Negative refractive index in a Doppler broadened three-level Λ-type atomic medium

We have achieved a negative refractive index with significantly reduced absorption in a three-level Λ-type atomic gas medium under Doppler broadening. It shows that the conditions for obtaining negative refractive index in the presence of Doppler broadening are very different from those of Doppler broadening absent. In particular, in order to obtain negative refractive index in the case of Doppler broadening the coupling laser intensity must be approximately ten times greater than that when the Doppler broadening is ignored. Meanwhile, the frequency band of negative refractive index with Doppler broadening is significantly expanded (about a hundred times) compared to that without Doppler broadening, however, the amplitude of negative refractive index decreases with increasing temperature (or Doppler width). Even in some cases as temperature (Doppler width) increases, the left-handedness of the material can disappear. In addition, we also show that the amplitude and the frequency band of negative refractive index can be changed by adjusting the intensity and the frequency of coupling laser. Our theoretical investigation can be useful for selection of laser parameters under different temperature conditions to achieve negative refractive index in experimental implementation.

Forward Modeling of Simulated Transverse Oscillations in Coronal Loops and the Influence of Background Emission

We simulate transverse oscillations in radiatively cooling coronal loops and forward-model their spectroscopic and imaging signatures, paying attention to the influence of background emission. The transverse oscillations are driven at one footpoint by a periodic velocity driver. A standing kink wave is subsequently formed and the loop cross section is deformed due to the Kelvin–Helmholtz instability, resulting in energy dissipation and heating at small scales. Besides the transverse motions, a long-period longitudinal flow is also generated due to the ponderomotive force induced slow wave. We then transform the simulated straight loop to a semi-torus loop and forward-model their spectrometer and imaging emissions, mimicking observations of Hinode/EIS and SDO/AIA. We find that the oscillation amplitudes of the intensity are different at different slit positions, but are roughly the same in different spectral lines or channels. X-t diagrams of both the Doppler velocity and the Doppler width show periodic signals. We also find that the background emission dramatically decreases the Doppler velocity, making the estimated kinetic energy two orders of magnitude smaller than the real value. Our results show that background subtraction can help recover the real oscillation velocity. These results are helpful for further understanding transverse oscillations in coronal loops and their observational signatures. However, they cast doubt on the spectroscopically estimated energy content of transverse waves using the Doppler velocity.

Effect of separate initial conditions on the lyman-α forest in simulations

ABSTRACT Using a set of high resolution simulations, we quantify the effect of species-specific initial transfer functions on probes of the intergalactic medium (IGM) via the Lyman-α forest. We focus on redshifts 2–6, after H i reionization. We explore the effect of these initial conditions on measures of the thermal state of the low density IGM: the curvature, Doppler width cutoff, and Doppler width distribution. We also examine the matter and flux power spectrum, and potential consequences for constraints on warm dark matter models. We find that the curvature statistic is at most affected at the $\approx 2{{\ \rm per\ cent}}$ level at z = 6. The Doppler width cutoff parameters are affected by $\approx 5{{\ \rm per\ cent}}$ for the intercept, and $\approx 8{{\ \rm per\ cent}}$ for the fit slope, though this is subdominant to sample variation. The Doppler width distribution shows a $\approx 30{{\ \rm per\ cent}}$ effect at z = 3, however the distribution is not fully converged with simulation box size and resolution. The flux power spectrum is at most affected by $\approx 5{{\ \rm per\ cent}}$ at high redshift and small scales. We discuss numerical convergence with simulation parameters.

Radiative-transfer modeling of nebular-phase type II supernovae

Nebular phase spectra of core-collapse supernovae (SNe) provide critical and unique information on the progenitor massive star and its explosion. We present a set of one-dimensional steady-state non-local thermodynamic equilibrium radiative transfer calculations of type II SNe at 300 d after explosion. Guided by the results obtained from a large set of stellar evolution simulations, we craft ejecta models for type II SNe from the explosion of a 12, 15, 20, and 25 M⊙ star. The ejecta density structure and kinetic energy, the 56Ni mass, and the level of chemical mixing are parametrized. Our model spectra are sensitive to the adopted line Doppler width, a phenomenon we associate with the overlap of Fe II and O I lines with Ly α and Ly β. Our spectra show a strong sensitivity to 56Ni mixing since it determines where decay power is absorbed. Even at 300 d after explosion, the H-rich layers reprocess the radiation from the inner metal rich layers. In a given progenitor model, variations in 56Ni mass and distribution impact the ejecta ionization, which can modulate the strength of all lines. Such ionization shifts can quench Ca II line emission. In our set of models, the [O I] λλ 6300, 6364 doublet strength is the most robust signature of progenitor mass. However, we emphasize that convective shell merging in the progenitor massive star interior can pollute the O-rich shell with Ca, which would weaken the O I doublet flux in the resulting nebular SN II spectrum. This process may occur in nature, with a greater occurrence in higher mass progenitors, and this may explain in part the preponderance of progenitor masses below 17 M⊙ that are inferred from nebular spectra.

The Optogalvanic Spectrum of Neutral Lanthanum between 5610 and 6110 Å

We report on a complete optogalvanic spectrum of a discharge burning in a La-Ar gas mixture, in the spectral range 5610–6110 Å (17,851 to 16,364 cm−1). About 1900 overlapping laser scans, each between 1 and 1.5 cm−1 wide, were necessary to cover this range. The resolution of the spectra is limited by the Doppler width of the spectral features to about 0.03 cm−1 (or ca. 0.01 Å) and is comparable with a Fourier-transform spectrum, but the sensitivity is much higher. Indeed, we could find more than 1800 lines, from which about 800 could be classified as transitions between known energy levels. The main focus of the investigations was to discover previously unknown energy levels by means of excitation of unclassified spectral features.

Recent progress of laser spectroscopy experiments on antiprotonic helium

The Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration is currently carrying out laser spectroscopy experiments on antiprotonic helium atoms at CERN’s Antiproton Decelerator facility. Two-photon spectroscopic techniques have been employed to reduce the Doppler width of the measured resonance lines, and determine the atomic transition frequencies to a fractional precision of 2.3–5 parts in 10 9 . More recently, single-photon spectroscopy of buffer-gas cooled has reached a similar precision. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as , which agrees with the known proton-to-electron mass ratio with a precision of 8×10 −10 . The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should enable further improvements in the experimental precision. This article is part of the Theo Murphy meeting issue ‘Antiproton physics in the ELENA era’.

Precision laser spectroscopy experiments on antiprotonic helium

At CERN‘s Antiproton Decelerator (AD) facility, the Atomic Spectroscopyand Collisions Using Slow Antiprotons (ASACUSA) collaboration is carrying out precise laser spectroscopy experiments on antiprotonic helium (p̅He+ ≡ p̅+He2++e−) atoms. By employing buffer-gas cooling techniquesin a cryogenic gas target, samples of atoms were cooled to temperatureT = 1.5–1.7 K, thereby reducing the Doppler width in the single-photon resonance lines. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as Mp̅/me = 1836.1526734(15). This agreed with the known proton-to-electron mass ratio with a precision of 8 . 1010. Further improvements in the experimental precision are currently being attempted. The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should further increase the experimental precision.