7,852 research outputs found
Photocathode Quantum Efficiency of Ultra-Thin Cs2Te Layers On Nb Substrates
The quantum efficiencies (QE) of photocathodes consisting of bulk Nb
substrates coated with thin films of Cs2Te are reported. Using the standard
recipe for Cs2Te deposition developed for Mo substrates (220 {\AA} Te
thickness), a QE ~11% - 13% at light wavelength of 248 nm is achieved for the
Nb substrates, consistent with that found on Mo. Systematic reduction of the Te
thickness for both Mo and Nb substrates reveals a surprisingly high residual QE
~ 6% for a Te layer as thin as 15 {\AA}. A phenomenological model based on the
Spicer 3-Step model along with a solution of the Fresnel equations for
reflectance, R, leads to a reasonable fit of the thickness dependence of QE and
suggests that layers thinner than 15 {\AA} may still have a relatively high QE.
Preliminary investigation suggests an increased operational lifetime as well.
Such an ultra-thin, semiconducting Cs2Te layer may be expected to produce
minimal ohmic losses for RF frequencies ~ 1 GHz. The result thus opens the door
to the potential development of a Nb (or Nb3Sn) superconducting photocathode
with relatively high QE and minimal RF impedance to be used in a
superconducting radiofrequency (SRF) photoinjector.Comment: 12 pages, 3 figure
Femtosecond-laser-irradiation-induced structural organization and crystallinity of Bi2WO6
Controlling the structural organization and crystallinity of functional oxides is key to enhancing
their performance in technological applications. In this work, we report a strong enhancement of
the structural organization and crystallinity of Bi2WO6 samples synthetized by a microwave-assisted
hydrothermal method after exposing them to femtosecond laser irradiation. X-ray difraction, UVvis and Raman spectroscopies, photoluminescence emissions, energy dispersive spectroscopy, feld
emission scanning electron microscopy, and transmission electron microscopy were employed to
characterize the as-synthetized samples. To complement and rationalize the experimental results, frstprinciples calculations were employed to study the efects of femtosecond laser irradiation. Structural
and electronic efects induced by femtosecond laser irradiation enhance the long-range crystallinity
while decreasing the free carrier density, as it takes place in the amorphous and liquid states. These
efects can be considered a clear cut case of surface-enhanced Raman scattering
Optical properties of graphene antidot lattices
Undoped graphene is semi-metallic and thus not suitable for many electronic
and optoelectronic applications requiring gapped semiconductor materials.
However, a periodic array of holes (antidot lattice) renders graphene
semiconducting with a controllable band gap. Using atomistic modelling, we
demonstrate that this artificial nanomaterial is a dipole-allowed direct gap
semiconductor with a very pronounced optical absorption edge. Hence, optical
infrared spectroscopy should be an ideal probe of the electronic structure. To
address realistic experimental situations, we include effects due to disorder
and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.
Electron energy loss and induced photon emission in photonic crystals
The interaction of a fast electron with a photonic crystal is investigated by
solving the Maxwell equations exactly for the external field provided by the
electron in the presence of the crystal. The energy loss is obtained from the
retarding force exerted on the electron by the induced electric field. The
features of the energy loss spectra are shown to be related to the photonic
band structure of the crystal. Two different regimes are discussed: for small
lattice constants relative to the wavelength of the associated electron
excitations , an effective medium theory can be used to describe the
material; however, for the photonic band structure plays an
important role. Special attention is paid to the frequency gap regions in the
latter case.Comment: 12 pages, 7 figure
Neural reflectance transformation imaging
Reflectance transformation imaging (RTI) is a computational photography technique widely used in the cultural heritage and material science domains to characterize relieved surfaces. It basically consists of capturing multiple images from a fixed viewpoint with varying lights. Handling the potentially huge amount of information stored in an RTI acquisition that consists typically of 50\u2013100RGB values per pixel, allowing data exchange, interactive visualization, and material analysis, is not easy. The solution used in practical applications consists of creating \u201crelightable images\u201d by approximating the pixel information with a function of the light direction, encoded with a small number of parameters. This encoding allows the estimation of images relighted from novel, arbitrary lights, with a quality that, however, is not always satisfactory. In this paper, we present NeuralRTI, a framework for pixel-based encoding and relighting of RTI data. Using a simple autoencoder architecture, we show that it is possible to obtain a highly compressed representation that better preserves the original information and provides increased quality of virtual images relighted from novel directions, especially in the case of challenging glossy materials. We also address the problem of validating the relight quality on different surfaces, proposing a specific benchmark, SynthRTI, including image collections synthetically created with physical-based rendering and featuring objects with different materials and geometric complexity. On this dataset and as well on a collection of real acquisitions performed on heterogeneous surfaces, we demonstrate the advantages of the proposed relightable image encoding
- âŠ