Sign Of Circular Polarization Research Articles

Near‐Infrared Circularly Polarized Electroluminescence with Switchable Handedness in Organic LEDs

AbstractIn this study, the development of the first fully‐organic circularly polarized near‐infrared organic light–emitting diode (CP‐NIR‐OLED) is presented. The devices are made by using a co‐assembled active layer consisting of a commercially available NIR‐emitting indacenodithienothiophene‐based molecule (ITIC) and a chiral inducer. Here, ITIC, which is traditionally utilized as non‐fullerene acceptor in organic photovoltaics for its superior characteristics, is demonstrated to act as an effective NIR emitter for solution‐processed OLEDs. By incorporating the chiral additive into the active layer, the emission is successfully polarized, achieving CP‐NIR‐OLEDs with dissymmetry values up to 0.074 and a maximum radiance of 1.65 Wsr−1m−2. Additionally, this study shows that by manipulating the position of the radiative exciton recombination zone within the active layer, the circular polarization sign can be switched, while maintaining and even enhancing device performance. This CP‐NIR‐OLEDs also exhibit remarkable operational stability, indicating their potential for short‐term applications. This work provides a significant step forward in the development of CP‐OLEDs and opens new avenues for their use in advanced near‐infrared optoelectronic devices.

Photon Ring Polarimetry with Next-generation Black Hole Imaging. I. M87*

The near-horizon region of a black hole impacts linear (LP) and circular polarization (CP) through strong lensing of photons, adding large-scale symmetries and anti-symmetries to the polarized image. To probe the signature of lensing in polarimetry, we utilize a geometric model of concentric Gaussian rings of equal radius to investigate the transition in the Fourier plane at which the photon ring signal begins to dominate over the direct image. We find analytic, closed-form expressions for the transition radii in total intensity, LP, and CP, wherein the resultant formulae are composed of ratios of tunable image parameters, with the overall “scale” set primarily by the thickness of the direct image. Using these formulae, we compute the transition radii for time-averaged images of M87* simulations at 230 GHz, studying both magnetically arrested disk (MAD) and standard and normal evolution configurations for various spin and electron heating models. We compare geometric values to radii obtained directly from the simulations through a coherent averaging scheme. We find that nearly all MAD models have a photon ring-dominated CP signal on long baselines shorter than Earth's diameter at 230 GHz. Across favored models for the M87* accretion flow identified by the Event Horizon Telescope (EHT) polarimetric constraints, we quantify the sensitivity and antenna size requirements for the next-generation EHT and the Black Hole Explorer orbiter to detect these features. We find that the stringent requirements for CP favor explorations using long baselines on the ground, while LP remains promising on Earth-space baselines.

Full Stokes-vector Inversion of the Solar Mg ii h and k Lines

The polarization of the Mg ii h and k resonance lines is the result of the joint action of scattering processes and the magnetic field–induced Hanle, Zeeman, and magneto-optical effects, thus holding significant potential for the diagnostic of the magnetic field in the solar chromosphere. The Chromospheric LAyer Spectro-Polarimeter sounding-rocket experiment, carried out in 2019, successfully measured at each position along the 196″ spectrograph slit the wavelength variation of the four Stokes parameters in the spectral region of this doublet around 280 nm, both in an active-region plage and in a quiet region close to the limb. We consider some of these CLASP2 Stokes profiles and apply to them the recently developed HanleRT Tenerife Inversion Code, which assumes a one-dimensional model atmosphere for each spatial pixel under consideration (i.e., it neglects the effects of horizontal radiative transfer). We find that the nonmagnetic causes of symmetry breaking, due to the horizontal inhomogeneities and the gradients of the horizontal components of the macroscopic velocity in the solar atmosphere, have a significant impact on the linear polarization profiles. By introducing such nonmagnetic causes of symmetry breaking as parameters in our inversion code, we can successfully fit the Stokes profiles and provide an estimation of the magnetic field vector. For example, in the quiet region pixels, where no circular polarization signal is detected, we find that the magnetic field strength in the upper chromosphere varies between 1 and 20 G.

Experimental Characterization of Polarized Light Backscattering in Fog Environments

This paper focuses on the experimental characterization of the polarization behavior of light backscattered through fog. A polarimetric orthogonal state contrast imager and an active, purely polarized white illuminator system are used to evaluate both linear and circular polarization signals. The experiments are carried out in a macro-scale fog chamber under controlled artificial fog conditions. We explore the effect of backscattering in each imaging channel, and the persistence of both polarization signals as a function of meteorological visibility. We confirm the presence of the polarization memory effect with circularly polarized light, and, as a consequence, the maintenance of helicity in backscattering. Moreover, the circular cross-polarized channel is found to be the imaging channel less affected by fog backscattering. These results are useful and should be taken into account when considering active polarimetric imaging techniques for outdoor applications under foggy conditions.

Nature of the Eclipsing Polar 1RXS J184542.4+483134

We have carried out a comprehensive study of the poorly investigated eclipsing polar1RXS J184542.4+483134 with a short orbital period Porb ≈ 79 min. An analysis of its long-term lightcurves points to a change in the position and sizes of the accretion spot as the accretion rate changes.Narrow and broad components, which are probably formed on the ballistic segment of the accretion streamand on the magnetic trajectory, respectively, are identified in the emission line profiles. An inversion ofthe line profiles from emission to absorption due to the obscuration of the accretion spot by the accretionstream is observed. Based on the eclipse duration and the radial velocities of the narrow line component,we impose constraints on the white dwarf mass, 0.49 ≤ M1/M ≤ 0.89, and the orbital inclination,79.7◦ ≤ i ≤ 84.3◦. An analysis of the cyclotron spectra points to the presence of two accretion spotswith magnetic field strengths B1 = 28.4+0.1−0.2 MG and B2 = 30 − 36 MG. The main spot has a complexstructure that apparently has a dense core and a less dense periphery emitting a spectrum with cyclotronharmonics. Polarization observations reveal a circular polarization sign reversal during the orbital periodand an anticorrelation of the polarization with the brightness of the polar. Our modeling of polarizationobservations using the simple model of an accreting white dwarf shows that the polarization propertiescan be interpreted in terms of two-pole accretion with different optical depths of the accretion spots(τ1/τ2 ∼ 10). An analysis of the Swift/XRT observations points to a predominance of bremsstrahlungin the X-ray radiation from the system.

DKIST Unveils the Serpentine Topology of Quiet Sun Magnetism in the Photosphere

We present the first quiet Sun spectropolarimetric observations obtained with the Visible SpectroPolarimeter at the 4 m Daniel K. Inouye Solar Telescope. We recorded observations in a wavelength range that includes the magnetically sensitive Fe i 6301.5/6302.5 Å doublet. With an estimated spatial resolution of 0.″08, this represents the highest spatial resolution full-vector spectropolarimetric observations ever obtained of the quiet Sun. We identified 53 small-scale magnetic elements, including 47 magnetic loops and four unipolar magnetic patches, with linear and circular polarization detected in all of them. Of particular interest is a magnetic element in which the polarity of the magnetic vector appears to change three times in only 400 km and which has linear polarization signals throughout. We find complex Stokes V profiles at the polarity inversion lines of magnetic loops and discover degenerate solutions, as we are unable to conclusively determine whether these arise due to gradients in the atmospheric parameters or smearing of opposite-polarity signals. We analyze a granule that notably has linear and circular polarization signals throughout, providing an opportunity to explore its magnetic properties. On this small scale, we see the magnetic field strength range from 25 G at the granular boundary to 2 kG in the intergranular lane (IGL) and sanity-check the values with the weak and strong field approximations. A value of 2 kG in the IGL is among the highest measurements ever recorded for the internetwork.

Magnetic Field Information in the Near-ultraviolet Fe ii Lines of the CLASP2 Space Experiment

We investigate theoretically the circular polarization signals induced by the Zeeman effect in the Fe ii lines of the 279.3–280.7 nm spectral range of the CLASP2 space experiment and their suitability to infer solar magnetic fields. To this end, we use a comprehensive Fe ii atomic model to solve the problem of the generation and transfer of polarized radiation in semiempirical models of the solar atmosphere, comparing the region of formation of the Fe ii spectral lines with those of the Mg ii h and k and the Mn i resonance lines. These are present in the same near-ultraviolet (near-UV) spectral region and allowed the mapping of the longitudinal component of the magnetic field (B L) through several layers of the solar chromosphere in an active region plage. We compare our synthetic intensity profiles with observations from the IRIS and CLASP2 missions, proving the suitability of our model atom to characterize these Fe ii spectral lines. The CLASP2 observations show two Fe ii spectral lines at 279.79 and 280.66 nm with significant circular polarization signals. We demonstrate the suitability of the weak-field approximation applied to the Stokes I and V profiles of these Fe ii lines to infer B L in the plage atmosphere. We conclude that the near-UV spectral region of CLASP2 allows us to determine B L from the upper photosphere to the top of the chromosphere of active region plages.

Magnetic Fields in Solar Plage Regions: Insights from High-sensitivity Spectropolarimetry

Plage regions are patches of concentrated magnetic field in the Sun’s atmosphere where hot coronal loops are rooted. While previous studies have shed light on the properties of plage magnetic fields in the photosphere, there are still challenges in measuring the overlying chromospheric magnetic fields, which are crucial to understanding the overall heating and dynamics. Here, we utilize high-sensitivity, spectropolarimetric data obtained by the 4 meter Daniel K. Inouye Solar Telescope to investigate the dynamic environment and magnetic field stratification of an extended, decaying plage region. The data show strong circular polarization signals in both plage cores and surrounding fibrils. Notably, weak linear polarization signals clearly differentiate between plage patches and the fibril canopy, where they are relatively stronger. Inversions of the Ca II 8542 Å spectra show an imprint of the fibrils in the chromospheric magnetic field, with typical field strength values ranging from ∼200 to 300 G in fibrils. We confirm the weak correlation between field strength and cooling rates in the lower chromosphere. Additionally, we observe supersonic downflows and strong velocity gradients in the plage periphery, indicating dynamical processes occurring in the chromosphere. These findings contribute to our understanding of the magnetic field and dynamics within plages, emphasizing the need for further research to explore the expansion of magnetic fields with height and the three-dimensional distribution of heating rates in the lower chromosphere.

The Magnetic Sensitivity of the (250–278 nm) Fe ii Polarization Spectrum

This paper presents a theoretical investigation of the polarization and magnetic sensitivity of the near-ultraviolet (near-UV) solar spectral lines of Fe ii between 250 and 278 nm. In recent years, UV spectropolarimetry has become key to uncovering the magnetism of the upper layers of the solar chromosphere. The unprecedented data obtained by the CLASP2 suborbital space experiment across the Mg ii h and k lines around 280 nm are a clear example of the capabilities of near-UV spectropolarimetry for magnetic field diagnostics throughout the whole solar chromosphere. Recent works have pointed out the possible complementary diagnostic potential of the many Fe ii lines in the unexplored spectral region between 250 and 278 nm, but no quantitative analysis of the polarization and magnetic sensitivity of those spectral lines has been carried out yet. To study the polarization signals in these spectral lines, we create a comprehensive atomic model including all the atomic transitions resulting in strong spectral lines. We then study the magnetic sensitivity of the linear and circular polarization profiles in a semiempirical model representative of the quiet Sun. We present a selection of Fe ii spectral lines with significant linear and circular polarization signals and evaluate their diagnostic capabilities by studying their formation heights and magnetic sensitivity through the action of the Hanle and Zeeman effects. We conclude that when combined with the CLASP2 spectral region these Fe ii lines are of interest for the inference of magnetic fields throughout the solar chromosphere, up to near the base of the corona.

Evidence for the Operation of the Hanle and Magneto-optical Effects in the Scattering Polarization Signals Observed by CLASP2 across the Mg ii h and k Lines

Radiative transfer investigations of the solar Mg ii h and k resonance lines around 280 nm have shown that, while their circular polarization (Stokes V) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects. Using the unprecedented observations of the Mg ii and Mn i resonance lines obtained by the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization signals at different wavelengths (i.e., at the center, and at the near and far wings of the k line) vary with the longitudinal component of the magnetic field (B L ) at their approximate height of formation. The B L is estimated from the V signals in the aforementioned spectral lines. Particular attention is given to the following quantities that are expected to be influenced by the presence of magnetic fields through the Hanle and MO effects: the sign of the U signals, the total linear polarization amplitude (LP) and its direction (χ) with respect to a reference direction. We find that at the center and near wings of the k line, the behavior of these quantities is significantly different in the observed quiet and plage regions, and that both LP and χ seem to depend on B L . These observational results are indicative of the operation of the Hanle effect at the center of the k line and of the MO effects at the near wings of the k line.

Light scattering from spiral particles

We calculate the light-scattering Mueller matrix elements of left-handed and right-handed spiral particles that have morphological chirality. For such orientation-averaged systems, none of the Mueller matrix elements are equal to zero, although basic relations do exist between enantiomers. The magnitude of the circular-polarization signal can be greater than 10%; however, in the backscattering region it can be significantly greater due to a manifestation of the coherent-backscattering mechanism. The magnitude of this circular-polarization opposition effect decreases with particle absorption and its width decreases with decreasing wavelength, which is consistent with the linear-polarization opposition effect and the intensity opposition effect.

Photon–phonon entanglement in the acousto-optic interaction of vector beams

We demonstrate that acousto-optic diffraction under certain conditions can be accompanied by inter-system photon–phonon entanglement. A photon diffracted during Stokes acousto-optic interaction of acoustic and optical vector beams represents a vector beam that manifests the intra-system entanglement between its states characterized by the different signs of circular polarization and orbital angular momentum. At the same time, a diffracted photon and an acoustic phonon emitted in the process of acousto-optic interaction demonstrate the inter-system entanglement between their circularly polarized vortex states.

Switchable Circularly Polarized Signals with High Asymmetric Factor Triggered by Dual Photonic Bandgap Structure (Small 49/2022)

Smart Photonic Materials In article number 2204199, Bin Wang, Jinpeng Li, Guangdong Ying, and co-workers construct a dual photonic bandgap structure to achieve flexible switching of intensity, wavelength, and direction of circularly polarized luminescence accompanied by ultra-high asymmetry factor values. This work provides insights into the real-time manipulation of circularly polarized signals by chiral photonic materials.

Switchable Circularly Polarized Signals with High Asymmetric Factor Triggered by Dual Photonic Bandgap Structure.

Currently, the smart photonic materials that can switch circularly polarized signals in real-time have attracted extensive attention due to numerous potential applications in information storage and photonics displays. However, the dynamically reversible switching of circularly polarized signals requires precise structural reconfiguration, which is rarely achieved in traditional biomaterials. Herein, a dual photonic bandgap (PBG) structure is constructed based on the optical propagation principle of cellulose-based photonic crystals, enabling the flexible switching of the intensity, wavelength, and direction of circularly polarized luminescence (CPL). By adjusting the fluorescence intensity and the matching degree of chiral structure, the asymmetric factor value of dual PBG structure is up to -1.47, far exceeding other cellulose-based materials. Importantly, it is demonstrated that dual CPL emission can be efficiently induced by two different PBGs, opening a new approach for on-demand switching of single and dual CPL emission. In addition, the dual PBG structure exhibits dual circularly polarized reflected signals under the circular polarizer, which perfectly embodies the applicability of multiple encryptions in QR codes. This work provides new insights into the real-time manipulation of circularly polarized signals by chiral photonic materials.

The polarization signals of the solar K I D lines and their magnetic sensitivity

Aims. This work aims to identify the relevant physical processes in shaping the intensity and polarization patterns of the solar K I D lines through spectral syntheses, placing particular emphasis on the D2 line. Methods. The theoretical Stokes profiles were obtained by numerically solving the radiative transfer problem for polarized radiation considering one-dimensional semi-empirical models of the solar atmosphere. The calculations account for scattering polarization, partial frequency redistribution (PRD) effects, hyperfine structure (HFS), J- and F-state interference, multiple isotopes, and magnetic fields of arbitrary strength and orientation. Results. The intensity and circular polarization profiles of both D lines can be suitably modeled while neglecting both J-state interference and HFS. The magnetograph formula can be applied to both lines, without including HFS, to estimate weak longitudinal magnetic fields in the lower chromosphere. By contrast, modeling the scattering polarization signal of the D lines requires the inclusion of HFS. The amplitude of the D2 scattering polarization signal is strongly depolarized by HFS, but it remains measurable. A small yet appreciable error is incurred in the scattering polarization profile if PRD effects are not taken into account. Collisions during scattering processes have a clear depolarizing effect, although a quantitative analysis is left for a forthcoming publication. Finally, the D2 scattering polarization signal is particularly sensitive to magnetic fields with strengths around 10 G and it strongly depends on their orientation. Despite this, its center-to-limb variation relative to the amplitude at the limb is largely insensitive to the field strength and orientation. Conclusions. These findings highlight the value of the K I D2 line polarization for diagnostics of the solar magnetism, and show that the linear and circular polarization signals of this line are primarily sensitive to magnetic fields in the lower chromosphere and upper photosphere, respectively.

Spectral Lines in FUV and EUV for Diagnosing Coronal Magnetic Field

The diagnostic capabilities of spectral lines in far ultraviolet (FUV) and extreme ultraviolet (EUV) wavelength range are explored in terms of their Hanle and Zeeman sensitivity to probe vector magnetic field in the solar corona. The temperature range covered is log$_{10}(T)=5.5-6.3$. The circular polarization signal due to longitudinal Zeeman effect is estimated for spectral lines in the wavelength range of 500 to 1600 \r{A}. The Stokes $V/I$ signal for a FUV line is found to be in the order of 10$^{-4}$ for a longitudinal field strength of 10 Gauss, which further reduces to 10$^{-5}$ for wavelengths below 1200 \r{A}. Due to such low signals, the present study aims to find combination of spectral lines having different Hanle sensitivity but with identical peak formation temperature to probe coronal magnetic field vector. The combination of Hanle sensitive lines is better suited because the Hanle signals are stronger by at least an order of magnitude compared to Zeeman signals. The linear polarization signals due to Hanle effect from at least two spectral lines are required to derive information on the full vector. It is found from this study that there is always a pair of Hanle sensitive lines for a given temperature range suitable for probing coronal vector magnetic field and they are located in close proximity with each other in terms of their wavelength.

DNA-like Helices as Nanosized Polarizers of Electromagnetic Waves

The possibility of using a conducting double DNA-like helix as the basis of an electromagnetic wave polarizer, which converts an incident linearly polarized wave into a reflected wave with circular polarization, has been shown. A high-frequency resonance is studied, at which the wavelength of the incident radiation is approximately equal to the length of a helical turn. The simulation of a double DNA-like helix has been carried out. The electric currents arising in the helical strands under waves with circular polarization at high-frequency resonance have been analyzed. Fundamentally different behavior of the double DNA-like helix concerning waves with right-hand or left-hand circular polarization has been established, which can be called the effect of polarization selectivity. This effect is manifested in the fact that a double DNA-like helix at high-frequency resonance can create a reflected wave having only one sign of circular polarization. The electric vector of the reflected wave produces a turn in space with the opposite winding direction compared to the double helix. These studies also highlight the electromagnetic forces of interaction between helical strands. The equilibrium of the double DNA-like helix has been studied, including as an element of metamaterials and as an object with a high potential for use in nanotechnology.

Spectropolarimetric observations of the solar atmosphere in the Hα 6563 Å line

We present novel spectropolarimetric observations of the hydrogen Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno (IRSOL). The linear polarization is clearly dominated by the scattering of anisotropic radiation and the Hanle effect, while the circular polarization is dominated by the Zeeman effect. The observed linear polarization signals show a rich spatial variability, the interpretation of which would open a new window for probing the solar chromosphere. We study their spatial variation within coronal holes, finding a different behaviour for the U/I signals near the north and south solar poles. We identify some spatial patterns, which may facilitate the interpretation of the observations. In close-to-the-limb regions with sizable circular polarization signals, we find similar asymmetric Q/I profiles. We also show examples of net circular polarization profiles (NCP), along with the corresponding linear polarization signals. The application of the weak field approximation to the observed circular polarization signals gives 10 G (40–60 G) close to the limb quiet (plage) regions for the average longitudinal field strength over the spatio-temporal resolution element.

The polarization angle in the wings of Ca I 4227: A new observable for diagnosing unresolved photospheric magnetic fields

Context. When observed in quiet regions close to the solar limb, many strong resonance lines show conspicuous linear polarization signals, produced by scattering processes (i.e., scattering polarization), with extended wing lobes. Recent studies indicate that, contrary to what was previously believed, the wing lobes are sensitive to the presence of relatively weak longitudinal magnetic fields through magneto-optical (MO) effects. Aims. We theoretically investigate the sensitivity of the scattering polarization wings of the Ca I 4227 Å line to the MO effects, and we explore its diagnostic potential for inferring information on the longitudinal component of the photospheric magnetic field. Methods. We calculate the intensity and polarization profiles of the Ca I 4227 Å line by numerically solving the problem of the generation and transfer of polarized radiation under non-local thermodynamic equilibrium conditions in one-dimensional semi-empirical models of the solar atmosphere, taking into account the joint action of the Hanle, Zeeman, and MO effects. We consider volume-filling magnetic fields as well as magnetic fields occupying a fraction of the resolution element. Results. In contrast to the circular polarization signals produced by the Zeeman effect, we find that the linear polarization angle in the scattering polarization wings of Ca I 4227 presents a clear sensitivity, through MO effects, not only to the flux of the photospheric magnetic field, but also to the fraction of the resolution element that the magnetic field occupies. Conclusions. We identify the linear polarization angle in the wings of strong resonance lines as a valuable observable for diagnosing unresolved magnetic fields. Used in combination with observables that encode information on the magnetic flux and other properties of the observed atmospheric region (e.g., temperature and density), it can provide constraints on the filling factor of the magnetic field.

Polarization properties of FRB 20201124A from detections with the Effelsberg 100-m radio telescope

ABSTRACT The repeating fast radio burst (FRB) source, FRB 20201124A, was found to be highly active in 2021 March and April. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 2021 April 9 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multicomponent bursts in our sample yields a DM of 411.6 ± 0.6 pc cm−3. We find that the rotation measure (RM) of the bursts varies around their weighted mean value of −601 rad m−2 with a standard deviation of 11.1 rad m−2. This RM magnitude is 10 times larger than the expected Galactic contribution along this line of sight (LoS). We estimate an LoS magnetic field strength of 4–6 µG, assuming that the entire host galaxy DM contributes to the RM. Further polarization measurements will help determine FRB 20201124A’s RM stability. The bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating FRB. Their polarization position angles (PAs) are flat across the burst envelopes and vary between bursts. We argue that the varying polarization fractions and PAs of FRB 20201124A are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the RM variability, is hard to explain with Faraday conversion. The high linear polarization fractions, flat PAs, and downward drift from FRB 20201124A bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters.