Absorption Linewidth Research Articles

Spectroscopy of a Sample of RV Tauri Stars Without IR Excess

We observed high-resolution optical spectra of 11 RV Tauri stars without IR excess, with the primary goal of searching for chemical depletion patterns. Using equivalent widths of absorption lines, we calculated the photospheric parameters and chemical element abundances for five stars in the sample: HD 172,810, V399 Cyg, AA Ari, V457 Cyg, and V894 Per. Only the abundance pattern of V457 Cyg suggests depletion. In the spectrum of this star, TiO lines are also observed in the emission, in addition to metal emissions. V457 Cyg is likely a binary system that was once surrounded by a circumbinary disc. In the spectrum of V894 Per, we find a set of spectral lines that appear to belong to another star, corroborating that it is an eclipsing variable rather than an RV Tauri star. The high overabundance of sodium may result from mass transfer within the binary system.

Multivariate Predictors of Lyman Continuum Escape. I. A Survival Analysis of the Low-redshift Lyman Continuum Survey* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs GO-15626, GO-13744,

To understand how galaxies reionized the Universe, we must determine how the escape fraction of Lyman continuum (LyC) photons (f esc) depends on galaxy properties. Using the z ∼ 0.3 Low-redshift Lyman Continuum Survey (LzLCS), we develop and analyze new multivariate predictors of f esc. These predictions use the Cox proportional hazards model, a survival analysis technique that incorporates both detections and upper limits. Our best model predicts the LzLCS f esc detections with an rms scatter of 0.31 dex, better than single-variable correlations. According to ranking techniques, the most important predictors of f esc are the equivalent width (EW) of Lyman-series absorption lines and the UV dust attenuation, which track line-of-sight absorption due to H i and dust. The H i absorption EW is uniquely crucial for predicting f esc for the strongest LyC emitters, which show properties similar to weaker LyC emitters and whose high f esc may therefore result from favorable orientation. In the absence of H i information, star formation rate surface density (ΣSFR) and [O iii]/[O ii] ratio are the most predictive variables and highlight the connection between feedback and f esc. We generate a model suitable for z > 6, which uses only the UV slope, ΣSFR, and [O iii]/[O ii]. We find that ΣSFR is more important in predicting f esc at higher stellar masses, whereas [O iii]/[O ii] plays a greater role at lower masses. We also analyze predictions for other parameters, such as the ionizing-to-nonionizing flux ratio and Lyα escape fraction. These multivariate models represent a promising tool for predicting f esc at high redshift.

Toward liquid cell quantum sensing: Ytterbium complexes with ultranarrow absorption.

The energetic disorder induced by fluctuating liquid environments acts in opposition to the precise control required for coherence-based sensing. Overcoming fluctuations requires a protected quantum subspace that only weakly interacts with the local environment. We report a ytterbium complex that exhibited an ultranarrow absorption linewidth in solution at room temperature with a full width at half maximum of 0.625 milli-electron volts. Using spectral hole burning, we measured an even narrower linewidth of 410 pico-electron volts at 77 kelvin. Narrow linewidths allowed low-field magnetic circular dichroism at room temperature, used to sense Earth-scale magnetic fields. These results demonstrated that ligand protection in lanthanide complexes could substantially diminish electronic state fluctuations. We have termed this system an "atomlike molecular sensor" (ALMS) and proposed approaches to improve its performance.

Where is the supervirial hot gas? I: a pilot study with sightlines to Galactic X-ray binaries

ABSTRACT Hot, $\log (T/\mathrm{K})$$\sim$ 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study, we investigate absorption features of S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ in the spectra of three XRBs, namely 4U 1735–44, 4U 1820–30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2$\sigma$ upper limits for S$_{\mathrm{XVI}}$ K$\alpha$ and Si$_{\mathrm{XIV}}$ K$\alpha$ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at $\log (T/\mathrm{K})$$\gt\ 7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended Circumgalactic Medium.

Fluorescence Enhancement of Single V2 Centers in a 4H-SiC Cavity Antenna.

Solid state quantum emitters are a prime candidate in distributed quantum technologies since they inherently provide a spin-photon interface. An ongoing challenge in the field, however, is the low photon extraction due to the high refractive index of typical host materials. This challenge can be overcome using photonic structures. Here, we report the integration of V2 centers in a cavity-based optical antenna. The structure consists of a silver-coated, 135 nm-thin 4H-SiC membrane functioning as a planar cavity with a broadband resonance yielding a theoretical photon collection enhancement factor of ∼34. The planar geometry allows us to identify over 20 single V2 centers at room temperature with a mean (maximum) count rate enhancement factor of 9 (15). Moreover, we observe 10 V2 centers with a mean absorption line width below 80 MHz at cryogenic temperatures. These results demonstrate a photon collection enhancement that is robust to the lateral emitter position.

Narrow absorption lines from intervening material in supernovae

Narrow absorption features in nearby supernova (SN) spectra are a powerful diagnostic of the slow-moving material in the line of sight: they are extensively used to infer dust extinction from the host galaxies, and they can also serve in the detection of circumstellar material originating from the SN progenitor and present in the vicinity of the explosion. Despite their wide use, very few studies have examined the biases of the methods to characterize narrow lines, and not many statistical analyses exist. This is the first paper of a series in which we present a statistical analysis of narrow lines of SN spectra of various resolutions. We developed a robust automated methodology to measure the equivalent width (EW) and velocity of narrow absorption lines from intervening material in the line of sight of SNe, including Na I D, Ca II H&K, K I, and diffuse interstellar bands. We carefully studied systematic biases in heterogeneous spectra from the literature by simulating different signal-to-noise, spectral resolution, size and orientation of the slit, and we present the real capabilities and limitations of using low- and mid-resolution spectra to study these lines. In particular, we find that the measurement of the EW of the narrow lines in low-resolution spectra is highly affected by the evolving broad P-Cygni profiles of the SN ejecta, both for core-collapse and type Ia SNe, inducing a conspicuous apparent evolution. Such pervading non-physical evolution of narrow lines might lead to wrong conclusions on the line-of-sight material, for example concerning circumstellar material ejected from the SN progenitors. We thus present an easy way to detect and exclude those cases to obtain more robust and reliable measurements. Finally, after considering all possible effects, we analysed the temporal evolution of the narrow features in a large sample of nearby SNe to detect any possible variation in their EWs over time. We find no time evolution of the narrow line features in our large sample for all SN types.

Impact of disc magnetisation on MHD disc wind signature

Context. Observations of blue-shifted X-ray absorption lines indicate the presence of wind from the accretion disc in X-ray binaries. Magnetohydrodynamic (MHD) driving is one possible wind-launching mechanism. Recent theoretical developments have made self-similar magnetic accretion-ejection solutions much more generalised, showing that wind can be launched at a much lower magnetisation than the equipartition value, which had previously been the only possibility. Aims. In this work, we model the transmitted spectra through MHD-driven photoionised wind models with different levels of magnetisation. We investigate the possibility of detecting absorption lines by upcoming instruments, such as XRISM and Athena. We investigate the robustness of the method of fitting asymmetric line profiles by multiple Gaussians. Methods. We used the photoionisation code XSTAR to simulate the transmitted model spectra. To cover the extensive range of velocity and density of the wind spanned over a large distance (∼105 gravitational radii), we divided the wind into slabs following a logarithmic radial grid. Fake observed spectra are finally produced by convolving model spectra with instrument responses. Since the line asymmetries are apparent in the convolved spectra as well, this can be used in future XRISM and Athena spectra as an observable diagnostic to fit for. We applied some amount of rigor in assessing the equivalent widths of the major absorption lines, including the Fe XXVI Lyα doublets, which will be clearly distinguishable thanks to the superior quality of future high-resolution spectra. Results. Disc magnetisation stands as another crucial MHD variable that can significantly alter the absorption line profiles. Pure MHD outflow models at low magnetisation are dense enough to be observed by the existing or upcoming instruments. Therefore, these models can serve as simpler alternatives to MHD-thermal models. Fitting with multiple Gaussians is a promising method for handling asymmetric line profiles, as well as the Fe XXVI Lyα doublets.

Enhanced optical nonlinearity of epsilon-near-zero metasurface by quasi-bound state in the continuum

Bound states in the continuum (BICs) provide a powerful way to enhance the nonlinear properties of materials, epsilon-near-zero (ENZ) materials are considered as promising candidates with strong nonlinearities. However, the realization of BIC based on ENZ materials in the near-infrared (NIR) is very challenging due to the large loss in the NIR. Here, a high-quality quasi-BIC based on the ENZ metasurface is proposed for the first time, which is composed of patterned ENZ films embedded in a dielectric-metal hybrid structure, and realizes destructive interference between the Berreman mode and photonic mode to form the Friedrich-Wintergen BIC (FW-BIC). The electric field is strongly confined in the ENZ film, resulting in considerable field enhancement, and the nonlinear refractive index coefficient is 1.63 × 10−12 m2/W, which is three orders of magnitude larger than that of the ITO film. The instantaneous response time is 600 fs and extremely high modulation speed up to the THz level. Benefiting from the perfect absorption and narrow linewidth of quasi-BIC and the change in refractive index of the metasurface induced by Kerr nonlinearity, the absolute modulation is from near-zero to 92% with an extinction ratio of 23.2 dB. It provides a promising platform for the development of integrated ultrafast high-speed photonics.

Influence of Halogen-Solvent Hydrogen Bonding on Gold Nanocluster Photoluminescence.

The influence of gold nanocluster-solvent interactions on nanostructure optical properties was determined. Using [Au11(BINAP)4X2]+, where X = Cl or Br, as a model system, the dramatic influence of halogen-solvent hydrogen bonding on nanocluster optical properties was resolved. The creation of a nanocluster-solvent hydrogen-bond network yielded intense photoluminescence (PL) and an accompanying 2-fold reduction in vibration-mediated nonradiative decay rates. PL was quenched for systems that did not support hydrogen bonding. As reflected by absorption line widths, Raman scattering, and transient absorption spectroscopy measurements, the hydrogen-bond network increased nanocluster structural rigidity and reduced nonradiative carrier decay rates. The results highlight the significant role of the nanocluster-solvent interface in determining the properties of structurally precise materials.

Elucidating ultranarrow 2F7/2 to 2F5/2 absorption in ytterbium(iii) complexes.

Achieving ultranarrow absorption linewidths in the condensed phase enables optical state preparation of specific non-thermal states, a prerequisite for quantum-enabled technologies. The 4f orbitals of lanthanide(iii) complexes are often referred to as "atom-like," reflecting their isolated nature, and are promising substrates for the optical preparation of specific quantum states. To better understand the photophysical properties of 4f states and assess their potential for quantum applications, theoretical building blocks are required for rapid screening. In this study, an atomic-level perturbative calculation (i.e., spin-orbit crystal field, SOCF) is applied to various Yb(iii) complexes to investigate their linear absorption and emission through a fitting mechanism of their experimentally determined transition energies and oscillator strengths. In particular, the optical properties of (thiolfan)YbCl(THF) (thiolfan = 1,1'-bis(2,4-di-tert-butyl-6-thiomethylenephenoxy)ferrocene), a recently reported complex with an ultranarrow optical linewidth, are computed and compared to those of other Yb(iii) compounds. Through a transition energy sampling study, major contributors to the optical linewidth are identified. We observe particularly isolated f-f transitions and narrow linewidths, which we attribute to two distinct factors. Firstly, the ultra-high atomic similarity of the orbitals involved in the optical transition, along with the presence of an anisotropic crystal field, collectively contribute to the observed narrow transitions. Secondly, we note highly correlated excited-ground energy fluctuations that serve to greatly suppress inhomogeneous line-broadening. This article illustrates how SOCF can be used as a low-cost method to probe the influence of crystal field environment on the optical properties of Yb(iii) complexes to assist the development of novel lanthanide series quantum materials.

MANIFESTATION OF STELLAR EVOLUTION IN METAL-DEFICIENT STARS

Metal-poor stars allow us to establish the early history of the Milky Way, furthermore the stars on advanced evolution stage (e.g. giants, AGB stars etc.) give the opportunity to research the peculiarities of stellar evolution at low metallicity. On the base of the 11-m Southern African Large Telescope (SALT) spectra obtained using HRS fibre-fed echelle-spectrograph during 2018–2020, the atmospheric parameters and elemental abundances of four metal-poor stars HE 1523-0901, HD 6268, HD 121135, and HD 195636 ( [Fe/H] ~ -1.5 – -3.0) have been obtained. The iron abundance was determined based on the equivalent widths of absorption lines. The carbon abundance was obtained using the molecular synthesis fitting for the CH (4300-4330 ÅÅ) region, nitrogen from CN at 3883 Å, oxygen from [O] line at 6300 Å and IR triplet at 7770 Å The relationship between the chemical enrichment of stars and their stellar evolution is considered. It may be associated with the processes of mixing inside the stars, and the mechanisms of matter transfer during stellar evolution.

Absorption in narrow and wide gap materials

The absorption spectrum of a material reveals the absorbed light frequencies, characteristic peaks, and the line width of absorption bands. This information is critical for understanding the energy levels involved in the absorption process as well as the material's electronic structure. In this study, an equation connecting the absorption line width with the static dielectric function is derived for narrow and wide gap materials. It is then compared with the Penn model. It has been found that the constant in the Penn model has a value that is restricted to the range of 0.5 to 1. Application of this equation to various narrow and wide gap materials is then discussed.

WAFER: A new method to retrieve sun-induced fluorescence based on spectral wavelet decompositions

Sun-induced fluorescence (SIF) as a close remote sensing based proxy for photosynthesis is accepted as a useful measure to remotely monitor vegetation health and gross primary productivity. It is therefore important to develop methods that allow for its precise and reliable retrieval from radiance measurements with spectral resolutions that have been increasing over the past few years. Retrieval methods are catching up to the increasing complexity of the available datasets making use of their whole information extent (spectral, spatial and temporal) but the comparability of different SIF retrievals and consistency across scales is still limited.In this work we present the new retrieval method WAFER (WAvelet decomposition FluorEscence Retrieval) based on wavelet decompositions of the measured spectra of reflected radiance as well as a reference radiance not containing fluorescence. By comparing absolute absorption line depths by means of the corresponding wavelet coefficients, a relative reflectance is retrieved independently of the fluorescence, i.e. without introducing a coupling between reflectance and fluorescence. The fluorescence can then be derived as the remaining offset. This method can be applied to arbitrary chosen wavelength windows in the whole spectral range, such that all the spectral data available is exploited, including the separation into several frequency (i.e. width of absorption lines) levels and without the need of extensive training datasets.At the same time, the assumptions about the reflectance shape are minimal and no spectral shape assumptions are imposed on the fluorescence, which not only avoids biases arising from wrong or differing fluorescence models across different spatial scales and retrieval methods but also allows for the exploration of this spectral shape for different measurement setups.WAFER is tested on a synthetic dataset as well as several diurnal datasets acquired with a field spectrometer (FloX) over an agricultural site. We compare the WAFER method to two established retrieval methods, namely the improved Fraunhofer line discrimination (iFLD) method and spectral fitting method (SFM) and find a good agreement with the added possibility of exploring the true spectral shape of the offset signal and free choice of the retrieval window. On our synthetic dataset, WAFER seems to outperform the SFM and works best in a spectral window only containing solar Fraunhofer lines where we achieve a relative retrieval error of 10% on average. Applied to the real dataset, the method returns reasonable diurnal cycles for SIF and can, due to the decoupling of reflectance and fluorescence retrieval, reveal interesting trends at times when vegetation canopies may experience a midday depression that remain largely unobserved with current methods.

Size-Dependent Optical Properties of InP Colloidal Quantum Dots.

Indium phosphide colloidal quantum dots (CQDs) are the main alternative for toxic and restricted Cd based CQDs for lighting and display applications. Here we systematically report on the size-dependent optical absorption, ensemble, and single particle photoluminescence (PL) and biexciton lifetimes of core-only InP CQDs. This systematic study is enabled by improvements in the synthesis of InP CQDs to yield a broad size series of monodisperse core-only InP CQDs with narrow absorption and PL line width and significant PL quantum yield.

Radiatively Driven Clumpy X-Ray Absorbers in the NLS1 Galaxy IRAS 13224-3809

Recent radiation-magnetohydrodynamic simulations of active galactic nuclei predict the presence of the disk winds, which may become unstable and turn into fragmented clumps far from the central black hole. These inner winds and the outer clumps may be observed as ultrafast outflows (UFOs) and partial absorbers, respectively. However, it is challenging to observationally constrain their origins because of the complicated spectral features and variations. To resolve such degeneracies of the clumpy absorbers and other components, we developed a novel spectral-ratio model fitting technique that estimates the variable absorbing parameters from the ratios of the partially absorbed spectra to the non-absorbed one, canceling the complex non-variable spectral features. We applied this method to the narrow-line Seyfert 1 galaxy IRAS 13224-3809 observed by XMM-Newton in 2016 for ∼1.5 Ms. As a result, we found that the soft spectral variation is mostly caused by changes in the partial covering fraction of the mildly ionized clumpy absorbers, whose outflow velocities are similar to those of the UFO (∼0.2–0.3c). Furthermore, the velocities of the clumpy absorbers and UFOs increase similarly with the X-ray fluxes, consistent with the change in the UV-dominant continuum flux. We also discovered a striking correlation between the clump covering fraction and the equivalent width of the UFO absorption lines, which indicates that increasing the outflow in the line of sight leads to more prominent UFOs and more partial absorption. These findings strongly suggest that the clumpy absorbers and the UFO share the same origin, driven by the same UV-dominant continuum radiation.

The smallest PbS nanocrystals pervasively show decreased brightness, linked to surface-mediated decay on the average particle.

PbS semiconductor nanocrystals (NCs) have been heavily explored for infrared optoelectronics but can exhibit visible-wavelength quantum-confined optical gaps when sufficiently small (⌀ = 1.8-2.7nm). However, small PbS NCs traditionally exhibited very broad ensemble absorption linewidths, attributed to poor size-heterogeneity. Here, harnessing recent synthetic advances, we report photophysical measurements on PbS ensembles that span this underexplored size range. We observe that the smallest PbS NCs pervasively exhibit lower brightness and anomalously accelerated photoluminescence decays-relative to the idealized photophysical models that successfully describe larger NCs. We find that effects of residual ensemble size-heterogeneity are insufficient to explain our observations, so we explore plausible processes that are intrinsic to individual nanocrystals. Notably, the anomalous decay kinetics unfold, surprisingly, over hundreds-of-nanosecond timescales. These are poorly matched to effects of direct carrier trapping or fine-structure thermalization but are consistent with non-radiative recombination linked to a dynamic surface. Thus, the progressive enhancement of anomalous decay in the smallest particles supports predictions that the surface plays an outsized role in exciton-phonon coupling. We corroborate this claim by showing that the anomalous decay is significantly remedied by the installation of a rigidifying shell. Intriguingly, our measurements show that the anomalous aspect of these kinetics is insensitive to temperature between T = 298 and 77K, offering important experimental constraint on possible mechanisms involving structural fluctuations. Thus, our findings identify and map the anomalous photoluminescence kinetics that become pervasive in the smallest PbS NCs and call for targeted experiments and theory to disentangle their origin.

Ultra-narrowband, electrically switchable, and high-efficiency absorption in monolayer graphene resulting from lattice plasmon resonance

We propose a novel approach to obtain the ultra-narrowband, electrically switchable, and high-efficiency absorption in the monolayer graphene. The monolayer graphene is sandwiched between the silica substrate and a square array of silver nanospheres, which is covered by a very thick silica layer. The fundamental role of the thick silica cover layer is to homogenize the surrounding medium of the silver nanospheres, and thus efficiently exciting the lattice plasmon resonance. The lattice plasmon resonance arising from the coupling between the dipolar plasmon resonance of silver nanospheres and the zero-order diffraction wave at Wood anomaly can enhance the electromagnetic fields at the graphene surface, and thus greatly improve the near-infrared light absorption of the monolayer graphene with the maximum absorption efficiency up to 45%. Owing to the low radiation loss of the lattice plasmon resonance, the absorption linewidth of the monolayer graphene can be largely compressed to only several nanometers (3.2 nm ∼ 7.4 nm). Moreover, the absorption of the monolayer graphene in our proposed nanostructures has a nearly 100% modulation depth to exhibit an excellent switching property. Our work will be helpful for some graphene-based photoelectric devices.

2.85-kW cryogenic Nd:YAG slab laser operating at 946 nm

A several-kilowatt level laser at 946 nm is demonstrated in a cryogenically cooled diode-pumped Nd:YAG slab system. The oscillator realizes the record of the output power of 2.85 kW in a compact cavity with the homemade cryogenically cooled Nd:YAG crystal slab. The pulse repetition rate and pulse width of the laser are 400 Hz and 200 µs, respectively, and the center wavelength under full power is 946.2 nm with a bandwidth of about 0.5 nm. The absorption linewidth at 808 nm is a little narrowed and blue shifts at the cryogenic temperature, while the cooling temperature decreases from 300 to 77 K. The measured thermal conductivity increases from 10 to 70 W/mˑK, and the thermal expansion coefficient decreases from 7.5*10−6 to 1.5*10−6/K. These results could guide us in simulating the temperature gradient and thermal-induced stress distributions in the crystal slab.

Millimeter-Wave Ferromagnetic Resonance of a Saturated Magnetic Transmission Line

Saturated ferrites exhibit microwave and millimeter wave absorption because they consist of nano- magnetic harmonic oscillators. The absorption frequency and line width are dictated by the strength and range of interactions between the magnetic dipoles. The forced orientation of magnetic dipoles along the magnetic bias results in Zeeman splitting in the energy levels. The incoming electromagnetic wave can excite dipoles to transition between these energy levels. In order to investigate the effects of input frequency and Gilbert damping constant on the electromagnetic properties, a magnetic transmission line model is presented for a saturated ferrite. The enhanced power loss during ferromagnetic resonance is contributed to the strong spike of longitudinal magnetic admittance and transverse magnetic impedance.

Interference fringe suppression in tunable diode laser absorption spectroscopy based on BPNN

To reduce the influence of interference fringes in gas detection based on tunable diode laser absorption spectroscopy (TDLAS) and improve measurement accuracy, an interference fringe filtering method based on a back propagation neural network (BPNN) was developed in this study. First, a set of numerical simulation data of interference fringes was generated by periodic sinusoidal functions with different frequencies and phases. The topology, input, and output parameters in the BPNN were determined and optimized by the simulation data. A varying temperature WMS experimental system with a dual optical path was built, by which signals and reference concentrations of CH4 in different concentration ranges were measured. The experimental results verified the excellent performance of the BPNN in suppressing interference fringes with different frequencies and phases. When the ratio of the fringe spectral range to the absorption line width was 1.37, the standard deviation of BPNN was reduced by 4.9 times, and the relative error was reduced by 4.5 times compared with the traditional least squares (LS) method. The obtained results sufficiently demonstrated that the BPNN can effectively reduce interference fringe noise in TDLAS.