Photon Absorption Effects Research Articles

Photon-assisted mesoscopic transport through a toroidal carbon nanotube coupled to normal metal leads

The mesoscopic transport through a toroidal carbon nanotube (TCN) system applied with ac fields to the electrodes has been investigated by employing the nonequilibrium Green's function (NGF) technique. The Landauer–Büttiker-like formula is presented for numerical calculations of differential conductance and tunneling current. The conductance resonance takes place due to the electrons resonating in the quantum levels of TCN and side-band caused by the external ac fields. The photon-assisted transport can be observed both in conductance oscillation and current oscillation with respect to the magnetic flux. The side peaks and current suppressions are the main effect of photon absorption and emission in the transport procedure. The stair-like current–voltage characteristics are resulted from the quantum nature of TCN and applied microwave fields. The photon–electron pumping effect can be obtained by applying the microwave fields to the leads.

Chalcogenide glasses as solid state optical limiters at 1.064 μm

Abstract Chalcogenide glasses present high values for third-order non-linear optical properties, i.e., high non-linear refractive indices (800 times as high as the non-linearity of silica glass at 1.064 μm) and a high non-linear absorption coefficient, which can reach more than 5 cm/GW. We study here the optical limiting properties of several chalcogenide glasses at 1.064 μm with a picosecond pulsed Nd:YAG laser and we observed a real optical limiting behavior. In the case of GeAs2Se2, glass for example, the decrease of the transmitted intensity is around 75% compared to the theoretical linear transmission, for incident pulses of 5 GW/cm2. The comparison between the theoretical non-linear transmission induced by the two photon absorption effect and the experimental non-linear transmittance shows that the optical limiting properties are mainly due to non-linear absorption.

All optical switching and continuum generation in silicon waveguides

First demonstration of cross phase modulation based interferometric switch is presented in silicon on insulator waveguides. By using Mach-Zehnder interferometric configuration we experimentally demonstrate switching of CW signal ~25 nm away from the pump laser. We present the effect of free carrier accumulation on switching. Additionally, we theoretically analyze the transient effects and degradations due to free carrier absorption, free carrier refraction and two photon absorption effects. Results suggest that at low peak power levels the system is governed by Kerr nonlinearities. As the input power levels increase the free carrier effects becomes dominant. Effect of free carrier generation on continuum generation and power transfer also theoretically analyzed and spectral broadening factor for high input power levels is estimated.

Estimation of the local statistical noise in positron emission tomography revisited: practical implementation

The purpose of this report is to implement novel modifications to overcome the limitations of an existing algorithm for estimating the local statistical noise in a positron emission tomography (PET) image without performing repeated measures. The original algorithm is based on a modification of the filtered back-projection algorithm that allows the variance to be estimated using only a single sinogram. In addition, the effects of photon absorption, random coincidences, radioactive decay, and detector nonuniformity are taken into account. However, there are some limitations when applying this method with modern scanners. In particular, it is common practice to interleave the projections in the sinogram (to increase the sampling rate along each projection) and to perform an interpolation when actually back-projecting to reconstruct the images. Both of these procedures introduce covariance among the elements of the projections, which is cumbersome and impractical to deal with using the existing technique for creating a variance image. An alternative image reconstruction scheme that is shown to be equivalent to image reconstruction using traditional filtered back-projection greatly simplifies the estimation of the variance image. The proposed methods were tested by Monte Carlo simulations and by using repeated scans of a uniform phantom filled with F-18. Results demonstrate that the proposed methods are very rigorous and stable when compared to calculations of the local variance using either repeated measures with a large number of measurements, or region-of-interest estimates of the variance, assuming homogeneous variance structure. In addition, strategies for extending the proposed technique are discussed that would permit the estimation of the variance due to measurement error of a pixel in a brain map from both single subjects and pooled group data.

Signal transduction in the photoactive yellow protein. I. Photon absorption and the isomerization of the chromophore.

Molecular dynamics simulation techniques together with time-dependent density functional theory calculations have been used to investigate the effect of photon absorption by a 4-hydroxy-cinnamic acid chromophore on the structural properties of the photoactive yellow protein (PYP) from Ectothiorodospira halophila. The calculations suggest that the protein not only modifies the absorption spectrum of the chromophore but also regulates the subsequent isomerization of the chromophore by stabilizing the isomerization transition state. Although signaling from PYP is thought to involve partial unfolding of the protein, the mechanical effects accompanying isomerization do not appear to directly destabilize the protein.

Simultaneous measurements of electric and thermal fields utilizing an electrooptic semiconductor probe

A method to simultaneously measure electric and thermal fields with a single probe is presented in this paper. The Pockels effect is employed within a gallium-arsenide probe to measure electric fields, and the effect of photon absorption due to bandtail states in the semiconductor is used to determine temperature. The measured optical power is found to be inversely related to temperature, in agreement with theory, and experimental results demonstrate a temperature sensitivity of 0.31 /spl mu/W//spl deg/C at 25/spl deg/C and an accuracy of /spl plusmn/0.5/spl deg/C between 20/spl deg/C-60/spl deg/C. The minimum detectable electric field is 1.24/spl plusmn/0.06 V/m using a 300-ms electrical bandwidth. Temporal phase stability of /spl plusmn/3/spl deg//h is achieved through the implementation of a system phase reference channel. The invasiveness of the probe is quantified by examining the change in the characteristic impedance and capacitance per unit length of a planar transmission line. Measured and simulated data show that the effect is equivalent to a lumped shunt capacitance on the order of a few femtofarads. The examination of a monolithic microwave integrated circuit in an X-band quasi-optical power-combining array and the calibration of electric-field data that was corrupted by temperature-dependent effects inherent to the electrooptic probe demonstrate the capability of this combined electrothermal measurement technique.

Comparison of Different Methods for Attenuation Correction in Brain PET: Influence on the Calculation of the Metabolic Glucose Rate

Zusammenfassung Ziel: Für die Korrektur des Effektes der im Körpergewebe gestreuten oder absorbierten Photonen (Schwächungskorrektur) existieren bei der Positronen-Emissionstomographie (PET) verschiedene methodische Ansätze. Die vorliegende Arbeit untersucht den Einfluß der kalten (KT), heißen (PIT), segmentierten Transmission (ST) sowie eines rechnerischen Verfahrens (RV) auf die Schwächungskorrektur anhand der absoluten Bestimmung des regionalen zerebralen Glukosemetabolismus (rMRGIu). Methodik: 6 Patienten mit unterschiedlichen zerebralen Erkrankungen wurden mit einem ECAT Exact 922/47 unter Ruhebedingungen untersucht. Die durch die KT, PIT, RV und ST schwächungskorrigierten Daten wurden dann zur Beurteilung des regionalen Glukosemetabolismus in 16 ROIs (regions of interest) absolut quantifiziert. Ergebnisse: Bis auf drei Regionen (temporookzipital links und rechts sowie parietookzipital rechts) konnte kein signifikanter Unterschied des mit Hilfe der absoluten Quantifizierung in 16 ROIs ermittelten Glukosemetabolismus zwischen der KT und dem RV festgestellt werden. Im Gegensatz zum RV wurde sowohl bei der PIT als auch bei der ST in allen 16 ROIs ein signifikant niedrigerer Wert der regionalen zerebralen Glukoseutilisation gegenüber der KT nachgewiesen. Aufgrund dieser Resultate ergibt sich sowohl in der linken als auch in der rechten Hirnhälfte eine kontinuierliche Abnahme der metabolischen Werte (rMRGIu) in der Reihenfolge KT, RV, PIT und ST. Der globale, zerebrale Glukosemetabolismus zeigte keinen signifikanten Unterschied zwischen der KT und dem RV (p = 0,2489), während mit PIT (p = 0,03) und ST (p = 0,03) ein signifikant erniedrigter Glukosemetabolismus gegenüber der-KT nachzuweisen war. Schlußfolgerung: Bei den für die zerebralen PET-Studien am Patienten durchgeführten Schwächungskorrekturverfahren läßt sich bis auf drei Regionen quantitativ kein signifikanter Unterschied zwischen KT und RV nachweisen. Die rMRGIu der durch PIT und ST schwächungskorrigierten Bilder sind signifikant niedriger als in KT korrigierten Aufnahmen. Um eine zerebrale PET-Akquisition zu verkürzen, erscheint für die klinische Routine das RV als Alternative zur KT am besten geeignet.

Photoinduced optical absorption and 400-nm luminescence in low-germanium-content optical fiber preforms irradiated with ArF and KrF excimer-laser light

The optical absorption spectrum and the 400-nm photoluminescence (PL) of a 1.4-mol. % Ge photosensitive optical fiber preform subjected to high fluence of 193-nm ArF and 248-nm KrF excimer-laser irradiation are measured. The largest absorption increases occur near 200 nm in both cases, but a small net bleaching of absorption is obtained near the laser wavelength for KrF irradiations. The blue PL decreases during ArF exposure but increases with the KrF laser. In similarly excited 9-mol. % Ge fiber preforms the blue PL always decreases. A study of the PL intensity as a function of irradiating laser light intensity shows no evidence of multiple photon absorption effects.

Theory of spectral line shape. III. The Fano operator from near to far wing

In this third part of a theory of spectral line shape, applicable to conditions of atmospheric transmission, an evaluation of the frequency dependence of the complete Fano collision operator has been made using the collision time theory developed earlier. This results in a considerable reduction of the terms in the Fano operator. An analysis of the spectral line wing shows that there are two different regimes: the near wing and the far wing. These regimes are demarcated by a turning point at a frequency displacement of kT/h. In the very far wing, beyond the turning point, the behavior of the Fano operator changes and the effects of binary collision overlap and photon absorption into the binary collision complex become important. The line wing behavior is corroborated qualitatively by experimental results for carbon dioxide near 4.2 μm.

Influence of higher-order photon absorption on bistable-soliton propagation and switching.

The effect of weak higher-order photon absorption on bistable-soliton propagation and switching in a fiber whose intensity-dependent refractive index is described by the linear-plus-smooth-step model is investigated numerically. The choice of model allows the application of the adiabatic assumption and a clear analytic interpretation of all of the observed results.

Estimation of the Local Statistical Noise in Emission Computed Tomography

A simple modification of the filtered backprojection algorithm is presented for the computation of the local statistical noise in emission computed tomography. The technique is general in that any distribution of radioactivity may be accommodated. When applied to positron emission tomography, it is shown that the effects of photon absorption, random coincidences, radioactive decay, and detector nonuniformity may be included. Calculations have shown the effects of resolution, object size, and photon absorption on the statistical noise of disk-shaped emitters. Comparison of calculation and experiment show close agreement both in magnitude and spatial variation. Measurements of the noise level in tomograms of the brain obtained during continuous inhalation of 150-CO2 demonstrate that estimates of radioactivity concentration with a precision of a few percent are readily attainable.

The effect of photon absorption by breast tissue in myocardial imaging with thallium 201

The effect of overlying breast tissue on 201Tl imaging of the myocardium has been studied experimentally and in two patients in whom normal images were only obtained after moving the breast out of the field of view.

Interactions of picosecond laser pulses with organic molecules I. Two-photon fluorescence quenching and singlet states excitation in Rhodamine dyes

Intensity dependent quenching and reversal of the two-photon fluorescence patterns in Rhodamine 6G and DPA, of picosecond pulses from a mode-locked ruby laser have been investigated by measurements of the two-photon fluorescence efficiencies of these dyes. While for Rhodamine 6G there was a marked departure from the square law dependence at high laser intensities, the experimental curve for DPA showed no evidence of quenching. When excited by a mode-locked neodymium: glass laser Rhodamine 6G fluorescence was not quenched at fluxes as high as 5 x 10 30 photons cm -2 s -1 but in Rhodamine B quenching appeared at a laser flux of 3 x 10 27 photons cm -2 s -1 . These quenching results and measurements of the absorption of pulses by Rhodamine 6G, previously excited by second harmonic pulses, are explained by the effects of single photon absorption and stimulated emission from the S 1 and S 2 excited singlet states. A square pulse approximation has been employed to solve the general rate equations and the fitting of the calculated curves to the experimental results gave values for the stimulated emission and absorption cross-sections of the S 1 and S 2 states of Rhodamine 6G and for the relaxation time (~ 2 ps) between the vibrational manifolds of these excited states. Taking into account random phase and amplitude fluctuations of the picosecond pulses, time and space averaged two-photon fluorescence profiles, using these values of the dye parameters, showed quenching and reversal of the patterns for the laser pulse intensities at which these effects were experimentally observed. The possibilities of frequency tunable pulses, of transform-limited durations, from mode-locked dye lasers employed with an electro-optical streak camera of time-resolution equal to that of the pulse durations (~ 2 ps) for time-resolved excited state molecular spectroscopy are briefly considered.

Measurements of superficial absorbed dose with 2 MV x rays used at glancing angles.

A photographic film method of investigating the variation of absorbed dose near an air-tissue interface is described, which involves the use of a microdensitometer. The corrections necessary to allow for the effect of direct photon absorption in the film are discussed. Application of the method to a situation in which the angle of incidence of the X-ray beam varies from 0 to 180 deg. shows that at 2MV the 80 per cent depth varies from 0·15 to 1·1 mm below the surface. It is suggested that this allows adequate treatment of superficial tissue, while retaining some degree of skin saving even when the angle of incidence to the normal is 90 deg.