Intrinsic Circular Dichroism Research Articles

Steering high-Q intrinsic chiral quasi-bound states in the continuum via engineered 2.5D phase-change metasurfaces.

High-Q intrinsic quasi-bound states in the continuum (QBICs) require three-dimensional (3D) geometries with both in-plane and out-of-plane mirror symmetry breakings, hindering practical implementations due to the complex architectures. Here we demonstrate that high-Q intrinsic QBICs can be flexibly controlled by using the engineered 2.5D phase-change metasurfaces of Ge2Sb2Te5 (GST). By introducing the additional out-of-plane perturbations of slant angle θ and azimuthal angle φ, highly efficient and high-Q intrinsic circular dichroism (CD) for both reflection and transmission can be realized. The spinning-selected magnetic dipole (MD) is responsible for the high-Q intrinsic chirality. The high-Q intrinsic CD is robust to the variation of structural parameters, and its Q-factor and resonance location can be tuned through the phase transition of GST.

Enhanced intrinsic chiroptical response of resonant metallic metasurfaces.

The physics of resonant metasurfaces underpins many electromagnetic functionalities with enhanced performance by virtue of resonant excitations. Resonances originating from bound states in the continuum (BICs) were recently recognized in photonics for their superior optical properties, strong local field enhancement, and suppression of radiative losses. Very recently, a concept of intrinsically chiral dielectric BIC metasurfaces was proposed that combines strong narrowband resonant features with the polarization control of scattered light. Here, we design a resonant chiral metallic metasurface supporting a BIC resonance in the microwave wavelength range. In our structure, the metasurface units (meta-atoms) are characterized with rotational and mirror spatial symmetries. We numerically characterize metasurface mode properties in eigenmode calculations and scattering spectra for linearly polarized excitation under oblique incidence. Then, we investigate intrinsic chiroptical effects for transmission of normally propagating excitation beams by breaking the meta-atom in-plane mirror symmetries. We predict that the intrinsic circular dichroism in such structures may exceed 0.74.

Tailoring intrinsic chiroptical responses via twisted bilayer α-MoO3 separated by a VO2 film

Flexible control of intrinsic chiroptical responses within compact nanostructures is crucial for flat optics, topological photonics, and chiroptics. However, previous approaches require complicated patterns with both in-plane and out-of-plane mirror symmetry breaking to achieve intrinsic chirality, and their chiroptical responses cannot be dynamically controlled as well. Herein, we demonstrated that near-perfect intrinsic circular dichroism (CD) can be achieved within a lithography-free structure consisting of the twisted bilayer α-MoO3 separated by a vanadium dioxide (VO2) film. By twisting the bilayer α-MoO3, dual-band intrinsic chiroptical responses can be realized due to the excitations of the hyperbolic phonon polaritons modes in the mid-infrared. It is the spin-selected average electric-field enhancement instead of the chiral absorption that is responsible for the intrinsic CD of the device. In addition, the chiroptical responses are insensitive to the variation of the thickness of the structure as well as the incident angle, and high contrast CD can be dynamically tuned by varying the volume fraction of VO2.

Large exchange-driven intrinsic circular dichroism of a chiral 2D hybrid perovskite

In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.

Surface Lattice Resonances in 3D Chiral Metacrystals for Plasmonic Sensing.

Chiral lattice modes are hybrid states arising from the chiral plasmonic particles assembled in ordered arrays with opportune periodicity. These resonances exhibit dependence on excitation handedness, and their observation in plasmonic lattices is strictly related to the chiroptical features of the fundamental plasmonic unit. Here, the emergence of chiral surface lattice resonances (c-SLRs) is shown in properly engineered arrays of nanohelices (NHs), fully three dimensional (3D)chiral nano-objects fabricated by focused ion beam processing. By tuning the relative weight of plasmonic and photonic components in the hybrid mode, the physical mechanism of strong diffractive coupling leading to the emergence of the lattice modes is analyzed, opening the way to the engineering of chiral plasmonic systems for sensing applications. In particular, a coupling regime is identified where the combination of a large intrinsic circular dichroism (CD) of the plasmonic resonance with a well-defined balance between the photonic quality factor (Q factor) and the plasmonic field enhancement (M) maximizes the capability of the system to discriminate refractive index (RI) changes in the surrounding medium. The results lay the foundation for exploiting CD in plasmonic lattices to high performance refractometric sensing.

Circularly Polarized Photoluminescence of Chiral 2D Halide Perovskites at Room Temperature.

Chiral halide perovskites have attracted considerable attention because of their chiroptical, second-harmonic generation, and ferroelectricity properties and their potential application in chiroptoelectronics and chiral spintronics. However, the fundamental research of these properties is insufficient. In this work, chiral perovskites were synthesized using precursor solutions with various stoichiometric ratios ⟨n⟩. The chiral perovskite film prepared from the solution with ⟨n⟩ = 1 is composed of (R-/S-/rac-MBA)2PbBr4, whereas the films prepared from the solutions with ⟨n⟩ larger than 1 are a mixture of (R-/S-/rac-MBA)2(CsMA)n-1PbnBr3n+1 with n = 1 and large n values. A photoluminescence quantum yield of approximately 90 was obtained. Symmetric circular dichroism (CD) spectra were observed without an external magnetic field. Under various magnetic fields, magnetic field-induced CD features are superimposed with the intrinsic chirality-induced CD features. For the ⟨n⟩ = 1 chiral perovskite film, the energy level splitting induced by chiral molecules are a few 10 μeV, whereas the energy level splitting induced by magnetic fields are at the range of ∼-250 to ∼250 μeV. Circularly polarized photoluminescence spectra were observed at room temperature and associated with the spin-preserved energy funneling from highly energetic phases to the lower energetic phases.

Active Ag/Co Composite Chiral Nanohole Arrays

The magneto-chiroptical properties of Ag/Co composite chiral nanohole arrays (CCNAs) are investigated. The intrinsic circular dichroism of CCNA samples was induced by in-plane symmetry breaking by a glancing angle deposition method, and it can be enhanced strongly by surface plasmon resonance when Ag is a major component. On the other hand, the Co component allows for magnetic tuning circular dichroism, with the maximum modulation amplitude up to 0.35°. Remarkably, the enhanced local electric field induced by Wood’s anomalies gives rise to sharper and stronger magnetic circular dichroism (normalized for the same Co content) of samples with high Ag concentration. The result demonstrates the possibility of fabricating large-scale active chiroptical metasurfaces using CCNAs, which is promising for imaging and sensing applications.

Giant intrinsic circular dichroism of enantiomorphic flat Chern bands and flatband devices

Circular dichroism (CD) is generally observed in the optically active chiral molecules that originate from macroscopic electric and magnetic dipoles, which is usually quite small. In solid states, the so-called valley CD may arise microscopically from interband transitions between two chiral electronic valley bands of nonzero Berry curvatures at a given $k$ point. However, generally, two sets of $K$ and ${K}^{\ensuremath{'}}$ valleys coexist in the Brillouin zone with opposite chiral selectivities, so that the net CD is zero for the whole material. Here, we demonstrate a giant CD originating from photoexcitation between two chiral Chern flat bands of opposite Chern numbers, namely, the enantiomorphic flat Chern bands. The dissymmetry factor $g$ of such flat CD can reach the theoretical maximum value of 2 with the optimal spin-orbit coupling strength. Based on first-principles calculations, we identify that the Li intercalated bilayer $\ensuremath{\pi}$-conjugated nickel-bis(dithiolene) hosts a set of yin-yang kagome bands with an estimated large $g=0.74$ under magnetic field. Furthermore, based on the flat-CD mechanism, we propose two flatband devices of topological photodetectors and circularly polarized lasers.

Dynamic tuning of enhanced intrinsic circular dichroism in plasmonic stereo-metamolecule array with surface lattice resonance

Abstract Enhancing the circular dichroism signals of chiral plasmonic nanostructures is vital for realizing miniaturized functional chiroptical devices, such as ultrathin wave plates and high-performance chiral biosensors. Rationally assembling individual plasmonic metamolecules into coupled nanoclusters or periodic arrays provides an extra degree of freedom to effectively manipulate and leverage the intrinsic circular dichroism of the constituent structures. Here, we show that sophisticated manipulation over the geometric parameters of a plasmonic stereo-metamolecule array enables selective excitation of its surface lattice resonance mode either by left- or right-handed circularly polarized incidence through diffraction coupling, which can significantly amplify the differential absorption and hence the intrinsic circular dichroism. In particular, since the diffraction coupling requires no index-matching condition and its handedness can be switched by manipulating the refractive index of either the superstrate or the substrate, it is therefore possible to achieve dynamic tuning and active control of the intrinsic circular dichroism response without the need of modifying structure parameters. Our proposed system provides a versatile platform for ultrasensitive chiral plasmonics biosensing and light field manipulation.

Mueller matrix study of the dichroism in nanorods dimers: rod separation effects.

We have studied the optical response of chiral metastructures composed of a disordered array of couples of plasmonic Au nanorods helically piled along the vertical direction. The fabrication is based on the use of multiaxial and multimaterial evaporation of the different metastructure building blocks through nanohole masks. From the analysis of the Mueller Matrix elements of the system, obtained both experimentally and from dedicated numerical simulations in forward and backward illumination conditions, we have been able to determine the linear and circular dichroic response of the system, as well as to sort out the optical anisotropy and intrinsic circular dichroism contributions to the circular differential extinction. We have also analyzed the dependence of the optical properties as a function of the angle between the rods and of the thickness of the dielectric separator. The study of quasi-planar as well as three-dimensional structures allows unraveling the role played by interactions between the constituting building blocks and, in particular, the distance between rods. We have experimentally and theoretically observed a decrease of the circular dichroic contribution and a change of the optical anisotropic contribution when the structures evolve from non-planar to planar.

Enantiomer-Selective Molecular Sensing Using Racemic Nanoplasmonic Arrays.

Building blocks of life show well-defined chiral symmetry which has a direct influence on their properties and role in Nature. Chiral molecules are typically characterized by optical techniques such as circular dichroism (CD) where they exhibit signatures in the ultraviolet frequency region. Plasmonic nanostructures have the potential to enhance the sensitivity of chiral detection and translate the molecular chirality to the visible spectral range. Despite recent progress, to date, it remains unclear which properties plasmonic sensors should exhibit to maximize this effect and apply it to reliable enantiomer discrimination. Here, we bring further insight into this complex problem and present a chiral plasmonic sensor composed of a racemic mixture of gammadions with no intrinsic CD, but high optical chirality and electric field enhancements in the near-fields. Owing to its unique set of properties, this configuration enables us to directly differentiate phenylalanine enantiomers in the visible frequency range.

Giant intrinsic circular dichroism of prolinol-derived squaraine thin films

Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.

Theoretical Investigation of Single-Molecule Sensing Using Nanotube-Enhanced Circular Dichroism.

First-principles calculations have been used to investigate the potential use of circular dichroism (CD) spectroscopy in single-molecule sensing. Using a real-space implementation of time-dependent density functional theory (TDDFT), several systems involving single-walled carbon nanotubes (SWCNT) and small molecules have been studied to evaluate their CD response. Large induced CD (ICD) effects, differing for each test molecule, were observed in all SWCNT-molecule complexes. As the SWCNT used in this study shows no intrinsic CD response, the ICD spectra are the result of interaction with the small molecules. This finding is general and independent of the (a)chiral nature of the adsorbed molecule. Our results indicate that it is possible to design a system that uses SWCNT for detection of molecules using the change in CD spectrum of the system induced by adsorption of the molecule onto the SWCNT surface.

Optimization of expression and purification of human mortalin (Hsp70): Folding/unfolding analysis

Human mortalin is a Hsp70 mitochondrial protein that plays an essential role in the biogenesis of mitochondria. The deregulation of mortalin expression and its functions could lead to several age-associated disorders and some types of cancers. In the present study, we optimized the expression and purification of recombinant human mortalin by the use of two-step chromatography. Low temperature (18°C) and 0.5mM (IPTG) was required for optimum mortalin expression. Chaperone activity of mortalin was assessed by the citrate synthase and insulin protection assay, which suggested their protective role in mitochondria. Folding and unfolding assessments of mortalin were carried out in the presence of guanidine hydrochloride (GdnHCl) by intrinsic fluorescence measurement, ANS (8-analino 1-nephthlene sulfonic acid) binding and CD (circular dichroism) analysis. Under denaturing conditions, mortalin showed decrease in tryptophan fluorescence intensity along with a red shift of 11nm. Moreover, ANS binding studies illustrated decrease in hydrophobicity. CD measurement of mortalin showed a predominant helical structure. However, the secondary structure was lost at low concentration of GdnHCl (1M). We present a simple and robust method to produce soluble mortalin and warranted that chaperones are also susceptible to unfolding and futile to maintain protein homeostasis.

Angle-resolved photoemission with circularly polarized light in the nodal mirror plane of underdoped Bi2Sr2CaCu2O8+δ superconductor

Unraveling the nature of pseudogap phase in high-temperature superconductors holds the key to understanding their superconducting mechanisms and potentially broadening their applications via enhancement of their superconducting transition temperatures. Angle-resolved photoemission spectroscopy (ARPES) experiments using circularly polarized light have been proposed to detect possible symmetry breaking state in the pseudogap phase of cuprates. The presence (absence) of an electronic order which breaks mirror symmetry of the crystal would in principle induce a finite (zero) circular dichroism in photoemission. Different orders breaking reflection symmetries about different mirror planes can also be distinguished by the momentum dependence of the measured circular dichroism. Here, we report ARPES experiment on an underdoped Bi2Sr2CaCu2O8+δ (Bi2212) superconductor in the Γ (0,0)-Y (π,π) nodal mirror plane using circularly polarized light. No circular dichroism is observed on the level of ∼2% at low temperature, which places a clear constraint on the forms of possible symmetry breaking orders in this sample. Meanwhile, we find that the geometric dichroism remains substantial very close to its perfect extinction such that a very small sample angular offset is sufficient to induce a sizeable dichroic signal. It highlights the importance to establish a perfect extinction of geometric dichroism as a prerequisite for the identification of any intrinsic circular dichroism in this material.

The Role of Heme Chirality in the Circular Dichroism of Heme Proteins

The rotational strength (R) of the Soret transition in sperm-whale myoglobin (SW Mb), the hemoglobin from Chironomus thummi thummi (CTT Hb), and human hemoglobin (hHb) has been calculated using 20 high-resolution (< 1:5 Å) crystal structures. The intrinsic rotational strength due to heme non-planarity was calculated using π-electron theory and time-dependent density functional theory (TDDFT). Calculations on model protoporphyrins with a planar nucleus and with various torsional angles for the 2- and 4-vinyl substituents showed maximum R of ±0.70 Debye-Bohr magneton (1 DBM = 0.9273 · 10-38 cgs units). Viewing the heme so that the 2- and 4-vinyls are in a counterclockwise relationship, if a vinyl points toward the viewer, it contributes positively to R. Calculations of the intrinsic R for explicit heme geometries of SW Mb, CTT Hb, and hHb gave averages of 0.40±0.09, ±0:44±0.04, and +0.32±0.11 DBM, respectively. Coupling of the Soret transition with aromatic side-chain and peptide backbone transitions was also considered. For SW Mb, the magnitudes of the contributions decreased in the order Rint > Raro > Rpep. For CTT Hb and hHB, the orders were, respectively, Rint > Rpep > Raro and Rint > Raro ≈ Rpep. Human Hb ɑ chains showed the same trend as CTT Hb. Only in the hHb β chains did Raro predominate, with the order Raro > Rint > Rpep. The total predicted Rtot for SW Mb, CTT Hb, and hHb averaged +0.77±0.10 (0.56 - 0.80), -0.37±0.12 (-0.5), and +0.31±0.17 DBM (0.23 - 0.50), respectively. (Values in parentheses are experimental values.) Thus, contrary to the currently accepted view, coupling with aromatic side-chain or peptide transitions is not the dominant factor in the Soret circular dichroism (CD) of these proteins. The Soret CD is dominated by intrinsic CD of the heme chromophore, of which vinyl torsion is the major determinant. This result suggests an explanation for the large effect of heme isomerism on the Soret CD of Mb and Hb. Rotation about the ɑ-γ axis may be associated with large changes in vinyl torsion and thus substantially alter the intrinsic CD, even reversing its sign.

Promotion of Activity and Thermal Stability of Chloroperoxidase by Trace Amount of Metal Ions (M2+/M3+)

The effect of M(2+) (Zn(2+), Cu(2+), Cd(2+), Mn(2+), Pb(2+)) and M(3+) (Cr(3+), La(3+), Fe(3+), Ce(3+), Y(3+), Al(3+)) metal ions on the activity and thermal stability of chloroperoxidase (CPO) was investigated in this work. It was found that the lower concentration of metal ions was favorable to CPO activity whereas the higher concentration reversed the results. CPO activity could be increased to 116.4-127.1% in the presence of a trace amount of these M(2+)/M(3+) metal ions at a concentration range of 0-25 μmol L(-1) after 2 h of incubation at 25 °C. The activating effect of M(3+) is better than that of M(2+), and Cr(3+) was mostly efficient. The thermal stability of the enzyme was also improved significantly. Only 30.3% of CPO activity was retained at 50 °C whereas 82.6% of CPO activity was maintained in the presence of Cr(3+) after 2 h of incubation at the same temperature. The activation of CPO by metal ions at their low concentration was studied through intrinsic fluorescence, circular dichroism (CD), and UV-Vis spectra assay. A favorable environment around the active site was achieved in the presence of metal ions. Intrinsic fluorescence and CD spectra indicated that the α-helix structure of CPO was strengthened in metal ion-contained media. More exposure of the heme ring was achieved for easy access of the substrate, which was suggested by UV-Vis spectrum analysis. This strategy for enhancing CPO activity is very simple and useful. It will be favorable to the practical application of this enzyme.

Multispectroscopic methods reveal different modes of interaction of anti cancer drug mitoxantrone with Poly(dG-dC).Poly(dG-dC) and Poly(dA-dT).Poly(dA-dT)

The interaction of mitoxantrone with alternating Poly(dG-dC).Poly(dG-dC) and Poly(dA-dT).Poly(dA-dT) duplex has been studied by absorption, fluorescence and Circular Dichroism (CD) spectroscopy at Drug to Phosphate base pair ratios D/P=20.0–0.04. Binding to GC polymer occurs in two distinct modes: partial stacking characterized by red shifts of 18–23nm at D/P=0.2–0.8 and external binding at D/P=1.0–20.0 whereas that to AT polymer occurs externally in the entire range of D/P. The binding constant and number of binding sites is 3.7×105M−1, 0.3 and 1.3× 104M−1, 1.5 in GC and AT polymers, respectively at low D/P ratios. CD binding isotherms show breakpoints at D/P=0.1, 0.5 and 0.25, 0.5 in GC and AT polymers, respectively. The intrinsic CD bands indicate that the distortions in GC polymer are significantly higher than that in AT polymer. Docking studies show partial insertion of mitoxantrone rings between to GC base pairs in alternating GC polymer. Side chains of mitoxantrone interact specifically with base pairs and DNA backbone. The studies are relevant to the understanding of suppression or inhibition of DNA cleavage on formation of ternary complex with topoisomerase-II enzyme and hence the anti cancer action.

Aromatic side‐chain cluster of biotin binding site of avidin allows circular dichroism spectroscopic investigation of its ligand binding properties

Promiscuous ligand binding by hen egg-white avidin has been demonstrated and studied by using circular dichroism (CD) spectroscopy complemented by molecular docking calculations. It has been shown that the biotin-binding pocket of avidin is able to accommodate a wide variety of chemical compounds including therapeutic drugs (e.g., thalidomide, NSAIDs, antihistamines), natural compounds (bilirubin, myristic acid), and synthetic agents (xanthenone dyes). The cluster of aromatic residues located at the biotin-binding pocket renders the intrinsic CD spectrum of avidin sensitive to ligand binding that results in the increase of the vibronic components of the (1) L(b) transition of the Trp residues. Extrinsic (induced) CD bands measured with chemically diverse avidin ligands are generated by intramolecular coupled oscillator (e.g., bilirubin) or by intermolecular ligand-Trp exciton coupling mechanism [e.g., 2-(4'-hydroxyazobenzene)-benzoic acid (HABA)]. Among the compounds of which avidin-binding affinity constants have been calculated, two novel high-affinity ligands, flufenamic acid and an enzyme inhibitor thiazole derivative have been identified (K(d) ≈ 1 μM). Avidin binding mode of the ligand molecules has been discussed in the light of docking results. The induced CD profile of the thiazole derivative has been correlated with the stereochemistry of its docked conformation. The important role in the ligand binding of a polar side-chain cluster at the bottom of the biotin-binding cavity as well as the analogous avidin-binding mode of HABA and fenamic acid type NSAIDs have been proposed.

Thermodynamic and structural study of phenanthroline derivative ruthenium complex/DNA interactions: Probing partial intercalation and binding properties

The binding of [Ru(PDTA-H 2)(phen)]Cl (PDTA = propylene-1,2-diaminetetra-acetic acid; phen = 1,10 phenanthroline) with ctDNA (=calf thymus DNA) has been investigated through intrinsic and induced circular dichroism, UV–visible absorption and fluorescence spectroscopies, steady-state fluorescence, thermal denaturation technique, viscosity and electrochemical measurements. The latter indicate that the cathodic and anodic peak potentials of the ruthenium complex shift to more positive values on increasing the DNA concentration, this behavior being a direct consequence of the interaction of both the reduced and oxidized form with DNA binding. From spectrophotometric titration experiments, the equilibrium binding constant and the number of monomer units of the polymer involved in the binding of one ruthenium molecule (site size) have been quantified. The intrinsic circular dichroism (CD) spectra show an unwinding and a conformational change of the DNA helix upon interaction of the ruthenium complex. Quenching process, thermal denaturation experiments and induced circular dichroism (ICD) are consistent with a partial intercalative binding mode.