Abstract: We report on a high-intensity source of polarization-entangled photon pairs with high momentum definition. Type-II noncollinear phase matching in parametric down conversion produces true entanglement: No part of the wave function must be discarded, in contrast to previous schemes. With two-photon fringe visibilities in excess of 97%, we demonstrated a violation of Bell's inequality by over 100 standard deviations in less than 5 min. The new source allowed ready preparation of all four of the EPR-Bell states.

Abstract: A two-photon optical imaging experiment was performed based on the quantum nature of the signal and idler photon pairs produced in spontaneous parametric down-conversion. An aperture placed in front of a fixed detector is illuminated by the signal beam through a convex lens. A sharp magnified image of the aperture is found in the coincidence counting rate when a mobile detector is scanned in the transverse plane of the idler beam at a specific distance in relation to the lens.

Abstract: Monte Carlo simulations of photon propagation offer a flexible yet rigorous approach toward photon transport in turbid tissues. This method simulates the “random walk” of photons in a medium that contains absorption and scattering. The method is based on a set of rules that govern the movement of a photon in tissue. The two key decisions are (1) the mean free path for a scattering or absorption event, and (2) the scattering angle. Figure 4.1 illustrates a scattering event. At boundaries, a photon is reflected or moves across the boundary. The rules of photon propagation are expressed as probability distributions for the incremental steps of photon movement between sites of photon—tissue interaction, for the angles of deflection in a photon’s trajectory when a scattering event occurs, and for the probability of transmittance or reflectance at boundaries. Monte Carlo light propagation is rigorous yet very descriptive. However, this method is basically statistical in nature and requires a computer to calculate the propagation of a large number of photons. To illustrate how photons propagate inside tissues, a few photon paths are shown in Fig. 4.2.

Abstract: 1 Quantum Wells and Superlattices.- 2 Crystal Symmetry.- 2.1 Symmetry Operations, Groups.- 2.2 Point-Group Classification.- 2.3 Space Groups.- 2.4 Group Representations, Characters.- 2.5 Point-Group Representations.- 2.6 Spinor Representations.- 2.7 Representations of Space Groups.- 2.8Invariance Under Time Inversion.- 2.9 Selection Rules.- 2.10 Determination of Linearly Independent Components of Material Tensors.- 3 Electron Spectrum in Crystals, Quantum Wells and Superlattices.- 3.1 The k-p Method.- 3.2 The Effective-Mass Method Deformation Potential.- 3.3 Method of Invariants.- 3.4 Electron and Hole Spectrum in Diamond-and Zincblende-Type Cubic Crystals.- 3.5 Electron Spectra of Quantum Wells and Superlattices.- 3.6 Hole Spectrum in Quantum Wells and Superlattices for Degenerate Bands.- 3.7 Deformed and Strained Superlattices.- 3.8 Quantum Wells and Superlattices in a Magnetic Field.- 3.9 Spectrum of Quantum Wells and Superlattices in an Electric Field.- 4 Vibrational Spectra of Crystals and Superlattices Electron-Phonon Interaction.- 4.1 Normal Vibrations: Distribution in Irreducible Representations.- 4.2 Vibrational Spectra of Superlattices.- 4.3 Electron-Phonon Interaction.- 5 Localized Electron States and Excitons in Heterostructures.- 5.1 Shallow Impurity Centers.- 5.2 Localized States at Superlattice Defects.- 5.3 Excitons.- 5.4 Exchange Splitting of Exciton Levels.- 6 Interband Optical Transitions.- 6.1 Optical Superlattices.- 6.2 Interband Transitions and Dielectric Susceptibility of a Periodic Heterostructure.- 6.3 Coulomb Interaction Between the Electron and the Hole.- 6.4 Exciton Polaritons in an Optical Superlattice.- 6.5 Light Reflection.- 6.6 Electro-Optical Effects in Interband Transitions.- 6.7 Magneto-Optical Spectra.- 7 ntraband Transitions.- 7.1 Cyclotron Resonance and Effective Electron Mass.- 7.2 Intersubband Absorption.- 7.3 Electron-Spin Resonance.- 7.4 IR Reflection in an Undoped Superlattice.- 8 Light Scattering.- 8.1 Theory of Light Scattering in Semiconductors.- 8.2 Scattering by Intersubband Excitations.- 8.3 Scattering by Acoustical Phonons with a Folded Dispersion Law.- 8.4 Scattering by Optical Phonons in Heterostructures.- 8.5 Acceptor Spin-Flip Raman Scattering.- 9 Polarized Luminescence in Quantum Wells and Superlattices.- 9.1 Luminescence as a Tool to Study Electronic Spectra and Kinetic Processes in Two-Dimensional Systems.- 9.2 Luminescence in the Quantum Hall Regime, Quantum Beats.- 9.3 Optical Spin Orientation and Alignment of Electron Momenta.- 9.4 Optical Orientation and Alignment of Excitons.- 9.5 Polarized Luminescence of Excitons and Impurities in an External Magnetic Field.- 10 Nonlinear Optics.- 10.1 Two-Photon Absorption.- 10.2 Photoreflectance.- 10.3 Diffraction from a Light-Induced Spatial Grating.- 10.4 Third-Harmonic Generation.- 10.5 Linear and Circular Photogalvanic (Photovoltaic) Effects.- 10.6 Current of Optically Oriented Electrons.- 10.7 Photon Drag Current.

Abstract: The scientific fields of confined electrons and photons have become areas of major efforts worldwide. Their appeal originates in the many facets they offer in fundamental and applied science, in technology and device development, and to high technology, large-scale industries.

Abstract: We have observed longitudinal acceleration of free electrons by photon scattering in the low-energy Thomson regime. This is the first observation of the transition between the Thomson and Compton regimes of electron scattering due to higher-order photon interactions.

Abstract: A diffusion model of noninvasive absorption spectroscopy was used to determine how the change in signal resulting from a point absorber depends on the position of that absorber relative to the source and detector. This is equivalent to calculating the relative probability that a photon will visit a certain location in tissue before its detection. Experimental mapping of the point-target response in tissue-simulating materials confirmed the accuracy of the model. For steady-state spectroscopy a simple relation was derived between the mean depth visited by detected photons, the source-detector separation, and the optical penetration depth. It was also demonstrated theoretically that combining a pulsed source with time-gated detection provides additional control over the spatial distribution of the photon-visit probability.

Abstract: Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

Abstract: Analysis of nonlinear Thomson scattering of intense lasers from relativistic electron beams is extended to describe off-axis scattering geometries. Electron trajectories are calculated for the case of a plane electromagnetic wave of arbitrary intensity, either circularly or linearly polarized, interacting with a relativistic electron beam at an arbitrary interaction angle. The trajectories are used to derive analytic expressions for the intensity distribution of the scattered radiation. These expressions are valid in the nonlinear regime (arbitrary laser intensity) and include the generation of harmonics. The effect of interaction angle on the intensity distribution is discussed and spectra are plotted numerically for the specific cases of head-on and transverse scattering. The dependence of x-ray frequency, pulse duration, and photon flux on interaction geometry are also examined. Applications to the laser synchrotron source are discussed. There are potential advantages of both head-on and transverse interaction geometries: head-on scattering results in the generation of higher frequencies and higher photon fluxes; normal incidence scattering can result in ultrashort x-ray pulses.

Abstract: Based on microscopic transition probabilities and on the principle of detailed balance, absorption and emission rates are calculated for indirect transitions. It is found that the emission rate at a photon energy ħω can be expressed by the absorption coefficient for the same photon energy in the same way as for direct transitions. This relation is quite generally valid including cases where the electrons in the exited state (conduction band) and the electrons in the ground state (valence band) are distributed according to two different quasi-Fermi distributions. A generalized Planck's law is formulated for luminescence which contains a nonzero chemical potential of the emitted photons as the only difference to the description of thermal radiation.

Abstract: Gravitons in a squeezed vacuum state, the natural result of quantum creation in the early Universe or by black holes, will introduce metric fluctuations. These metric fluctuations will introduce fluctuations of the light cone. It is shown that when the various two-point functions of a quantized field are averaged over the metric fluctuations, the light cone singularity disappears for distinct points. The metric-averaged functions remain singular in the limit of coincident points. The metric-averaged retarded Green's function for a massless field becomes a Gaussian which is nonzero both inside and outside of the classical light cone. This implies some photons propagate faster than the classical light speed, whereas others propagate slower. The possible effects of metric fluctuations upon one-loop quantum processes are discussed and illustrated by the calculation of the one-loop electron self-energy.

Abstract: A low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) is used to excite photon emission from Au(110) surfaces. In the detected photon intensity the (1 \ifmmode\times\else\texttimes\fi{} 2) reconstruction of the Au surface is clearly resolved. This first observation of atomic resolution in STM-induced photon emission is interpreted in terms of local variations of the electromagnetic interaction of tip and sample occurring at constant tunneling current. Similar effects are expected to affect other scanning probe methods, in particular those involving photons.

Abstract: In two-photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the energetic position and the lifetime of the intermediate state. Image states are bound states of electrons in front of metal surfaces in the potential well, built by the image potential and the surface. They have been studied extensively by two-photon photoemission. Experimental results on the binding energies of image states on (111) and (100) surfaces of Ag, Au, Cu, and Pd agree with calculations by a one-dimensional scattering model. The results for Fe, Co, and Ni are not yet understood. Lifetimes of image states between 4 and 180 fs have been measured. Particular emphasis is placed on image states on metal overlayers. Information on film growth and surface morphology has been obtained. Lateral localization and coupling to quantum well states has been observed. Dielectric overlayers have also been investigated.

Abstract: A dedicated beamline with a broad energy range of photons and with high flux and a high degree of circular polarization has been designed for the ELETTRA storage ring. The beamline exploits the circularly polarized radiation produced by an electromagnetic elliptical wiggler and cover the (5–1200) eV photon energy range using a double‐incidence spherical grating monochromator.

Abstract: The subject contrast of bony anatomy in megavoltage medicalradiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x‐ray detection process, is x‐ray energy absorption noise. X‐ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x‐ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these ‘‘absorbed energy distributions (AEDs)’’, the x‐ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1–4 mm) and x‐ray energies (0.1–10 MeV). A number of conclusions can be drawn from these investigations. (i) The x‐ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1–10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%–5% of the incident photons are attenuated. (iii) The metal ‘‘conversion’’ plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x‐ray absorption noise.

Abstract: In this work we present a theoretical calculation of the photon emission spectrum via direct and phonon-assisted intra-conduction-band transitions, assessing the impact of the approximations usually made in theoretical analysis of the phenomenon. We have compared our results to a previous work, finding a marked disagreement. Our phonon-assisted emission spectrum has a much lower effective temperature, so that it overwhelms the contribution of direct processes over the range of photon energies spanning from infrared to near-UV. We discuss qualitative arguments supporting our results presenting also a simplified model suitable for a much more efficient implementation of the spectrum calculation.

Abstract: Intrinsic infrared (IR) detectors in the long wavelength range (8-20 Am) are based on an optically excited interband transition, which promotes an electron across the band gap (E(sub g)) from the valence band to the conduction band as shown. These photoelectrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (h(sub upsilon)) must be higher than the E(sub g) of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by controlling the E(sub g) of the photosensitive material. Examples for such materials are Hg(1-x), Cd(x), Te, and Pb(1-x), Sn(x), Te, in which the energy gap can be controlled by varying x. This means detection of very-long-wavelength IR radiation up to 20 microns requires small band gaps down to 62 meV. It is well known that these low band gap materials, characterized by weak bonding and low melting points, are more difficult to grow and process than large-band gap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersub-band transitions in multiquantum well (MQW) structures made of more refractory large-band gap semiconductors. The idea of using MQW structures to detect IR radiation can be explained by using the basic principles of quantum mechanics. The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent Schroudiner equation.

Abstract: A technique for tomographic imaging of electron density using the energy spectrum of Compton scattered gamma-rays is described. The energy-angle relationship for Compton scattering is utilized to determine the direction of scattered photons. A single-scattering "forward mapping" model is constructed to relate electron density to the detector response. The nonlinearity of the model, caused by accounting for pre- and post-scattering radiation attenuation, is dealt with by solving the "inverse mapping" iteratively. In order to assure convergence of the image reconstruction problem to a nonnegative and bounded solution, a variety of "regularization" algorithms are examined. The capabilities and limitations of these algorithms are demonstrated by reconstructing a number of images from Monte Carlo simulated measurements. Part II of this paper presents the experimental aspects of the technique.

Abstract: A mathematical model for photon behavior within a spherical integrating-cavity absorption meter (ICAM) that does not depend on the assumption of a homogeneous energy density within the cavity has been developed. Explicit expressions for the proportion of emitted or reflected photons that survive a single transit across the cavity, the average number of collisions with the wall per photon, and the average path length per photon, are derived for an absorbing nonscattering medium. Monte Carlo modeling shows that operation of the ICAM is essentially unaffected by scattering, in agreement with the experimental observations of Fry et al. [Appl. Opt. 31, 2055 (1992)]. Calculations for the performance of the absorption meter as a function of the cavity diameter, the absorption coefficient of the medium, and the reflectivity of the cavity are presented.

Abstract: We develop a theory of photon‐echo phenomena in harmonic vibrational modes. Although classical harmonic oscillators cannot produce any nonlinear optical signal in the linear response limit, we demonstrate that quantum harmonic oscillators that are coupled to any physically reasonable bath can give rise to novel nonlinear optical behavior, even in the perturbative limit. We show that photon echoes in high‐frequency vibrational modes are strongly affected by both population relaxation and pure dephasing. The time dependence of the echo signal is shown to be highly sensitive to the amount of inhomogeneous broadening in the vibrational line. As an example, we develop the simple model of population relaxation resulting from linear coupling to the bath and pure dephasing resulting from quadratic coupling to the bath. Counter to the classical picture, echo signal is present when the only coupling to the bath is linear, but absent when the only coupling is quadratic.

Abstract: We consider the spectra of Thomson-thick, geometrically-thin accretion discs around Galactic black hole candidates in the reflection model and compute their iron K edges and iron K$\alpha$ lines. We compare the smeared iron K edge profiles that we compute with observation and find them to be a satisfactory description of the data. We find that a combination of Doppler broadening and resonant Auger destruction of line photons can make iron K$\alpha$ lines very difficult to detect in highly ionized inclined discs. We detail the physics of resonant Auger destruction at the level it is currently understood and point out its implications.

Abstract: In this paper a method of modeling the distribution of scattered events in emission projection data is developed and applied. This method is based on the use of a transmission map to define the inhomogeneous scattering object. The key point is the use of the set of line integrals calculated as part of the attenuation correction technique, as the basis of a model of the distribution of scattered events. The probability of a photon being scattered through a given angle and being detected in the emission energy window is approximated using a Gaussian function. The parameters of this Gaussian are determined using Monte Carlo generated parallel‐beam scatter line spread functions from a nonuniformly attenuating phantom. The model is incorporated into a two‐dimensional projector–backprojector and used with the Expectation‐Maximization‐Maximum‐Likelihood algorithm for the reconstruction of fan‐beam phantom data. The correction is shown to perform well for a phantom that varies slowly in the axial direction. For the more clinically realistic situation of a torso phantom, the method produces improvements in terms of blood pool to myocardium contrast, but does not restore the contrast to the level exhibited in a reconstruction from ‘‘scatter free’’ data.

Abstract: Some considerations about the importance of coherence effects for bremsstrahlung processes in non--equilibrium dense matter (Landau - Pomeranchuk - Migdal - effect) are presented. They are of particular relevance for the application to photon - and di-lepton production from high energy nuclear collisions, to gluon radiation in QCD transport, or parton kinetics and to neutrino and axion radiation from supernova explosion and from hot neutron stars. The soft behavior of the bremsstrahlung from a source described by classical transport models is discussed and pocket correction formulas for the in-matter radiation cross sections are suggested in terms of standard transport coefficients. The radiation rates are also discussed within a non--equilibrium quantum field theory (Schwinger - Kadanoff - Baym - Keldysh) formulation. A classification of diagrams and corresponding resummation in physically meaningful terms is proposed, which considers the finite damping width of all source particles in matter. This way each diagram in this expansion is already free from the infra--red divergences. Both, the correct quasi--particle and quasi--classical limits are recovered from this subset of graphs. Explicit results are given for dense matter in thermal equilibrium. The diagrammatic description may suggest a formulation of a transport theory that includes the propagation of off--shell particles in non--equilibrium dense matter.

Abstract: A covariant expression for the instantaneous radiation damping force acting on an accelerated charged particle is derived within the frame of classical electrodynamics. The radiation pressure of the wave emitted by the charge is averaged on a sphere of radius $R$ to obtain the net force due to the photon momentum recoil, and the limit is taken when $R$ tends to zero, assuming no internal structure of the particle. The relativistic Doppler effects break the symmetry of the instantaneous rest frame dipole radiation pattern, and the Abraham-Becker damping force is obtained.

Abstract: We demonstrate use of a charge-coupled device (CCD) with sub-electron readnoise performance as a non-dispersive X-ray spectrometer. The exceptionally low readnoise (0.9 electrons rms) was achieved by applying a floating gate output amplifier with 16 readouts per pixel. The soft X-ray quantum efficiency was enhanced over other front-side illuminated devices by using a novel thin-poly gate structure. The combination of sub-electron noise and good soft X-ray quantum efficiency have enabled us to detect photons in the EUV energy range (E

Abstract: Multidimensional off resonant spectroscopy of crystals using a train of optical pulses can be effectively used to probe nuclear dynamics in solids. We predict a clear photon echo signal even in the absence of inhomogeneous broadening. This technique may be used for studying phonon dynamics in solids, in structurally frozen systems such as glasses, and in systems where the validity of the concept of phonons is not equally well established, such as liquids and gases. With this technique it is also possible to obtain information on the lifetime of phonons. We predict a long time tail of the nonlinear signal proportional to t−n/5, where n is the order of the response function studied.

Abstract: A simple, physically consistent model has been proposed that seeks to explain in a unified way the X-ray spectra and rapid X-ray variability of the so-called Z sources and other accreting neutron stars in low-mass systems. Here we summarize the results of detailed numerical calculations of the X-ray spectra of the Z sources predicted by this model. Our computations show that in the Z sources, photons are produced primarily by electron cyclotron emission in the neutron star magnetosphere. Comptonization of these photons by the hot central corona and radial inflow produces X-ray spectra, color-color tracks, and countrate variations like those observed in the Z sources.

Abstract: Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation effects near the interfaces of different media, where charged particle equilibrium is not established, remain unsolved. Furthermore, changes in atomic number, which affect the multiple Coulomb scattering of the secondary electrons, are not accounted for by current photon dose calculation algorithms. As local interface effects are mainly due to the perturbation of secondary electrons, a photon–electron cascade model is proposed which incorporates explicit electron transport in the calculation of the primary photon dose component in heterogeneous media. The primary photonbeam is treated as the source of many electron pencil beams. The latter are transported using the Fermi–Eyges theory. The scatteredphoton dose contribution is calculated with the dose spread array [T. R. Mackie, J. W. Scrimger, and J. J. Battista, Med. Phys. 12, 188–196 (1985)] approach. Comparisons of the calculation with Monte Carlo simulation and TLD measurements show good agreement for positions near the polystyrene–aluminum interfaces.