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PubMed2005

Extrinsic optical scattering loss in photonic crystal waveguides: role of fabrication disorder and photon group velocity.

Formulas are presented that provide clear physical insight into the phenomenon of extrinsic optical scattering loss in photonic crystal waveguides due to random fabrication imperfections such as surface roughness and disorder. Using a photon Green-function-tensor formalism, we derive explicit expressions for the backscattered and total transmission losses. Detailed calculations for planar photonic crystals yield extrinsic loss values in overall agreement with experimental measurements, including the full dispersion characteristics. We also report that loss in photonic crystal waveguides scales inversely with group velocity, at least, thereby raising serious questions about future low-loss applications based on operating frequencies that approach the photonic band edge.

Journal Article↗
PubMed2005

Heavy photon states in photonic chains of resonantly coupled cavities with supermonodispersive microspheres.

We propose and demonstrate a "bottom-up" approach to constructing photonic structures for photon manipulation. Supermonodispersive polymer microspheres are used as building blocks and a size uniformity better than 0.05% could be obtained by sorting the spheres using spectroscopic methods. The spheres are positioned in a V groove on a silicon substrate and form a photonic chain with resonant coupling of the optical whispering-gallery modes. Photonic band modes are clearly observed in fluorescence and resonant scattering spectra, and an excellent agreement with a tight-binding model calculation is found. Heavy photon states and a group index as high as 40 are obtained.

Journal Article↗
PubMed2006

Measurement of identified and inclusive photon second-harmonic parameter and implications for direct photon production in [FORMULA: SEE TEXT] Au+Au.

The azimuthal distribution of identified pi0 and inclusive photons has been measured in [FORMULA: SEE TEXT] Au+Au collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter (nu2) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured pT range 1-6 GeV/c. An attempt is made to extract nu2 of direct photons.

Journal Article↗
PubMed2003

Single-photon tunneling in photonic crystals with deep defect states.

A new design for a single-photon tunneling device based on tunneling of light through a gap in a photonic crystal linear waveguide containing a deep defect state is suggested. Unlike recently observed single-photon tunneling through nanometer scale nonlinear pinholes in a thick gold film, in the device described here the photon current should be strictly conserved. By appropriate design of the barrier width, defect level position in the bandgap, and defect spatial position in the barrier, a desired photon tunneling rate can readily be engineered.

Journal Article↗
PubMed1981

Contamination of high-energy photon beams by scattered photons.

Absorbed dose and energy distribution of scattered photons that contaminate high-energy photon beams have been calculated for 6-MV and 21-MV bremsstrahlung sources using the Monte Carlo method. The percentage of the absorbed dose at the central axis is less than 3%. The results show that the main sources of scattered photons are the primary collimator for 6-MV and the flattening filter for 21-MV bremsstrahlung sources respectively. The absorbed dose from the phantom scattered photons has been calculated by a semi-empirical method. These calculations show that the dominating contribution to the absorbed dose outside the primary beam is due to these phantom scattered photons. In the surface region of the phantom, however, electrons from the air and collimators may give rise to a high absorbed dose.

Electrons↗
PubMed1997

Photonic force microscope based on optical tweezers and two-photon excitation for biological applications.

A new scanning probe microscope, the photonic force microscope (PFM), based on optical tweezers and two-photon absorption processes for biological applications is described. Optical tweezers are used to trap a fluorescent latex bead with a diameter of 200 nm in an aqueous solution in all three dimensions. The fluorescent dye is chosen to fulfill the two-photon absorption criterion for the 1064-nm line of a Nd:YVO4 laser. The intensity of the fluorescence emission is utilized as a very sensitive position sensor along the optical axis. Two-dimensional images are formed by laterally scanning the trapped latex bead across biological samples while recording the two-photon-induced fluorescences intensity. A scanning probe image of the outer surface of a small neurite from a cultured rat hippocampal neuron is shown, which is hardly visible under differential interference contrast microscopy. The lateral resolution is given by the bead diameter; the axial resolution is 40 nm. Under the experimental conditions the maximal imaging force applied by the probe is below 5 pN.

Animals↗
PubMed2006

The influence of photon depth of interaction and non-collinear spread of annihilation photons on PET image spatial resolution.

PURPOSE: The quality of PET imaging is impaired by parallax errors. These errors produce misalignment between the projected location of the true origin of the annihilation event and the line of response determined by the coincidence detection system. Parallax errors are due to the varying depths of photon interaction (DOI) within the scintillator and the non-collinear (NC) emission of the annihilation photons. The aim of this work was to address the problems associated with the DOI and the NC spread of annihilation photons and to develop a quantitative model to assess their impact on image spatial resolution losses for various commonly used scintillators and PET geometries. METHODS: A theoretical model based on Monte Carlo simulations was developed to assess the relative influence of DOI and the NC spread of annihilation photons on PET spatial resolution for various scintillator materials (BGO, LSO, LuAP, GSO, NaI) and PET geometries. RESULTS: The results demonstrate good agreement between simulated, experimental and published overall spatial resolution for some commercial systems, with maximum differences around 1 mm in both 2D and 3D mode. The DOI introduces an impairment of non-stationary spatial resolution along the radial direction, which can be very severe at peripheral positions. As an example, the radial spatial resolution loss due to DOI increased from 1.3 mm at the centre to 6.7 mm at 20 cm from the centre of a BGO camera with a 412-mm radius in 2D mode. Including the NC, the corresponding losses were 3.0 mm at the centre and 7.3 mm 20 cm from the centre. CONCLUSION: Without a DOI detection technique, it seems difficult to improve PET spatial resolution and increase sensitivity by reducing the detector ring radius or by extending the detector in the axial direction. Much effort is expended on the design and configuration of smaller detector elements but more effort should be devoted to the DOI complexity.

Artifacts↗
PubMed2000

Critical energies for ssb and dsb induction in plasmid DNA by vacuum-UV photons: an arrangement for irradiating dry or hydrated DNA with monochromatic photons.

PURPOSE: Theoretical modelling techniques are often used to simulate the action of ionizing radiations on cells at the nanometre level. Using monoenergetic vacuum-UV (VUV) radiation to irradiate DNA either dry or humidified, the action spectra for the induction of DNA damage by low energy photons and the role of water and can be studied. These data provide inputs for the theoretical models. METHODS: Various combinations of monochromator, grating and VUV window have been used to obtain monochromatic photons from the 2 GeV electron synchrotron at the CLRC, Daresbury Laboratory. A sample chamber containing plasmid DNA is installed at the end of the beamline. The chamber can be evacuated or water can be introduced (as water vapour or humidified helium). In this way, DNA can be irradiated either dry or humidified. RESULTS: An arrangement for irradiating dry or humidified DNA using monoenergetic photons from 7 eV to 150 eV has been developed. At the energies used, exposure rates vary from about 5 x 10(10) to 3 x 10(12) photons cm(-2) s(-1) over a 1 cm2 sample area. At all but the lowest energies this is sufficient to produce significant levels of DNA damage in just a few minutes. The measured dose variation over the sample area is typically 30%, but this is reduced significantly using sample scanning techniques.

DNA Damage↗
PubMed1996

Photon-absorbed fractions for cylindrical geometry: a TLD photon-absorbed fraction model.

The purpose of this study is to develop a mathematical model and calculate photon-absorbed fractions in a homogeneous nonradioactive cylinder placed inside off-center and outside a cylindrical homogeneous distribution of activity. In the second case, both the radioactive cylinder and the nonradioactive one are placed in a tissue-equivalent nonradioactive medium. The values of the photon-absorbed fractions are investigated for various geometrical configurations using water as the material filling the cylinders and the medium in between and an isotope commonly used in Nuclear Medicine, 99mTc. The calculations for off-center cylinders allows for modeling inhomogeneous distributions of activity within a tumor by placing several "cold" cylinders of various sizes in a radioactive finite cylinder. This three-dimensional model calculates photon-absorbed fractions for inhomogeneous activity distributions that can be used in quantitative nuclear medicine for self-absorption correction, thus introducing a more realistic correction than the one-dimensional corrections. These calculations are also used to model the response of a cylindrical TLD (thermoluminiscent dosimeter) placed inside a homogeneous radioactive cylinder and outside the homogeneous radioactive cylinder, in an absorbing nonradioactive surrounding medium. The purpose of these calculations is to evaluate the photon-absorbed fraction in the TLD as an instrument of measuring the time-integrated activity of a homogeneous radioactive source versus an inhomogeneous one. The dependence of the TLD-absorbed fraction on the position of the TLD with respect to the radioactive cylinder is investigated.

Biophysical Phenomena↗
PubMed1997

Abutting orthogonal electron and photon beams in the head and neck region using asymmetrical photon beam edges.

Irradiation of target volumes within the ethmoid sinus and nasal cavity region has, at our institution, been performed using one anterior electron beam, abutted to two opposing lateral photon beams, in order to exclude the orbit from the irradiated volume. However, this technique give rise to larger dose hetereogeneities within the target volume, as well as undesired hot spots in the chiasma region. To reduce the inhomogenious distribution, we have introduced asymmetrical photon beam edges to broaden the penumbra of the two opposing photon beams resembling the dose gradient of the perpendicular electron beam. The achieved reduction in dose heterogeneity, measured from the differential dose-volume-histogram, is in the order of 30-40% when applying the asymmetrical abutting technique compared with the conventional two opposing photon beam technique. As demonstrated by calculating the tumour control probability (TCP), the increase in dose homogeneity within the target volume, may be of clinical significance.

Dose-Response Relationship, Radiation↗
PubMed1999

Fluorescence spectral properties of the anticancer drug topotecan by steady-state and frequency domain fluorometry with one-photon and multi-photon excitation.

Topotecan is an antitumor agent with activity against a variety of cancers. We examined the steady-state and time-resolved fluorescence spectral properties of topotecan with one- and two-photon excitation. Topotecan was found to display a high two-photon cross section near 20 GM for wavelengths within the fundamental output of a Ti:sapphire laser, 800-880 nm. In frozen solution the anisotropies of topotecan are near the theoretical maxima for one-photon and two-photon excitation with colinear electronic transitions. The intensity and anisotropy decays of topotecan fluorescence were found to be homogeneous (single exponentials) in phosphate-buffered saline and propylene glycol. The steady-state and time-resolved data indicate that topotecan binds to a double-helical DNA oligomer d(AT)10 resulting in increased anisotropies and multiexponential intensity and anisotropy decays. Subnanosecond components in the anisotropy decay of the DNA-topotecan complex suggest loose binding of the drug to DNA. Loose binding of topotecan to DNA is also revealed by accessibility of topotecan to collisional quenching by iodide.

Antineoplastic Agents↗
PubMed2004

Three-dimensional differentiation of photo-autotrophic biofilm constituents by multi-channel laser scanning microscopy (single-photon and two-photon excitation).

A simple microscopic method to three-dimensionally differentiate between various members in photo-autotrophic biofilm systems is described. By dual-channel single-photon (confocal) and two-photon laser scanning microscopy, the signals in the red and far red channels as well as their combination can be simultaneously recorded. The method takes advantage of the autofluorescent signal of cyanobacteria-recorded in the red and far red channel and the autofluorescent signal of the green algae-recorded in the far red channel only. The differentiation is based on the specific pigment composition of cyanobacteria and green algae in combination with the appropriate filter settings for detection of the autofluorescent emission signals. The method allows the non-destructive, three-dimensional examination of fully hydrated interfacial microbial communities at high resolution as well as the clear separation between autofluorescent signals of cyanobacteria and green algae. Furthermore, there is a third option to record additional signals simultaneously such as nucleic acid stained bacteria, bacteria labeled with phylogenetic probes or glycoconjugates stained by using lectins. With state of the art laser scanning microscopes, even a fourth channel is available for recording yet another parameter, e.g. in the reflection (single-photon only) or fluorescence (single- and two-photon) mode. Thus the approach represents a convenient tool to study multiple parameters of complex photo-autotrophic biofilm systems.

Biofilms↗
PubMed2002

Critical appraisal of treatment techniques based on conventional photon beams, intensity modulated photon beams and proton beams for therapy of intact breast.

PURPOSE: To analyse different treatment techniques with conventional photon beams, intensity modulated photon beams, and proton beams for intact breast irradiation for patients in whom conventional irradiation would cause potentially dangerous lung irradiation. MATERIALS AND METHODS: Five breast cancer patients with highly concave breast tissue volume around the lung were considered at planning level in order to assess the suitability of different irradiation techniques. Three-dimensional dose distributions for conventional two-field tangential photon treatment, two-field intensity modulated radiotherapy (IMRT), three-field non-IMRT, three-field IMRT, and single-field proton treatment were investigated, aiming at assessing the possibility to reduce lung irradiation below risk levels. Analysis of dose-volume histograms and related physical and biological parameters (significant minimum, maximum and mean doses, conformity indexes and equivalent uniform dose (EUD)) for planned target volume (PTV) and lung was carried out. Dose plans were compared with the conventional two-field tangential photon technique. RESULTS: PTV coverage was comparable for non-IMRT and IMRT techniques (EUD from 47.1 to 49.4 Gy), and improved with single-field proton treatment (EUD=49.8 Gy). Lung irradiation was reduced, in terms of mean dose, with three-field (9.5 Gy) and proton technique (3.5 Gy), with respect to the conventional two-field treatment (12.9 Gy); also a reduction of the lung volume irradiated at high doses was observed. Better results could be achieved with protons. In addition, cardiac irradiation was also reduced with those techniques. CONCLUSIONS: Geometrically difficult breast cancer patients could be irradiated with a three-field non-IMRT technique thus reducing the dose to the lung which is proposed as standard for this category of patients. Intensity modulated techniques were only marginally more successful than the corresponding non-IMRT treatments, while protons offer excellent results.

Breast↗
PubMed2004

Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores.

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Journal Article↗
PubMed2002

Square spiral photonic crystals: robust architecture for microfabrication of materials with large three-dimensional photonic band gaps.

We provide a blueprint for microfabricating photonic crystals with very large and robust three-dimensional photonic band gaps (PBG's). These crystals are based on interleaving polygonal spiral posts and can be efficiently manufactured on a large scale in a one-step process using the recently introduced technique of glancing angle deposition. We classify various families of spiral photonic crystals based on (i) the parent three-dimensional (3D) point lattice to which they are most closely related, (ii) the crystallographic axis of the parent lattice around which the spiral posts wind, and (iii) the set of points of the parent lattice which the spiral arms connect or nearly connect. We obtain optimal geometries for the spiral photonic crystals by detailed mapping of the size and location of the PBG within a multidimensional parameter space which characterizes the shape of each spiral post. For the optimum PBG, the spiral arms and elbows may deviate significantly from the points of the original point lattice. The largest 3D PBG's are obtained for square spiral posts that wind around the [001] axis of diamond (or face centered cubic) lattice and in which the spiral arm segments approximately connect either the fifth or first nearest-neighbor points of the parent lattice. In the case of silicon posts (with a dielectric constant of 11.9) in an air background, whose arm segments nearly connect fifth nearest-neighbor point of the diamond lattice, the full PBG can be as large as 16% of the gap center frequency. For the corresponding air posts in a silicon background, the maximum PBG is 24% of the gap center frequency. We compute both the total electromagnetic density of states and the electromagnetic field distributions near the PBG. It is suggested the PBG in an optimized structure is highly tolerant to various forms of disorder that may arise during the manufacturing process.

Journal Article↗
PubMed2004

Radiation pattern of a classical dipole in a photonic crystal: photon focusing.

An asymptotic analysis of the radiation pattern of a classical dipole in a photonic crystal possessing an incomplete photonic bandgap is presented. The far-field radiation pattern demonstrates a strong modification with respect to the dipole radiation pattern in vacuum. Radiated power is suppressed in the direction of the spatial stop band and strongly enhanced in the direction of the group velocity, which is stationary with respect to a small variation of the wave vector. An effect of radiated power enhancement is explained in terms of photon focusing. A numerical example is given for a square-lattice two-dimensional photonic crystal. Predictions of asymptotic analysis are substantiated with finite-difference time-domain calculations, revealing a reasonable agreement.

Journal Article↗
PubMed2004

Distinguishing genuine entangled two-photon-polarization sfrom independently generated pairs of entangled photons.

A scheme to distinguish entangled two-photon-polarization (ETP) states from two independent entangled one-photon-polarization (EOP) states is proposed. Using this scheme, the experimental generation of ETP by parametric down-conversion is confirmed through the anticorrelations among three orthogonal two-photon-polarization states. The estimated fraction of ETP among the correlated photon pairs is 37% in the present experimental setup.

Journal Article↗
PubMed2004

Nonlinear photonic crystals as a source of entangled photons.

Nonlinear photonic crystals can be used to provide phase matching for frequency conversion in optically isotropic materials. The phase-matching mechanism proposed here is a combination of form birefringence and phase velocity dispersion in a periodic structure. Since the phase matching relies on the geometry of the photonic crystal, it becomes possible to use highly nonlinear materials. This is illustrated considering a one-dimensional periodic Al0.4Ga0.6As/air structure for the generation of 1.5 microm light. We show that phase-matching conditions used in schemes to create entangled photon pairs can be achieved in photonic crystals.

Journal Article↗