Circularly Polarized Emission Research Articles

Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition.

Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are promising semiconductors for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS2 single crystals. By illuminating WS2 with off-resonance circularly polarized light and measuring the resulting spatially resolved circularly polarized emission (Pcirc), we find significantly large circular polarization (Pcirc up to 60% and 45% for α- and β-domains, respectively) already at 300 K, which increases to nearly 90% in the α-domains at 80 K. Studies of spatially resolved photoluminescence (PL) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin-probe force microscopy, and conductive atomic force microscopy reveal direct correlation among the PL intensity, defect densities, and chemical potential, with the α-domains showing lower defect densities and a smaller work function by 0.13 eV than the β-domains. This work function difference indicates the occurrence of type-two band alignments between the α- and β-domains. We adapt a classical model to explain how electronically active defects may serve as nonradiative recombination centers and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidence for tungsten vacancies (WVs) being the primary defects responsible for the suppressed PL and circular polarization in WS2. These results therefore suggest a pathway to control the opto-valleytronic properties of TMDCs by means of defect engineering.

Spin-dependent directional emission from a quantum dot ensemble embedded in an asymmetric waveguide.

In this study, we examine a photonic wire waveguide embedded with an ensemble of quantum dots (QDs) that directionally emits into the waveguide depending on the spin state of the ensemble. The directional emission is facilitated by the spin-orbit interaction of light. The waveguide has a two-step stair-like cross section and QDs are embedded only in the upper step, such that the circular polarization of emission from the spin-polarized QDs controls the direction of the radiation. We numerically verify that more than 70% of the radiation from the ensemble emitter is toward a specific direction in the waveguide. We also examine a microdisk resonator with a stair-like edge, which supports selective coupling of the QD ensemble radiation into a whispering gallery mode that rotates unidirectionally. Our study provides a foundation for spin-dependent optoelectronic devices.

Inverting the Handedness of Circularly Polarized Luminescence from Light-Emitting Polymers Using Film Thickness.

The emission of circularly polarized light is central to many applications, including data storage, optical quantum computation, biosensing, environmental monitoring, and display technologies. An emerging method to induce (chiral) circularly polarized (CP) electroluminescence from the active layer of polymer light-emitting diodes (polymer OLEDs; PLEDs) involves blending achiral polymers with chiral small-molecule additives, where the handedness/sign of the CP light is controlled by the absolute stereochemistry of the small molecule. Through the in-depth study of such a system we report an interesting chiroptical property: the ability to tune the sign of CP light as a function of active layer thickness for a fixed enantiomer of the chiral additive. We demonstrate that it is possible to achieve both efficient (4.0 cd/A) and bright (8000 cd/m2) CP-PLEDs, with high dissymmetry of emission of both left-handed (LH) and right-handed (RH) light, depending on thickness (thin films, 110 nm: gEL = 0.51, thick films, 160 nm: gEL = -1.05, with the terms "thick" and "thin" representing the upper and lower limits of the thickness regime studied), for the same additive enantiomer. We propose that this arises due to an interplay between localized CP emission originating from molecular chirality and CP light amplification or inversion through a chiral medium. We link morphological, spectroscopic, and electronic characterization in thin films and devices with theoretical studies in an effort to determine the factors that underpin these observations. Through the control of active layer thickness and device architecture, this study provides insights into the mechanisms that result in CP luminescence and high performance from CP-PLEDs, as well as demonstrating new opportunities in CP photonic device design.

Induced Optical Chirality and Circularly Polarized Emission from Achiral CdSe/ZnS Quantum Dots via Resonantly Coupling with Plasmonic Chiral Metasurfaces

AbstractFunctionalization of quantum dots to induce circular dichroism as well as the direct emission of circularly polarized light is the subject of intensive studies for the obvious importance not only in practical applications but also in gaining a deeper understanding of quantum semiconductor heterostructures. In this work, remarkable chiral features in achiral CdSe/ZnS colloidal quantum dots (CQDs) induced by plasmonic chiral metasurfaces are reported. Not only giant circular dichroism, but also a high degree of circular polarization in the light emitted by the CQDs hybridized with the chiral metasurfaces is observed. By probing the response of CQDs combined with tunable chiral metasurfaces, it is shown that the induced chiral features are closely linked to the correlation between the absorption bands of CQDs and the chiral bands of the metasurfaces. The findings indicate that plasmonic chiral metasurface is an effective medium in reshaping CQDs with useful chiral properties.

Single-Feed Circularly Polarized Flared-U Fractal Boundary Microstrip Antenna

ABSTRACT A new compact Flared-U-type fractal patch antenna is studied for linear and circular polarization (CP). For linear polarization, patch sides are replaced with symmetrical Flared-U fractal curves. By interchanging the edges of a square structure with asymmetrical Flared-U-type fractal curves in two perpendicular directions, two degenerated orthogonal modes are generated for CP emission. To design compact CP antennas, indentation parameters of the Flared-U-type fractal boundary curves are optimized. All the simulations are carried out using finite-element-method-based HFSS electromagnetic simulation tool. Measured results depict that 10-dB return loss and 3-dB axial ratio bandwidths of the presented iteration order (IO) 1 Flared-U CP antenna 2 are 6.17% and 1.8%, respectively, at operating frequencies of 2300 MHz. Obtained results demonstrate that by deploying fractal boundary curves as sides of the square structure, an excellent CP is achieved.

Circularly Polarized Phosphorescent Electroluminescence with a High Dissymmetry Factor from PHOLEDs Based on a Platinahelicene.

Circularly polarized (CP) light is of interest in areas such as quantum optical computing, optical spintronics, biomedicine, and high efficiency displays. Direct emission of CP light from organic light-emitting diodes (OLEDs) has been a focus of research as it has the immediate application of increasing efficiency and simplifying device architecture in OLED based displays. High dissymmetry (gEL) factor values have been reported for devices employing fluorescent polymers, but these CP-OLEDs are limited in their ultimate efficiencies by the type of emissive electronic transitions involved. In contrast, phosphorescent OLEDs (PHOLEDs) can emit light from triplet excited states and can therefore achieve very high efficiencies. However, CP-PHOLEDs are significantly understudied, and the two previous reports suffered from very low brightness or gEL values. Here, we use a platinahelicene complex to construct a CP-PHOLED that achieves both a display level brightness and a high gEL factor. The dissymmetry of CP emission reached with this proof-of-concept single-layer helicene-based device is sufficient to provide real-world benefits over nonpolarized emission and paves the way toward chiral metal complex-based CP-PHOLED displays.

Single layer single probe feed circularly polarized triple band fractal boundary microstrip antenna for wireless applications

Triple band circularly polarized fractal boundary microstrip antenna is proposed and experimentally studied. The two orthogonal sides of the square patch are replaced with asymmetrical fractal curves for circular polarization (CP) radiation. A 45° rotated asymmetrical fractal slot is embedded at the center of the fractal patch for triband CP operation. The indentation factors of the fractal curves are optimized for better CP emission. The inserted fractal slot redistributes the current elements on the patch for triband CP radiation. The proposed antenna is implemented over a single layer with a single probe feed, so it is simple to fabricate and is cost effective when compared with the existing stacked layered multiband antennas in the open literature. The measured 3 dB axial ratio bandwidths of the proposed antenna at 2.4, 3.4, and 5.8 GHz are 2, 1, and 1.8%, respectively, so it is well suited for WLAN and Wi-MAX wireless applications.

ChemInform Abstract: Synthesis of Optically Active, X‐Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence.

Optically active, Frechet-type dendrimers containing an emissive X-shaped π-electron system as the core unit were synthesized. Gram-scale optical resolution and transformations of 4,7,12,15-tetrasubstituted [2.2]paracyclophanes were also carried out. The high-generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X-shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.

Synthesis of Optically Active, X-Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence.

Optically active, Fréchet-type dendrimers containing an emissive X-shaped π-electron system as the core unit were synthesized. Gram-scale optical resolution and transformations of 4,7,12,15-tetrasubstituted [2.2]paracyclophanes were also carried out. The high-generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X-shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.

Synthesis of Optically Active, X‐Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence

AbstractInvited for the cover of this issue is Yasuhiro Morisaki at Kwansei Gakuin University together with the group of Yoshiki Chujo at Kyoto University. The image depicts the circularly polarized luminescence of the dendrimer films described in their Full Paper. Read the full text of the article at 10.1002/chem.201504270.

Cover Picture: Synthesis of Optically Active, X‐Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence (Chem. Eur. J. 7/2016)

Cover Picture: Synthesis of Optically Active, X‐Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence (Chem. Eur. J. 7/2016)

Applications of spoof planar plasmonic waveguide to frequency-scanning circularly polarized patch array

Spoof planar plasmonic waveguides (SPPWs) show potential applications in many microwave and terahertz systems, e.g. imaging, communication and sensing, due to their capability of highly confining spoof surface plasmon polaritons (SPPs) and relatively low propagation loss. Meanwhile, efficient electromagnetic emission from the SPPWs is one of the dominant factors in the practical applications. In this article, we demonstrate a beam-scanning circularly polarized (CP) antenna based on the SPPW of blind-hole array, which can efficiently convert spoof SPPs propagating in the SPPW into free-space emissions. Several centre-inclined rectangular patches are adopted to achieve the CP emission. The antenna array was designed and fabricated in the microwave frequency band, and the measured results validate the good emission performance of the proposed array, e.g. high gain and wide-scan angle. Finally, the proposed idea can be further extended into the terahertz frequency band.

MnSb‐based spin LED with side‐wall emission

AbstractCombination of the spin‐dependent optical transition and the spin current would lead us to the opportunity of developing photo‐ and light‐emitting diodes that respectively senses and emits circularly polarized light. Here, we describe preparation of spin LED consisting of an epitaxial MnSb spin injection layer and GaAs‐AlGaAs double hetero‐structures, and show characteristics of EL from a side wall emission. Taking advantage of remnant state of ferromagnet, circular polarization emission has been realized without application of an external field. Demonstration of electrical switching of circular polarization has been carried out with alternative switching between two spin‐LEDs having antiparallel magnetization configuration. While the technique used in the present demonstration was a crude one, it certainly show the feasibility of building a compact and robust light source whose circular polarization is controlled electrically. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Circular polarization emission from an external cavity diode laser

An external cavity diode laser (ECDL) containing a chiral sculptured-thin-film (STF) mirror for very pure circularly polarized (CP) emission was fabricated and its single-mode, left-handed CP lasing performance was observed. The extinction ratio of the CP output was found to increase rapidly near the threshold of the injection current for the laser diode. The Jones-matrix representation of a chiral STF mirror was used to calculate the eigenstates in the external cavity of the ECDL with the transfer matrix method, the results suggesting that the only resonant mode in the external cavity of the laser is CP with the same handedness as the structural handedness of the chiral STF mirror.

Circularly polarized emission from colloidal nanocrystal quantum dots confined in microcavities formed by chiral mirrors

Colloidal semiconductor nanocrystal quantum dots (NQDs) confined in a microcavity device emit resonance-enhanced, circularly polarized (CP) light, when the device consists of a pair of chiral sculptured-thin-film mirrors separated by layers of emissive nanocrystals and spacers that form a λ∕2-resonant cavity. The resonant emission from the NQDs within the microcavity is strongly directed normally to the mirror surfaces. It is speculated that the enhanced coupling between the NQD excitons and the confined electromagnetic field resulted in the observed CP emission from the embedded NQDs with significant improvements in the light output's directionality and color purity.

Spin polarization in self-assembled quantum dots

The spin properties of carriers in self-assembled quantum dots are studied theoretically using an empirical tight-binding method including spin-orbit coupling. Electrons and holes are found to reveal distinctive spin polarization: electron spin is found to follow the direction of the external magnetic field while hole spin is polarized along the growth direction of the dots. As a result, quasispins of electrons and holes are polarized along two perpendicular directions in a Voigt configuration and the circular polarization of emission from quantum dots quenches. The spin polarization of hole is responsible for a zero in-plane $g$ factor of holes in quantum dots. Our theoretical results successfully explain experiments.

On the relationship between chromospheric oscillations of radio brightness at 1.76 cm with periods from minutes to hours and magnetic field changes

We present the results of Fourier and wavelet time series analysis for the high-cadence observations of an active region NOAA 8011, obtained with the Nobeyama Radioheliograph (NoRH) at the wavelength of 1.76 cm on 17.01.1997. Oscillations in brightness are found to be present with periods in the range from minutes to hours. The relationship between the active region oscillations in the microwave total intensity (I) and circular polarization (V) emission and changes of the magnetic field, deduced from the high-resolution magnetograms from MDI/SoHO, is investigated. We concentrate on the identification of different oscillation modes, the temporal behavior of the oscillations and the spatial distribution of different oscillating frequencies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)

We present multifrequency, high-resolution, Very Long Baseline Array circular polarization images of the radio source 3C 84 in the center of NGC 1275. Our images reveal a complex distribution of circular polarization in the inner parsec of the radio jet, with local levels exceeding 3% polarization, the highest yet detected with VLBI techniques. The circular polarization changes sign along the jet, making 3C 84 also the first radio jet to show both signs of circular polarization simultaneously. The spectrum and changing sign of the circular polarization indicate that it is unlikely to be purely intrinsic to the emitted synchrotron radiation. The Faraday conversion process makes a significant and perhaps dominant contribution to the circular polarization, and the observed spectrum suggests that the conversion process is near saturation. The sign change in the circular polarization along the jet may result from this saturation or may be due to a change in magnetic field order after an apparent bend in the jet. From the small spatial scales probed here, 0.15 pc, and the comparably high levels of circular polarization inferred for the intraday variable source PKS 1519-273, we suggest a connection between small spatial scales and efficient production of circular polarization.

Spatially resolved microwave oscillations above a sunspot

Using high quality VLA observations, we detected for the first time spatially resolved oscillations in the microwave total intensity ( I ) and circular polarization ( V ) emission of a sunspot–associated gyroresonance source. Oscillations were detected at 8.5 and 5 GHz during several time intervals of our 10–hour–long dataset. The oscillations are intermittent: they start suddenly and are damped somehow more gradually. Despite their transient nature when they are observed they show significant positional, amplitude and phase stability. The spatial distribution of intensity variations is patchy and the location of the patches of strong oscillatory power is not the same at both frequencies. The strongest oscillations are associated with a small region where the 8.5 GHz emission comes from the second harmonic of the gyrofrequency while distinct peaks of weaker oscillatory power appear close to the outer boundaries of the 8.5 and 5 GHz g–r sources, where the emissions come from the third harmonic of the gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5 GHz oscillations (the rms amplitudes of the I oscillations are 1.3–2.5 $\times 10^4$ K and 0.2–1.5 $\times 10^5$ K, respectively). At both frequencies the oscillations have periods in the three–minute range: the power spectra show two prominent peaks at 6.25–6.45 mHz and 4.49–5.47 mHz. Our models show that the microwave oscillations are caused by variations of the location of the third and/or second harmonic surfaces with respect to the base of the chromosphere–corona transition region (TR), i.e. either the magnetic field strength or/and the height of the base of the TR oscillates. The best–fit model to the observed microwave oscillations can be derived from photospheric magnetic field strength oscillations with an rms amplitude of 40 G or oscillations of the height of the base of the TR with an rms amplitude of 25 km. Furthermore, small variations of the orientation of the magnetic field vector yield radio oscillations consistent with the observed oscillations.

Circular dichroism and circularly polarized luminescence of highly oriented Langmuir-Blodgett films of poly(p-phenylene vinylene)

Langmuir-Blodgett films of a poly(p-phenylene vinylene) (PPV) precursor were prepared using an amphiphilic precursor, where the counter-ion chloride was partially exchanged by a long chain dodecylbenzenesulfonate (DBS) ion. These LB-PPV films are strongly ordered along the dipping direction as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission. The consequence of this anisotropy is a strong circular dichroism (100 mdeg) in the π-π band and a circular polarization emission with an asymmetry factor g e =0.84.