140 research outputs found
Thin-disk laser scaling limit due to thermal-lens induced misalignment instability
We present an obstacle in power scaling of thin-disk lasers related with
self-driven growth of misalignment due to thermal lens effects. This
self-driven growth arises from the changes of the optical phase difference at
the disk caused by the excursion of the laser eigen-mode from the optical axis.
We found a criterion based on a simplified model of this phenomenon which can
be applied to design laser resonators insensitive to this effect. Moreover, we
propose several resonator architectures which are not affected by this effect.Comment: 19 pages, 13 figure
Spatial hole burning in thin-disk lasers and twisted-mode operation
Spatial hole burning prevents single-frequency operation of thin-disk lasers
when the thin disk is used as a folding mirror. We present an evaluation of the
saturation effects in the disk for disks acting as end-mirrors and as
folding-mirrors explaining one of the main obstacles towards single-frequency
operation. It is shown that a twisted-mode scheme based on a multi-order
quarter-wave plate combined with a polarizer provides an almost complete
suppression of spatial hole burning and creates an additional wavelength
selectivity that enforces efficient single-frequency operation.Comment: 14 pages, 16 figure
Passive alignment stability and auto-alignment of multipass amplifiers based on Fourier transforms
The stability properties of Fourier-based multipass amplifier to
misalignments (tilts) of its optical components has been investigated. For this
purpose, a method to quantify the sensitivity to tilts based on the amplifier
small signal gain has been elaborated and compared with measurements. To
improve on the tilt stability by more than an order of magnitude a simple
auto-alignment system has been proposed and tested. This study, combined with
other investigations devoted to the stability of the output beam to variations
of aperture and thermal lens effects of the active medium, qualifies the
Fourier-based amplifier for the high-energy and the high-power sector.Comment: 10 pages, 11 figure
Role of copper interstitials in CuInSeâ: First-principles calculations
Formation enthalpies and migration barriers of copper interstitials and Frenkel pairs in CuInSeâ (CIS) are determined by first-principles calculations within density functional theory using the nonlocal screened exchange Heyd-Scuseria-Ernzerhof (HSE06) functional. Interstitials occur on four symmetrically inequivalent sites with formation enthalpies of 0.17â0.38 eV, which are much lower than previously reported values based on local approximations. A direct interstitial and indirect interstitialcy diffusion mechanism with migration barriers as low as 0.22 and 0.34 eV are identified. The results provide evidence that the fast interstitial diffusion of copper is important for understanding metastabilities, Fermi-level pinning at interfaces, electric-field-induced creation of p-n junctions, and widely varying experimentally measured diffusion coefficients in CIS devices
Formation rates of iron-acceptor pairs in crystalline silicon
The characteristic association time constant describing the formation of iron-acceptor pairs in crystalline silicon has been measured for samples of various p-type dopant concentrations and species (B, Ga, and In) near room temperature. The results show that the dopant species has no impact on the pairing kinetics, suggesting that the pairing process is entirely limited by iron diffusion. This conclusion was corroborated by measurement of the activation energy of pair formation, which coincides with the migration enthalpy of interstitial iron in silicon. The results also indicate that the pair-formation process occurs approximately twice as fast as predicted by a commonly used expression.This work has been supported by the Australian Research
Council and the State of Lower Saxony
Observation of oscillatory relaxation in the Sn-terminated surface of epitaxial rock-salt SnSe topological crystalline insulator
Topological crystalline insulators have been recently predicted and observed
in rock-salt structure SnSe thin films. Previous studies have
suggested that the Se-terminated surface of this thin film with hydrogen
passivation, has a reduced surface energy and is thus a preferred
configuration. In this paper, synchrotron-based angle-resolved photoemission
spectroscopy, along with density functional theory calculations, are used to
demonstrate conclusively that a rock-salt SnSe thin film
epitaxially-grown on \ce{Bi2Se3} has a stable Sn-terminated surface. These
observations are supported by low energy electron diffraction (LEED)
intensity-voltage measurements and dynamical LEED calculations, which further
show that the Sn-terminated SnSe thin film has undergone a surface
structural relaxation of the interlayer spacing between the Sn and Se atomic
planes. In sharp contrast to the Se-terminated counterpart, the observed Dirac
surface state in the Sn-terminated SnSe thin film is shown to yield a
high Fermi velocity, m/s, which suggests a potential mechanism
of engineering the Dirac surface state of topological materials by tuning the
surface configuration.Comment: 12 pages, 13 figures, supplementary materials include
A compact 20-pass thin-disk multipass amplifier stable against thermal lensing effects and delivering 330 mJ pulses with
We report on an Yb:YAG thin-disk multipass amplifier delivering 50 ns long
pulses at a central wavelength of 1030 nm with an energy of 330 mJ at a
repetition rate of 100 Hz. The beam quality factor at the maximum energy was
measured to be . The small signal gain is 20, and the gain
at 330 mJ was measured to be 6.9. The 20-pass amplifier is designed as a
concatenation of stable resonator segments in which the beam is alternately
Fourier transformed and relay-imaged back to the disk by a 4f-imaging optical
scheme stage. The Fourier transform propagation makes the output beam robust
against spherical phase front distortions, while the 4f-stage is used to
compensate the thermal lens of the thin-disk and to reduce the footprint of the
amplifier
Improved X-ray detection and particle identification with avalanche photodiodes
Avalanche photodiodes are commonly used as detectors for low energy x-rays.
In this work we report on a fitting technique used to account for different
detector responses resulting from photo absorption in the various APD layers.
The use of this technique results in an improvement of the energy resolution at
8.2 keV by up to a factor of 2, and corrects the timing information by up to 25
ns to account for space dependent electron drift time. In addition, this
waveform analysis is used for particle identification, e.g. to distinguish
between x-rays and MeV electrons in our experiment.Comment: 6 pages, 6 figure
The Lamb shift in muonic hydrogen and the proton radius
By means of pulsed laser spectroscopy applied to muonic hydrogen (ÎŒâ p) we have measured the 2S F = 1 1/2 â 2PF = 2 3/2 transition frequency to be 49881.88(76) GHz. By comparing this measurement with its theoretical prediction based on bound-state QED we have determined a proton radius value of rp = 0.84184 (67) fm. This new value is an order of magnitude preciser than previous results but disagrees by 5 standard deviations from the CODATA and the electronproton scattering values. An overview of the present effort attempting to solve the observed discrepancy is given. Using the measured isotope shift of the 1S-2S transition in regular hydrogen and deuterium also the rms charge radius of the deuteron rd = 2.12809 (31) fm has been determined. Moreover we present here the motivations for the measurements of the ÎŒ 4He + and ÎŒ 3He + 2S-2P splittings. The alpha and triton charge radii are extracted from these measurements with relative accuracies of few 10 â 4. Measurements could help to solve the observed discrepancy, lead to the best test of hydrogen-like energy levels and provide crucial tests for few-nucleon ab-initio theories and potentials
The Lamb shift in muonic hydrogen
The long quest for a measurement of the Lamb shift in muonic hydrogen is over. Last year we measured the 2S1/2F=1â2P3/2F=2 energy splitting (Pohl et al., Nature, 466, 213 (2010)) in ÎŒp with an experimental accuracy of 15 ppm, twice better than our proposed goal. Using current QED calculations of the fine, hyperfine, QED, and finite size contributions, we obtain a root-mean-square proton charge radius of rpâ=â0.841â84â(67) fm. This value is 10 times more precise, but 5 standard deviations smaller, than the 2006 CODATA value of rp. The origin of this discrepancy is not known. Our measurement, together with precise measurements of the 1Sâ2S transition in regular hydrogen and deuterium, gives improved values of the Rydberg constant, Rââ=â10â973â731.568â160â(16) mâ»Âč and the rms charge radius of the deuteron rdâ=â2.128â09â(31) fm
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