We study, apparently for the first time, the threshold conditions for the time-harmonic natural modes of the micro-to-nanosize plasmonic laser shaped as a circular quantum wire with a flat graphene ...strip, placed symmetrically inside it, in the H-polarization case. We suppose that the quantum wire is made of a nonmagnetic gain material, characterized with the aid of the "active" imaginary part of the complex refractive index. The emergence of lasers integrating plasmonic effects marks a significant trend in contemporary photonics. Here, the graphene offers a promising alternative to the noble metals as it exhibits the capacity to sustain plasmon-polariton natural surface waves across the infrared and terahertz (THz) spectra. The used innovative approach is the lasing eigenvalue problem (LEP), which is classical electromagnetic field boundary-value problem, adapted to the presence of active region. It is tailored to deliver both the mode-specific emission frequency, which is purely real at the threshold, and the value of the gain index of the active region, necessary to make the frequency real-valued. The conductivity of graphene is characterized using the quantum Kubo formalism. We reduce the LEP for the considered nanolaser to a hyper-singular integral equation for the current on the strip and discretize it by the Nystrom-type method. This method is meshless and computationally economic. After discretization, a matrix equation is obtained. The sought for mode-specific pairs {the frequency and the threshold gain index} correspond to the zeros of the matrix determinant. It should be noted that the convergence to exact LEP eigenvalues is guaranteed mathematically if the discretization order is taken progressively larger. Two families of modes are identified and studied: the modes of the quantum wire, perturbed by the presence of the graphene strip and the plasmon modes of the strip. The frequencies of all plasmon modes and the lowest mode of the quantum wire are found to be well-tuned by changing the chemical potential of graphene. Engineering analytic formulas for the plasmon-mode frequencies and thresholds are derived. We believe that the presented results can be used in the creation of single-mode tunable micro and nanolasers.
FOCUS (Fast Monte CarlO approach to Coherence of Undulator Sources) is a new GPU‐based simulation code to compute the transverse coherence of undulator radiation from ultra‐relativistic electrons. ...The core structure of the code, which is written in the language C++ accelerated with CUDA, combines an analytical description of the emitted electric fields and massively parallel computations on GPUs. The combination is rigorously justified by a statistical description of synchrotron radiation based on a Fourier optics approach. FOCUS is validated by direct comparison with multi‐electron Synchrotron Radiation Workshop (SRW) simulations, evidencing a reduction in computation times by up to five orders of magnitude on a consumer laptop. FOCUS is then applied to systematically study the transverse coherence in typical third‐ and fourth‐generation facilities, highlighting peculiar features of undulator sources close to the diffraction limit. FOCUS is aimed at fast evaluation of the transverse coherence of undulator radiation as a function of the electron beam parameters, to support and help prepare more advanced and detailed numerical simulations with traditional codes like SRW.
FOCUS (Fast Monte CarlO approach to Coherence of Undulator Sources), a new GPU‐based code to compute the transverse coherence of X‐ray radiation from undulator sources as a function of the electron beam parameters, is described. FOCUS is validated with the Synchrotron Radiation Workshop (SRW) and SPECTRA codes. Examples of application to coherence studies in third‐ and fourth‐generation light sources are shown.
In this paper we report on recent two-dimensional (2D) electron beam size measurements with a nonconventional synchrotron radiation interferometric technique based on x-ray heterodyne near field ...speckles (HNFS). The method relies on Fourier analysis of the random speckle patterns generated by a water suspension of nanospheres to assess the full 2D transverse coherence of the incoming x rays. The horizontal and vertical electron beam sizes are then retrieved by means of statistical optics approaches. The manuscript thoroughly describes the HNFS technique, and shows experimental results obtained at the ALBA Synchrotron Light Source. By changing the machine coupling, beam sizes as small as5μmare measured, thus improving on past measurements reported in the literature and proving the HNFS diagnostics suitable for low-emittance particle beams.
In this paper we extensively describe the heterodyne near field speckle method (HNFS) to characterize both spatial and temporal coherence of synchrotron radiation (SR). The method relies on Fourier ...analysis of near field speckles generated by scattering from nanoparticles suspended in a liquid. A criterion based on master curves of power spectra is introduced and validated by measurements on the visible light produced by the ALBA bending dipole. While spatial coherence measurements with HNFS have been reported, we present for the first time measurements of the temporal coherence of SR wavefronts with the HNFS method both for narrowband and white light beams. In the former case, using a band-pass filter, a coherence time of 40±10fs is measured, in good agreement with the expected value of 43 fs for the filter inverse linewidth. Moreover, by exploiting the self-reference scheme of the technique, we show that coherence areas propagate carrying nonvanishing curvature. In the latter case, the measured coherence time of the incident SR without any monochromator is 1.6±0.4fs , corresponding to a bandwidth of 240 nm at a peak wavelength of 350 nm. Exploiting the Wiener-Kintchine theorem, we also retrieve the SR power spectral density at the sample position from the measured temporal coherence function. Results are in good agreement with the measurements performed using a standard spectrometer, yielding a coherence time of 1.4 fs.
Collimators with embedded beam position monitor (BPM) button electrodes will be installed in the Large Hadron Collider (LHC) during the current long shutdown period. For the subsequent operation, ...BPMs will allow the collimator jaws to be kept centered around the beam orbit. In this manner, a better beam cleaning efficiency and machine protection can be provided at unprecedented higher beam energies and intensities. A collimator alignment algorithm is proposed to center the jaws automatically around the beam. The algorithm is based on successive approximation and takes into account a correction of the nonlinear BPM sensitivity to beam displacement and an asymmetry of the electronic channels processing the BPM electrode signals. A software implementation was tested with a prototype collimator in the Super Proton Synchrotron. This paper presents results of the tests along with some considerations for eventual operation in the LHC.
Beam feasibility study of a collimator with in-jaw beam position monitors Wollmann, Daniel; Nosych, Andriy A.; Valentino, Gianluca ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2014, Letnik:
768
Journal Article
Recenzirano
Odprti dostop
At present, the beam-based alignment of the LHC collimators is performed by touching the beam halo with both jaws of each collimator. This method requires dedicated fills at low intensities that are ...done infrequently and makes this procedure time consuming. This limits the operational flexibility, in particular in the case of changes of optics and orbit configuration in the experimental regions. The performance of the LHC collimation system relies on the machine reproducibility and regular loss maps to validate the settings of the collimator jaws. To overcome these limitations and to allow a continuous monitoring of the beam position at the collimators, a design with jaw-integrated Beam Position Monitors (BPMs) was proposed and successfully tested with a prototype (mock-up) collimator in the CERN SPS. Extensive beam experiments allowed to determine the achievable accuracy of the jaw alignment for single and multi-turn operation. In this paper, the results of these experiments are discussed. The non-linear response of the BPMs is compared to the predictions from electromagnetic simulations. Finally, the measured alignment accuracy is compared to the one achieved with the present collimators in the LHC.
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