Showing papers on "Photon energy published in 2002"

Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics

Abstract: We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...

Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code

Abstract: A recent paper analyzed the sensitivity to various simulation parameters of the Monte Carlo simulations of nine beams from three major manufacturers of commercial medical linear accelerators, ranging in energy from 4-25 MV. In this work the nine models are used: to calculate photon energy spectra and average energy distributions and compare them to those published by Mohan et al. [Med. Phys. 12, 592-597 (1985)]; to separate the spectra into primary and scatter components from the primary collimator, the flattening filter and the adjustable collimators; and to calculate the contaminant-electron fluence spectra and the electron contribution to the depth-dose curves. Notwithstanding the better precision of the calculated spectra, they are similar to those calculated by Mohan et al. The three photon spectra at 6 MV from the machines of three different manufacturers show differences in their shapes as well as in the efficiency of bremsstrahlung production in the corresponding target and filter combinations. The contribution of direct photons to the photon energy fluence in a 10 x 10 field varies between 92% and 97%, where the primary collimator contributes between 0.6% and 3.4% and the flattening filter contributes between 0.6% and 4.5% to the head-scatter energy fluence. The fluence of the contaminant electrons at 100 cm varies between 5 x 10(-9) and 2.4 x 10(-7) cm(-2) per incident electron on target, and the corresponding spectrum for each beam is relatively invariant inside a 10 x 10 cm2 field. On the surface the dose from electron contamination varies between 5.7% and 11% of maximum dose and, at the depth of maximum dose, between 0.16% and 2.5% of maximum dose. The photon component of the percentage depth-dose at 10 cm depth is compared with the general formula provided by AAPM's task group 51 and confirms the claimed accuracy of 2%.

Subbandgap laser-induced single event effects: carrier generation via two-photon absorption

Abstract: Carrier generation based on subbandgap two-photon absorption is demonstrated and shown to be a viable alternative to the conventional single-photon excitation approach in laser-induced single event effects. The two-photon approach exhibits characteristics distinct from those of single-photon excitation, and may be advantageous for a range of single-event effect investigations. The charge track produced by two-photon absorption more closely resembles that of heavy-ion irradiation and, because the photon energy is subbandgap, backside injection through bulk silicon wafers is straightforward and three-dimensional mapping is possible.

Atmospheres and Spectra of Strongly Magnetized Neutron Stars II: Effect of Vacuum Polarization

Abstract: We study the effect of vacuum polarization on the atmosphere structure and radiation spectra of neutron stars with surface magnetic fields B=10^14-10^15 G, as appropriate for magnetars. Vacuum polarization modifies the dielectric property of the medium and gives rise to a resonance feature in the opacity; this feature is narrow and occurs at a photon energy that depends on the plasma density. Vacuum polarization can also induce resonant conversion of photon modes via a mechanism analogous to the MSW mechanism for neutrino oscillation. We construct atmosphere models in radiative equilibrium with an effective temperature of a few \times 10^6 K by solving the full radiative transfer equations for both polarization modes in a fully ionized hydrogen plasma. We discuss the subtleties in treating the vacuum polarization effects in the atmosphere models and present approximate solutions to the radiative transfer problem which bracket the true answer. We show from both analytic considerations and numerical calculations that vacuum polarization produces a broad depression in the X-ray flux at high energies (a few keV \la E \la a few tens of keV) as compared to models without vacuum polarization; this arises from the density dependence of the vacuum resonance feature and the large density gradient present in the atmosphere. Thus the vacuum polarization effect softens the high energy tail of the thermal spectrum, although the atmospheric emission is still harder than the blackbody spectrum because of the non-grey opacities. We also show that the depression of continuum flux strongly suppresses the equivalent width of the ion cyclotron line and therefore makes the line more difficult to observe.

X-ray emission from betatron motion in a plasma wiggler.

Abstract: The successful utilization of an ion channel in a plasma to wiggle a 28.5 GeV electron beam to obtain broad band X-ray radiation is reported. The ion channel is induced by the electron bunch as it propagates through an under-dense 1.4 meter long lithium plasma. The quadratic density dependence of the spontaneously emitted betatron X-ray radiation and the divergence angle of (1 {approx} 3) x 10{sup -4} radian of the forward emitted X-rays as a consequence of betatron motion in the ion channel are in good agreement with theory. The absolute photon yield and the peak spectral brightness at 1.42 KeV photon energy are estimated.

Generation and characterization of ultrafast white-light continuum in condensed media

Abstract: The generation of white-light continuum by femtosecond laser pulses in transparent condensed media is investigated comprehensively with 262-, 393-, and 785-nm pump wavelengths We find that the ratio of the medium’s bandgap energy to the photon energy of the incident wavelength determines the amount of anti-Stokes broadening, independently of the pump wavelength and the medium’s bandgap It is also shown that, although the amount of anti-Stokes broadening is greater for the longer pump wavelength, the shorter pump wavelength is more advantageous for generating shorter-wavelength continua in the UV region In addition, a self-induced change in polarization of the white-light continuum that is generated is observed in an isotropic material with a cubic crystal structure, such as CaF2 and LiF After the investigation of polarization, the frequency chirp of the continuum is characterized by the Kerr-gate method with 70-fs temporal and 10-nm wavelength resolution

Long Polaron Lifetime in InAs/GaAs Self-Assembled Quantum Dots

Abstract: We have investigated the polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots by pump-probe midinfrared spectroscopy. A long T1 polaron decay time is measured at both low temperature and room temperature, with values around 70 and 37 ps, respectively. The decay time decreases for energies closer to the optical phonon energy. The relaxation is explained by the strong coupling for the electron-phonon interaction and by the finite lifetime of the optical phonons. We show that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.

Mechanisms of photo double ionization of helium by 530 eV photons.

Abstract: We have measured fully differential cross sections for photo double ionization of helium 450 eV above the threshold using the COLTRIMS technique. We have found an extremely asymmetric energy sharing between the two electrons and an angular asymmetry parameter β ≃ 2 for the fast electrons and β ≃ 0 for the slow electrons. The asymmetric energy sharing together with the asymmetry parameter β ≃ 2 for the fast electron indicate that the fast electron absorbs not only most of the photon energy but also its angular momentum. The electron angular distributions show that the very slow electrons (about 2 eV) are emitted isotropically with a slight backward emission to the momentum of the fast electron which is expected from the shake-off mechanism. At the same time, the slow electrons at higher energies are mostly emitted at 90 degree to the fast electron indicating an (e,2e) like collision between the two electrons. All our data are in good agreement to CCC calculations.

Origin of the peak-dip-hump line shape in the superconducting-state (pi,0) photoemission spectra of Bi2Sr2CaCu2O8.

Abstract: From detailed high-resolution measurements of the photon energy dependence of the � �; 0� superconducting-state photoemission spectrum of the bilayer Bi high-temperature superconductors, we show that the famous peak-dip-hump line shape is dominated by a superposition of spectral features originating from different electronic states which reside at different binding energies, but are each describable by essentially identical single-particle spectral functions. The previously identified bilayersplit CuO2 bands are the culprit: with the ’’superconducting’’ peak being due to the antibonding band, while the hump is mainly formed by its bonding bilayer-split counterpart.

Experimental studies of the conduction-band structure of GaInNAs alloys

Abstract: In this paper, we carry out a comprehensive review of the nitrogen-induced modifications of the electronic structure of Ga1−yInyNxAs1−x alloys. We study in detail the behaviour of the conduction-band effective mass as a function of Fermi energy, nitrogen content and pressure. From measurements of the plasma frequency for samples with different electron concentrations we have determined the dispersion relation for the lowest conduction band. We have also studied composition, temperature and pressure dependent optical absorption spectra on free-standing layers of Ga1−yInyNxAs1−x (0 ≤ x ≤ 0.025 and 0 ≤ y ≤ 0.09) lattice-matched to GaAs. Spectroscopic ellipsometry measurements performed in a wide photon energy range from 1.5 to 5.5 eV have been used to determine the energy dependence of the dielectric function as well as the energies of E1, E0' and E2 critical point transitions. Experiments have shown that nitrogen has a large effect on the dispersion relations and on the optical spectra for the conduction-band states close to the Γ point. A much smaller effect has been observed for X and L minima as well as for the valence-band states. We have compared our results with other available experimental data. The results are analysed in terms of the analytical band anti-crossing model as well as the local density approximation calculations and empirical pseudopotential models.

Coherent optical phonon oscillations in bulk GaN excited by far below the band gap photons.

Abstract: We report on generation of coherent optical phonon oscillations in 150 microm thick bulk GaN. With photon energy far below the band gap, the generation mechanisms of coherent phonon modes of A1(LO), high- and low-frequency E2 are revealed to be the impulsive stimulated Raman scattering. We find that one among the two degenerate E2 modes is selectively detected with a proper choice of probe polarization. Dephasing times range from 1.5 to 70 ps for different modes, and phonon-three-photon absorbed carrier interactions are compared between the A1(LO) and the E2 mode.

Dosimetry of therapeutic photon beams using an extended dose range film

Abstract: For intensity modulated radiation therapy (IMRT) dose distribution verification, multidimensional measurements are required to quantify the steep dose-gradient regions. High resolution, two-dimensional dose distributions can be measured using radiographic film. However, the photon energy response of film is known to be a function of depth, field size, and photon beam energy, potentially reducing the accuracy of dose distribution measurements. The dosimetric properties of the recently developed Kodak EDR2 film were investigated and compared to those of Kodak XV film. The dose responses of both film types to 6 MV and 18 MV photon beams were investigated for depths of 5 cm, 10 cm, and 15 cm and field sizes of 4x4 cm2 and 15x15 cm2. This analysis involved the determination of sensitometric curves for XV and EDR2 films, the determination of dose profiles from exposed XV and EDR2 films, and comparison of the film-generated dose profiles to ionization chamber measurements. For the combinations of photon beam energy, depth, and field size investigated here, our results indicate that the sensitometric curves are nearly independent of field size and depth of calibration. For a field size of 4x4 cm2, a single sensitometric curve for either EDR2 and XV film can be used for the determination of relative dose profiles. For the larger field size, the sensitometric curve for EDR2 film is superior to XV film in regions where the dose falls below 20% of the central axis dose, due to the effects that the increased low energy scattered photon contributions have on film response. The limited field size and depth dependence of sensitometric data measured using EDR2 film, along with the inherently wide linear dose-response range of EDR2 film, makes it better suited to the verification of IMRT dose distributions.

Precursor states for charge carrier generation in conjugated polymers probed by ultrafast spectroscopy.

Abstract: Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. ${\mathrm{S}}_{1}$ excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of $\ensuremath{\approx}10%$. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.

Electron Spectra and the RBE of X Rays

Abstract: For an assessment of the possible difference in effectiveness between mammography X rays and conventional X rays, the energy and LET spectra of the released electrons are examined. At photon energies below 20 keV and above 100 keV, the energy of the electrons increases with increasing photon energy, which implies that higher-energy photons produce less densely ionizing radiation and are therefore somewhat less effective per unit dose. However, in the intermediate energy range from 20 keV to 100 keV-the range that is relevant to medical diagnostics-the change from the photoelectric effect to the Compton effect causes a transient decrease of electron energies. The ionization density is therefore similar for 200 kVp X rays and 30 kVp mammography X rays, and the distributions of dose in LET suggest an RBE of 30 kVp mammography X rays compared to 200 kVp X rays of up to 1.3. This is in line with an earlier assessment by Brenner and Amols in terms of microdosimetric data, but it is strongly at variance with a recent claim that X rays for mammography are about four times more effective at small doses than conventional X rays and that they cause a correspondingly greater risk for breast cancer. Since LET need not be the only relevant factor, general response functions are examined here that specify-at low dose-the effect per electron of initial energy E and account, for example, for a particular role of the electron range. It is shown that, with any response per electron track that is a nondecreasing function of its starting energy, the low-dose RBE of the mammography X rays relative to the 200 kVp X rays must be substantially less than 2. The Auger electron that accompanies most photoelectrons, but only a minority of the Compton electrons, may increase the effectiveness of the mammography X rays somewhat, but it cannot explain the reported high values of the RBE.

The Photoionization Cross‐Section of Impurities in Quantum Dots

Abstract: In quantum dot structures, photoionization has been considered as an optical transition from the impurity ground state to the conduction subbands. Using a variational approach, we have calculated the photon energy dependence of the photoionization cross-section for a hydrogenic donor impurity in an infinite barrier GaAs quantum dot as a function of the sizes of the dot and the impurity position. The results we have obtained show that the photoionization cross-section is strongly affected by the quantum size effects and the position of the impurity and its overall shape seems to be a signature of the quantum dot system.

In-depth resolution of the magneto-optical Kerr effect in ferromagnetic multilayers

Abstract: How does one determine the magnetization state and hysteresis loop corresponding to one of the ferromagnetic (FM) layers located at a given depth in a stack of FM/non-FM layers by means of the magneto-optical Kerr effect? For this purpose the representation of the Kerr effect in the complex rotation-ellipticity plane is introduced. A depth sensitivity function controlling the Kerr effect is defined and its dependence on the photon energy and angle of incidence is studied. A general way to determine the in-depth location of the FM layer, from which the Kerr signal originates, is proposed. In the case of a FM bilayer structure, previous proposed solutions are discussed within a unified formalism. For a system with three or more FM layers two approaches are proposed to extract selectively the magneto-optical signals originating at individual FM layers: the parallel Kerr vector and cascade numerical projection methods. These methods are successively checked experimentally on simple multilayer structures. Finally, on the basis of the developed approaches a readout solution for multivalued magneto-optical recording in a four-storage-layer structure is proposed.

Quantum electronic stability and spectroscopy of ultrathin Pb films on Si ( 111 ) 7 × 7

Abstract: The growth of Pb on Si(111)737 has been studied with photoelectron spectroscopy. At low temperature ~110 K!, Pb grows in a quasi layer-by-layer mode that allows for the direct observation of discrete quantum well states. The quantum well states are analyzed in terms of the Bohr-Sommerfeld phase quantization model using a phenomenological phaseshift function and reduced quantum numbers. Fermi-level crossings occur when the film thicknessNd5n(l F/2), where d is the atomic layer spacing and l F the bulk Fermi wavelength (N,n are integers!. The photoemission intensity from the quantum well states shows a strong modulation with photon energy which can be interpreted on the basis of the matrix elements for direct transitions in bulk Pb~111!. The in-plane effective mass of the quantum well states is greatly enhanced in the vicinity of the substrate band edge. The present results provide important elements for understanding the growth morphology of Pb films in recent STM studies.

The level position of a deep intrinsic defect in 4H-SiC studied by photoinduced electron paramagnetic resonance

Abstract: Photoinduced electron paramagnetic resonance studies performed on nominally semi-insulating, high purity 4H-SiC have revealed charge transfer from an intrinsic defect (ID) to both the shallow boron acceptor and nitrogen donor. At 4 K, incident photon energy between 1.0 and 1.7 eV produces an increase in paramagnetic boron of approximately the same magnitude as the decrease in the paramagnetic defect concentration. For T<80 K, both spectra remain unchanged after removing the light. Illumination with energy greater than 1.8 eV at 4 K increases the nitrogen, boron, and ID spectra simultaneously, but after blocking the light all three signals return to the pre-illumination level. A model based on excitation to and from the bandedges places the defect level 1.1±0.2 eV above the valence bandedge.

The X‐ray spectrum of Cyg X‐2

Abstract: ABSTRA C T The spectra of disc accreting neutron stars generally show complex curvature, and individual components from the disc, boundary layer and neutron star surface cannot be uniquely identified. Here we show that much of the confusion over the spectral form derives from inadequate approximations for Comptonization and for the iron line. There is an intrinsic lowenergy cut-off in Comptonized spectra at the seed photon energy. It is very important to model this correctly in neutron star systems as these have expected seed photon temperatures (from either the neutron star surface, inner disc or self-absorbed cyclotron) of <1 keV, clearly within the observed X-ray energy band. There is also reflected continuum emission which must accompany the observed iron line, which distorts the higher energy spectrum. We illustrate these points by a reanalysis of the Ginga spectra of Cyg X-2 at all points along its Z track, and show that the spectrum can be well fitted by models in which the low-energy spectrum is dominated by the disc, while the higher energy spectrum is dominated by Comptonized emission from the boundary layer, together with its reflected spectrum from a relativistically smeared, ionized disc.

Kα-radiation from relativistic laser-produced plasmas

Abstract: We present systematic investigations on the generation of titanium Kα-radiation (E = 4.5 keV) from plasmas produced with ultrashort, high-intensity laser pulses. A maximum Kα-yield appears at a laser intensity of about 3·1017 W/cm2, corresponding to a plasma electron temperature of a few times the Kα photon energy. We observe a second increase in the Kα-emission yield at intensities higher than a few times 1018 W/cm2, due to the increase in the K-shell ionization cross-section for relativistic electrons. The intensity dependence of the Kα-yield is modeled by an analytical expression, only including cross-section, pathlength and electron energy distribution.

Barriers to Quantitative Electron Probe X-Ray Microanalysis for Low Voltage Scanning Electron Microscopy

Abstract: Low voltage x-ray microanalysis, defined as being performed with an incident beam energy ≤5 keV, can achieve spatial resolution, laterally and in depth, of 100 nm or less, depending on the exact selection of beam energy and the composition of the target. The shallow depth of beam penetration, with the consequent short path length for x-ray absorption, and the low overvoltage, the ratio of beam energy to the critical ionization energy, both contribute to minimizing the matrix effects in quantitative x-ray microanalysis when the unknown is compared to pure element standards. The low beam energy restricts the energy of the atomic shells that can be excited, forcing the analyst to choose unfamiliar shells/characteristic peaks. The low photon energy shells are subject to low fluorescence yield, so that the peak-to-continuum background is reduced, severely limiting detectability. The limited resolution of semiconductor energy dispersive spectrometry results in frequent peak interference situations and further exacerbates detection limits. Future improvements to the x-ray spectrometry limitations are possible with x-ray optics-augmented wavelength dispersive spectrometry and microcalorimeter energy dispersive spectrometry.

Photoionization of C2+ ions: time-reversed recombination of C3+ with electrons

Abstract: We have investigated photoionization (PI) of the 1S ground state and 3Po metastable states of C2+ ions in the photon energy range 40.8-56.9 eV at a resolution of 30 meV. Absolute PI cross sections have been measured using a photon-ion merged beam arrangement at the Advanced Light Source. Detailed calculations using the semi-relativistic Breit-Pauli R-matrix approach suggest a fraction of 40% of metastable ions in the primary beam of the experiment. The present results are discussed in the light of previous electron-C3+-ion photorecombination (PR) studies. As an example, the role of the intermediate C2+(2p4d 1P) resonance in both PI and PR is analysed.

Polaron effects on the third-order nonlinear optical susceptibility in a quantum disk

Abstract: Theoretical investigation of polaron effects on the third-harmonic generation due to intersubband transition in a quantum disk are presented. Contributions from the bulk longitudinal optical and the surface optical phonon modes are considered separately. The analytic expression for the third-harmonic generation is derived by using the density matrix method. The numerical calculations are performed on a typical GaAs quantum disk with different incident photon energy and under various disk thicknesses. The results demonstrate that the polaron effects are quite important especially around the peak value of the third-order nonlinear optical susceptibility. The maximum χ3ω(3) under the parameters we choose is 7.8×10−16(m/V)2, which is around two order higher than that in the bulk GaAs.

Microdosimetric Interpretation of the Photon Energy Response of LiF:Mg,Ti Detectors

Abstract: Photon energy response of MTS-N (LiF:Mg,Ti) detectors (TLD Poland) and of MTS-N detectors sensitised with 200 Gy of 60Co gamma rays, followed by UV irradiation (sMTS-N), has been determined using X rays with narrow energy spectra, in the energy range from 20 to 300 keV. The over-response of LiF:Mg,Ti detectors for X rays (relative TL efficiency eta = 1.1) can be explained as an ionisation density effect. Low energy X rays produce short electron tracks, which locally deposit a high radiation dose and, consequently, lead to an enhanced (supralinear) response. This over-response has not been observed in sensitised MTS-N where supralinearity in the response after gamma ray doses above 1 Gy is not seen. Using the dose-response curves measured for MTS-N detectors after 137Cs gamma ray irradiation and local doses calculated using Monte Carlo generated electron tracks, it was possible to predict the relative TL effectiveness for different X ray energies. The calculation procedure can be applied to predict the photon energy response of LiF:Mg,Ti detectors in an arbitrary photon field.

Unexpected surface sensitivity at high energies in angle-resolved photoemission

Abstract: Angle-resolved photoemission data from Al(001) taken with photon energies up to several hundred electron volts show an unexpected surface sensitivity at high energies. The surface state at the center of the surface Brillouin zone can be clearly observed up to photon energies higher than 700 e V. The surface to bulk intensity ratio appears to increase with photon energy, despite the longer inelastic mean-free path of the electrons and the increased vibrational amplitude at the surface. We explain this surprising behavior by considering the effect of phonon excitation in the photoemission event.

Study of Two-Photon Absorption Spectral Property of a Novel Nonlinear Optical Chromophore Using Femtosecond Continuum

Abstract: The degenerate two-photon absorption (TPA) spectrum has been studied for a novel organic chromophore (AF389) that exhibits extremely high TPA cross-section values around the ∼800 nm range in both nanosecond and femtosecond domains. To measure the direct degenerate TPA spectrum of AF389 in the femtosecond regime, we have developed a new technique that is based on the use of a single continuum-generation beam as an intense coherent white-light source for TPA excitation. The different spectral components of the input continuum beam are spatially dispersed and then pass the sample through different pathways so that nondegenerate TPA processes among different spectral components can be avoided. Our results show that the TPA spectrum of AF389 is different from the linear absorption spectrum on the scale of absorbing photon energy. There is a significant blue shift of the absorption peak position for the TPA spectrum with respect to the linear absorption spectrum, indicating that the transition pathways are diff...