holey fiber with high Raman gain coefficient
No full textWe investigate efficient broadband infrared supercontinuum generation in meter-length single-mode small-core tellurite holey fiber. The fiber is pumped by 1.06µm picosecond pulses in the normal dispersion region. The high Raman gain coefficient and the broad Raman gain bands of the tellurite glass are exploited to generate a cascade of Raman Stokes orders, which initiate in the highly normal dispersion region and quickly extend to longer wavelengths across the zero dispersion wavelength with increasing pump power. A broadband supercontinuum from 1.06µm to beyond 1.70µm is generated. The effects of the pump power and of the fiber length on the spectrum and on the power conversion efficiency from the pump to the supercontinuum are discussed. Power scaling indicates that using this viable normal dispersion pumping scheme, 9.5 W average output power of infrared supercontinuum and more than 60% conversion efficiency can be obtained from a 1 m long tellurite fiber with a large mode area of 500µm2
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