Continuum Radiation Research Articles

Bridging High-density Electron Beam Coronal Transport and Deep Chromospheric Heating in Stellar Flares

The optical and near-ultraviolet (NUV) continuum radiation in M-dwarf flares is thought to be the impulsive response of the lower stellar atmosphere to magnetic energy release and electron acceleration at coronal altitudes. This radiation is sometimes interpreted as evidence of a thermal photospheric spectrum with T ≈ 104 K. However, calculations show that standard solar flare coronal electron beams lose their energy in a thick target of gas in the upper and middle chromosphere (log10 column mass/[g cm−2] ≲ −3). At larger beam injection fluxes, electric fields and instabilities are expected to further inhibit propagation to low altitudes. We show that recent numerical solutions of the time-dependent equations governing the power-law electrons and background coronal plasma (Langmuir and ion-acoustic) waves from Kontar et al. produce order-of-magnitude larger heating rates than those that occur in the deep chromosphere through standard solar flare electron beam power-law distributions. We demonstrate that the redistribution of beam energy above E ≳ 100 keV in this theory results in a local heating maximum that is similar to a radiative-hydrodynamic model with a large, low-energy cutoff and a hard power-law index. We use this semiempirical forward-modeling approach to produce opaque NUV and optical continua at gas temperatures T ≳ 12,000 K over the deep chromosphere with log10 column mass/[g cm−2] of −1.2 to −2.3. These models explain the color temperatures and Balmer jump strengths in high-cadence M-dwarf flare observations, and they clarify the relation among atmospheric, radiation, and optical color temperatures in stellar flares.

Multispectral tunable symmetry-protected bound states in the continuum in all-dielectric split-ring resonator metasurfaces

Symmetry incompatible bound states in the continuum (BICs) are the non-radiating states that remain decoupled from the radiation continuum due to the symmetry mismatch. However, it has always been challenging to observe multiple symmetry-protected BICs in single resonator dielectric metasurfaces. This work reports multiple symmetry-protected BICs for both linear polarizations in all-dielectric metasurfaces made up of split-ring resonators (SRRs). Intensive numerical simulations predict that SRR can support a high-quality factor (Q) of 106 for standalone and with substrate in presence of asymmetry. However, for a realistic and practical scenario, a Q-factor of 8000 for standalone structures is achieved, which reduces by a factor of 1.25 in the presence of silica substrate. Even if the out-of-plane asymmetry is introduced either in the geometry or variation in the angle of incidence, BICs are transformed into quasi-BICs possessing Fano resonance. Eigenmode and multipole decomposition analysis confirm multiple BIC resonances; notable dominant contributions are from magnetic dipole, toroidal dipole, and electric and magnetic quadrupoles. Integrating monolayer graphene adds tunability to the metasurface, where distinct quasi-BIC modes can be employed to facilitate an excellent switching.

Massive-Star Feedback at Low Metallicity

AbstractEarly cosmic epochs are characterized by low metallicity and high specific star-formation rates. These conditions are dominated by massive-star feedback that may be dramatically different than the traditional model dominated by hot, thermal superwinds driven by supernova explosions. Instead, metal-poor feedback from massive stars may be radiation-dominated, with weak mechanical feedback, possibly aiding the escape of Lyα and Lyman continuum radiation. I will describe our understanding that is emerging from observations of starburst galaxies in the local universe.

Acoustic embedded eigenstates in metasurface-based structures

Energy confinement plays an important role in improving wave–matter interactions, enabling applications such as sensing, lasing, and filtering. One convenient solution for achieving large energy-confinement is based on embedded eigenstates, i.e., non-radiating eigenmodes supported by open structures. While the analysis of these modes is quite consolidated in electromagnetics and optics, their relevance in acoustics has been less explored, despite their wide application potential. In this work, we explore acoustic embedded scattering eigenstates within the radiation continuum enabled by resonant metasurface pairs. At resonance, each metasurface strongly reflects the input wave, but as their surface resonances interfere with each other through coupling with a longitudinal resonance, an embedded eigenstate emerges. Through both theoretical and numerical analysis, we show the conditions for such an exotic resonant state to emerge and its implication for acoustic systems.

An extreme blue nugget, UV-bright starburst at z = 3.613 with 90 per cent of Lyman continuum photon escape

ABSTRACT We present the discovery and analysis of J1316+2614 at z = 3.6130, a UV-bright star-forming galaxy (MUV ≃ −24.7) with large escape of Lyman continuum (LyC) radiation. J1316+2614 is a young (≃10 Myr) star-forming galaxy with SFR ≃ 500 M⊙ yr−1 and a starburst mass of log(M⋆/M⊙) ≃ 9.7. It shows a very steep UV continuum, βUV = −2.59 ± 0.05, consistent with residual dust obscuration, E(B − V) ≃ 0. LyC emission is detected with high significance (≃17σ) down to 830 Å, for which a very high relative (absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ (≃0.87) is inferred. The contribution of a foreground or active galactic nucleus contamination to the LyC signal is discussed, but is unlikely. J1316+2614 is the most powerful ionizing source known among the star-forming galaxy population, both in terms of production (QH ≈ 1056 s−1) and escape of ionizing photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly α, H β, and other rest-frame optical lines are detected, but these are weak ($EW_{0} \rm [H\beta ] \simeq 35$ Å), with their strengths reduced roughly by $\simeq 90{{\ \rm per\ cent}}$. J1316+2614 is the first case known where the effect of large escape of ionizing photons on the strength of nebular lines and continuum emission is clearly observed. Gas inflows are detected in J1316+2614 from the blue-dominated peak Ly α emission (with a blue-to-red peak line ratio Iblue/Ired ≃ 3.7) and redshifted interstellar medium absorption (≃100 km s−1). Our results suggest that J1316+2614 is undergoing a gas compaction event, possibly representing a short-lived phase in the evolution of massive and compact galaxies, where strong gas inflows have triggered an extreme star formation episode and nearly 100 per cent LyC photons are escaping.

Stellar feedback in M 83 as observed with MUSE

Context. Energy and momentum injected by young, massive stars into the surrounding gas play an important role in regulating further star formation and in determining the galaxy’s global properties. Before supernovae begin to explode, stellar feedback consists of two main processes: radiation pressure and photoionisation. Aims. We study pre-supernova feedback and constrain the leakage of Lyman continuum (LyC) radiation in a sample of ∼4700 H II regions in the nearby spiral galaxy M 83. We explore the impact that the galactic environment and intrinsic physical properties (metallicity, extinction, and stellar content) have on the early phases of H II region evolution. Methods. We combined VLT/MUSE observations of the ionised gas with young star cluster physical properties derived from HST multiwavelength data. We identified H II regions based on their Hα emission, and cross-matched the sample with planetary nebulae and supernova remnants to assess contaminant sources and identify evolved H II regions. We also spectroscopically identified Wolf-Rayet (WR) stars populating the star-forming regions. We estimated the physical properties of the H II regions (luminosity, size, oxygen abundance, and electron density). For each H II region, we computed the pressure of ionised gas (Pion) and the direct radiation pressure (Pdir) acting in the region, and investigated how they vary with galactocentric distance, with the physical properties of the region, and with the pressure of the galactic environment (PDE). For a subset of ∼500 regions, we also investigated the link between the pressure terms and the properties of the cluster population (age, mass, and LyC flux). By comparing the LyC flux derived from Hα emission with the one modelled from their clusters and WRs, we furthermore constrained any escape of LyC radiation (fesc). Results. We find that Pion dominates over Pdir by at least a factor of 10 on average over the disk. Both pressure terms are strongly enhanced and become almost comparable in the central starburst region. In the disk (R ≥ 0.15 Re), we observe that Pdir stays approximately constant with galactocentric distance. We note that Pdir is positively correlated with an increase in radiation field strength (linked to the negative metallicity gradient in the galaxy), while it decreases in low extinction regions, as is expected if the amount of dust to which the momentum can be imparted decreases. In addition, Pion decreases constantly for increasing galactocentric distances; this trend correlates with the decrease in extinction – indicative of more evolved and thus less compact regions – and with changes in the galactic environment (traced by a decrease in PDE). In general, we observe that H II regions near the centre are underpressured with respect to their surroundings, whereas regions in the rest of the disk are overpressured and hence expanding. We find that regions hosting younger clusters or those that have more mass in young star clusters have a higher internal pressure, indicating that clustered star formation likely plays a dominant role in setting the pressure. Finally, we estimate that only 13% of H II regions hosting young clusters and WR stars have fesc ≥ 0, which suggests that star formation taking place outside young clusters makes a non-negligible contribution to ionising H II regions.

Enhanced bound states in the continuum in a DBR-assisted photonic crystal slab.

Bound states in the continuum (BICs) are perfectly confined resonances within the radiation continuum. The novel characteristics of single BICs have been studied in great detail in various wave systems, including electromagnetic waves, acoustic waves, water waves, and elastic waves in solids. In practice, the performance of BICs is limited by the finite size of the structure, while the combination of multiple BICs can further improve the localization of resonances. In this study, we experimentally demonstrate the combination of Fabry-Perot and symmetry-protected BICs at near infrared wavelengths by employing a compound photonic crystal system composed of a photonic crystal slab and a distributed Bragg reflector, resulting in an enhanced high quality factor.

Inverted level populations of hydrogen atoms in ionized gas

Context. Level population inversion of hydrogen atoms in ionized gas may lead to stimulated emission of hydrogen recombination lines, and the level populations can in turn be affected by powerful stimulated emissions. Aims. In this work the interaction of the radiation fields and the level population inversion of hydrogen atoms is studied. The effect of the stimulated emissions on the line profiles is also investigated. Methods. Our previous nl-model for calculating level populations of hydrogen atoms and hydrogen recombination lines is improved. The effects of line and continuum radiation fields on the level populations are considered in the improved model. By using this method the properties of simulated hydrogen recombination lines and level populations are used in analyses. Results. The simulations show that hydrogen radio recombination lines are often emitted from the energy level with an inverted population. The widths of Hnα lines can be significantly narrowed by strong stimulated emissions to be even less than 10 km s−1. The amplification of hydrogen recombination lines is more affected by the line optical depth than by the total optical depth. The influence of stimulated emission on the estimates of electron temperature and density of ionized gas is evaluated. We find that comparing multiple line-to-continuum ratios is a reliable method for estimating the electron temperature, while the effectiveness of the estimation of electron density is determined by the relative significance of stimulated emission.

Introducing a Symmetry‐Breaking Coupler into a Dielectric Metasurface Enables Robust High‐Q Quasi‐BICs

Dielectric metasurfaces supporting quasibound states in the continuum (quasi‐BICs) exhibit very high‐quality factor resonances and electric field confinement. However, accessing the high‐Q end of the quasi‐BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high‐Q quasi‐BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to high‐symmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. This approach is validated by adding a ≈100 nm diameter cylinder between two reflection‐symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. It is found that high‐Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. This metasurface's second harmonic generation capability in the optical range is further explored. Displacing the coupler as much as a full diameter from a BIC condition produces record‐breaking normalized conversion efficiencies >102 W−1. The strategy of enclosing a disruptive element between multiple high‐symmetry points in a BIC metasurface can be applied to construct robust high‐Q quasi‐BICs in many geometrical designs.

Terahertz metasurface with multiple BICs/QBICs based on a split ring resonator.

Bound state in the continuum (BIC) refers to the trapped state in the radiation continuum of a system. In the terahertz band, BIC provides a unique and feasible method to design devices with ultra-high quality factor (Q factor) and to achieve intense terahertz-matter interaction, which is of great value to terahertz science and technology. Here, multiple BICs protected by the resonance symmetry in the terahertz metasurface consisting of metallic split ring resonators (SRR) is demonstrated. The evolution from the BIC to the quasi-BIC (QBIC) is induced by changing the gap width of the SRRs. The proposed BICs are experimentally demonstrated and analyzed by the coupled mode theory along with the numerical simulation. It is found that the leakage behavior of these QBICs is strongly affected by the intrinsic Ohmic loss in the SRRs while it is quite robust to the tilted incidence.

Ultraviolet supercontinuum generation driven by ionic coherence in a strong laser field

Supercontinuum (SC) light sources hold versatile applications in many fields ranging from imaging microscopic structural dynamics to achieving frequency comb metrology. Although such broadband light sources are readily accessible in the visible and near infrared regime, the ultraviolet (UV) extension of SC spectrum is still challenging. Here, we demonstrate that the joint contribution of strong field ionization and quantum resonance leads to the unexpected UV continuum radiation spanning the 100 nm bandwidth in molecular nitrogen ions. Quantum coherences in a bunch of ionic levels are found to be created by dynamic Stark-assisted multiphoton resonances following tunneling ionization. We show that the dynamical evolution of the coherence-enhanced polarization wave gives rise to laser-assisted continuum emission inside the laser field and free-induction decay after the laser field, which jointly contribute to the SC generation together with fifth harmonics. As proof of principle, we also show the application of the SC radiation in the absorption spectroscopy. This work offers an alternative scheme for constructing exotic SC sources, and opens up the territory of ionic quantum optics in the strong-field regime.

Fano effect induced giant and robust enhancement of photon correlations in cavity QED systems.

The Fano effect arising from the interference between two dissipation channels of the radiation continuum enables tuning of the photon statistics. Understanding the role of the Fano effect and exploiting it to achieve strong photon correlations are of both fundamental and applied significance. We present an analytical description of Fano-enhanced photon correlations based on cavity quantum electrodynamics to show that the Fano effect in atom-cavity systems can improve the degree of antibunching by over four orders of magnitude. The enhancement factors and the optimal conditions are explicitly given, and found to relate to the Fano parameter q. Remarkably, the Fano enhancement manifests robustness against the decoherence processes and can survive in the weak coupling regime. We expect our work to provide insight to tuning the photon statistics through the Fano effect, which offers a new, to the best of our knowledge, route to enhance the photon correlations, as well as the possibility of generating nonclassical light in a wider diversity of systems without the need of a strong light-matter interaction.

Interplay of Leakage Radiation and Protection in Topological Photonic Crystal Cavities

AbstractThe introduction of topological concepts to the design of photonic crystal cavities holds great promise for applications in integrated photonics due to the prospect of topological protection. This study examines the signatures of topological light confinement in the leakage radiation of 2D topological photonic crystal cavities. The cavities are implemented in an all‐dielectric platform that features the photonic quantum spin Hall effect at telecom wavelengths and supports helical edge states that are weakly coupled to the radiation continuum. The modes of resonators scaling down to single point defects in the surrounding bulk lattice are characterized via spectral position and multipolar nature of the eigenstates. The mode profiles in real and momentum space are mapped using far‐field imaging and Fourier‐spectropolarimetry, revealing how certain properties of the cavity modes reflect on their origin in the topological bandstructure. This includes band‐inversion‐induced confinement and inverted scaling of mode spectra for trivial and topological defect cavities. Furthermore, hallmarks of topological protection in the loss rates are demonstrated, which are largely unaffected by cavity shape and size. The results constitute an important step toward the use of radiative topological cavities for on‐chip confinement of light, control of emitted wave fronts, and enhancement of light–matter interactions.

The Host Galaxy and Rapidly Evolving Broad-line Region in the Changing-look Active Galactic Nucleus 1ES 1927+654

Changing-look active galactic nuclei (AGNs) present an important laboratory to understand the origin and physical properties of the broad-line region (BLR). We investigate follow-up optical spectroscopy spanning ∼500 days after the outburst of the changing-look AGN 1ES 1927+654. The emission lines displayed dramatic, systematic variations in intensity, velocity width, velocity shift, and symmetry. Analysis of optical spectra and multiband images indicates that the host galaxy contains a pseudobulge and a total stellar mass of . Enhanced continuum radiation from the outburst produced an accretion disk wind, which condensed into BLR clouds in the region above and below the temporary eccentric disk. Broad Balmer lines emerged ∼100 days after the outburst, together with an unexpected, additional component of narrow-line emission. The newly formed BLR clouds then traveled along a similar eccentric orbit (e ≈ 0.6). The Balmer decrement of the BLR increased by a factor of ∼4–5 as a result of secular changes in cloud density. The drop in density at late times allowed the production of He i and He ii emission. The mass of the black hole cannot be derived from the broad emission lines because the BLR is not virialized. Instead, we use the stellar properties of the host galaxy to estimate . The nucleus reached near or above its Eddington limit during the peak of the outburst. We discuss the nature of the changing-look AGN 1ES 1927+654 in the context of other tidal disruption events.

Bound states in the continuum on flatbands of symmetry-broken photonic crystal slabs

Bound states in the continuum (BICs) are localized states despite in a radiation continuum, rendering the ultra-high quality factor for enhanced light–matter interactions and supporting exotic topological properties. So far, most of studied BICs in photonic crystal (PhC) slabs are only vertically localized, i.e. allowing propagations in the plane of slab therein but forbidding radiations, and the density of optical states (DOS) at their frequencies is limited due to the steep dispersion characteristics. Here, we report a BIC existing on a flatband of symmetry-broken PhC slab. The flatband, associated with largely reduced group velocity, significantly sharpens the DOS at the frequency of BIC, which can be realized via finely controlling broken vertical symmetry in the PhC slab. The effect of broken symmetry is revealed in a simple effective Hamiltonian near the second-order Γ point of such system. Our results show the simultaneous engineering of dispersion and leaky characteristics of modes, offering new opportunities to boost light–matter interactions and to enhance the performance of photonic devices.

A cautionary tale of Ly C escape fraction estimates from high-redshift galaxies

ABSTRACTMeasuring the escape fraction, fesc, of ionizing, Lyman continuum (Ly C) radiation is key to our understanding of the process of cosmic reionization. In this paper, we provide a methodology for recovering the posterior probability distribution of the Ly C escape fraction, $f_{\rm esc}^{\rm PDF}$, considering both the observational uncertainties and ensembles of simulated transmission functions through the intergalactic medium (IGM). We present an example of this method applied to a VUDS galaxy at z = 3.64 and find $f_{\rm esc}^{\rm PDF}$ = 0.51$^{+0.33}_{-0.34}$ and compare this to the values computed assuming averaged IGM transmission with and without consideration of detection bias along average sightlines yielding $f_{\rm esc}^{\langle T \rangle }$ = 1.40$^{+0.80}_{-0.42}$ and $f_{\rm esc}^{\rm bias}$ = 0.82$^{+0.33}_{-0.16}$. Our results highlight the limitations of methods assuming average, smooth transmission functions. We also present MOSFIRE data for a sample of seven Ly C candidates selected based on photometric redshifts at z > 3.4, but find that all seven have overestimated photometric redshifts by Δz ∼ 0.2 making them unsuitable for Ly C measurements. This results likely due to a bias induced by our selection criteria.

Realization of Polaritonic Topological Charge at Room Temperature Using Polariton Bound States in the Continuum from Perovskite Metasurface (Advanced Optical Materials 6/2022)

In article number 2102386, Hai Son Nguyen and co-workers demonstrate experimentally polariton bound states in the continuum (“pol-BICs”) in an excitonic metasurface based on 2D halide perovskite. These hybrid light–matter states, operating at room temperature, exhibit all peculiar features of BICs: a total uncoupling from the radiative continuum associated to an infinite radiative quality factor, and a topological charge associated to a polarization singularity in the farfield emission.

Realization of Polaritonic Topological Charge at Room Temperature Using Polariton Bound States in the Continuum from Perovskite Metasurface

AbstractExciton‐polaritons are mixed light–matter excitations resulting from the strong coupling regime between an active excitonic material and photonic resonances. Harnessing these hybrid excitations provides a rich playground to explore fascinating fundamental features, as out‐of‐equilibrium Bose–Einstein condensation and quantum fluids of light, plus novel mechanisms to be exploited in optoelectronic devices. The formation of exciton‐polaritons arising from the mixing between hybrid inorganic–organic perovskite excitons and an optical bound state in a continuum (BIC) of a subwavelength‐scale metasurface, are experimentally investigated at room temperature. These polaritonic eigenmodes, hereby called polariton BICs (pol‐BICs) are revealed in reflectivity, resonant scattering, and photoluminescence measurements. Although pol‐BICs only exhibit a finite quality factor bounded by the nonradiative losses of the excitonic component, they fully inherit BIC peculiar features: a full uncoupling from the radiative continuum in the vertical direction, which is associated to a locally vanishing farfield radiation in momentum space. Most importantly, the experimental results confirm that the topological nature of the photonic BIC is perfectly transferred to the pol‐BIC. This is evidenced by the observation of a polarization vortex in the farfield of polaritonic emission. The results pave the way to engineer BIC physics of interacting bosons and novel room temperature polaritonic devices.

Group theory based formation mechanism and evolution of multiple Fano resonances in dielectric nanohole arrays with lattice-perturbed

Based on the electromagnetic properties of all-dielectric optical metamaterial, an all-dielectric metasurface of lattice-perturbed nanohole array is proposed to excite a multiple Fano resonance in the near-infrared region. Combined with the group theory, the formation mechanism and evolution law of multiple Fano resonances in this structure when its unit cell is a square lattice configuration and the square lattice symmetry is broken are explored in depth. The results show that double degenerate mode directly excited by the normal incident plane wave is coupled to vertical free-space radiation continuum to form double Fano resonance when unit cell is symmetrical, while the uncoupled non-degenerate modes excited by the normal incident plane wave is coupled to vertical free-space radiation continuum to form triple Fano resonance with higher <i>Q</i> factor when the symmetry is broken. Numerical simulation is used to explore the influences of <i>x</i>-polarized and <i>y</i>-polarized plane wave on the above Fano resonances, and the results show that the Fano resonance of double degenerate resonance is polarization independent, while the non-degenerate resonance is polarization dependent. The findings in this work can provide an effective theoretical reference for designing other square lattice metasurface to realize the excitation and evolution of multiple Fano resonances.

Ion-photon emission from titanium target under ion beam sputtering

Ion photon emission in the wavelength range of 280 - 420 nm resulting from 5 Kr+ ion beam sputtering from titanium in the presence and the absence of oxygen was studied experimentally. The observed spectra consist of a series of discrete lines superimposed with a broadband continuum. Discrete lines are attributed to excited neutral Ti I and excited ions Ti II. The differences in the observed intensities of spectral lines are discussed in terms of the electron-transfer processes between the excited sputtered atom and electronic levels of the solid. The radiative dissociation process and breaking of chemical bonds seem to contribute to the enhancement of emitted photons intensity. Continuum radiation was observed and is very probably related to the electronic structure of titanium. The collective deactivation of 3d-shell electrons appears to play a role in the emission of this radiation.