Browsing by Subject "Photons"
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Item Human Wavelength Discrimination of Monochromatic Light Explained by Optimal Wavelength Decoding of Light of Unknown Intensity(Public Library of Science, 2011-05-20) Zhaoping, Li; Geisler, Wilson S.; May, Keith A.We show that human ability to discriminate the wavelength of monochromatic light can be understood as maximum likelihood decoding of the cone absorptions, with a signal processing efficiency that is independent of the wavelength. This work is built on the framework of ideal observer analysis of visual discrimination used in many previous works. A distinctive aspect of our work is that we highlight a perceptual confound that observers should confuse a change in input light wavelength with a change in input intensity. Hence a simple ideal observer model which assumes that an observer has a full knowledge of input intensity should over-estimate human ability in discriminating wavelengths of two inputs of unequal intensity. This confound also makes it difficult to consistently measure human ability in wavelength discrimination by asking observers to distinguish two input colors while matching their brightness. We argue that the best experimental method for reliable measurement of discrimination thresholds is the one of Pokorny and Smith, in which observers only need to distinguish two inputs, regardless of whether they differ in hue or brightness. We mathematically formulate wavelength discrimination under this wavelength-intensity confound and show a good agreement between our theoretical prediction and the behavioral data. Our analysis explains why the discrimination threshold varies with the input wavelength, and shows how sensitively the threshold depends on the relative densities of the three types of cones in the retina (and in particular predict discriminations in dichromats). Our mathematical formulation and solution can be applied to general problems of sensory discrimination when there is a perceptual confound from other sensory feature dimensions.Item Interaction of intense laser fields with carbon nanotubes(2006) Hsu, Han; Reichl, L. E.The main topic addressed in this dissertation is the interaction of intense laser fields with the π electrons of single-walled carbon nanotubes. Since it is very difficult to carry out ab initio calculations for nanotubes in the presence of intense laser fields, we first propose a simple but realistic model to simulate the π electrons in a graphene layer and single-walled carbon nanotubes. We replace the atomic potential of each carbon atom of graphene or nanotubes by a two-dimensional attractive regularized δ function. By adjusting the parameters in this simple δ-potential model, we successfully reproduce the band structures and wave functions for graphene and nanotubes calculated by ab initio methods. With its simplicity and accuracy, we can use δ-potential model to study the interaction of intense laser fields with nanotubes. Combining the δ-potential model and Floquet-Bloch theory, we calculate the electronic states and the electron motion in an armchair nanotube driven by monochromatic intense laser fields with polarization parallel to the nanotube axis. The intensity and frequency (photon energy) of the applied laser fields are varied so their effect on the electrons can be understood. In each case, Floquet-Bloch theory is used to calculate the Floquet-Bloch states, quasienergy band, mean energy band, and electron current density. By summing up the current density of all occupied Floquet-Bloch states, the harmonic generation spectrum can be determined. We demonstrate that the deformation of quasienergy band and mean energy band is related to high-order harmonic generation. Only the states deviating from field-free eigenstates may contribute to high-order harmonic generation.Item On the application of quantum perturbation theory to gravitational interactions(1950) DeWitt, Bryce S. (Bryce Seligman), 1923-2004Part 1: The formalism preliminary to a quantum perturbation treatment of the interaction of wave fields with gravitation is here developed. Since spinor fields are of importance, a resumé is given of Pauli’s treatment of spinors in general coordinates, involving the introduction of generalized Dirac operators. The essential points of the Einstein-Mie theory are outlined, and spin angular momentum is discussed from the general coordinate viewpoint with the aid of the generalized orthogonal group. The commutation law for covariant differentiation is obtained for arbitrary fields. The symmetric stress tensor can be constructed either from the canonical energy-momentum tensor together with the spin angular momentum tensor or directly through variation of the metric tensor. The concepts of energy, momentum and spin angular momentum, and hence the Hamiltonian formalism itself, can be introduced for the gravitational field only with respect to a "background space" which has a flat metric of no physical geometrical significance. In the background space only Lorentz transformations have immediate invariant significance, and general coordinate transformations appear as "gauge transformations" in the gravitational and accompanying matter fields. Only the total integrated energy, momentum and spin angular momentum quantities are invariant under these “gauge transformations.”Item A search for the rare decay of a [B meson to two photons](2010-05) Ruland, Andrew Michael; Ritchie, Jack L., 1955-; Lang, Karol; Schwitters, Roy; Hoffmann, Gerald; Lambert, DavidThis thesis describes a search for the rare radiative decay of a B meson to two photons. where the charged congugate mode is implied throughout. These decays are highly suppressed in the Standard Model where the branching fraction is expected to be of order 10^-8. In some new physics scenarios this could be enhanced by up to an order of magnitude to 10^-7. Therefore an observation of a significant signal above the Standard Model prediction could be a sign of new physics. The search for this rare decay was performed using the data collected with the BaBar detector at the SLAC National Accelerator Laboratory PEP-II storage ring operating at the Upsilon(4S) resonance. The analysis uses a dataset with an integrated luminosity of 425.7 fb-1 corresponding to 467 million BB pairs. A signal yield of 21.3 +12.8 -11.8 events with a significance of 1.88 sigma was measured using an unbinned extended maximum likelihood fit. An upper limit on the branching fraction is set at the 90% confidence level of less than 3.2 times 10^-7. This is about two times more stringent than the best upper limit of less than 6.2 times 10^-7 published by the Belle collaboration.