Chiroptical Response Research Articles

Optical Asymmetry and Structural Complexity in Hierarchically Organized Chiral CuO Nanostructures: Insight into the Geometric and Crystallographic Effects on Cooperative Chirality.

Optical asymmetry and structural complexity across different length scales were realized in flower-shaped CuO nanostructures, prepared through refluxing an aqueous solution of copper acetate, sodium hydroxide, and D-tartaric acid, as well as in their toroid-like forms obtained on calcination at 600 °C. Atomic scale chirality in the flower morphology could be visualized as putative Boerdijk-Coexter-Bernal like tetrahelical fragments, while that in the toroid form could be identified as screw dislocation-driven helicity. The fraction of asymmetry in the nanostructures has been evaluated from their chiroptical responses based on Kuhn asymmetry factor (g) from circular dichroism (CD) spectroscopy in the entire UV-vis range. The origin of chirality in the two CuO nanostructures has been assigned to the helical arrangement of the Cu-O-Cu network in accordance with their microscopic and spectroscopic observations. Attempts have been made to interpret the crystallographic and geometric chiralities in the two CuO nanostructures based on the redshift and augmented intensity of the CD signal along with an increase in their corresponding anisotropic factor on calcination. Further, the diverse interaction of the toroid-shaped CuO nanostructures with enantiomeric tryptophan moieties has been illustrated from the measurement of their corresponding thermodynamic parameters.

Chiroptical Recognition of Carboxylates with Charge-Neutral Double-Stranded Zinc(II) Helicates.

Chirality analysis of small molecules for the determination of their enantiopurity is nowadays ruled by streamlined chromatographic methods which utilize chiral stationary phases. Chiroptical probes which rely on host-guest interactions are so far overshadowed by the latter but have the benefit of depending only on common spectroscopic techniques such as CD spectroscopy to distinguish enantiomers and to quantify their ratio. Interest into this receptor-based approach is constantly rising because non-invasive high-throughput screenings with a minimal waste production can be performed. In this study we investigate the possibility to utilize metal-based containers in form of charge-neutral helicates able to recognize anions for this purpose. Key building block of the helicates are triazole units which show rotational freedom and give rise to either a meso-structure or a racemic mixture of the right- and left-handed complex. A chiroptical response of the probe is observed upon recognition of chiral mono- or dicarboxylates and chirality analysis of tartrate is conducted by CD spectroscopy.

Chiral Metasurfaces: A Review of the Fundamentals and Research Advances

Chirality, the absence of mirror symmetry, is predominant in nature. The chiral nature of the electromagnetic field behaves differently with chiral matter for left circularly polarized and right circularly polarized light. The chiroptical behavior in the sensing of naturally occurring chiral objects is weak, and improving the chiroptical response enhances the chiral sensing platform. This review covers the fundamental concepts of chiral metasurfaces and various types of single- and multi-layered chiral metasurfaces. In addition, we discuss tunable and deep-learning-based chiral metasurfaces. Tunability is achieved by manipulating the meta-atom’s property in response to external stimuli for applications such as optical modulation, chiral photonics, advanced sensing, and adaptive optics. Deep-learning modeling techniques, such as CNNs and GANs, offer efficient learning of the complex relationships in data, enabling the optimization and accurate prediction of chiral metasurface properties. The challenges in the design and fabrication of chiral metasurface include achieving broadband performance and scalability and addressing material limitations. Chiral metasurface performance is evaluated by optical rotation, circular dichroism enhancement, and tunability, which are quantified through the spectroscopic measurement of circular dichroism and optical rotation. Chiral metasurface progress enables applications, including metaholography, metalenses, and chiral sensing. Chiral sensing improves the detection of pharmaceuticals and biomolecules, increasing the sensitivity and accuracy of analytical diagnostics.

Two-Dimensional Chiral Metasurfaces Obtained by Geometrically Simple Meta-atom Rotations.

Two-dimensional chiral metasurfaces seem to contradict Lord Kelvin's geometric definition of chirality since they can be made to coincide by performing rotational operations. Nevertheless, most planar chiral metasurface designs often use complex meta-atom shapes to create flat versions of three-dimensional helices, although the visual appearance does not improve their chiroptical response but complicates their optimization and fabrication due to the resulting large parameter space. Here we present one of the geometrically simplest two-dimensional chiral metasurface platforms consisting of achiral dielectric rods arranged in a square lattice. Chirality is created by rotating the individual meta-atoms, making their arrangement chiral and leading to chiroptical responses that are stronger or comparable to more complex designs. We show that resonances depending on the arrangement are robust against geometric variations and behave similarly in experiments and simulations. Finally, we explain the origin of chirality and behavior of our platform by simple considerations of the geometric asymmetry and gap size.

Coherent multipolar amplification of chiroptical scattering and absorption from a magnetoelectric nanoparticle

Background-free detection of inherently weak chiroptical signals remains one of the great challenges in research communities and industries. We demonstrate coherent multipolar amplification of chiroptical responses via a magnetoelectric nanoparticle capped with an optically active monolayer encapsulated in a lossless background medium. Such an achiral nanoparticle can simultaneously support both electric and magnetic Mie-type resonances. We show how the combined excitation of orthogonal multipolar modes of the same order boosts the magnetoelectric coupling induced by the adsorbed chiral molecules, thus enabling coherently enhanced chiroptical responses from the ligand-capped magnetoelectric nanoparticle and allowing for absolute chirality measurements, in comparison with non-magnetoelectric nanoparticles. Furthermore, we develop rigorous expressions to separate relative contributions of chiral and nonchiral portions of circular differential absorption cross section, and analyzed the chirality-dependent far-field radiation patterns at different overlapped multipolar modes, providing a theoretical framework to understand the underlying enhancement mechanism of the magnetoelectric-assisted sensing of molecular chirality.

Chiroptical Response Inversion and Enhancement of Room-Temperature Exciton-Polaritons Using 2D Chirality in Perovskites.

Although chiral semiconductors have shown promising progress in direct circularly polarized light (CPL) detection and emission, they still face potential challenges. A chirality-switching mechanism or approach integrating two enantiomers is needed to discriminate the handedness of a given CPL; additionally, a large material volume is required for sufficient chiroptical interaction. These two requirements pose significant obstacles to the simplification and miniaturization of the devices. Here, room-temperature chiral polaritons fulfilling dual-handedness functions and exhibiting a more-than-two-order enhancement of the chiroptical signal are demonstrated, by embedding a 40 nm-thick perovskite film with a 2Dchiroptical effect into a Fabry-Pérot cavity. By mixing chiral perovskites with different crystal structures, a pronounced 2Dchiroptical effect is accomplished in the perovskite film, featured by an inverted chiroptical response for counter-propagating CPL. This inversion behavior matches the photonic handedness switch during CPL circulation in the Fabry-Pérotcavity, thus harvesting giant enhancement of the chiroptical response. Furthermore, affected by the unique quarter-wave-plate effects, the polariton emission achieves a chiral dissymmetry of ±4% (for the emission from the front and the back sides). The room-temperature polaritons with the strong dissymmetric chiroptical interaction shall have implications on a fundamental level and future on-chip applications for biomolecule analysis and quantum computing.

Synthesis And Conformational Behaviors of Unnatural Peptides Alternating Chiral And Achiral α,α-Disubstituted α-Amino Acid Units.

The development of peptidomimetics to modulate the conformational profile of peptides has been extensively studied in the fields of biological and medicinal chemistry. However, large-scale synthesis of peptidomimetics with both an ordered sequence and a controlled secondary structure is highly challenging. In this paper, the framework of peptidomimetics has been designed to be alternating an achiral α,α-disubstituted α-amino acid unit and a chiral α-methylphenylalanine unit. The polymers were synthesized via our invented Ugi reaction-based polycondensation technique. The chiral higher-order structures of the alternating peptides were evaluated mainly through CD spectroscopy. The UV-vis and CD spectra of the polymers in three solvents were systematically measured at various temperatures. The anisotropic factors of CD (gCD ) values were calculated to know the chiroptical response. The results indicate the characteristic conformational behaviors. In a polar solvent, the hydrogen bonds between the N-H group of MePhe unit and the C = O of α,α-diphenylglycine unit outweigh the intraresidue hydrogen bonds in α,α-diphenylglycine unit, leading to the formation of a prevailing preferred-handed 310 -helical conformation. On the other hand, in a less polar solvent, the intrachain hydrogen bonds switch to intraresidue hydrogen bonds in α,α-diphenylglycine unit, which make the polymer adopting a prevailing extended planar C5 -conformation. This article is protected by copyright. All rights reserved.

Plexcitonic optical chirality in the chiral plasmonic structure-microcavity-exciton strong coupling system.

Chiral plexcitonic systems exhibit a novel chiroptical phenomenon, which can provide a new route to design chiroptical devices. Reported works focused on the two-mode strong coupling between chiral molecules and nanoparticles, while multiple-mode coupling can provide richer modulation. In this paper, we proposed a three-mode coupling system consisting of a chiral Au helices array, a Fabry-Pérot cavity, and monolayer WSe2, which can provide an extra chiral channel, a more widely tunable region, and more tunable methods compared to two-mode coupled systems. The optical response of this hybrid system was investigated based on the finite element method. Mode splitting observed in the circular dichroism (CD) spectrum demonstrated that the chiroptical response successfully shifted from the resonant position of the chiral structure to three plexcitons through strong coupling, which provided a new route for chiral transfer. Furthermore, we used the coupled oscillator model to obtain the energy and Hopfield coefficients of the plexciton branches to explain the chiroptical phenomenon of the hybrid system. Moreover, the tunability of the hybrid system can be achieved by tuning the temperature and period of the helices array. Our work provides a feasible strategy for chiral sensing and modulation devices.

High‐Q and Strong Chiroptical Responses in Planar Metasurfaces Empowered by Mie Surface Lattice Resonances

AbstractPlanar chiral metasurfaces have emerged as a versatile platform for manipulating chiral light‐matter interactions in diverse applications. However, it remains challenging to achieve high quality factors together with strong chirality. Here, chiral all‐dielectric metasurfaces with a high Q factor of 1220 and relatively large planar chirality of circular dichroism −0.37 under normal incidence, or with a high Q factor of 872 and large extrinsic 3D chirality of circular dichroism 0.7, based on Mie surface lattice resonances (SLRs) are proposed and experimentally demonstrated. Both SLR‐induced planar chirality under normal incidence and SLR‐assisted extrinsic 3D chirality under oblique incidence are achieved by perturbing the square lattice and by slightly tilting the incidence angle, respectively. It shows that the high‐Q and strong chirality arises from the different excitation efficiencies of Mie SLRs under the incidences of left‐ and right‐handed circularly polarized light. With these findings, this study provides a new direction for realizing high‐performance chiral metasurfaces and holds great promise for chiral nanophotonic applications in biochemical sensing, emission or lasing, and nonlinear optics.

Nanoparticles with Chiroptical Responses

Nanoparticles with Chiroptical Responses

High Q Chiroptical Responses Driven by Toroidal Dipole Resonance in All-Dielectric Metasurfaces

High Q Chiroptical Responses Driven by Toroidal Dipole Resonance in All-Dielectric Metasurfaces

Polarization Resolved Fluorescence Detection of Carbon Nanotube Corona Phases

Circular dichroism measures how chiral particles absorb right- and left- circularly polarized light differently to distinguish pairs of enantiomers. It can also elucidate differential photoabsorption of chiral fluorescent species via their emissions. However, this method has generally been restricted to bulk samples. Hence, these methods conceal information on the specific chiroptical response of a single particle. Furthermore, circular dichroism has been restricted in time- resolution as well as spectral range. In this work, we observed fluorescence detected circular dichroism (FDCD) of individually deposited single walled carbon nanotube (SWNT) corona. A custom-built setup compromising a linear polarizer (LP) and a quarter waveplate (QWP) was used to induce circularly polarized light (CPL). The light polarization state at the sample plane was analyzed using Stoke’s parameter. Experimental results showed that linear dichroism of the dichroic filter inside the microscope changes the polarization state of the incoming beam drastically, and thus inducing elliptically polarized light at the sample plane. A liquid crystal variable retarded (LCVR) was used to correct the polarization state of light and yield CPL at the sample plane. Singly-dispersed SWNT with different wrapping polymers were deposited onto a glass substrate. Alternating cycles of right- and left-handed circularly polarized light was delivered to excite the sample. The fluorescence intensities of each individual SWNT were tracked. The dissymmetry (g) parameters: g = 2(IL-IR)/(IL+IR) (where I L and I R are the measured fluorescence intensities with left and right circularly polarized light, respectively) were analyzed from the fluorescent signals to understand photoabsorption differences. A non-zero value of dissymmetry parameter provides information on how the corona modulates SWNT photoluminescence, an outcome not observable by bulk characterization methods. This observation can potentially enable the measurement of chiral species local interactions without a separations process.

(Invited) Tuning Optoelectronic and Chiroptical Properties of Carbon Dots Based Hybrids

Carbon dots (CDs) are a rising star in carbon nanomaterials due to their easy preparation and numerous potential applications. With a diameter from a few to tens of nanometers, and a tunable structure, composition, and morphology, they integrate the merits of water solubility, low biological toxicity, and diversity of opto-electronic properties. Commonly, they are categorized into different subgroups, where graphene quantum dots (GQDs) and carbon nanodots (CNDs) can be found. GQDs are two-dimensional nanoparticles of one or few-layer graphene sheets formed during the exfoliation processes of graphitic materials, whereas CNDs are amorphous nanoparticles fabricated from easily affordable organic precursors, natural products, or waste.In recent years, we have set-up methodologies for the synthesis of GQDs and CNDs, carried out a detailed characterization of the obtained structures, and explored the modulation of properties by chemical modification of these carbon nanostructures, as part of the objectives of our research.Special attention was dedicated to customize the optoelectronic properties of GQDs and CNDs, and to investigate the electronic communication in related functional materials.1 Both, electron-donor and electron-acceptor character of GQDs and CNDs, make their charge-transfer chemistry rather versatile. Charge-separated states are observed in the photoexcitation of CNDs previously endowed with electron-donating extended tetrathiafulvalenes (exTTF) or electron-accepting 11,11,12,12-tetracyano-9,10-anthra-p-quinodimethanes (TCAQ).In a different approach, the top-down synthesis of chiral GQDs by covalent functionalization of GQDs with chiral ligands has been explored.2 Chirality can actually be transferred to a supramolecular structure built with pyrene molecules, where the chiral-GQDs/pyrene ensembles show a characteristic chiroptical response depending on the configuration of the organic ligands introduced on the GQDs structure. Furthermore, amide hydrogen bonding and π-π stacking interactions could drive the self-aggregation process to construct supramolecular polymers at the nanoscale (Figure).Within this contribution, we will present and discuss the most recent advances on the use of CDs in our group. References a) Ferrer-Ruiz, A.; Scharl, T.; Haines, P.; Rodríguez-Pérez, L.; Cadranel, A.; Herranz, M. A.; Guldi, D. M.; Martín, N. Angew. Chem. Int. Ed. 2018, 57, 1001-1005; b) Scharl, T.; Ferrer-Ruiz, A.; Saura-Sanmartín, A.; Rodríguez-Pérez, L.; Herranz, M. A.; Martín, N.; Guldi, D. M. Chem. Commun., 2019, 55, 3223-3226; c) Ferrer-Ruiz, A.; Scharl, T.; Rodríguez-Pérez, L.; Cadranel, A.; Herranz, M. A.; Martín, N.; Guldi, D. M. J. Am. Chem. Soc. 2020, 142, 20324−20328.a) Vázquez-Nakagawa, M.; Rodríguez-Pérez, L.; Herranz, M. A.; Martín, N. Chem. Commun., 2016, 52, 665-668, b) Vázquez-Nakagawa, M.; Rodríguez-Pérez, L:, Martín, N.; Herranz, M. A.; Angew. Chem. Int. Ed., 2022, 61, e202211365. Figure 1

Temperature-modulated inversion and switching of chiroptical responses in dynamic side-by-side oligomers of gold nanorods

Temperature-modulated inversion and switching of chiroptical responses in dynamic side-by-side oligomers of gold nanorods

Surface Topographical Engineering of Chiral Au Nanocrystals with Chiral Hot Spots for Plasmon-Enhanced Chiral Discrimination.

Surface roughness in chiral plasmonic nanostructures generates asymmetrical localized electromagnetic fields, which hold great promise for applications in chiral recognition, chiroptical spectroscopic sensing, and enantioselective photocatalysis. In this study, we develop a surface topographical engineering approach to precisely manipulate the surface structures of chiral Au nanocrystals. Through carefully controlling the amounts of l- or d-cystine (Cys) and the seed solution in the growth process, we successfully synthesize chiral Au nanocrystals with highly disordered, ordered, and less ordered wrinkled surfaces. An underlying principle governing the relationship between surface roughness, orderliness, and chiroptical response is also proposed. More importantly, the chiral ordered wrinkles on the surfaces of the nanocrystals generate asymmetrical localized electronic fields with enhanced intensity, which achieve excellent plasmon-enhanced chiral discrimination ability for penicillamine (Pen) enantiomers. This work offers exciting prospects for manipulating the surface structures of chiral nanocrystals and designing highly sensitive plasmon-enhanced enantioselective sensors with chiral hot spots.

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks.

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Multi-mode optical chirality extremizations on the incident momentum sphere.

We study the momentum-space evolutions for chiral optical responses of multi-mode resonators scattering plane waves of varying incident directions. It was revealed, in our previous study [Phys. Rev. Lett.126, 253901 (2021)10.1103/PhysRevLett.126.253901], that for single-mode resonators the scattering optical chiralities characterized by circular dichroism (CD) are solely decided by the third Stokes parameter distributions of the quasi-normal mode (QNM) radiations: CD = S3. Here we extend the investigations to multi-mode resonators, and explore numerically the dependence of optical chiralities on incident directions from the perspectives of QNM radiations and their circular polarization singularities. In contrast to the single-mode regime, for multi-mode resonators it is discovered that CDs defined in terms of extinction, scattering and absorption generally are different and cannot reach the ideal values of ±1 throughout the momentum sphere. Though the exact correspondence between CD and S3 does not hold anymore in the multi-mode regime, we demonstrate that the positions of the polarization singularities still serve as an efficient guide for identifying those incident directions where the optical chiralities can be extremized.

Characterization of Chiral Nanostructured Surfaces Made via Colloidal Lithography.

Optically anisotropic materials were produced via colloidal lithography and characterized using scanning electronic microscopy (SEM), confocal microscopy, and polarimetry. A compact hexagonal array mask composed of silica sub-micron particles was fabricated via the Langmuir-Blodgett self-assembly technique. Subsequently, the mask pattern was transferred onto monocrystalline silicon and commercial glass substrates using ion beam etching in a vacuum. Varying the azimuthal angle while etching at oblique incidence carved screw-like shaped pillars into the substrates, resulting in heterochiral structures depending on the azimuthal angle direction. To enhance the material's optical properties through plasmon resonance, gold films were deposited onto the pillars. Polarimetric measurements were realized at normal and oblique incidences, showing that the etching directions have a clear influence on the value of the linear birefringence and linear dichroism. The polarimetric properties, especially the chiroptical responses, increased with the increase in the angle of incidence.

Tunable Optical Activity in Twisted Anisotropic Two-Dimensional Materials.

Twisted van der Waals structures exhibit a variety of unusual electrical and optical phenomena and could provide a powerful means for designing nanodevices with tunable chiral properties. However, programming intrinsic chiral properties of the film on the atomic scale remains a great challenge due to the limitations of fabrication and measurement techniques. Here, we report a highly tunable large optical activity of twisted anisotropic two-dimensional (2D) materials, including black phosphorus (BP), ReS2, PdSe2, and α-MoO3, by varying the twist angle between the stacked layers. The chirality can be deliberately tailored through the engineering of the symmetry, band structure, and anisotropy of 2D materials, demonstrating the high tunability of the chirality. The results show the highest thickness-normalized ellipticity value (13.8 deg μm-1, twisted ReS2) and ellipticity value (1581 mdeg, twisted BP) among the systems based on 2D materials. It is also shown that the chiroptical response exists in an extremely large spectral range from the visible to the infrared. Furthermore, the twisted ReS2 enabled spin-selective control of the information transformation. These results show that highly controllable chirality in twisted 2D anisotropic materials has considerable potential in on-chip polarization optics, nano-optoelectronics, and biology.

Collectively Enhanced Giant Circular Dichroism of Germanium Nanohelix Square Lattice Arrays

Circular dichroism is a unique chiroptical signature of the chirality of a system and is a prevalent way to characterize and distinguish systems on a fundamental level and for their technological applicability. Thus, engineering and maximizing the chiroptical response of a single chiral object or a metasurface composed of chiral entities is a formidable task. Current efforts strongly focus on individual metallic nanostructures and their periodic ensembles to harvest on (resonant) plasmonic properties and interactions. This route, however, waives the advantages of high‐refractive‐index nanoscale materials embracing low dissipative losses at optical wavelengths and electromagnetic fields penetrating and propagating in such materials. Herein, a strong circular dichroism is demonstrated in square lattices of nanohelices made of the high‐refractive‐index semiconductor germanium, with dissymmetry factors outperforming metal‐based ensembles. The observation of a much higher dissymmetry emerges for illumination with spatially coherent light, in comparison to spatially incoherent light. High dissymmetry is attributed to cooperative coupling between single helices, resulting from the combination of dielectric resonances of both the individual helical building blocks and the highly ordered lattice.