Circularly Polarized Emission Research Articles

Achiral Dichroic Dyes-mediated Circularly Polarized Emission Regulated by Orientational Order Parameter through Cholesteric Liquid Crystals.

It is noteworthy that cholesteric liquid crystal (CLC) platforms have been witnessed in high-performance circularly polarized luminescence (CPL) behaviors through the highly organized chiral co-assembled arrangement of achiral dyes. However, most CPL-active design strategies are closely relative to the helix co-assembly structure of CLC rather than achiral dyes. Herein, we developed an intriguing regulation strategy for CPL-active CLC materials. They were regulated using the orientational order parameter (SF) of achiral dichroic dyes as an incisive probe for the order arrangement degree of achiral dyes in CLC media. The I-shaped phenothiazine derivative PHECN dye (SF =0.30) emitted a strong CPL signal (|glum| = 0.47). In contrast, the T-shaped derivative (PHEBen) dye (SF =0.09) showed a weak circular polarization level (|glum| = 0.07) at similar CLC textures. Most interestingly, this kind of dichroic PHECN dye with a higher SF could greatly improve the contrast ratio of CPL (Δglum = 0.47) and emission intensity (ΔFL = 46.0%) at direct-current electric field compared with the T-shaped PHEBen (Δglum = 0.07 and ΔFL = 1.0%) in CLCs. This work demonstrates that an induced CPL emission can be mediated using achiral dichroic dye, which will open a new avenue for developing excellent CPL-active display materials.

Achiral Dichroic Dyes‐mediated Circularly Polarized Emission Regulated by Orientational Order Parameter through Cholesteric Liquid Crystals

AbstractIt is noteworthy that cholesteric liquid crystal (CLC) platforms have been witnessed in high‐performance circularly polarized luminescence (CPL) behaviors through the highly organized chiral co‐assembled arrangement of achiral dyes. However, most CPL‐active design strategies are closely relative to the helix co‐assembly structure of CLC rather than achiral dyes. Herein, we developed an intriguing regulation strategy for CPL‐active CLC materials. They were regulated using the orientational order parameter (SF) of achiral dichroic dyes as an incisive probe for the order arrangement degree of achiral dyes in CLC media. The I‐shaped phenothiazine derivative PHECN dye (SF=0.30) emitted a strong CPL signal (|glum|=0.47). In contrast, the T‐shaped derivative (PHEBen) dye (SF=0.09) showed a weak circular polarization level (|glum|=0.07) at similar CLC textures. Most interestingly, this kind of dichroic PHECN dye with a higher SF could greatly improve the contrast ratio of CPL (Δglum=0.47) and emission intensity (ΔFL=46.0 %) at direct‐current electric field compared with the T‐shaped PHEBen (Δglum=0.07 and ΔFL=1.0 %) in CLC. This work demonstrates that an induced CPL emission can be mediated using achiral dichroic dye, which will open a new avenue for developing excellent CPL‐active display materials.

Formamidine Engineering the Lattice Distortion of Chiral Halide Perovskites for Efficient Blue Circularly Polarized Emission

AbstractChiral halide perovskites have emerged as promising circularly polarized light sources due to their great potential applications in 3D displays, optical data storage, and optical communication devices. However, to date, chiral halide perovskites with efficient blue circularly polarized luminescence (CPL) are scarcely unearthed. Herein, through formamidine engineering to decrease lattice distortion, the first new types of chiral corrugated 2D halide perovskites are presented (R/S)‐(BPEA)FAPbBr4·H2O (2‐R/S) (BPEA = 4‐bromophenylethylaminium, FA = formamidinium). Interestingly, driven by the incorporation of FA organic cations, the chiral perovskite layer realizes the transformation from (100)‐oriented (R/S)‐(BPEA)2PbBr4·H2O (1‐R/S) to (110)‐oriented 2‐R/S accompanying with reduced lattice distortion. More importantly, (110)‐oriented chiral perovskites 2‐R exhibit remarkable blue CPL with large asymmetry factors (glum =8.4 × 10−3). This work offers an exciting approach to realizing efficient blue CPL and promotes their further applications in photoelectronic devices.

Angular Control of Circularly Polarized Emission from Achiral Molecules via Magnetic Dipoles Sustained in a Chiral Metamirror.

Circularly polarized emission (CPE) plays an important role in the designs of advanced displays and photonic integrated circuits. Unfortunately, the control of CPE handedness is limited by the chiral metasurfaces employed to emit chiral light. Particularly, the switching of the handedness with chiral metasurfaces relies on flipping the metasurfaces, which adds some constraints to practical applications. Herein, we propose an angle-sensitive chiral metamirror with Mie resonators to realize handedness switching. The Mie resonator supports a magnetic dipole having large field enhancement. This chiral metamirror is applied to excite CPEs with opposite handedness at emission angles within 10°. In contrast to the conventional methods, this work proposes a more efficient approach to manipulate the handedness of CPE.

Compact spin-valley-locked perovskite emission.

Circularly polarized light sources with free-space directional emission play a key role in chiroptics1, spintronics2, valleytronics3 and asymmetric photocatalysis4. However, conventional approaches fail to simultaneously realize pure circular polarization, high directionality and large emission angles in a compact emitter. Metal-halide perovskite semiconductors are promising light emitters5-8, but the absence of an intrinsic spin-locking mechanism results in poor emission chirality. Further, device integration has undermined the efficiency and directionality of perovskite chiral emitters. Here we realize compact spin-valley-locked perovskite emitting metasurfaces where spin-dependent geometric phases are imparted into bound states in the continuum via Brillouin zone folding, and thus, photons with different spins are selectively addressed to opposite valleys. Employing this approach, chiral purity of 0.91 and emission angle of 41.0° are simultaneously achieved, with a beam divergence angle of 1.6°. With this approach, we envisage the realization of chiral light-emitting diodes, as well as the on-chip generation of entangled photon pairs.

Enhancing Circularly Polarized Emission by a Planar Chiral Dielectric Metasurface

AbstractCircularly polarized emission (CPE) has attracted great interest in optoelectronics, biosensing, etc. Here, a resonant planar chiral dielectric metasurface that can be constructed to provide a preferable way for CPE is demonstrated. The planar metasurface is easy to fabricate, and the corresponding far‐field circular polarization degree is extremely high and angle‐insensitive. The designed metasurface is made of a periodic array of in‐plane mirror‐asymmetric silicon pillar dimers and supports a low‐leakage chiral band‐edge mode. Experimentally, the photoluminescence of a thin film of PbS/CdS quantum dots coated on such a metasurface is enhanced by 57 times with the far‐field degree of circular polarization as high as 0.74. The planar platform provides a flexible and efficient way for manipulating CPE, in which other periodic coupling effects may also be adopted to enhance the light–matter interaction, such as quasi‐bound states in the continuum and lattice resonance.

Impact of chiral ligands on photophysical and electro-optical properties of β-diketonate europium complexes in circularly polarized OLEDs.

Luminescent lanthanide complexes exhibiting chiroptical properties are attracting attention for their application in chiral optoelectronics and photonics, thanks to their unique optical properties, allied to intraconfigurational f-f transitions, which are generally electric-dipole-forbidden and can be magnetic dipole-allowed, which in an appropriate environment can lead to high dissymmetry factors and strong luminescence, in the presence of an antenna ligand. However, because luminescence and chiroptical activity are governed by different selection rules, their successful application in commonly used technologies is still an expectation. Recently, we showed that europium complexes bearing β-diketonates acting as luminescence sensitizers, and chiral bis(oxazolinyl) pyridine derivatives as the chirality inducer, reasonably perform in circularly polarized (CP) organic light-emitting devices (OLEDs). Indeed, europium β-diketonate complexes are an interesting molecular starting point, given their strong luminescence and their established use in conventional (i.e., nonpolarized) OLEDs. In this context, it is interesting to investigate in detail the impact of the ancillary chiral ligand on complex emission properties and the performances of corresponding CP-OLEDs. Here we show that, by incorporating the chiral compound as emitter in the architecture of solution processed electroluminescent devices, CP emission is retained, and the efficiency of the device is comparable to reference unpolarized OLED. The observed remarkable dissymmetry values strengthen the position of chiral lanthanide-OLEDs as CP-emitting devices.

Superhelicene with NIR Circularly Polarized Emission

Key words superhelicenes - NIR - circularly polarized luminescence - hexabenzocoronene

The Polarized Emission of AGN at Millimeter Wavelengths as Seen by POLAMI

We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at 3 mm and that the linear polarization is also more variable than the total flux. The linear polarization angle is highly variable in all sources with excursions > 180°; and for the case of the polarization angle, also the 1 mm variations appear to be faster than those at 3 mm. These results are fully compatible with recent multi-zone turbulent jet models, and they definitively rule out the popular single-zone models for blazars. They also further confirm that the short-wavelength (inner) emitting regions have better ordered magnetic fields than the long-wavelength ones (further downstream). Moreover, the POLAMI program has shown statistical evidence that, for most of the monitored sources, circular polarization emission is displayed the majority of the time. The circular polarization detection rate and the maximum degree of circular polarization found are comparable with previous surveys at much longer wavelengths, thus, opening a new window for circular polarization and jet composition studies in the mm range. The process generating circular polarization must not be strongly wavelength-dependent. The widespread presence of circular polarization in the POLAMI sample is likely due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jets. The peculiar behavior of circular polarization in 3C 66A, which we consider a hallmark of circular polarization generation by Faraday conversion in helical fields, is discussed.

Reciprocal and Non‐reciprocal Chiroptical Features in Thin Films of Organic Dyes

AbstractThe study of chiroptical properties in thin films of chiral organic π‐conjugated dyes is more complex than isotropic solutions, due to the interference of macroscopic anisotropies often present in the solid state. The experimental circularly polarized (CP) absorption (electronic circular dichroism, ECD) and CP emission (circularly polarized luminescence, CPL) of a thin film can be expressed as the sum of several terms, some of which are non‐reciprocal upon film flipping. In this review, we shall discuss the fundamental theoretical concepts about reciprocal and non‐reciprocal chiroptical features in thin films of organic dyes, describing their correlation with the solid‐state structural organization and suggesting a practical toolbox to quantify their relative weight on an experimental ECD or CPL spectrum. Moreover, we shall also take into account the literature on thin films of organic π‐conjugated dyes showing strong non‐reciprocal CP absorption and/or CP emission, due to their potential high interest in the flourishing field of chiral photonics and electronics.

Highly Tunable Circularly Polarized Emission of an Aggregation-Induced Emission Dye Using Helical Nano- and Microfilaments as Supramolecular Chiral Templates.

Aggregation-induced emission (AIE)-based circularly polarized luminescence (CPL) has been recognized as a promising pathway for developing chiroptical materials with high luminescence dissymmetry factors (|glum|). Here, we propose a method for the construction of a thermally tunable CPL-active system based on a supramolecular self-assembly approach that utilizes helical nano- or microfilament templates in conjunction with an AIE dye. The CPL properties of the ensuing ensembles are predominantly determined by the intrinsic geometric differences among the various filament templates such as their overall dimensions (width, height, and helical pitch) and the area fraction of the exposed aromatic segments or sublayers. The proposed mechanism is based on the collective data acquired by absorption, steady state and time-resolved fluorescence, absolute quantum yield, and CPL measurements. The highest |glum| value for the most promising dual-modulated helical nanofilament templates in the present series was further enhanced, reaching up to |glum| = 0.25 by confinement in the appropriate diameter of anodized aluminum oxide (AAO) nanochannels. It is envisioned that this methodology will afford new insights into the design of temperature-rate indicators or anti-counterfeiting tags using a combination of structural color by the nano- and microfilament templates and the AIE property of the guest dye.

Strong Circularly-Polarized Room-Temperature Phosphorescence from a Feasibly Separable Scaffold of Bidibenzo[b,d]furan with Locked Axial Chirality.

Novel accessible scaffold featuring circularly-polarized room-temperature phosphorescence (RTP) is attractive but challenging. Herein, we report a new feasibly separable bidibenzo[b,d]furan compound with strong circularly-polarized RTP when doping into the rigid polymer matrix. The simple silica-gel column can separate the absolute chiral R-isomer with excellent chiroptical properties. The experimental data reveal that the treated films exhibit an RTP efficiency of 14.8 %, a largest dissymmetric factor of 0.12, and a longest lifetime of 0.56 s under ambient conditions. It is found that reducing the nonradiative decays boosts the intrinsic circularly-polarized RTP emission. The impressive results indicate that the locked axial chirality skeleton endows the potential of achieving superior circularly-polarized emission for the small organic optoelectronic molecules.

Circularly Polarized Emission from Mixture of Vacuum‐Evaporated Mesogenic Luminophores

AbstractDirect circular polarized (CP) emission from a twisted mesogenic luminophore is one of the promising approaches for generating high degree of CP light since device parameters governing the degree of CP emission are easily controlled. Most high CP emissions have been performed by solution process since a twisted structure of mesogenic luminophore is easily achieved. However, since solution process has been impeded by the limitation of low resolution and efficiency, vacuum evaporation has been commonly used in display industries to obtain high performance and resolution. Here, high CP electroluminescence (CPEL) with a dissymmetric factor of 0.914 is demonstrated through the sequential vacuum evaporation of two mesogenic luminophores and chiral agent. The sequentially evaporated luminophores and chiral agent are converted into a mixture by thermal annealing, and the luminophore mixture constructs a twisted structure and generates resultantly high CPEL. The electroluminescence device fabricated with the mixture exhibits three times higher luminance and two times higher efficiency than the devices with each single luminophore. Further, the dissymmetric factors according to the device parameters based on the Stokes parameter analysis are theoretically investigated. It is anticipated that the approach paves the way toward improving CPEL performance.

Metamaterial Loaded Circularly Polarized Fractal Antenna for 2.4 GHz Frequency Applications

A metamaterial loaded circularly polarized fractal boundary patch antenna is investigated for applications at the ISM band. The suggested patch comprises of Minkowski fractal over metamaterial inspired reactive impedance substrate. Initially, asymmetrical Minkowski fractal curves are deployed at the edges of a square patch for generating the circular polarization (CP). Later, using metamaterial reactive impedance substrate as antenna loading, CP bandwidth is enhanced. The characteristics of the Minkowski fractals are adjusted to obtain the pure CP radiation. The metamaterial loaded Koch fractal antenna is also studied for CP emission. Minkowski antenna with fractal reactive impedance surface as metamaterial with iteration 1 is experimentally examined. The achieved 10-dB return loss and 3-dB axial ratio bandwidths are 40.7% and 7.47%, respectively.

Highly Efficient Inverted Circularly Polarized Organic Light-Emitting Diodes

Circularly polarized (CP) electroluminescence has been demonstrated as a strategy to improve the performance of organic light-emitting diode (OLED) displays. CP emission can be generated from both small-molecule and polymer OLEDs (SM-OLEDs and PLEDs), but to date, these devices suffer from low dissymmetry factors (g-factor < 0.1), poor device performance, or a combination of the two. Here, we demonstrate the first CP-PLED employing an inverted device architecture. Through this approach, we demonstrate a highly efficient CP-PLED, with a current efficiency of 16.4 cd/A, a power efficiency of 16.6 lm/W, a maximum luminance of over 28,500 cd/m2, and a high EL dissymmetry (gEL) of 0.57. We find that the handedness of the emitted light is sensitive to the PLED device architecture: the sign of CP-EL from an identically prepared active layer reverses between inverted and conventional devices. The inverted structure affords the first demonstration of CP-PLEDs exhibiting both high efficiency and high dissymmetry-the two figures of merit which, until now, have been difficult to achieve at the same time. We also highlight device architecture and associated internal electric field to be a previously unexplored means to control the handedness of CP emission. Our findings significantly broaden the versatility of CP emissive devices and should enable their further application in a variety of other CP-dependent technologies.

Emergent Nonreciprocal Circularly Polarized Emission from an Organic Thin Film.

Controlling circularly polarized (CP) emission is key for both fundamental understanding and applications in the field of chiral photonics and electronics. Here, a completely new way to achieve this goal is presented. A luminescent thin film, made from a chiral conjugated phenylene bis-thiophenylpropynone able to self-assemble into ordered structures, emits highly circularly polarized light with opposite handedness from its two opposite faces. Such emergent nonreciprocal behavior in CP emission, so far unprecedented, represents a fundamental advance, opening new opportunities in design, preparation, and applications of CP emitting materials.

Search for Zeeman-splitting of OH 6.035 GHz line in the young planetary nebula K 3-35

ABSTRACT Magnetic field could play a role in the formation and early evolution of non-spherical planetary nebulae (PNe). The predominant source of information of the magnetic fields in PNe is the polarization observations of maser emission. To date, distinct and/or possible Zeeman pairs have only been reported towards four PNe by measuring the OH ground-state transitions at 1.6–1.7 GHz. With the C-band (4–8 GHz) receiving system of the Shanghai TianMa 65-m radio telescope, we aim to search for possible Zeeman pairs of the PNe towards which the OH excited-state 6.035 GHz maser lines have been detected. For the young PN K 3-35, a new emission component near VLSR = 20.5 km s−1, which is currently the strongest (Ipeak ∼ 0.3 Jy) among the four components towards K 3-35 is detected. A clear S-shaped feature corresponding to this new emission component is observed in the Stokes V spectrum. Frequency shifts are seen between the fitted left-hand circular polarization and right-hand circular polarization emission peaks for the two emission components near VLSR = 19.7 and 20.5 km s−1. If the S-shaped profile and the frequency shifts are the results of Zeeman-splitting, the line-of-sight magnetic field strengths of +2.9 ± 0.6 and +4.5 ± 0.4 mG can be inferred for these two emission components, respectively.

Magnetic and Electric Control of Circularly Polarized Emission through Tuning Chirality-Generated Orbital Angular Momentum in Organic Helical Polymeric Nanofibers.

Circularly polarized light emission promotes the development of smart photonic materials for advanced applications in chiral sensing and information storage. The orbital angular momentum is a unique property for organic chiral helical materials. In this work, a type of organic chiral polymeric nanowires is designed with strong chirality induced orbital angular momentum. Under the stimulus of an external magnetic field of 600 mT, circularly polarized emission from the chiral polymeric nanowire becomes more pronounced, where the g factor increases from 0.21 to 0.3. The observed phenomena mainly originate from the chirality-dependent orbital angular momentum. Moreover, the orbital angular momentum in helical chiral nanowire structures can be suppressed by inhibiting electron transport in a helical way to diminish circularly polarized light emission at room temperature.

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.