Degree Of Circular Polarization Research Articles

Optical Detection of Dynamic Polarization of Nuclei in Semiconductors

Much research has been performed by now on optical pumping of spin-polarized electrons in semiconductors [1–6]. In these experiments, the spin-oriented carriers were excited from the valence band into the conduction band by absorption of circularly-polarized light. The most convenient for the detection of the spin orientation of the electrons is an optical method [2, 3] with investigation of the recombination-luminescence polarization. For III–V crystals, the selection rules for interband transitions yield a simple connection S = 0.5P between the degree of circular polarization of the luminescence (S) and the degree of stationary orientation of the electrons (P) at the bottom of the conduction band. We have investigated, also by an optical method, a number of effects that arise when the electron spins interact strongly with the magnetic moments of the nuclei in the crystal.

Biaxial strain tuning of exciton energy and polarization in monolayer WS2

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of −130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation, we measure a monotonic decrease in the circular polarization degree under the applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from the biaxial strain. The analysis shows that the suppression of the circular polarization degree under the biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.

Circularly‐Polarized Laser Equivalent to Two Phase‐Shifted Linearly‐Polarized Sources

AbstractCompact circularly‐polarized (CP) light sources are important for chirality‐based applications at the chip level. The CP light is conventionally generated by passing the linearly‐polarized (LP) one through quarter wave plates which are too thick for packed integrations. In this work, the in‐plane submicron phase shifter substitutes the quarter‐wave plate in the creation of CP light. After the lasing field of a holed microdisk is coupled into the nearby waveguide and phase shifter, it is guided into two grating couplers that output orthogonally LP waves vertically. The two LP waves are phase‐shifted by and combine into the coherent CP radiation in the far‐field zone. Experimentally, the fabricated devices exhibit a degree of circular polarization as high as 74%. This device scheme is simple and effective in that it relies on no extrinsic materials, large‐area auxiliary structures, or specialized operation conditions critical for CP radiation. The compatibility issues for integrated photonic systems are of less concern in such schemes.

Dichroism Effects in the Ionization of Polarized Atoms by Short Circularly Polarized Laser Pulses

The dichroism effects, i.e., asymmetries of the ionization probability with respect to the inversion of either the atomic orientation (magnetic dichroism, MD) or the circular polarization degree of the photon beam (circular dichroism, CD), are investigated using the time-dependent perturbation theory (PT). It is shown that the magnitude of these effects depends not only on the polarization states of the atom and laser pulse but also on the intensity and duration of the latter. We find that the CD can also be observed in the ionization of oriented initial bound states, which is impossible in long-pulse ionization. Predictions of our PT analysis are supported by the results obtained by numerical solutions of the time-dependent Schrödinger equation (TDSE) describing the ionization of the excited 2P-states of the hydrogen atom.

Image‐Based Classification of Intense Radio Bursts From Spectrograms: An Application to Saturn Kilometric Radiation

AbstractSaturn Kilometric Radiation (SKR) is a non‐thermal auroral emission with peak emission occurring at 100–400 kHz. Its properties have been extensively studied since Cassini's arrival at Saturn until mission end with its Radio and Plasma Wave Science (RPWS) experiment. Low Frequency Extensions (LFEs) of SKR which consist of global intensifications of SKR accompanied by extensions of the main SKR band down to lower frequencies have been studied in particular. Low Frequency Extensions result from internally driven tail reconnection and from solar wind compressions of the magnetosphere, which also trigger tail reconnection. They have been cataloged through visual inspection with two approaches, using an intensity threshold for LFEs in 2006 (Reed et al., 2018, https://doi.org/10.1002/2017ja024499) and more recently O’Dwyer et al. (2023a, https://doi.org/10.25546/103103) produced a sample of LFEs detected by Cassini/RPWS by fitting their exact frequency‐time coordinates with polygons. In this study we use the latter catalog of LFEs as a training set for an image based machine learning algorithm to classify all LFEs detected by Cassini/RPWS. The inputs to the model are multi‐channel images consisting of spectrogram images in flux density and degree of circular polarization. The outputs of the model are binary masks showing the exact location of the LFE in frequency‐time space. The median Intersection Over Union across the testing and training set were calculated to be 0.97 and 0.98, respectively. The output of this study is a list of all 4,874 LFEs detected using this method. The list of LFE frequency‐time coordinates is available for use amongst the scientific community.

FAST Observations of FRB 20220912A: Burst Properties and Polarization Characteristics

We report the observations of FRB 20220912A using the Five-hundred-meter Aperture Spherical radio Telescope. We conducted 17 observations totaling 8.67 hr and detected a total of 1076 bursts with an event rate up to 390 hr−1. The cumulative energy distribution can be well described using a broken power-law function with the lower- and higher-energy slopes of −0.38 ± 0.02 and −2.07 ± 0.07, respectively. We also report the L-band (1–1.5 GHz) spectral index of the synthetic spectrum of FRB 20220912A bursts, which is −2.6 ± 0.21. The average rotation measure value of the bursts from FRB 20220912A is −0.08 ± 5.39 rad m−2, close to 0 rad m−2 and was relatively stable over 2 months. Most bursts have nearly 100% linear polarization. About 45% of the bursts have circular polarization with Signal-to-Noise ratio > 3, and the highest circular polarization degree can reach 70%. Our observations suggest that FRB 20220912A is located in a relatively clean local environment with complex circular polarization characteristics. These various behaviors imply that the mechanism of circular polarization of FRBs likely originates from an intrinsic radiation mechanism, such as coherent curvature radiation or inverse Compton scattering inside the magnetosphere of the FRB engine source (e.g., a magnetar).

(Invited) Proximity Induced Chiral Quantum Light Generation in Strain-Engineered WSe2/NiPS3 Heterostructures

Quantum light emitters (QEs) capable of generating single photons of well-defined circular polarization could enable non-reciprocal single photon devices and deterministic spin-photon interfaces critical for realizing complex quantum networks. To date, emission of such chiral quantum light has been achieved via the application of intense external magnetic fields electrical/optical injection of spin polarized carriers/excitons, or coupling with complex photonic/meta-structures. Here we report free-space generation of highly chiral single photons from QEs created via nanoindentation of monolayer WSe2 - NiPS3 heterostructures at zero external magnetic field. These QEs emit in the 760-800 nm range with a degree of circular polarization and single photon purity as high as 0.89 and 80% respectively, independent of pump laser polarization. Scanning diamond nitrogen vacancy microscopy and temperature dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in antiferromagnetic (AFM) order of NiPS3, both of which are co-localized by the strain field associated with the nanoscale indentations.

Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures.

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin-photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.

Enhancing the circular polarization degree of NIR-II circularly polarized light of cellulose nanocrystal films

Enhancing the circular polarization degree of NIR-II circularly polarized light of cellulose nanocrystal films

Metasurface reflector enables room-temperature circularly polarized emission from VCSEL

While most lasers are linearly polarized, circularly polarized laser sources are crucial components for many optical applications such as biosensing, quantum technologies, and AR/VR. However, conventional methods for generating chiral light have limitations in device miniaturization. Vertical-cavity surface-emitting lasers (VCSELs), with their small footprint and surface emission feature, can be integrated with ultrathin metasurfaces for light manipulation, offering an opportunity to realize ultracompact chiral lasers. Here we report, to our knowledge, the first demonstration of chiral lasing from electrically pumped VCSELs at room temperature, without spin injection. This is enabled by incorporating a high-contrast chiral metasurface reflector as the VCSEL top mirror, favoring one specific circular polarization for lasing. Our demonstrated 940 nm VCSELs show stable single-mode chiral lasing and achieve a circular-polarization degree of up to 59%. This study provides a scalable approach for circular-polarization control of VCSELs and holds great promise for applications that desire ultracompact chiral emitters.

Polarization formalism for ALP-induced X-ray emission from magnetars

Missions like NASA's Imaging X-ray Polarimetry Explorer (IXPE) are poised to provide an unprecedented view of the Universe in polarized X-rays. Polarization probes physical anisotropies, a fact exploited by particle physicists to look for the anisotropic a E·B operator in the axion-like-particle (ALP) Lagrangian. Such studies have typically focused on polarization in the radio and microwaves, through local or cosmic birefringence effects. To such polarization studies we add X-rays emanating from magnetars — a class of neutron stars with near-critical strength magnetic fields — that are important targets for IXPE. ALPs produced in the neutron star core convert to X-rays in the magnetosphere; such X-rays are polarized along the direction parallel to the dipolar magnetic field at the point of conversion. We develop the full theoretical formalism for ALP-induced polarization in the presence of dipolar magnetic fields. For uncorrelated photon and ALP production mechanisms, we completely disentangle the ALP contributions to the Stokes parameters in terms of the ALP intensity, the ALP-to-photon conversion probability, and the ALP-induced birefringence. In the proper limit, our results demonstrate that the inclusion of ALPs suppresses the observed degree of circular polarization compared to its pure astrophysical value. Our results can also be used to impose limits on ALP couplings with IXPE polarization data from magnetars 4U 0142+61 and 1RXS J170849.0-400910, the subject of upcoming work.

Chiral single photons from deterministic quantum emitter arrays via proximity coupling to van der Waals ferromagnets

Chiral single photons are highly sought to enhance encoding capacities or enable propagation-dependent routing in nonreciprocal devices. Unfortunately, most semiconductor quantum emitters (QEs) produce only linear polarized photons unless external magnets are applied. Magnetic proximity coupling utilizing 2D ferromagnets promises to make bulky external fields obsolete. Here we directly grow Fe-doped MoS2 (Fe:MoS2) via chemical vapor deposition that displays pronounced hard ferromagnetic properties even in monolayer form. This approach with monolayer ferromagnets enables full utilization of the strain from the pillar stressor to form QE in WSe2 deterministically. The Fe:MoS2/WSe2 heterostructures display strong hysteretic magneto-response and high-purity chiral single photons with a circular polarization degree of 92 ± 1% (74% average) without external magnetic fields. Furthermore, the chiral single photons are robust against uncontrolled twist-angle and external stray-fields. This ability to manipulate quantum states and transform linear polarized photons into high-purity chiral photons on-chip enables nonreciprocal device integration in quantum photonics.

Scalar induced gravitational waves from Chern-Simons gravity during inflation era

We investigate the scalar induced gravitational waves (SIGWs) in the Chern-Simons (CS) gravity with a dynamical scalar field during slow roll inflation. Due to the parity violation in the CS term, the SIGWs are generally polarized, which are effectively characterized by the degree of circular polarization. We derive the semianalytic expression to evaluate the power spectra and the degree of circular polarization of the SIGWs, which receive contributions from the general relativity and the parity-violating term, respectively. We find that the correction from the parity-violating CS term is negligible on large scales, which means that the degree of circular polarization of SIGWs is very small.

Chiral electroluminescence from thin-film perovskite metacavities.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

Polarized laser speckle images produced by calibrated polystyrene microspheres suspensions: comparison between backscattering and transmission experimental configurations

In this paper, we analyze polarized laser speckle images produced by calibrated polystyrene microspheres suspensions and discuss the results obtained while considering two different experimental configurations for light detection: speckle-transmission setup and speckle-backscattering one. The degree of light polarization (DOP) and speckle grain size (dx) were monitored in both cases. Results show that, when backscattered photons are detected, the degree of circular polarization allows a fine monitoring of the variation of scatterers concentration in a sample. A helicity flipping is detected as the scatterers concentration increases, showing that a transition from a simple scattering regime to a multiple scattering one is taking place. When transmitted scattered photons are detected dx is a better parameter than DOP for following changes in scattering regime: dx values initially increase slightly then decrease as the scattering coefficient increases. The observed maximum corresponds to the shift from a simple to a multiple scattering regime.

Generation of high circular polarization of interstellar Lyman α radiation triggering biological homochirality

ABSTRACT The homochirality of biological molecules on Earth is a long-standing mystery regarding the origin of life. Circularly polarized ultraviolet (UV) light could induce the enantiomeric excess of biological molecules in the interstellar medium, leading to the homochirality on Earth. By performing 3D radiation transfer simulations with multiple scattering processes in interstellar dusty slabs, we study the generation of the circular polarization (CP) of UV light at Lyα ($\lambda = 0.1216~{\rm \mu m}$) as well as at near-infrared (NIR, $\lambda = 2.14~{\rm \mu m}$) wavelengths. Our simulations show that the distributions of CP exhibit a symmetric quadrupole pattern, regardless of wavelength and viewing angle. The CP degree of scattered light from a dusty slab composed of aligned grains is ∼15 per cent for Lyα and ∼3 per cent at NIR wavelengths in the case of oblate grains with an Mathis-Rumpl-Nordsieck size distribution. We find that the CP degree of Lyα is well correlated with that in the NIR regardless of viewing angle, whilst being a factor of ∼5 higher. Thus, a high CP of Lyα is expected at sites where NIR CP is detected. We suggest that such circularly polarized Lyα may initiate the enantiomeric excess of biological molecules in space.

Broadband, angle-dependent optical characterization of asymmetric self-assembled nanohole arrays in silver

Plasmonic nanostructured materials made of nanohole arrays in metal are significant plasmonic devices exhibiting resonances and strong electromagnetic confinement in the visible and near-infrared range. As such, they have been proposed for use in many applications such as biosensing and communications. In this work, we introduce the asymmetry in nanoholes, and investigate its influence on the electromagnetic response by means of broadband experimental characterization and numerical simulations. As a low-cost fabrication process, we use nanosphere lithography, combined with tilted silver evaporation, to obtain a 2D hexagonal array of asymmetric nanoholes in Ag. Our experimental set-up is based on a laser, widely tunable in the near-infrared range, with precise polarization control in the input and in the output. We next resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. We attribute the disbalance of left and right transmitted light to the asymmetry of the nanohole, which we support by numerical simulations. We believe that the optimization of such simple plasmonic geometry could lead to multifunctional flat-optic devices.

MULTISTABILITY IN A CHIRAL SEMICONDUCTOR MICROCAVITY

The features of the bi- and multistability effects in the semiconductor Bragg microcavity with chiral photonic crystal slab on the upper mirror are investigated theoretically. It is shown that the response of such a chiral structure under a linearly polarized coherent resonant pump demonstrates sharp multistable transitions with abrupt jumps of the exciton intensity and degree of circular polarization. It is shown that of the thresholds of bistable transitions in the system with different sense of circular polarization differ slightly, i.e. in case of a non-optimized structure, we can expect to obtain even a larger amplitude of the jumps of the degree of circular polarization of the excitonic response due to the multistability than in a specially optimized chiral structure with a high degree of circular polarization at low pump intensity.

Investigation on the Formation of Herringbone Structure in Type II Solar Radio Bursts

We report detailed observation of the “herringbone” of a Type II solar radio burst that occurred on 2010 November 3rd. Data from the Space Weather Prediction Center, National Oceanic and Atmospheric Administration, e-CALLISTO, and Nançay RadioHeliograph are analyzed. We determine the brightness temperature and degree of circular polarization of the “herringbone” burst. Correlations between the physical parameters and the “herringbone” are examined. Based on the relationship, this is the first study that suggested this “herringbone” was generated through fundamental plasma.

Polarization of fast radio bursts: radiation mechanisms and propagation effects

ABSTRACT Fast radio bursts (FRBs) are observed to be highly polarized. Most have high linear polarization but a small fraction shows significant circular polarization. We systematically investigate a variety of polarization mechanisms of FRBs within the magnetar theoretical framework considering two emission sites inside and outside the magnetosphere. For each site, we discuss both intrinsic radiation mechanisms and propagation effects. Inside the magnetosphere, we investigate the polarization properties of both coherent curvature radiation and inverse Compton scattering by charged bunches and conclude that both mechanisms produce 100 per cent linear polarization at an on-axis geometry but can produce circular polarization if the viewing angle is off axis. The lack of circular polarization for the majority of bursts requires that the bunches have a large transverse dimension size. Resonant cyclotron absorption within magnetosphere may produce high circular polarization if electrons and positrons have an asymmetric Lorentz factor distribution. Outside the magnetosphere, the synchrotron maser emission mechanism in general produces highly linearly polarized emission. Circular polarization would appear at off-beam angles but the flux is greatly degraded and such bursts are not detectable at cosmological distances. Synchrotron absorption in a nebula with ordered magnetic field may reduce the circular polarization degree. Cyclotron absorption in a strongly magnetized medium may generate significant circular polarization. Faraday conversion in a medium with field reversal can convert one polarization mode to another. The two absorption processes require stringent physical conditions. Significant Faraday conversion may be realized in a magnetized dense environment involving binary systems or supernova remnants.