Circular Dichroism Values Research Articles

Vibrational Mode-Dependent Circular Dichroism of Jet-Cooled Styrene Oxide.

Vibrational mode-dependent circular dichroism (CD) effects in vibrational CD spectra are often used to determine the handedness of molecules with multiple chiral centers. Here, we investigate vibrational mode-dependent CD effects in the electronic CD spectra of jet-cooled (R)-(+)- and (S)-(-)-styrene oxide obtained by using resonant two-photon ionization CD (R2PICD) spectroscopy. Although most fundamental vibronic bands demonstrated CD signatures consistent with that of the origin band, the overtone and combination bands exhibited opposite CD signatures. Theoretical calculations suggest that CD in fundamental bands are primarily governed by Franck-Condon contributions, whereas Herzberg-Teller contributions, which reflect the influences of vibrational modes on CD values, become more significant in overtone and combination bands. These results underscore the potential of mode-dependent vibronic CD signals in R2PICD spectra for determining the absolute configurations of molecules with multiple chiral centers.

High-Q terahertz chirality enhancement using elliptical hole metasurface

Circular dichroism (CD) technology has garnered significant attention due to its extensive applications in ultra-sensitive biosensing and spin-selective optical frequency conversion. However, existing terahertz chiral structures are constrained by linewidth, which limits their effectiveness in narrowband signal processing. In this study, we propose the notion of quasi-bound states in the continuum (quasi-BIC) within a planar elliptical hole all-silicon terahertz metasurface that exhibits broken mirror symmetry. This approach achieves a CD value as high as 0.97, with a linewidth below 0.5 GHz and a Quality (Q)-factor reaching up to 107 in the 1.3 THz to 1.55 THz band, thereby enabling ultra-narrowband terahertz chirality. This method significantly enhances the Q-factor of optical resonant systems, reduces linewidth, and achieves strong CD while addressing the trade-off between high Q-factor and high CD observed in existing structures. The theoretical foundations for achieving ultra-narrow linewidth are established through band structure calculations and far-field polarization analysis. Additionally, the Q-factor of quasi-BIC can be flexibly optimized through parameter tuning, rendering it more practical than perfect BIC in real-world applications. This study presents a novel solution for terahertz narrowband chirality and optical filters, potentially advancing technologies in related fields.© 2024 Optica Publishing Group under the terms of the Optica Publishing Group Open Access Publishing Agreement.

Tunable and strong circular dichroism metamaterial absorber based on gold-VO2 hybrid structure

The existing terahertz chiral absorber faces a series of issues, including low efficiency, un-tunable absorption, and complex structure. We present a chiral metamaterial absorber integrated with gold and phase-change material vanadium dioxide (VO2) to address the issues above. Our designed metamaterial absorber’s maximum circular dichroism (CD) value, as an ideal chirality-selective meta-absorber, reaches 0.95 when VO2 behaves as a metallic state. However, when VO2 is in the insulator state, the CD value is merely 0.003. Notably, the intrinsic mechanisms of the CD effect are revealed by analyzing the difference in electromagnetic response under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) illumination. The designed chiral metamaterial absorber supports the free switching of the CD signal between ‘on’ and ‘off’ states by continuously adjusting the conductivity of VO2 with a CD value ranging from 0 to 0.95. Moreover, the absorption performance of chiral metamaterials is affected by geometric parameters and the angle of incidence. To evaluate the capability of chiral structure in biosensing, three typical avian influenza viruses are sensed by detecting changes in their resonant frequencies and CD values. Finally, the presented chiral-selective absorber with phase-change materials has an important reference value in the terahertz range for constructing chiral light detectors, chiral thermal switching, biosensors, and tunable chiral photonics.

Giant circular dichroism in all-dielectric planar chiral Meta-GMR

In this paper, we introduce a simplified approach involving a single-layer dielectric meta-surface on top of planar waveguide that can achieve a high circular dichroism (CD) value of 0.95 under normal incidence. Within this intrinsic chiral structure, we harness the distinctive characteristics of guided mode resonance (GMR) [1] to induce out-of-plane electric/magnetic field vectors during lateral light propagation, thereby enhancing its optical chirality. Notably, the enhanced CD value becomes prominent at a half-wave plate thickness. This meta-GMR design enables giant CD in both of reflection and transmission, offering promising applications in biosensing, especially for chiral molecules.

Infrared phase-change chiral metasurfaces with tunable circular dichroism

Integrating phase-change materials in metasurfaces has emerged as a powerful strategy to realize optical devices with tunable electromagnetic responses. Here, phase-change chiral metasurfaces based on GST-225 material with the designed trapezoid-shaped resonators are demonstrated to achieve tunable circular dichroism (CD) responses in the infrared regime. The asymmetric trapezoid-shaped resonators are designed to support two chiral plasmonic resonances with opposite CD responses for realizing switchable CD between negative and positive values using the GST phase change from amorphous to crystalline. The electromagnetic field distributions of the chiral plasmonic resonant modes are analyzed to understand the chiroptical responses of the metasurface. Furthermore, the variations in the absorption spectrum and CD value for the metasurface as a function of the baking time during the GST phase transition are analyzed to reveal the underlying thermal tuning process of the metasurface. The demonstrated phase-change metasurfaces with tunable CD responses hold significant promise in enabling many applications in the infrared regime such as chiral sensing, encrypted communication, and thermal imaging.

High transmission of circularly polarized light and circular dichroism with all-dielectric metamaterial

The optical chirality of metamaterials gives birth to distinct responses of left circularly polarized (LCP) and right circularly polarized (RCP) light, leading to asymmetric transmission and circular dichroism (CD) of circularly polarized light. In this work, meta-atom structure composed of square and semicircular combinations were designed, which exhibits extreme high ratio of transmission, reflection and CD values in blue light range. In compare to other reported structures, the structure composed of semicircular and square combinations can achieve better asymmetric transmission. At the wavelength of 460 nm, the incident LCP light can be mostly converted to RCP light, and the incident RCP light can be almost completely reflected. Furthermore, the geometric phase can be controlled by designing the orientation angle of the meta-atom, and the metamaterial can function under any linear or circular polarization and achieving the regulation of the light field. The designed metamaterial provides promising insights into the high CD and possible applications in optical integrated devices, such as anomalously refracted, focusing, and holograms, etc.

Maximal circular polarization isolation and asymmetric polarization conversion by chiral meta-structure

Chiral objects hold immense significance in modern optical technology, particularly due to their ability to manipulate circularly polarized waves. The chiroptical effects observed in naturally known chiral structures are typically very weak, however, the use of engineered meta-structures has proven to be highly effective in overcoming these shortcomings. Despite extensive research efforts, the construction of chiroptical phenomena approaching maximum performance has proven to be challenging, mostly due to the lack of optimal design choices and the existence of material losses. Here we present a metasurface constituting S-shaped building blocks capable of realizing virtually maximum chiroptical phenomena. The structure demonstrates nearly full polarization transmission, conversion to an opposite spin state, and reflection of the opposite spin state at a wavelength of 1549 nm. As a result, the maximum circular dichroism (CD) value reaches up to ≈1 (0.9993) for a given polarization state. Furthermore, reduced symmetry enables the one-way flow of a given polarization state resulting in about unity (0.998) asymmetric transmission (AT) value. Through rigorous numerical simulations, we elucidate the underlying principles driving these extraordinary optical properties. The CD and AT values are record-high demonstrated so far. The single-layer design offers an ultrathin profile, making it compatible with integrated photonics and providing opportunities for applications in compact, lightweight optical devices such as circular polarizers, half-wave plates, and self-polarizing reflectors.

Weyl semimetal/dielectric/Weyl semimetal stack for highly circularly polarized thermal radiation.

Highly circularly polarized (CP) infrared thermal radiation is greatly in demand because of its significant potential in mid-infrared (mid-IR) applications. To exploit the magnitude and quality factor of circular dichroism (CD) simultaneously, a lithography-free platform consisting of a Weyl semimetal (WSM)/dielectric (Ge)/WSM stack sitting on a metallic substrate (Mo) is proposed. A chiral response and varying CD values from -1 to 0.957 have been demonstrated. The numerical results from a generalized 4 × 4 transfer matrix algorithm verify that the chiral structure manifests a remarkably high quality factor (Q-factor) of 605. The effect of the thickness of each layer in the stack on the CD value is investigated. Moreover, it is identified that the design results in an angle-independent performance. A dual-channel chiral absorber operating in the 18-21 μm wavelength range has also been achieved. Our simple yet powerful paradigm could offer a new way of manipulating the Q-factor and resonance wavelength of a chiral absorber while maintaining near-unity CD, offering a new approach for the advancement of more efficient and tunable chiral optical devices. The approach is generally applicable to other planar configurations with different WSMs and can be extended to other wavelengths.

Tunable and switchable multifunctional terahertz meta-mirror based on graphene and vanadium dioxide.

We design a multifunctional THz polarization modulation meta-mirror integrated with polarization conversion and dichroism functions switched by temperature and voltage. The meta-mirror is composed of two-layered graphene metasurfaces and a layer of vanadium dioxide (VO2) on a gold film substrate. Linear-to-linear polarization conversion and linear dichroism (LD) can be switched by temperature control in the VO2 film and Fermi level adjustments in the graphene metasurfaces, where the polarization conversion ratio (PCR) is higher than 0.9 in the range of 2.89 THz to 4.02 THz, LD value reached a maximum of 0.6 at 3.84 THz, and linear-to-circular polarization conversion and circular dichroism (CD) can also be tuned with ellipticity higher than 0.9 in the range of 2.32 THz to 2.69 THz and CD value as high as 0.71 at 2.45 THz. The proposed meta-mirror is the first THz metamaterial device integrating four switchable functions, including linear-to-linear polarization conversion, linear-to-circular polarization conversion, linear dichroism and circular dichroism. The meta-mirror is a promising design for compact system integration in THz imaging, sensing and biological detection applications.

BINOL induces chirality in polyfluorene-based electroluminescent polymers in solid state

Nowadays circularly polarized luminescence (CPL) gains much attraction because it can be applied to a variety of fields. An efficient method to achieve CPL is blending achiral emissive polymers with chiral small-molecule additives as a chiral inducer. In this work, we demonstrate that 1,1′-Bi-2-naphthol (BINOL) acts as a chiral-inducer for poly (9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) in a thin film. In the thin films of F8BT/BINOL before annealing, only circular dichroism (CD) signals of BINOL were observed. After the annealing at 120 °C, clear CD signals of F8BT were observed indicating that the chiroptical property was induced on the F8BT thin films. The CD signal intensity was enhanced by increasing the annealing temperature up to 200 °C. By optimizing the concentration of BINOL and the annealing temperature, we obtained a dissymmetry CD value of 0.012. Photoluminescence quantum yield of the F8BT/BINOL thin films was also increased after the annealing.

Terahertz dichroic device withy multi-functionality, multi-band operation and multi-dimensional tunability

Most existing dichroic metasurfaces have limitations such as single operating band, single functionality, fixed dichroic performance or only adjustable in a single dimension, as well as complex structure. Here, we propose a terahertz dichroic device based on patterned graphene layer and gold layer to address these limitations. By utilizing the anisotropy endowed by the patterned graphene layer and the gold layer, the proposed device can achieve both dual-band circular dichroism (CD) and linear dichroism (LD), with maximum CD values of 0.866 at 1.09 THz and −0.614 at 1.76 THz, and maximum LD values of 0.779 at 0.51 THz and −0.633 at 0.76 THz. Meanwhile, the dichroism of the device can be regulated through multiple dimensions, i.e., the rotation angle of the top patterned graphene layer, the Fermi level of graphene and the incident angle. The proposed dichroic device offers multi-functionality, multi-band operation and multi-dimensional adjustability with a relatively simple structure, and is expected to be widely used in the fields of terahertz imaging, polarization detection, optical sensing and biological spectroscopy.

Tunable circular dichroism in all-dielectric metasurface with efficient wavefront manipulation

In this paper, an all-dielectric metasurface based on phase-change material Ge2Sb2Se4Te1 (GSST) working in the mid-infrared band is designed, and combined with Pancharatnam-Berry (PB) phase to realize multifunctional metasurfaces. The metasurface can convert incident circularly polarized waves to transmitted or reflected waves of opposite chirality, regardless of GSST is in amorphous or crystalline state. Due to constructive interference and destructive interference, the metasurface obtains giant circular dichroism (CD). The maximum CD value reaches 0.997 at f = 64 THz when GSST is in amorphous state and 0.97 at f = 45 THz when GSST is in crystalline state. According to the principle of PB phase, three optical devices are realized by the rational arrangement of all-dielectric meta-atoms with different rotation angles, which are beam deflectors, lenses and vortex beam generators.

Electromagnetic heating-assisted metasurface for stably tunable, fast-responding chiroptics.

Herein, a graphene-dielectric metasurface with the function of stably tunable and fast responding on the chiroptics is theoretically investigated and numerically demonstrated. Via utilizing the intrinsic thermo-optical effect of the silicon, the circular dichroism (CD) peak position can be linearly scaled with a spectral sensitivity of up to 0.06 nm/K by artificially adjusting the temperature. Moreover, a perfectly adjusting manipulation with a wavelength shift of full width at half maximum for the resonant spectrum and the simultaneously maintained CD values can be realized by a slight temperature variation of ∼0.8 K. Additionally, we take a graphene layer as the heating source to actually demonstrate the ultra-fast thermal generation. Applying an input voltage of 2 V to the graphene with only 10 µs can rapidly increase the metasurface temperature of up to 550 K. Such performances hold the platform with wide applications in functional chiroptics and optoelectronics.

Highly and tunable full-Stokes chiral metasurface polarization response based on CH3NH3PbI3 perovskites

Owning to the unique optical chiral response, chiral metasurfaces are demand in a wide range of applications. Circular dichroism (CD) as an important index of optical chirality response has been widely concerned by researchers. However, the reported high CD values require complex metasurface structures. Here, we propose a simple structure, high chiral response, terahertz (THz) chiral metasurface based on CH3NH3PbI3(MAPbI3) perovskite regulated by light field. The full Stokes parameters of 0.1THz are obtained, including intensity, direction, and ovality. The results show that CD values of chiral metasurface are improved by integrating of MAPbI3. Moreover, the CD values (0.2–0.49) can be regulated by changing MAPbI3 thickness, metasurface geometric phase and conductivity. This work demonstrates that perovskite metasurface is a promising candidate for the biosensing, polarization control, polarization sensitive imaging and so on.

Gradient-assisted metasurface absorber with dual-band chiral switching and quasi-linearly tunable circular dichroism.

Chiral metasurfaces with tunable or switchable circular dichroism (CD) responses hold great potential for advanced optical devices. In this work, we theoretically propose and numerically demonstrate a chiral metasurface absorber composed of periodically serrated Ge2Sb2Te5 (GST) resonators. By harnessing strong plasmonic resonance using the gradient geometry, we achieve a strongly enhanced chiral response with a CD value of 0.98 at λ2 = 2359 nm and a CD value of 0.7 at λ1 = 2274 nm. Additionally, by controlling the gradient difference in the serrated GST resonator, we can modify the CD intensity in multiple dimensions and near-perfectly optimize the chiral properties. Furthermore, it is worth noting that the CD value can be strongly varied by adjusting the phase transition characteristics of GST in the range of 0.007 to 0.7 at λ1 and 0.002 to 0.98 at λ2, corresponding to a switch between "on" and "off" states. The findings give new insight into multi-functional chiroptics and hold wide applications.

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.

Chiral Quasi-Bound States in the Continuum of a Dielectric Metasurface for Optical Monitoring and Temperature Sensing

Chiral BIC can reach ultrahigh quality factors (Q-factor) based on its asymmetry, with broken mirror symmetries and in-plane inversion. Only by in-plane structural perturbation can chiral quasi-BIC (q-BIC) appear, so it is much more realizable and reasonable for the manufacturers in practical productions and fabrications considering the technology and means that are available. In this paper, we design a new dielectric metasurface employing H-shaped silica meta-atoms in the lattice, which is symmetrical in structure, obtaining chiral BIC with ultrahigh Q-factor (exceeding 105). In this process, we change the length of the limbs of the structure to observe the specific BICs. Previous scholars have focused on near-infrared-wavelength bands, while we concentrate on the terahertz wavelength band (0.8–1 THz). We found that there is more than one BIC, thus realizing multiple BICs in the same structure; all of them exhibit excellent circular dichroism (CD) (the maximum value of CD is up to 0.8127) for reflectance and transmittance, which provides significant and unique guidance for the design of multi-sensors. Meanwhile, we performed temperature sensing with chiral BIC; the sensitivity for temperature sensing can reach 13.5 nm/°C, which exhibits high accuracy in measuring temperature. As a consequence, the result proposed in this study will make some contributions to advanced optical imaging, chiral sensors with high frequency and spectral resolution, optical monitoring of environmental water quality, multiple sensors, temperature sensing, biosensing, substance inspection and ambient monitoring and other relevant optical applications.

Terahertz broadband tunable chiral metamirror based on VO2-metal hybrid structure.

Aiming at the problems of narrow working bandwidth, low efficiency, and complex structure of existing terahertz chiral absorption, we propose a chiral metamirror composed of C-shaped metal split ring and L-shaped vanadium dioxide (VO2). This chiral metamirror is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric layer and VO2-metal hybrid structure as the top. Our theoretical results led us to show that this chiral metamirror has a circular dichroism (CD) value greater than 0.9 at 5.70 to 8.55 THz and has a maximum value of 0.942 at f = 7.18 THz. In addition, by adjusting the conductivity of VO2, the CD value can be continuously adjustable from 0 to 0.942, which means that the proposed chiral metamirror supports the free switching of the CD response between the on and off states, and the CD modulation depth exceeds 0.99 in the range of 3 to 10 THz. Moreover, we discuss the influence of structural parameters and the change of incident angle on the performance of the metamirror. Finally, we believe that the proposed chiral metamirror has important reference value in the terahertz range for constructing chiral light detectors, CD metamirrors, switchable chiral absorbers and spin-related systems. This work will provide a new idea for improving the terahertz chiral metamirror operating bandwidth and promote the development of terahertz broadband tunable chiral optical devices.

Chiral metasurfaces of wavy rectangle resonators with tunable circular dichroism

Chiral metasurfaces operating at the mid-infrared frequencies are of great interest for chiral molecule vibrational sensing and other applications. Here, mid-infrared chiral metasurfaces of wavy rectangle resonators with strong chiroptical response and tunable circular dichroism (CD) are demonstrated. The chiral metasurface exhibits a high CD value of more than 0.57 as the chiral plasmonic resonance wavelength is tuned from 4.96 to 5.82 µm by enlarging the geometric dimensions of the unit cell. Furthermore, CD values can be continuously tuned from positive to zero and negative without altering the chiral plasmonic resonance wavelength, by simply relocating the cutting slot to change the geometric symmetry of the wavy rectangle structure. These results aim to advance the development of many promising applications in mid-infrared frequencies including protein molecule detection, thermal emission control, and infrared-light communication.

The role of lateral substituent size in azobenzene chromophores on photo- and chiro-optical properties of photoactive polymethacrylates

Properties of functional liquid crystalline (LC) macromolecular materials, especially those showing self-organizing behaviour and targeted for specific purposes, strongly rely on the molecular structure of the mesogens used as side-chains. In order to get inside this relationship, new series of chiral LC polymethacrylates (PMA6Azo-R) with azobenzene groups having different lateral substituents (R), namely the methyl (-Me) and ethyl (-Et) groups on the central benzene ring, were synthesized and their comparative study was carried out. The synthesized polymers consist of photochromic azobenzene fragments that differ in the size of the lateral substituent (R = -H, -Me, -Et). All polymers exhibit the cholesteric phase, whereas the polymer without lateral alkyl group in addition forms the low-temperature SmĈ* phase. Investigations of the photoorientation process under linearly polarised visible light irradiation reveal that an increase in the size of side substituent considerably reduces the values of maximum achievable photoinduced dichroism (from 0.89 for PMA6Azo-H to 0.33 for PMA6Azo-Et that is associated with a decrease in the degree of mesogen anisometry). The effect of the lateral substituent size in azobenzene photochrome on the chiro-optical properties and mesomorphic behaviour of studied polymers was established. The presence of the lateral substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature. Chiro-optical properties of the polymers and their photoswitching were studied, and it was found, that the introduction of the lateral substituent in the azobenzene fragment prevents the formation of exciton couples, that leads to a decrease in the circular dichroism (CD) values. UV irradiation and E-Z isomerisation of the azobenzene chromophores results in complete disappearance of CD signal; subsequent visible light irradiation does not lead to its full recovery. On the other hand, annealing of the films at temperatures higher than the glass transition almost fully recovers the initial CD values. The designed polymethacrylates can be considered as promising materials for various photonic applications.