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At least 73 records · Page 4Linked to original sources
PubMed2006

Efficient single-photon sources based on low-density quantum dots in photonic-crystal nanocavities.

An efficient single-photon source based on low-density InGaAs quantum dots in a photonic-crystal nanocavity is demonstrated. The single-photon source features the effects of a photonic band gap, yielding a single-mode spontaneous emission coupling efficiency as high as beta = 92% and a linear polarization degree up to p = 95%. This appealing performance makes it well suited for practical implementation of polarization-encoded schemes in quantum cryptography.

Journal Article↗
PubMed1982

Contamination of a 15-MV photon beam by electrons and scattered photons.

The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.

Electrons↗
PubMed2005

Influence of photon scattering and attenuation on ROI analysis in brain perfusion single-photon emission tomographic imaging of normal subjects.

OBJECTIVE: The aim of this study was to evaluate the effect of scatter and attenuation correction in region of interest (ROI) analysis in brain perfusion single-photon emission tomography (SPECT) and to assess the influence of selecting the reference area on semi-quantification. METHODS: Ten normal subjects were enrolled and injected with 123I-iodoamphetamine to undergo simultaneous emission and transmission scanning for scatter and attenuation correction. We reconstructed three SPECT images from common projection data of each subject: with scatter correction and non-uniform attenuation correction, with scatter correction and uniform attenuation correction, and with uniform attenuation correction applied to data without scatter correction. A program for automated ROI drawing was used to set ROIs on various regions in brain images. Regional count ratios were compared in images with different correction procedures by using three different reference areas. RESULTS: The effect of the combination of scatter and attenuation correction was marked in the precentral, temporal, posterior, hippocampus and especially in the cerebellum. In contrast, it was not appreciable in the central and parietal areas. When using the cerebellar ROI as the reference, the count ratio varied widely depending on the correction procedures. On the other hand, the whole brain reference offered the least variation in the count ratio. CONCLUSIONS: The influence of photon scattering and attenuation was dependent on regions. Since the count in the cerebellar ROI is greatly affected by photon scattering and attenuation, nonuniform attenuation correction combined with scatter correction deserves consideration when using the cerebellar ROI as the reference.

Algorithms↗
PubMed2002

Effective atomic numbers for photon energy absorption and photon attenuation of tissues from human organs.

Effective atomic numbers for photon energy- absorption (Z(PEA)eff) and photon interaction (ZPI(eff)) of human organs and tissues such as cortical bone, ovary, eye lens, testis, breast tissue, adipose tissue, lung tissue, soft tissue, soft tissue, (4-component), blood (whole), brain (grey/white matter), and skeletal muscle have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The ZPEAeff and ZPIeff values steadily increase, up to 8-50 keV, and steadily decrease up to 1.25-2.0 MeV for all of the substances studied. From 2.0 MeV, the values rise with the increase in energy, up to 20 MeV. Significant differences exist between the ZPIeff and ZPEAeff in the energy region of 20-400 keV and 3-20 MeV for cortical bone; 15-150 keV for soft tissue, ovary, testis, blood, brain, lung, and skeletal muscle; 15-100 keV for breast tissue, eye lens, and soft tissue (4-component); and 10-100 keV for adipose tissue. A maximum difference of 28.37% is observed at 100 keV for cortical bone, and 30.43% at 40 keV for adipose tissue. For ovary, eye lens, testis, breast tissue, lung tissue, soft tissue, soft tissue (4-component), blood (whole), brain (grey/white matter), and skeletal muscle, a maximum difference of 31.74%, 29.60%, 31.87%, 30.61%, 31.47%, 31.52%, 29.95%, 31.63%, 32.36%, and 31.42%, respectively, is seen at 50 keV. The energy positions at which the maximum of ZPEAeff and ZPIeff occurs differ. The single effective atomic number directly obtained using the program XMuDat (Z(XMUDATTeff)) are found to be higher compared to those of ZPEAeff and ZPIeff values. The effect of absorption edge on effective atomic numbers, and its variation with photon energy and the possibility of defining 2 set values of effective atomic numbers below the absorption edges of elements present in the organs and tissues, are discussed.

Absorptiometry, Photon↗
PubMed2006

Two-photon photosensitized production of singlet oxygen: optical and optoacoustic characterization of absolute two-photon absorption cross sections for standard sensitizers in different solvents.

Singlet molecular oxygen, O2(a1Deltag), can be produced upon resonant two-photon excitation of a photosensitizer. In the present study, two molecules that have received recent attention in studies of nonlinear organic materials were characterized for use as standard two-photon sensitizers: 2,5-dicyano-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, CNPhVB, and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, BrPhVB. Absolute two-photon absorption cross sections, delta, were independently determined for these molecules using two techniques that have heretofore not been applied to this problem: an optical technique (time-resolved detection of O2(a1Deltag) phosphorescence) and a nonoptical technique (a time-resolved laser-induced optoacoustic experiment). For experiments performed in toluene, a solvent commonly used for such nonlinear optical studies, appreciable absorption by the solvent itself complicates the measurements. In cyclohexane, however, delta values could be obtained without the interfering effects of solvent absorption. On the basis of these results, we discuss key aspects of the respective techniques used to quantify values of delta. The information reported herein provides some explanation for the lack of consensus that is routinely observed in published values of delta, certainly for experiments performed in aromatic solvents such as toluene and benzene.

Benzene Derivatives↗
PubMedDate not supplied

Comparison of relative photon flux from single cells of the bioluminescent marine bacteria Vibrio fischeri and Vibrio harveyi using photon-counting microscopy.

Photon counting microscopy was used to quantify relative photon flux from 616 single sessile cells of Vibrio harveyi and 180 single sessile cells of V. fischeri. V. fischeri cells all emitted light at nearly the same level (average 15.93+/-11.48 photons/min, range 0-58.9 photons/min) when bioluminescence was induced, whereas photon flux of V. harveyi single cells was extremely variable (average 10.51+/-20.60 photons/min, range 0-211 photons/min).

Evaluation Studies as Topic↗
PubMed2006

Two-photon photochromism of a diarylethene enhanced by Förster resonance energy transfer from two-photon absorbing fluorenes.

Resonance energy transfer from two-photon absorbing fluorene derivatives to the photochromic compound 3,4-bis-(2,4,5-trimethyl-thiophen-3-yl)furan-2,5-dione (PC 1) is investigated in hexane under one- and two-photon excitation. The quenching of the steady-state fluorescence of donor molecules in the presence of the diarylethene acceptor is used to study the nature of resonance energy transfer. The Förster distances and critical acceptor concentrations are determined for nonbound donor-acceptor pairs in homogeneous molecular ensembles. Quite significantly, up to a two-fold enhancement in the velocity of the photochromic transformation of 1, in the presence of two-photon absorbing fluorene derivatives, is demonstrated.

Ethylenes↗
PubMed1994

Analysis of photon beam exit dose using photon point kernels.

The Monte Carlo method is used to analyse the dose fall-off at the exit surface of a megavoltage photon beam. The convolution/superposition method of dose calculation using Monte-Carlo-generated homogeneous photon kernels is shown to be in error for exit dose calculation. Instead, photon kernels that incorporate modelling of the exit surface were generated, also using Monte Carlo, to analyse the problem, and the calculated dose fall-off using these kernels agrees well with measured data. In addition, the physics underlying the characteristics of the dose fall-off is analysed based on complete Monte Carlo modelling. Practical improvements to the convolution/superposition method are suggested.

Algorithms↗
PubMed2004

Photonic crystal fibre enables short-wavelength two-photon laser scanning fluorescence microscopy with fura-2.

We report on a novel and compact reliable laser source capable of short-wavelength two-photon laser scanning fluorescence microscopy based on soliton self-frequency shift effects in photonic crystal fibre. We demonstrate the function of the system by performing two-photon microscopy of smooth muscle cells and cardiac myocytes from the rat pulmonary vein and Chinese hamster ovary cells loaded with the fluorescent calcium indicator fura-2/AM.

Animals↗
PubMedDate not supplied

Two-photon laser scanning fluorescence microscopy using photonic crystal fiber.

We report the application of a simple yet powerful modular pulse compression system based on photonic crystal fibers that improves on incumbent two-photon laser scanning fluorescence microscopy techniques. This system provides more than a sevenfold increase in fluorescence yield when compared with a commercial two-photon microscopy system. From this, we infer pulses of IR radiation of less than 35 fs duration reaching the sample.

Animals↗
PubMed1998

Calculation of photon energy and dose distributions in a 50 MV scanned photon beam for different target configurations and scan patterns.

A method to characterize the energy distribution in the whole photon field is valuable when designing an accelerator for choosing target and flattening filter or scan pattern. Another field of application is beam characterization for treatment planning systems or other dosimetric purposes. This work is focused on the energy distribution in different 50 MV bremsstrahlung beams with different scanning of electrons on three different targets. Fluence differential in energy and angle at the exit of each target has been determined by Monte Carlo calculations for a narrow beam. Data for broad beams were obtained by convolution of the narrow beams with different scan patterns. Photon energy fluence differential in energy at SSD 100 were thus found to be rather different for the targets studied. The results are presented as mean energy profiles and narrow beam half-value layer (HVL) in water. Two different experimental setups were used to measure HVL at the central axis and at off-axis positions. The two methods gave results which differ by 5%-6% and the calculated data where within these experimental results. In conclusion, the presented method for characterization of the photon field energy distribution is well within the experimental results and can thus be used to improve accelerator design or dosimetric calculations, e.g., for treatment planning.

Biophysical Phenomena↗
PubMed2006

Theoretical investigation of one-photon and two-photon absorption properties for multiply N-confused porphyrins.

We have theoretically investigated a series of multiply N-confused porphyrins and their Zn or Cu complexes for the first time by using DFT(B3LYP/6-31G*) and ZINDO/SOS methods. The electronic structure, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that the OPA spectra of multiply N-confused porphyrins are red-shifted and the OPA intensities decrease compared to normal porphyrin. The maximum two photon absorption wavelengths lambda(max) are blue-shifted and the TPA cross sections delta(max) are increased 22.7-112.1 GM when the N atoms one by one are inverted from core to beta position to form multiply N-confused porphyrins. Especially delta(max) of N3CP get to 164.7 GM. The electron donors -C6F5s at meso-position can make the TPA cross section delta(max) increase. After forming metal complexes with Cu or Zn, the TPA properties of multiply N-confused porphyrins are further increased except for N3CP, N4CP. Our theoretical findings demonstrate that the multiply N-confused prophyrins as well as their metal complexes and derivatives are promising molecules that can be assembled series of materials with large TPA cross section, and are sure to be the subject of further investigation.

Journal Article↗
PubMed1983

The effect of photon scatter and consequent electron build-up in air on the calculation of dose equivalent quantities in the ICRU sphere for photon energies from 0.662 to 10 MeV.

A Monte Carlo computer program, DEIPHOS has been modified and employed to calculate depth-dose distributions within the 30 cm diameter ICRU tissue-equivalent sphere for initially parallel beams of photons (0.662 to 10 MeV) when the sphere is in an air medium. Dose equivalent values are computed as the sum of two contributions, the dose equivalent when the sphere is in vacuo and the dose equivalent from scattered photons and the corresponding electrons and positrons produced in an air column in front of the sphere. The dimensions of the air cylinder required to attain full electronic equilibrium were derived for each photon energy. The calculation of absorbed dose to air in air is described. Dose equivalents at 300 and 1000 mg cm-2 depths, the restricted dose equivalent indices, the average dose equivalent in the sphere as well as depth-dose equivalent and angle-dose equivalent distributions are presented, normalised both to fluence and absorbed dose to air.

Air↗
PubMed1984

Energy imparted to water slabs by photons in the energy range 5-300 keV. Calculations using a Monte Carlo photon transport model.

In diagnostic examinations of the trunk and head, the energy imparted to the patient is related to the radiation risk. In this work, the energy imparted to laterally infinite, 10-300 mm thick water slabs by 5-300 keV photons is calculated using a Monte Carlo photon transport model. The energy imparted is also derived for energy spectra of primary photons relevant to diagnostic radiology. In addition to values of energy imparted, values of backscattered and transmitted energies, quantities primarily obtained in the transport calculations, are reported. Assumptions about coherent scattering are shown to be important for values of backscattered and transmitted energies but unimportant with respect to values of energy imparted. Comparisons are made with other Monte Carlo results from the literature. Discrepancies of 10-20% in some calculated quantities can be traced back to the use of different tabulations of interaction cross-sections by various authors.

Humans↗
PubMed1999

Femtosecond measurements of the time of flight of photons in a three-dimensional photonic crystal.

We report on experimental measurements of the propagation behavior of short optical pulses in a three-dimensional photonic crystal in the visible spectrum. The propagation delay of 70 fs light pulses transmitted through a sample of a fcc synthetic opal at frequencies lying in a photonic stop band was measured directly using a time-of-flight technique. Taking into account spectral reshaping of the transmitted pulses as well as the residual frequency chirp of the incoming pulses, it is found that the pulses significantly slow down at the photonic band edges.

Journal Article↗
PubMed2005

Conductance of photons in disordered photonic crystals.

The conductance of photons in two-dimensional disordered photonic crystals is calculated using an exact multipole-plane wave method that includes all multiple scattering processes. Conductance fluctuations, the universal nature of which has been established for electrons in the diffusive regime, are studied for photons, in both principal polarizations and for varying disorder. Our simulations show that universal conductance fluctuations can be observed in H(||) (TE) polarization for weak and intermediate disorder while, for E(||) (TM) polarization, we show that the conductance variance is essentially independent of sample size but strongly dependent on disorder. The probability distribution of the conductance is also calculated in the diffusive and localized regimes, and also at their transition, for which the distributions for both polarizations are seen to be very similar.

Journal Article↗
PubMed2000

Orbital and intrinsic angular momentum of single photons and entangled pairs of photons generated by parametric down-conversion

States of light that are simultaneously eigenstates of orbital, intrinsic, and total angular momentum are found and eigenstates describing pairs of photons from spontaneous parametric down-conversion reveal classes of photon conversion with angular momentum conservation and cases where the initial angular momentum from the pump laser beam is shared between the converted photons and the generating medium. These angular momentum conservation laws open up a wide range of applications to be explored.

Journal Article↗
PubMed2002

Measurement of the photonic de broglie wavelength of entangled photon pairs generated by spontaneous parametric down-conversion.

Using a basic Mach-Zehnder interferometer, we demonstrate experimentally the measurement of the photonic de Broglie wavelength of entangled photon pairs (biphotons) generated by spontaneous parametric down-conversion. The observed interference manifests the concept of the photonic de Broglie wavelength. We also discuss the phase uncertainty obtained from the experiment.

Journal Article↗