Dichroism Signal Research Articles

Structural Origin of Enhanced Circularly Polarized Luminescence in Hybrid Manganese Bromides

AbstractChiral hybrid metal halides with a high dissymmetry factor (glum) and a superior photoluminescence quantum yield (PLQY) are promising candidates for circularly polarized luminescence (CPL) light sources. Here, we report eight new chiral hybrid manganese halides, crystallizing in the non‐centrosymmetric space group P212121 and showing intense CPL emissions. Oppositely‐signed circular dichroism (CD) and CPL signals are detected according to the R‐ and S‐configurations of the chiral alkanolammonium cations. Time‐resolved PL spectra show long averaged decay lifetimes up to 1 ms for (R‐3‐quinuclidinol)MnBr3 (R‐1). The glum of polycrystalline samples for coordinated structures (23×10−3) is more than doubled compared with the non‐coordinated ones (8.5×10−3), due to the structural variations. R‐1 exhibit both a high glum and a high PLQY (50.2 %). The effective chirality transfer mechanism through coordination bonds, with strongly emissive MnII centers, enables a new class of high‐performance CPL materials.

Structural Origin of Enhanced Circularly Polarized Luminescence in Hybrid Manganese Bromides.

Chiral hybrid metal halides with a high dissymmetry factor (glum ) and a superior photoluminescence quantum yield (PLQY) are promising candidates for circularly polarized luminescence (CPL) light sources. Here, we report eight new chiral hybrid manganese halides, crystallizing in the non-centrosymmetric space group P21 21 21 and showing intense CPL emissions. Oppositely-signed circular dichroism (CD) and CPL signals are detected according to the R- and S-configurations of the chiral alkanolammonium cations. Time-resolved PL spectra show long averaged decay lifetimes up to 1 ms for (R-3-quinuclidinol)MnBr3 (R-1). The glum of polycrystalline samples for coordinated structures (23×10-3 ) is more than doubled compared with the non-coordinated ones (8.5×10-3 ), due to the structural variations. R-1 exhibit both a high glum and a high PLQY (50.2 %). The effective chirality transfer mechanism through coordination bonds, with strongly emissive MnII centers, enables a new class of high-performance CPL materials.

Chiral Mesostructured Carbonate with Vibrational Circular Dichroism

AbstractThe absorption‐based optical activities (OA) of chiral inorganic materials without the assistance from organic molecules are limited in the plasmon of chiral metallic nanomaterials and the electron transition of chiral semiconductors at the ultraviolet–visible (UV–vis) region. In this work, chiral mesostructured manganese carbonate (CMMC) nano‐dumbbells with hierarchically chiral structures are synthesized, exhibiting vibrational circular dichroism (VCD) signals. CMMC powders are synthesized by a chiral‐molecule‐induced hydrothermal method. Three levels of hierarchical chirality exist in the CMMC, including primary twisted nanoflakes with distorted crystal lattices at the atomic level, secondary stacking of nanoflakes to form a nanoplate, and tertiary arranged nanoplates to form a twisting nano‐dumbbell. Antipodal CMMC samples exhibit antipodal VCD signals at the infrared region (IR) absorption 1400 and 860 cm–1, attributed to vibrational transitions in the asymmetric electric field and selective light harvesting in the hierarchically chiral structures.

Critical analysis of proximity-induced magnetism in MnTe/Bi2Te3 heterostructures

An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in $\mathrm{MnTe}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ layer. Furthermore, the interface between MnTe and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is not abrupt, but extending over $\ensuremath{\sim}2.2$ nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.

Picosecond time-resolved X-ray ferromagnetic resonance measurements at Shanghai synchrotron radiation facility

An experimental picosecond time-resolved X-ray ferromagnetic resonance (TR-XFMR) apparatus with a time resolution of 13 ps (RMS) or 31 ps (FWHM) was constructed and demonstrated in the 07U and 08U1A soft X-ray beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) using pump-probe detection and X-ray magnetic circular dichroism (XMCD) spectroscopy. Element and time-resolved ferromagnetic resonance was excited by continuous microwave phase-locking of the bunch clock within the photon beam during synchrotron radiation and was characterized by detecting the magnetic circular dichroism signals of the elements of interest in the magnetic films. Using this equipment, we measured the amplitude of the element-specific moment precession during ferromagnetic resonance (FMR) at 2 GHz in a single Ni81Fe19 layer.

Circularly Polarized Phosphorescence from Cocrystallization of Atomic Precise Silver Nanoclusters with Tartaric Acid

AbstractRecently, advanced optical materials with circularly polarized phosphorescence (CPP) have attracted much attention. However, such materials are limited due to the difficulty of preparation and the scarcity of precursors. Herein, CPP materials are constructed for the first time using water‐soluble, pseudo‐chiral silver nanoclusters with atomic precision ((NH4)9 [Ag9 (mba)9], H2mba = 2‐mercaptobenzoic acid, abbreviated to Ag9‐NCs hereafter). Induced by the complexation of l‐ or d‐tartaric acid (TrA) through hydrogen bonding, Ag9‐NCs crystallize from aqueous solutions, during which the chirality of the hybrid is amplified and the phosphorescence of Ag9‐NCs is enhanced. It is speculated that the chiral transfer from TrA to Ag9‐NCs is the main source of the well‐defined circular dichroism signals with good repeatability. The Ag9‐NCs/TrA cocrystals exhibit good CPP performance with dissymmetric factors up to 1.05 × 10−2, which is the highest value for metal NCs‐based CPP materials. This work is expected to be an inspiration for preparing metal NCs‐based CPP materials by supramolecular strategy. The so‐obtained organic–inorganic CPP materials can find a variety of applications in optoelectronic devices.

Enhanced circularly polarized luminescence from silica-coated self-assemblies of chiral tetraphenylethene-based amphiphiles

A pair of tetraphenylethene-based pyridinium salts with valine-containing attachments, TL and TD, were designed and synthesized. They emit faintly and give weak circular dichroism (CD) signals in dilute solutions, but show intense fluorescence and Cotton effects in aggregated state, displaying aggregation-induced emission (AIE) and CD (AICD) features. Their assemblies exhibit circular polarization luminescence (CPL) properties. Furthermore, the assemblies can be utilized as templates to fabricate twisted hybrid silica nanofibers through a supramolecular templating approach. Compared with the supramolecular assemblies, the obtained hybrid silicas show enhanced CPL activities and fluorescence efficiency due to the confinement effect of the silica frameworks.

Chiroptical Enhancement of Chiral Dicarboxylic Acids from Confinement in a Stereodynamic Supramolecular Cage.

The fundamental implications that chirality has in science and technology require continuous efforts for the development of fast, economic, and reliable quantitative methods for enantiopurity assessment. Among the different analytical approaches, chiroptical techniques in combination with supramolecular methodologies have shown promising results in terms of both costs and time analysis. In this article, a tris(2-pyridylmethyl)amines (TPMA)-based supramolecular cage is able to amplify the circular dichroism (CD) signal of a series of chiral dicarboxylic acids also in the presence of a complex mixture. This feature has been used to quantify tartaric acid in wines and to discriminate different matrixes using principal component analysis (PCA) of the raw CD data.

Light-induced ferromagnetism in moiré superlattices.

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena1-3. The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle1-3, electrical field4-6, moiré carrier filling7-11 and interlayer coupling12. Here we report that optical excitation can highly tune the spin-spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS2 /WSe2 moiré superlattices. Near the filling factor of -1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes9, and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter13-19.

Observing the orbital angular momentum of Fe and Co in chiral magnet Fe0.75Co0.25Si using soft x-ray magnetic circular dichroism

The intermetallic compound Fe1−xCoxSi has a helical magnetic order for 0.05<x<0.8, whereas its orbital angular momentum contributing to the occurrence of a Dzyaloshinskii–Moriya interaction has not yet been verified. We applied soft x-ray magnetic circular dichroism spectroscopy on the Fe L2,3 and Co L2,3 edges below Tc for Fe0.75Co0.25Si such that their orbital magnetic moments morb were evaluated independently. The dichroic signals provide direct experimental evidence that morb for both Fe and Co is coupled in a parallel manner with each spin counterpart mspin. The ratio of morb to mspin is independently estimated as morb/mspin∼3% for Fe and 9% for Co. By comparing the average of the two mspin values with the saturated magnetization using a commercial superconducting quantum interference device (SQUID) magnetometer, the hole number in the d bands for both Fe and Co is roughly estimated as approximately 1.5. This suggests that Fe and Co should be arranged closely for stabilizing the ferromagnetic moments.

Tunable Polarized Microcavity Characterized by Magnetic Circular Dichroism Spectrum.

Tunable resonator is a powerful building block in fields like color filtering and optical sensing. The control of its polarization characteristics can significantly expand the applications. Nevertheless, the methods for resonator dynamic tuning are limited. Here, a magnetically regulated circular polarized resonant microcavity is demonstrated with an ultrathin ferrimagnetic composite metal layer Ta/CoTb. We successfully tuned the cavity resonant frequency and polarization performance. A huge magnetic circular dichroism (MCD) signal (∼3.41%) is observed, and the microcavity valley position shifts 5.41 nm when a small magnetic field is applied. This resonant cavity has two-stable states at 0 T due to the magnetic remanence of CoTb film and can be switched using a tiny magnetic field (∼0.01 T). Our result shows that the ferrimagnetic film-based tunable microcavity can be a highly promising candidate for on-chip magneto-optical (MO) devices.

Can Weak Chirality Induce Strong Coupling between Resonant States?

Strong coupling between resonant states is usually achieved by modulating intrinsic parameters of optical systems, e.g., the refractive index of constituent materials or structural geometries. Externally introduced chiral enantiomers may couple resonances, but the extremely weak chirality of natural enantiomers largely prevents the system from reaching strong coupling regimes. Whether weak chirality could induce strong coupling between resonant states remains an open question. Here, we realize strong coupling between quasibound states in the continuum of a high-Q metasurface, assisted with externally introduced enantiomers of weak chirality. We establish a chirality-involved Hamiltonian to quantitatively describe the correlation between the coupling strength and the chirality of such systems, which provides an insightful recipe for enhancing the coupling of resonant states further in the presence of quite weak chirality. Consequently, high-sensitivity chiral sensing is demonstrated, in which the circular dichroism signal is enhanced 3 orders higher than the case without strong coupling. Our findings present a distinct strategy for manipulating optical coupling between resonances, revealing opportunities in chiral sensing, topological photonics, and quantum optics.

Anomalous circular phonon dichroism in transition metal dichalcogenides

A magnetic field can generally induce circular phonon dichroism based on the formation of Landau levels of electrons. Here, we study the magnetization-induced circular phonon dichroism in transition metal dichalcogenides, without forming Landau levels. We find that, instead of the conventional deformation potential coupling, pseudogauge-type electron-phonon coupling plays an essential role in the emergence of the phenomenon. As a concrete example, a large dichroism signal is obtained in monolayer MoTe2 on a EuO substrate, even without considering Rashba spin-orbit coupling. Due to the two-dimensional spin-valley-coupled band structure, MoTe2 shows a reciprocal and nonreciprocal absorption of circularly polarized acoustic phonons upon reversing the direction of phonon propagation and magnetization, respectively. By varying the gate voltage, a tunable circular phonon dichroism can be realized, which paves a way toward different physics and applications of two-dimensional acoustoelectronics.

All‐Optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces

AbstractOptical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all‐optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. This work theoretically predicts and experimentally demonstrates by pump–probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio‐temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out‐of‐equilibrium carriers and extinguished in a sub‐picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. Hence, nonlinear metasurfaces are here demonstrated to offer the flexibility to tailor their ultrafast optical response in a fully all‐optically reconfigurable platform.

Enhanced optical chirality with directional emission of Surface Plasmon Polaritons for chiral sensing applications

Chirality is a crucial aspect in life sciences, where systems capable of enhancing the chiroptical properties of molecules are highly demanded. In this work, we present a numerical proof of concept of a novel approach towards chiral sensing, consisting in the measurement of chiroptical properties via the directional emission of Surface Plasmon Polaritons (SPPs) on a metasurface. Based on the enhanced differential absorption between right and left circularly polarized light upon interaction with a metasurface made of high refractive index dielectric unit cells, a polarization-dependent SPP differential emission is obtained. Furthermore, the plasmonic emission direction is entirely dependent on the polarization handedness. Using FDTD numerical methods we report Circular Dichroism signals of around − 6° for the unit cell, with threefold dissymmetry factor enhancements in places accessible to analytes. We believe that this work sets a brand-new branch in chiral sensing towards faster, real-time measurements.

Ligand-Induced Ground- and Excited-State Chirality in Silicon Nanoparticles: Surface Interactions Matter.

Silicon-based light-emitting materials have emerged as a favorable substitute to various organic and inorganic systems due to silicon's high natural abundance, low toxicity, and excellent biocompatibility. However, efforts on the design of free-standing silicon nanoparticles with chiral non-racemic absorption and emission attributes are rather scare. Herein, we unravel the structural requirements for ligand-induced chirality in silicon-based nanomaterials by functionalizing with D- and L-isomers of a bifunctional ligand, namely, tryptophan. The structural aspects of these systems are established using high-resolution high-angle annular dark-field imaging in the scanning transmission electron microscopy mode, solid-state nuclear magnetic resonance, Fourier transform infrared, and X-ray photoelectron spectroscopy. Silicon nanoparticles capped with L- and D-isomers of tryptophan displayed positive and negative monosignated circular dichroic signals and circularly polarized luminescence indicating their ground- and excited-state chirality. Various studies supported by density functional theory calculations signify that the functionalization of indole ring nitrogen on the silicon surface plays a decisive role in modifying the chiroptical characteristics by generating emissive charge-transfer states. The chiroptical responses originate from the multipoint interactions of tryptophan with the nanoparticle surface through the indole nitrogen and -CO2- groups that can transmit an enantiomeric structural imprint on the silicon surface. However, chiroptical properties are not observed in phenylalanine- and alanine-capped silicon nanoparticles, which are devoid of Si-N bonds and chiral footprints. Thus, the ground- and excited-state chiroptics in tryptophan-capped silicon nanoparticles originates from the collective effect of ligand-bound emissive charge-transfer states and chiral footprints. Being the first report on the circularly polarized luminescence in silicon nanoparticles, this work will open newer possibilities in the field of chirality.

X-ray magnetic dichroism and tunnel magneto-resistance study of the magnetic phase in epitaxial CrVO x nanoclusters

Epitaxial clusters of chromium and chromium–vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr2O3-like phase, whereas usual Cr2O3 is known to be anti-ferromagnetic in the bulk. For mixed clusters, vanadium ions also contribute to the total magnetization and they are coupled to the chromium ion spins. By measuring the dichroic signal at different temperatures, we get insight into the possible spin configurations of vanadium and chromium ions: we propose that the magnetic dipoles observed in the clusters assembly could be related to ionic spins that couple at a very short range, as for instance in short one-dimensional spins chains.

Local controllability of hot electron and thermal effects enabled by chiral plasmonic nanostructures

AbstractThe control of hot electron (HE) and thermal effects induced by plasmonic nanostructures has recently attracted considerable attention. When illuminated by light with different circular polarization states, the circular dichroism signal of molecules adsorbed by plasmonic chiral nanostructures can control HE and thermal effects. These effects have the potential to enhance reaction rates and to change selectivity patterns in photothermal catalysis. Here, we propose an aluminum L-shaped chiral nanostructure system in which HE and thermal effects can be controlled in different regions of the nanostructure by changing the chirality of the excitation light. A large difference of 12.75% in the HE effect but a virtually identical thermal effect can be achieved in different regions of the nanostructure by selecting the appropriate probed region, while a large thermal effect difference of 65.67% but a virtually identical HE effect can be achieved in one region of the nanostructure by changing the polarization state of the excitation light. In addition, the HE and thermal chiral selectivity effects of double L-shaped nanostructures are investigated as these structures can be more easily controlled during asymmetric chiral growth and crystallization. This work combined with plasmonic chirality is beneficial for quantifying HE and thermal effects in photochemical reactions and provides theoretical support for designing catalysts and optimizing plasmonic platforms. Additionally, the local controllability of HE and thermal effects plays an essential role in high-resolution photochemical reactions, especially in single-molecule photochemical reactions.

Spin Selectivity of Chiral Mesostructured Iron Oxides with Different Magnetisms.

Spin selectivity physically depends on either magnetic materials with strong internal magnetic fields or symmetry-breaking materials with large spin-orbit coupling (SOC). However, the spin selectivity of symmetry-breaking magnetic materials is not understood. Herein, the spin selectivity of iron oxides with different magnetisms arising from varying spin alignment is investigated. Chiral mesostructured films of Fe3 O4 (CMFFs), γ-Fe2 O3 (CMγFs), and α-Fe2 O3 (CMαFs), which share the same mesostructure, are prepared by a controllable calcination process of chiral mesostructured FeOOH films (CMOFs) grown on the substrate via an amino acid-induced hydrothermal route. CMFFs and CMγFs with ferrimagnetism exhibit magnetic field-dependent and simultaneously chirality-independent magnetic circular dichroism (MCD) signals, while CMαFs with antiferromagnetism exhibit chirality-dependent, magnetic field-independent MCD signals. It is speculated that the competitive effect between the spin alignment-induced and chirality-induced effective magnetic fields determines the energy splitting of opposite spins in the materials with different magnetisms.

Engineering and Application of Pillar[6]arene Functionalized Chiral Surface in Selective Adsorption of R‐Adrenaline

Comprehensive SummaryChiral properties of biological surface is of importance as they dominate interactions with biomolecules (such as proteins, nucleic acids, hormones) in a living system. Inspired by this phenomenon, we designed and constructed a L‐/D‐N‐acetyl‐cysteine‐pillar[6]arene functionalized Si surface (L‐/D‐NACP surface) for studying the influence of superficial chirality on selective adsorption of R‐adrenaline. The fabricated chiral L‐/D‐NACP surface with consistency in hydrophilia and elementary composition exhibited a strong mirror‐symmetric circular dichroism signal. In the adsorption study, the contact‐angle of R‐adrenaline droplet on D‐NACP surface was more hydrophilic from macroscopical view, and the size and quantity of R‐adrenaline aggregation on D‐NACP surface was larger than L‐NACP surface, implying selective adsorption of R‐adrenaline appeared on the D‐NACP surface. The possible mechanism was due to the stronger host‐guest interaction between D‐NACP and R‐adrenaline, which facilitated the selective adsorption. Besides, the D‐NACP surface with good selectivity can be used for chiral separation of racemic adrenaline. The results enhance the understanding of hormones adsorption affected by surface chirality, and may provide some helpful inspiration for practical application of chiral surface.