Artificial Propeller Chirality and Counterintuitive Reversal of Circular Dichroism in Twisted Meta-molecules

Dynamic Chiral Folding for Arene-Perfluoroarene Force Driven Clamping

Peptide folding provides a feasible protocol to design intelligent chiral supramolecular systems toward sensing, recognition, and many other advanced functions. Here we conjugated dual short peptide arms on the aromatic core in a spatially close manner, allowing for intramolecular folding. Through controlling the aromatic core skeleton and appended luminophores, efficient chirality transfer to terminal pyrenes was realized. The through-space transferred chirality exhibited ultrahigh sensitivity toward solvent environments. Micropockets within dipeptides would accommodate the solvents specifically to initiate the chiroptical inversion expressed in the ground-state circular dichroism and the circularly polarized luminescence at photoexcited states. The dynamics also were depicted on the selective binding of guests through arene-perfluoroarene interactions. We successfully constructed a dynamic peptide-based clamp that could control the endo- and exo-type arene-perfluoroarene complexation. This work inspires the design and fabrication of peptide-based chiroptical materials with adaptability to external fields.

Chiral lasing enabled by strong coupling

Chiral quasi-bound states in the continuum are spin-dependent high-Q resonances in meta-photonic structures that are realized by perturbing symmetry-protected optical states by engineering in-plane and out-of-plane asymmetries, and they support chiral lasing in the vertical direction. Here, we explore the coupling between two resonances in a chiral metasurface and introduce a mechanism for high-purity chiral laser emission. We reveal that two resonances with nearly orthogonal polarizations become strongly coupled in an engineered chiral metasurface. The inherent phase difference of the resonances, associated with the coherent destruction on the decay channel, can endow high-Q factor and maximize chirality to one of the hybrid modes. We verify this approach experimentally by measuring transmission spectra, angle-resolved photoluminescence, and laser emission. We believe that this mechanism allows breaking restrictions on conventional chiral quasi-BIC lasing, enabling the realization of chiral emission at any designed direction.

Hierarchical assembly of Ag40 nanowheel ranging from building blocks to diverse superstructure regulation

Achieving precise and controllable hierarchical self-assembly of functional nanoclusters within crystal lattices to create distinct architectures is of immense significance, yet it creates considerable challenges. Here we successfully synthesized a silver nanowheel Ag40, along with its optically pure enantiomers S-/R-Ag40. Each species possesses an internal nanospace and exhibits host-guest interactions. These structures are constructed from primary building blocks (Ag9). By manipulating the surface anions and guest molecules, the nanowheels function as secondary building blocks, spontaneously organizing into complex double- and triple-helical crystalline superstructures or one-dimensional chains {Ag41}n through conformational matching and diverse noncovalent interactions. Moreover, we demonstrate that the water-mediated complex specifically assembled with uridine monophosphate nucleotides, resulting in chiral assemblies of Ag40 that exhibit chiroptical activity for specific recognition. Our findings provide insights into the efficient construction of assemblies with hollow frameworks and propose strategies for superstructure engineering by manipulating surface motifs.

Unraveling the Origin of Reverse Plasmonic Circular Dichroism from Discrete Bichiral Au Nanoparticles

Bichiral plasmonic nanoparticles exhibited intriguing geometry-dependent circular dichroism (CD) reversal; however, the crucial factor that dominates the plasmonic CD is still unclear. Combined with CD spectroscopy and theoretical multipole analysis, we demonstrate that plasmonic CD originates from the excitation of electric quadrupolar plasmons. Moreover, a comparative study of two distinct quadrupolar modes reveals the correlation between the sign of the CD and the local geometric handedness at the plasmonic hotspots, thereby establishing a structure-property relationship in bichiral nanoparticles. The reverse CD is attributed to the opposite directions of the wavelength shift of the two plasmon modes upon changing the particle geometry. By finely tuning the size of bichiral nanoparticles, we can further reveal that the dependence of plasmonic CD on the electric quadrupolar plasmons. Our work sheds light on the physical origin of plasmonic CD and provides important guidelines for the design of chiral plasmonic nanoparticles toward chirality-dependent applications.

Photonic and Nanomechanical Modes in Acoustoplasmonic Toroidal Nanopropellers

Non-conventional resonances, both acoustic and photonic, are found in metallic particles with a toroidal nanopropeller geometry, which is generated by sweeping a three-lobed 2D shape along a spiral with twisting angle α. For both optical and acoustic cases, the spectral location of resonances experiences a red-shift as a function of α. We demonstrate that the optical case can be understood as a natural evolution of resonances as the spiral length of the toroidal nanopropeller increases with α, implying a huge helicity-dependent absorption cross-section. In the case of acoustic response, two red-shifting breathing modes are identified. Additionally, even a small α allows the appearance of new low-frequency resonances, whose spectral dispersion depends on a competition between the length of the generative spiral and the pitch of the toroidal nanopropeller.

Optoelectronically navigated nano-kirigami microrotors

With the rapid development of micro/nanofabrication technologies, the concept of transformable kirigami has been applied for device fabrication in the microscopic world. However, most nano-kirigami structures and devices were typically fabricated or transformed at fixed positions and restricted to limited mechanical motion along a single axis due to their small sizes, which significantly limits their functionalities and applications. Here, we demonstrate the precise shaping and position control of nano-kirigami microrotors. Metallic microrotors with size of ~10 micrometers were deliberately released from the substrates and readily manipulated through the multimode actuation with controllable speed and direction using an advanced optoelectronic tweezers technique. The underlying mechanisms of versatile interactions between the microrotors and electric field are uncovered by theoretical modeling and systematic analysis. This work reports a novel methodology to fabricate and manipulate micro/nanorotors with well-designed and sophisticated kirigami morphologies, providing new solutions for future advanced optoelectronic micro/nanomachinery.

Multicomponent chiral plasmonic hybrid nanomaterials: recent advances in synthesis and applications

Chiral hybrid nanomaterials with multiple components provide a highly promising approach for the integration of desired chirality with other functionalities into one single nanoscale entity. However, precise control over multicomponent chiral plasmonic hybrid nanomaterials to enable their application in diverse and complex scenarios remains a significant challenge. In this review, our focus lies on the recent advances in the preparation and application of multicomponent chiral plasmonic hybrid nanomaterials, with an emphasis on synthetic strategies and emerging applications. We first systematically elucidate preparation methods for multicomponent chiral plasmonic hybrid nanomaterials encompassing the following approaches: physical deposition approach, galvanic replacement reaction, chiral molecule-mediated, chiral heterostructure, circularly polarized light-mediated, magnetically induced, and chiral assembly. Furthermore, we highlight emerging applications of multicomponent chiral plasmonic hybrid nanomaterials in chirality sensing, enantioselective catalysis, and biomedicine. Finally, we provide an outlook on the challenges and opportunities in the field of multicomponent chiral plasmonic hybrid nanomaterials. In-depth investigations of these multicomponent chiral hybrid nanomaterials will pave the way for the rational design of chiral hybrid nanostructures with desirable functionalities for emerging technological applications.

Thermal Emission Manipulation Enabled by Nano-Kirigami Structures

The nano-kirigami metasurfaces have controllable 3D geometric parameters and dynamic transformation functions and therefore provide a strong spectral regulation capability of thermal emission. Here, the authors propose and demonstrate a dynamic and multifunctional thermal emitter based on deformable nano-kirigami structures, which can be actuated by electronic bias or mechanical compression. Selective emittance and the variation of radiation intensity/wavelength are achieved by adjusting the geometric shape and the transformation of the structures. Particularly, a thermal management device based on a composite structure of nano-kirigami and polydimethylsiloxane (PDMS) thin film is developed, which can dynamically switch the state of cooling and heating by simply pressing the device. The proposed thermal emitter designs with strong regulation capability and multiple dynamic adjustment strategies are desirable for energy and sensing applications and inspire further development of infrared emitters.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Menezes L, Maier SA. 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.

Enhanced Faraday rotation by a Fano resonance in substrate-free three-dimensional magnetoplasmonic structures

Three-dimensional magnetoplasmonic nanostructures possess more novel and richer optical and magneto-optical (MO) behaviors compared with planar nanostructures, and exhibit attractive potential applications in micro-nano non-reciprocal photonic devices. However, fabrication of three-dimensional magnetoplasmonic nanostructures is difficult using the usual nanofabrication methods. This work constructs three-dimensional substrate-free Au/Co/Au structures prepared using focused ion beam (FIB) technology. In the three-dimensional split-ring structure, with y-polarized light normal incidence, a three-dimensional coupling current is formed between the vertical split-ring and the bottom square hole, which causes excitation of the Fano resonance. The Fano resonance causes a significant enhancement of the local magnetic field, resulting in a larger Faraday rotation (FR). The resonance also brings about a sign reversal of FR, which is related to the direction of the Lorentz force on electrons. Similar effects also exist in the three-dimensional nanopillar structure and the three-dimensional nanoring structure in the simulation results. Due to the high flexibility of FIB machining, the height and shape of the three-dimensional split-ring can be arbitrarily changed, which means the FR intensity and the position of the FR null point are tunable. The designed three-dimensional structures provide a new route to regulate the Faraday effect, and broaden the possibilities for the design and construction of MO devices.

Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells

Lattice resonances are collective electromagnetic modes supported by periodic arrays of metallic nanostructures. These excitations arise from the coherent multiple scattering between the elements of the array and, thanks to their collective origin, produce very strong and spectrally narrow optical responses. In recent years, there has been significant effort dedicated to characterizing the lattice resonances supported by arrays built from complex unit cells containing multiple nanostructures. Simultaneously, periodic arrays with chiral unit cells, made of either an individual nanostructure with a chiral morphology or a group of nanostructures placed in a chiral arrangement, have been shown to exhibit lattice resonances with different responses to right- and left-handed circularly polarized light. Motivated by this, here, we investigate the lattice resonances supported by square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, effectively functioning as 2.5-dimensional arrays. We find that these systems can support lattice resonances with almost perfect chiral responses and very large quality factors, despite the achirality of the unit cell. Furthermore, we show that the chiral response of the lattice resonances originates from the constructive and destructive interference between the electric and magnetic dipoles induced in the two nanostructures of the unit cell. Our results serve to establish a theoretical framework to describe the optical response of 2.5-dimensional arrays and provide an approach to obtain chiral lattice resonances in periodic arrays with achiral unit cells.

Broadband and high-efficiency polarization conversion with a nano-kirigami based metasurface

Nano-kirigami metasurfaces have attracted increasing attention due to their ease of three-dimension (3D) nanofabrication, versatile shape transformations, appealing manipulation capabilities and rich potential applications in nanophotonic devices. Through adding an out-of-plane degree of freedom to the double split-ring resonators (DSRR) by using nano-kirigami method, in this work we demonstrate the broadband and high-efficiency linear polarization conversion in the near-infrared wavelength band. Specifically, when the two-dimensional DSRR precursors are transformed into 3D counterparts, a polarization conversion ratio (PCR) of more than 90% is realized in wide spectral range from 1160 to 2030 nm. Furthermore, we demonstrate that the high-performance and broadband PCR can be readily tailored by deliberately deforming the vertical displacement or adjusting the structural parameters. Finally, as a proof-of-concept demonstration, the proposal is successfully verified by adopting the nano-kirigami fabrication method. The studied nano-kirigami based polymorphic DSRR mimic a sequence of discrete bulk optical components with multifunction, thereby eliminating the need for their mutual alignment and opening new possibilities.

Nano-kirigami enabled chiral nano-cilia with enhanced circular dichroism at visible wavelengths

Nano-kirigami method enables rich diversity of structural geometries that significantly broaden the functionalities of optical micro/nano-devices. However, the methodologies of various nano-kirigami are still limited and as a result, the chiral nano-kirigami structure has yet been pushed to the limit for operation at visible wavelength region. Here, the merits of the various nano-kirigami strategies are comprehensively explored and bio-inspired nano-cilia metasurface with enhanced circular dichroism at visible wavelengths is demonstrated. The stereo chiral nano-cilia metasurface is designed with three-fold rotational symmetry, which exhibits tuneable chiroptical responses when the nano-cilia are deformed to form strong chiral light-matter interactions. By employing electron-beam lithography (EBL) and focused ion beam (FIB) lithography, on-chip nano-cilia metasurfaces are experimentally realized in near-infrared wavelengths region and at visible wavelengths, respectively, successfully validating the giant circular dichroism revealed in simulations. Our work is useful to broaden the existing platform of micro/nano-scale manufacturing and could provide an effective method for the realization of versatile bioinspired nanostructures with profound chiroptical responses.

Observation of intrinsic chiral bound states in the continuum

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics¹, quantum optics², topological photonics³ and chiroptics⁴. Intrinsic chirality is weak in natural materials, and recent theoretical proposals^5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies⁸. Therefore, most of the experimental attempts^9-11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence^9,10 or structural anisotropy¹¹. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

High-Discrimination Circular Polarization Detection Based on Dielectric-Metal-Hybrid Chiral Metamirror Integrated Quantum Well Infrared Photodetectors

Circular polarization detection enables a wide range of applications. With the miniaturization of optoelectronic systems, integrated circular polarization detectors with native sensitivity to the spin state of light have become highly sought after. The key issues with this type of device are its low circular polarization extinction ratios (CPERs) and reduced responsivities. Metallic two-dimensional chiral metamaterials have been integrated with detection materials for filterless circular polarization detection. However, the CPERs of such devices are typically below five, and the light absorption in the detection materials is hardly enhanced and is even sometimes reduced. Here, we propose to sandwich multiple quantum wells between a dielectric two-dimensional chiral metamaterial and a metal grating to obtain both a high CPER and a photoresponse enhancement. The dielectric-metal-hybrid chiral metamirror integrated quantum well infrared photodetector (QWIP) exhibits a CPER as high as 100 in the long wave infrared range, exceeding all reported CPERs for integrated circular polarization detectors. The absorption efficiency of this device reaches 54%, which is 17 times higher than that of a standard 45° edge facet coupled device. The circular polarization discrimination is attributed to the interference between the principle-polarization radiation and the cross-polarization radiation of the chiral structure during multiple reflections and the structure-material double polarization selection. The enhanced absorption efficiency is due to the excitation of a surface plasmon polariton wave. The dielectric-metal-hybrid chiral mirror structure is compatible with QWIP focal plane arrays.

Tunable Reversal of Circular Dichroism in the Seed-Mediated Growth of Bichiral Plasmonic Nanoparticles

Plasmonic nanoparticles with an intrinsic chiral structure have emerged as a promising chiral platform for applications in biosensing, medicine, catalysis, separation, and photonics. Quantitative understanding of the correlation between nanoparticle structure and optical chirality becomes increasingly important but still represents a significantly challenging task. Here we demonstrate that tunable signal reversal of circular dichroism in the seed-mediated chiral growth of plasmonic nanoparticles can be achieved through the hybridization of bichiral centers without inverting the geometric chirality. Both experimental and theoretical results demonstrated the opposite sign of circular dichroism of two different bichiral geometries. Chiral molecules were found to not only contribute to the chirality transfer from molecules to nanoparticles but also manipulate the structural evolution of nanoparticles that synergistically drive the formation of two different chiral centers. By deliberately adjusting the concentration of chiral molecules and other synthetic parameters, such as the reducing agent concentration, the capping surfactant concentration, and the amount of Au precursor, we have been able to fine-tune the circular dichroism reversal of bichiral Au nanoparticles. We further demonstrate that the structure of chiral molecules and the crystal structure of Au seeds play crucial roles in the formation of Au nanoparticles with bichiral centers. The insights gained from this work not only shed light on the underlying mechanisms dictating the intriguing geometric and chirality evolution of bichiral plasmonic nanoparticles but also provide an important knowledge framework that guides the rational design of bichiral plasmonic nanostructures toward chiroptical applications.

Ultra-broadband and high-efficiency planar chiral metamaterial

To date, the helix-like assemblies are known for delivering the most broadband chiroptic response; however, as their dimensions shrink to the nanoscale, it becomes increasingly difficult to realize three-dimensional (3D) building blocks and accurate alignments. In addition, a continuous optical channel requirement hinders the downsizing for integrated photonics. Here, we introduce an alternative approach based on two assembled layers of dielectric-metal nanowires to demonstrate that chiroptic effects similar to helix-like metamaterials can be realized with an ultracompact planar structure by creating dissymmetry using orientation and making use of interference phenomena. We constructed two polarization filters for the near-(NIR) and the mid-infrared (MIR) spectrums that exhibit a broadband (0.835-2.11 µm and 3.84-10.64 µm) chiroptic response with maximum transmission and circular dichroism (CD) of approximately 0.965 and extinction ratio > 600. The structure is easy to fabricate, independent of alignments, and scalable from the visible to MIR range for applications including imaging, medical diagnostics, polarization conversion, and optical communication.

Metasurfaces Assisted Twisted α-MoO3 for Spinning Thermal Radiation

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer.

Angle-selective chiral absorption induced by diffractive coupling in metasurfaces

Here we report that a simple chiral metasurface with twisted metallic cut-wire arrays enables highly efficient and continuously tunable chiral absorption over a broad spectral range by scanning the incidence angle over a few degrees. The angle-selective chiral absorption results from the surface plasmon resonance (SPR) excited by diffractive effects of the metasurface. The diffraction-assisted chiral metasurface provides a straightforward strategy for achieving dynamically tunable chiral devices and offers intriguing possibilities for various applications in on-chip chiral detectors/emitters, chiral spectrometers, chiral lasers, and so on.

Realization of maximum optical intrinsic chirality with bilayer polyatomic metasurfaces

Optical chirality plays a key role in optical biosensing and spin-selective optical field manipulation. However, the maximum optical intrinsic chirality, which is represented by near-unity circular dichroism (CD), is yet to be achieved in a wide bandwidth range based on nanostructures. Here, we utilize dielectric bilayer polyatomic metasurfaces to realize the maximum optical intrinsic chirality over a wide bandwidth range. The CD efficiency of the two designed metasurfaces with opposite chirality is 99.9% at 1350 nm and over 98% from 1340 nm to 1361 nm. Our work provides a straightforward and powerful method for the realization of maximum optical intrinsic chirality, which has great potential in spin-selective optical wave manipulation.

Switchable Multifunctional Terahertz Metamaterials Based on the Phase-Transition Properties of Vanadium Dioxide

Currently, terahertz metamaterials are studied in many fields, but it is a major challenge for a metamaterial structure to perform multiple functions. This paper proposes and studies a switchable multifunctional multilayer terahertz metamaterial. Using the phase-transition properties of vanadium dioxide (VO2), metamaterials can be controlled to switch transmission and reflection. Transmissive metamaterials can produce an electromagnetically induced transparency-like (EIT-like) effect that can be turned on or off according to different polarization angles. The reflective metamaterial is divided into I-side and II-side by the middle continuous VO2 layer. The I-side metamaterials can realize linear-to-circular polarization conversion from 0.444 to 0.751 THz when the incident angle of the y-polarized wave is less than 30°. The II-side metamaterials can realize linear-to-linear polarization conversion from 0.668 to 0.942 THz when the incident angle of the y-polarized wave is less than 25°. Various functions can be switched freely by changing the conductivity of VO2 and the incident surface. This enables metamaterials to be used as highly sensitive sensors, optical switches, and polarization converters, which provides a new strategy for the design of composite functional metamaterials.