High-Performance Ultrathin Active Chiral Metamaterials

Chiral 3D structures through multi-dimensional transfer printing of multilayer quantum dot patterns

Three-dimensional optical nanostructures have garnered significant interest in photonics due to their extraordinary capabilities to manipulate the amplitude, phase, and polarization states of light. However, achieving complex three-dimensional optical nanostructures with bottom-up fabrication has remained challenging, despite its nanoscale precision and cost-effectiveness, mainly due to inherent limitations in structural controllability. Here, we report the optical characteristics of intricate two- and three-dimensional nanoarchitectures made of colloidal quantum dots fabricated with multi-dimensional transfer printing. Our customizable fabrication platform, directed by tailored interface polarity, enables flexible geometric control over a variety of one-, two-, and three-dimensional quantum dot architectures, achieving tunable and advanced optical features. For example, we demonstrate a two-dimensional quantum dot nanomesh with tuned subwavelength square perforations designed by finite-difference time-domain calculations, achieving an 8-fold enhanced photoluminescence due to the maximized optical resonance. Furthermore, a three-dimensional quantum dot chiral structure is also created via asymmetric stacking of one-dimensional quantum dot layers, realizing a pronounced circular dichroism intensity exceeding 20°.

Chirally Selective and Switchable Luminescence from Achiral Quantum Emitters on Suspended Twisted Stacking Metasurfaces

Dynamic control of circularly polarized photoluminescence has aroused great interest in quantum optics and nanophotonics. Chiral plasmonic metasurfaces enable the manipulation of the polarization state via plasmon-photon coupling. However, current plasmonic light-emitting metasurfaces for effective deterministic modulation of spin-dependent emission at near-infrared wavelengths are underexplored in terms of dissymmetry and tunability. Here, we demonstrate a microfluidic hybrid emitting system of a suspended twisted stacking metasurface coated with PbS quantum dots. The suspended metasurface is fabricated with a single step of electron beam exposure, exhibiting a strong optical chirality of 309° μm-1 with a thickness of less than λ/10 at key spectral locations. With significant chiral-selective interactions, enhanced photoluminescence is achieved with strong dissymmetry in circular polarization. The dissymmetry factor of the induced circularly polarized emission can reach 1.54. More importantly, altering the refractive index of the surrounding medium at the bottom surface of the metasurface can effectively manipulate the chiroptical responses of the hybrid system, hence leading to chirality-reversed emission. This active hybrid emitting system could be a resultful platform for chirality-switchable light emission from achiral quantum emitters, holding great potential for anticounterfeiting, biosensing, light sources, imaging, and displays.

Enhancing the Circularly Polarized Luminescence of Europium Coordination Polymers by Doping a Chromophore Ligand into Superhelices

Lanthanide-based molecular materials showing efficient circularly polarized luminescence (CPL) activity with a high quantum yield are attractive due to their potential applications in data storage, optical sensors, and 3D displays. Herein we present an innovative method to achieve enhanced CPL activity and a high quantum yield by doping a chromophore ligand into a coordination polymer superhelix. A series of homochiral europium(III) phosphonates with a helical morphology were prepared with the molecular formula S-, R-[Eu(cyampH)3-3n(nempH)3n]·3H2O (S/R-Eu-n, n = 0-5%). The doping of chromophore ligand S- or R-nempH2 into superhelices of S/R-Eu-0% not only turned on the CPL activity with the dissymmetry factor |glum| on the order of 10-3 but also increased the quantum yield by about 14-fold. This work may shed light on the development of efficient CPL-active lanthanide-based coordination polymers for applications.

Chiral Mesostructured Inorganic Materials with Optical Chiral Response

Fabricating chiral inorganic materials and revealing their unique quantum confinement-determined optical chiral responses are crucial tasks in the multidisciplinary fields of chemistry, physics, and biology. The field of chiral mesostructured inorganic materials started from the synthesis of individual nanocrystals and evolved to include their assembly from metals, semiconductors, ceramics, and inorganic salts endowed with various chiral structures ranging from atomic to micron scales. This tutorial review highlights the recent research on chiral mesostructured inorganic materials, especially the novel expression of mesostructured chirality and endowed optical chiral response, and it may inspire us with new strategies for the design of chiral inorganic materials and new opportunities beyond the traditional applications of chirality. Fabrication methods for chiral mesostructured inorganic materials are classified according to chirality type, scale, and symmetry-breaking mechanism. Special attention is given to highlight systems with original discoveries, exceptional phenomena, or unique mechanisms of optical chiral response for left- and right-handedness.

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.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Experimental probe of twist angle-dependent band structure of on-chip optical bilayer photonic crystal

Recently, twisted bilayer photonic materials have been extensively used for creating and studying photonic tunability through interlayer couplings. While twisted bilayer photonic materials have been experimentally demonstrated in microwave regimes, a robust platform for experimentally measuring optical frequencies has been elusive. Here, we demonstrate the first on-chip optical twisted bilayer photonic crystal with twist angle-tunable dispersion and great simulation-experiment agreement. Our results reveal a highly tunable band structure of twisted bilayer photonic crystals due to moiré scattering. This work opens the door to realizing unconventional twisted bilayer properties and novel applications in optical frequency regimes.

Terahertz biosensor integrated with Au nanoparticles to improve the sensing performance

A terahertz (THz) sensor is presented based on a metasurface integrated with Au nanoparticles (AuNPs). It was used to improve the sensitivity of the detection of whey protein and to enhance the sensing index of group delay. This demonstrates that AuNPs can improve the sensing performance of the biosensor. The internal mechanism can be explained by the modified perturbation theory and the coupled harmonic oscillator model. This study provides a means of enhancing the sensitivity of the THz biosensor.

Circularly Polarized Light Responsive Materials: Design Strategies and Applications

Circularly polarized light (CPL) with the end of optical vector traveling along circumferential trajectory shows left- and right-handedness, which transmits chiral information to materials via complicated CPL-matter interactions. Materials with circular dichroism respond to CPL illumination selectively with differential outputs that can be used to design novel photodetectors. Racemic or achiral compounds under CPL go through photodestruction, photoresolution, and asymmetric synthesis pathways to generate enantiomeric bias and optical activity. By this strategy, helical polymers and chiral inorganic plasmonic nanostructures are synthesized directly, and their intramolecular folding and subsequent self-assembly are photomodulable as well. In the aggregated state of self-assembly and liquid crystal phase, helical sense of the dynamic molecular packing is sensitive to enantiomeric bias brought by CPL, enabling the chiral amplification to supramolecular scale. In this review, the application-guided design strategies of CPL-responsive materials are aimed to be systematically summarized and discussed. Asymmetric synthesis, resolution, and property-modulation of small organic compounds, polymers, inorganic nanoparticles, supramolecular assemblies and liquid crystals are highlighted based on the important developments during the last decades. Besides, applications of light-matter interactions including CPL detection and biomedical applications are also referred.

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.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Ultrathin Suspended Chiral Metasurfaces for Enantiodiscrimination

Chiral metasurfaces can exhibit a strong circular dichroism, but it is limited by the complicated fabrication procedure and alignment errors. Here, a new type of self-aligned suspended chiral bilayer metasurface with only one-step electron beam lithography exposure is demonstrated. A significant optical chirality of 221° µm^-1 can be realized using suspended metasurfaces with a thickness of 100 nm. Furthermore, this study experimentally demonstrates that such a structure is capable of label-free discrimination of the chiral molecules at zeptomole level, exhibiting a much higher sensitivity (orders of magnitude) compared to the conventional circular dichroism spectroscopy. The fundamental principles for chiral sensing using molecules-metasurfaces interactions are explored. Benefiting from the giant chiroptical response, the proposed metadevice may offer promising applications for ultrathin circular polarizers, chiral molecular detectors, and asymmetry information processing.

Bubble-pen lithography: Fundamentals and applications: Nanoscience: Special Issue Dedicated to Professor Paul S. Weiss

Developing on-chip functional devices requires reliable fabrication methods with high resolution for miniaturization, desired components for enhanced performance, and high throughput for fast prototyping and mass production. Recently, laser-based bubble-pen lithography (BPL) has been developed to enable sub-micron linewidths, in situ synthesis of custom materials, and on-demand patterning for various functional components and devices. BPL exploits Marangoni convection induced by a laser-controlled microbubble to attract, accumulate, and immobilize particles, ions, and molecules onto different substrates. Recent years have witnessed tremendous progress in theory, engineering, and application of BPL, which motivated us to write this review. First, an overview of experimental demonstrations and theoretical understandings of BPL is presented. Next, we discuss the advantages of BPL and its diverse applications in quantum dot displays, biological and chemical sensing, clinical diagnosis, nanoalloy synthesis, and microrobotics. We conclude this review with our perspective on the challenges and future directions of BPL.

Flatband mode in photonic moiré superlattice for boosting second-harmonic generation with monolayer van der Waals crystals

We theoretically investigate boosting second-harmonic generation (SHG) of monolayer van der Waals crystals by employing flatband modes hosted by photonic moiré superlattices. Such a system with high quality factor and a monolayer crystal accommodated on the top of it, provides a unique opportunity to enhance and manipulate SHG emission. We show that employing a doubly resonant diagram on such a moiré superlattice system not only boosts the SHG, but also tunes the directional emission of the second-harmonic wave. Moreover, we demonstrate that a structured beam illumination could further boost SHG, with the phase structure retrieved through a two-beam second-harmonic interference configuration. These results suggest the flatband modes in moiré superlattice as a promising platform for boosting SHG with monolayer van der Waals crystals, offering new possibilities for developing compact nonlinear photonic devices.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

NEMS-tunable dielectric chiral metasurfaces

Active control of strong chiroptical responses in metasurfaces can offer new opportunities for optical polarization engineering. Plasmonic active chiral metasurfaces have been investigated before, but their tunable chiroptical responses is limited due to inherent loss of plasmonic resonances, thus stimulating research in low loss active dielectric chiral metasurfaces. Among diverse tuning methods, electrically tunable dielectric chiral metasurfaces are promising thanks to their potential for on-chip integration. Here, we experimentally demonstrate nano-electromechanically tunable dielectric chiral metasurfaces with reflective circular dichroism (CD). We show a difference between absolute reflection under circulary polarized incident light with orthogonal polarization of over 0.85 in simulation and over 0.45 experimentally. The devices enable continuous control of CD by induced electrostatic forces from 0.45 to 0.01 with an electrical bias of 3V. This work highlights the potential of nano-electromechanically tunable metasurfaces for scalable optical polarization modulators.

Halogen Bonded Chiral Emitters: Generation of Chiral Fractal Architecture with Amplified Circularly Polarized Luminescence

Self-assembled chiroptical materials have attracted considerable attention due to their great applications in wide fields. During the chiral self-assembly, it remains unknown how achiral molecules can affect the assembly process and their final chiroptical performance. Herein, we report an achiral molecule directed chiral self-assembly via halogen bonds, exhibiting not only an unprecedented chiral fractal architecture but also significantly amplified circularly polarized luminescence (CPL). Two axially chiral emitters with halogen bond sites co-assemble with an achiral 1,4-diiodotetrafluorobenzene (F₄ DIB) and well-ordered chiral fractal structures with asymmetry amplification are obtained. The enhancement of the dissymmetry factors of the assemblies was up to 0.051 and 0.011, which was approximately 100 folds than those of the corresponding molecules. It was found that both the design of the chiral emitter and the highly directional halogen bond played an important role in hierarchically chirality transfer from chiral emitters to the micrometer scale chiral fractal morphology and amplified dissymmetry factors. We hope that this strategy can give a further insight into the fabrication of structurally unique featured highly efficient chiroptical materials.

Highly Efficient Anisotropic Chiral Plasmonic Metamaterials for Polarization Conversion and Detection

Plasmonic chiral metamaterials have attracted broad research interest because of their potential applications in optical communication, biomedical diagnosis, polarization imaging, and circular dichroism spectroscopy. However, optical losses in plasmonic structures severely limit practical applications. Here, we present the design concept and experimental demonstration for highly efficient subwavelength-thick plasmonic chiral metamaterials with strong chirality. The proposed designs utilize plasmonic metasurfaces to control the phase and polarization of light and exploit anisotropic thin-film interference effects to enhance optical chirality while minimizing optical loss. Based on such design concepts, we demonstrated experimentally optical devices such as circular polarization filters with transmission efficiency up to 90% and extinction ratio >180, polarization converters with conversion efficiency up to 90%, as well as on-chip integrated microfilter arrays for full Stokes polarization detection with high accuracy over a broad wavelength range (3.5-5 μm). The proposed design concepts are applicable from near-infrared to Terahertz regions via structural engineering.

Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications

Surface plasmonic sensors have been widely used in biology, chemistry, and environment monitoring. These sensors exhibit extraordinary sensitivity based on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) effects, and they have found commercial applications. In this review, we present recent progress in the field of surface plasmonic sensors, mainly in the configurations of planar metastructures and optical-fiber waveguides. In the metastructure platform, the optical sensors based on LSPR, hyperbolic dispersion, Fano resonance, and two-dimensional (2D) materials integration are introduced. The optical-fiber sensors integrated with LSPR/SPR structures and 2D materials are summarized. We also introduce the recent advances in quantum plasmonic sensing beyond the classical shot noise limit. The challenges and opportunities in this field are discussed.

Mechano-tunable chiral metasurfaces via colloidal assembly

Dynamic control of circular polarization in chiral metasurfaces is being used in many photonic applications. However, simple fabrication routes to create chiral materials with considerable and fully tunable chiroptical responses at visible and near-infrared wavelengths are scarce. Here, we describe a scalable bottom-up approach to construct cross-stacked nanoparticle chain arrays that have a circular dichroism of up to 11°. Due to their layered design, the strong superchiral fields of the inter-layer region are accessible to chiral analytes, resulting in a tenfold enhanced sensitivity in a chiral sensing proof-of-concept experiment. In situ restacking and local mechanical compression enables full control over the entire set of circular dichroism characteristics, namely sign, magnitude and spectral position. Strain-induced reconfiguration opens up an intriguing route towards actively controlled pixel arrays using local deformation, which fosters continuous polarization engineering and multi-channel detection.

Strong extrinsic chirality in biaxial hyperbolic material α-MoO3 with in-plane anisotropy

Chirality has always been a hot research topic because it possesses potential applications in polarization optics, chemical and biosensing. In the previous works, intrinsic chirality has been extensively explored, but its development is limited due to the complexity in fabrication of chiral metamaterials. Therefore, there is an urgent need to simplify fabrication and design of compact devices with chiral response. Extrinsic chirality has shown great potential because it can be realized in nonchiral anisotropic planar structures with low-cost fabrication techniques. In this paper, the extrinsic chirality of biaxial hyperbolic material $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ with in-plane anisotropy has been investigated. By analyzing the effect of thickness of $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film, the angle of incidence, azimuth angle, and wavelength of incidence on the circular dichroism (CD), the maximum CD can reach 0.77. This strong extrinsic chirality of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film results from the mutual orientation of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film and the incident light. In addition, $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film can still maintain strong extrinsic chirality when the azimuthal angle ranges from approximately 20°-57° and the angle of incidence is from 55°-80°.

Chiro-optical response of a wafer scale metamaterial with ellipsoidal metal nanoparticles

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Theory for Twisted Bilayer Photonic Crystal Slabs

We analyze scattering properties of twisted bilayer photonic crystal slabs through a high-dimensional plane wave expansion method. The method is applicable for arbitrary twist angles and does not suffer from the limitations of the commonly used supercell approximation. We show strongly tunable resonance properties of this system which can be accounted for semianalytically from a correspondence relation to a simpler structure. We also observe strongly tunable resonant chiral behavior in this system. Our work provides the theoretical foundation for predicting and understanding the rich optical physics of twisted multilayer photonic crystal systems.

Liquid Optothermoelectrics: Fundamentals and Applications

Liquid thermoelectricity describes the redistribution of ions in an electrolytic solution under the influence of temperature gradients, which leads to the formation of electric fields. The thermoelectric field is effective in driving the thermophoretic migration of charged colloidal particles for versatile manipulation. However, traditional macroscopic thermoelectric fields are not suitable for particle manipulations at high spatial resolution. Inspired by optical tweezers and relevant optical manipulation techniques, we employ laser interaction with light-absorbing nanostructures to achieve subtle heat management on the micro- and nanoscales. The resulting thermoelectric fields are exploited to develop new optical technologies, leading to a research field known as liquid optothermoelectrics. This Invited Feature Article highlights our recent works on advancing fundamentals, technologies, and applications of optothermoelectrics in colloidal solutions. The effects of light irradiation, substrates, electrolytes, and particles on the optothermoelectric manipulations of colloidal particles along with their theoretical limitations are discussed in detail. Our optothermoelectric technologies with the versatile capabilities of trapping, manipulating, and pulling colloidal particles at low optical power are finding applications in microswimmers and nanoscience. With its intricate interfacial processes and tremendous technological promise, optothermoelectrics in colloidal solutions will remain relevant for the foreseeable future.

Decoding Optical Data with Machine Learning

Optical spectroscopy and imaging techniques play important roles in many fields such as disease diagnosis, biological study, information technology, optical science, and materials science. Over the past decade, machine learning (ML) has proved promising in decoding complex data, enabling rapid and accurate analysis of optical spectra and images. This review aims to shed light on various ML algorithms for optical data analysis with a focus on their applications in a wide range of fields. The goal of this work is to sketch the validity of ML-based optical data decoding. The review concludes with an outlook on unaddressed problems and opportunities in this emerging subject that interfaces optics, data science and ML.

Moiré Fringe Induced Gauge Field in Photonics

We realize moiré fringe induced gauge field in a double-layer photonic honeycomb metacrystal with mismatched lattice constants. Benefitting from the generated strong effective gauge field, we report direct measurement of the band diagrams of both Landau level flat bands and intermagnetic-domain edge states. Importantly, we observe the correlation between the momentum and orbital position of the Landau modes, serving as an evidence of the noncommuteness between orthogonal components of the momentum. Without complicated time driving mechanics and careful site-by-site engineering, moiré superlattices could emerge as a powerful means to generate effective gauge fields for photonics benefiting from its simplicity and reconfigurability, which can be applied to nonlinearity enhancement and lasing applications at optical frequencies.

Smart bio-gel optofluidic Mach-Zehnder interferometers multiphoton-lithographically customized with chemo-mechanical-opto transduction and bio-triggered degradation

Stimulus-responsive optical polymers, especially gels, are enabling new-concept energy-transducing "smart" optics. Full exploitation of their molecule-derived tuning and integration with traditional micro/nano-optics/optoelectronics rely on the implementation of devices by advanced "intelligent" micro/nano-manufacturing technologies, especially photolithographies with wide compatibility. In light of the increasing need for an organic combination of smart optical materials and digital micro/nano-manufacturing, novel "smart" optical micro-switches, namely, stimulus-actuated Mach-Zehnder interferometers as a proof-of-concept demonstration, were prototyped with protein-based hydrogels via aqueous multiphoton femtosecond laser direct writing (FsLDW). Protein-based Mach-Zehnder-interferometric smart optical devices here display a morphological quality sufficient for optical applications (average surface roughness ≤∼20 nm), nano-precision three-dimensional (3D) geometry of these millimeter-scale devices and purposely structured distribution of photo-crosslinking degree. Moreover, the device configuration was customized with unbalanced branches in which meticulous stimulus-responsive ability can be realized by simply tuning the surrounding chemical stimuli (i.e., Na₂SO₄ concentration here). The "heterogeneous" configuration with unbalanced branches (i.e., different optical and stimulus-responsive features) exhibits as-designed "smart" switching of propagated near-infrared light (∼808 nm). These capabilities, along with total biodegradation, indicate the application promise of this gel-based optic construction strategy towards novel "intelligent", bio/eco-friendly, self-tuning or sensing photonic integrated systems like optofluidics.

Artificial Double-Helix for Geometrical Control of Magnetic Chirality

Chirality plays a major role in nature, from particle physics to DNA, and its control is much sought-after due to the scientific and technological opportunities it unlocks. For magnetic materials, chiral interactions between spins promote the formation of sophisticated swirling magnetic states such as skyrmions, with rich topological properties and great potential for future technologies. Currently, chiral magnetism requires either a restricted group of natural materials or synthetic thin-film systems that exploit interfacial effects. Here, using state-of-the-art nanofabrication and magnetic X-ray microscopy, we demonstrate the imprinting of complex chiral spin states via three-dimensional geometric effects at the nanoscale. By balancing dipolar and exchange interactions in an artificial ferromagnetic double-helix nanostructure, we create magnetic domains and domain walls with a well-defined spin chirality, determined solely by the chiral geometry. We further demonstrate the ability to create confined 3D spin textures and topological defects by locally interfacing geometries of opposite chirality. The ability to create chiral spin textures via 3D nanopatterning alone enables exquisite control over the properties and location of complex topological magnetic states, of great importance for the development of future metamaterials and devices in which chirality provides enhanced functionality.

Spiranthes sinensis-Inspired Circular Polarized Luminescence in a Solid Block Copolymer Film with a Controllable Helix

Chiral materials with circular polarized luminescence (CPL) have attracted much interest because of their extensive optical information and remarkable sensitivity. Inspired by the helical template in Spiranthes sinensis, we propose here a general and flexible method for fabricating solid CPL materials using a block copolymer-formed helix as a template. A chiral arrangement of various nonchiral fluorescent molecules was obtained in the block copolymer-based hybrid film. An excimer chiralty rule was found for the CPL emission of nonchiral fluorescent molecules: a right-handed helix induced left-handed CPL emission and a left-handed helix induced right-handed CPL emission. A dissipative particle dynamics simulation showed that such an antihelical effect is related to the length between the adjacent interacting points of nonchiral fluorescent molecules along the helical structure. Furthermore, the fluorescent films had a high dissymmetric factor for CPL emission, and thus, the films provide a general and flexible platform for designing and preparing advanced functional chiroptical materials.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Spin-preserving chiral photonic crystal mirror

Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image. Structures made of nanoscale chiral elements can exhibit chiroptical effects, such as dichroism for left- and right-handed circularly polarized light, which makes these structures highly suitable for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition. At the same time, strong chiroptical effects have been challenging to achieve even in synthetic optical media, and chiroptical effects for light with normal incidence have been speculated to be prohibited in thin, lossless quasi-two-dimensional structures. Here, we report an experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror. Unlike conventional mirrors, our mirror selectively reflects only one spin state of light while preserving its handedness, with a near-unity level of circular dichroism. The operational principle of the photonic crystal mirror relies on guided-mode resonance (GMR) with a simultaneous excitation of leaky transverse electric (TE-like) and transverse magnetic (TM-like) Bloch modes in the photonic crystal slab. Such modes are not reliant on the suppression of radiative losses through long-range destructive interference, and even small areas of the photonic crystal exhibit robust circular dichroism. Despite its simplicity, the mirror strongly outperforms earlier reported structures and, contrary to a prevailing notion, demonstrates that near-unity reflectivity contrast for opposite helicities is achievable in a quasi-two-dimensional structure.

Giant optical activity in plasmonic chiral structure via double-layer graphene moiré stacking in mid-infrared region

The plasmonic metamaterials and metasurfaces play a critical role in manipulating lights in the mid-infrared spectral region. Here, we first propose a novel plasmonic chiral structure with the giant optical activity in the mid-infrared spectral region. The chiral structure consists of the moiré patterns, which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle. It is demonstrated that the graphene-based plasmonic structure with moiré patterns exhibits the strong circular dichroism. The giant chiroptical response can be precisely controlled by changing the rotation angle and Fermi level of graphene. Furthermore, a dielectric interlayer is inserted between two layers of graphene to obtain the stronger circular dichroism. Impressively, the strongest circular dichroism can reach 5.94 deg at 13.6 µm when the thickness of dielectric interlayer is 20 nm. The proposed structure with graphene-based moiré patterns can be superior to conventional graphene chiral metamaterials due to some advantage of rotation-dependent chirality, flexible tunability and cost-effective fabrication. It will advance many essential mid-infrared applications, such as chiral sensors, thermal imaging and chiroptical detectors.

Chiral nanohole arrays

Chiral nanohole array (CNA) films are fabricated by a simple and efficient shadow sphere lithography (SSL) method and achieve label-free enantiodiscrimination of biomolecules and drug molecules at the picogram level. The intrinsic mirror symmetry of the structure is broken by three subsequent depositions onto non-close packed nanosphere monolayers with different polar and azimuthal angles. Giant chiro-optical responses with a transmission as high as 45%, a chirality of 21°μm^-1, and a g-factor of 0.17, respectively, are generated, which are among the largest values that have been reported in the literature. Such properties are due to the local rotating current density generated by a surface plasmon polariton as well as a strong local rotating field produced by localized surface plasmon resonance, which leads to the excitation of substantial local superchiral fields. The 70 nm-thick CNAs can achieve label-free enantiodiscrimination of biomolecules and drug molecules at the picogram level as demonstrated experimentally. All these advantages make the CNAs ready for low-cost, high-performance, and ultracompact polarization converters and label-free chiral sensors.

Room-Temperature Active Modulation of Valley Dynamics in a Monolayer Semiconductor through Chiral Purcell Effects

Spin-dependent contrasting phenomena at K and K' valleys in monolayer semiconductors have led to addressable valley degree of freedom, which is the cornerstone for emerging valleytronic applications in information storage and processing. Tunable and active modulation of valley dynamics in a monolayer WSe₂ is demonstrated at room temperature through controllable chiral Purcell effects in plasmonic chiral metamaterials. The strong spin-dependent modulation on the spontaneous decay of valley excitons leads to tunable handedness and spectral shift of valley-polarized emission, which is analyzed and predicted by an advanced theoretical model and further confirmed by experimental measurements. Moreover, large active spectral tuning (≈24 nm) and reversible ON/OFF switching of circular polarization of emission are achieved by the solvent-controllable thickness of the dielectric spacer in the metamaterials. With the on-demand and active tunability in valley-polarized emission, chiral Purcell effects can provide new strategies to harness valley excitons for applications in ultrathin valleytronic devices.

Induced radiation force of an optical line source on a cylinder material exhibiting circular dichroism

The optical radiation force experienced by a cylinder material of circular cross section exhibiting circular dichroism (known also as rotary polarization) in an electric line source illumination is considered. An exact analytical expression for the radiation force (per length) valid for any frequency range is derived assuming an electric line source radiating cylindrically diverging TM-polarized waves without any approximations. The partial-wave series expansion method in cylindrical coordinates utilizing standard Bessel and Hankel functions is used to derive the electric and magnetic field expressions and a dimensionless radiation force function (or efficiency), which depends on the scattering coefficient of the cylinder as well as the distance from the radiating source. To illustrate the analysis, numerical computations for the dimensionless radiation force function for a perfect electromagnetic conductor (PEMC) cylinder are performed with emphasis on its dimensionless size parameter and source distance, which clearly draw attention to the contribution of the cross-polarized scattered waves (resulting from the rotary polarization effect) to the total force. The numerical predictions demonstrate the possibility to pull a circular-shaped cylinder material with rotary polarization toward the illuminating electric line source with TM-polarized waves using a curved wavefront depending on the PEMC material admittance, distance to the source, and size of the cylinder.

Chiral response of a metasurface composed of nanoholes and tilted nanorods

Circular dichroism (CD) of metasurfaces has been used in biological monitoring, analytical chemistry, and perfect polarization converters. In this work, a metasurface consisting of nanoholes and tilted nanorods is proposed to achieve the CD effect. Numerical calculations show that electrical current forms between the film and the tilted nanorods under circularly polarized light illumination, and CD effects originate from the coupling between the current oscillations at the film and those on the tilted nanorods. This electrical oscillation mode provides unique coupling mechanisms for the CD effect. In addition, CD is strongly dependent on the structural parameters, and the resonant modes can be tuned by modulating the currents on the film. These results are helpful for designing novel chiral optical structures and provide unique methods for circular polarizers.

Electromagnetically induced transparency-like metamaterials for detection of lung cancer cells

A biosensor based on electromagnetically induced transparent (EIT) metamaterials (MMs) is proposed owing to the low loss and high Q-factor. The theoretical sensitivity of the biosensor based on EIT-like MMs were evaluated up to 248.8 GHz/RIU (RIU, Refractive Index Unit). In experiments, the cancer cells A549, as an analyte, are cultured on EIT-like MMs surface. The results show that when the cell concentration increases from 0.5 × 10⁵ to 5 × 10⁵ cells/ml, the frequency shift Δf could change from 24 to 50 GHz. Moreover, the coupled oscillators model is applied to explain the effect of the refractive index of analyte in simulations and the cell concentration in experiments on the EIT-like MMs. The fitting results exhibit that the refractive index of analyte and cell concentration significantly affect the radiative damping of the bright mode resonator γ₁. The proposed EIT-like MMs biosensors show great potentials for cell measurement because any change that results in the lineshape variation in EIT-like MMs can only be attributed to the change of external dielectric environment due to the suppression of radiative losses.

Tunable chiroptical response of graphene achiral metamaterials in mid-infrared regime

We numerically investigate a tunable and extrinsic chiroptical response of a graphene achiral metamaterial in mid-infrared regime. The achiral metamaterial is composed of cascaded metallic split ring apertures and complementary graphene rings patterned on a dielectric layer. The strong extrinsic chiroptical responses of the metamaterial are allowed at oblique incidence and the integrated graphene can dynamically modulate extrinsic chirality by changing its Fermi level. The spectra of the chiroptical responses will show a blue shift with increasing the Fermi level of the patterned graphene. The maximal values of circular dichroism in the reflection and transmission modes can reach 80% and 50%, respectively. The maximal values of polarization rotation angle in the reflection and transmission modes can reach 80° and 60°, respectively. This graphene-based metamaterial design paves a way for a myriad of important terahertz (THz) and mid-infrared applications, such as optical modulators, absorbers and polarizers.

Tunable surface plasmon-polaritons based on quantum coherence

Tunable surface plasmons on the interface of a multilevel atomic medium with a cross coupling of the electric and magnetic components of a plasmonic field are investigated. The strong chirality resulting from the quantum coherence leads to some exciting properties of the surface plasmons. Compared to the traditional chiral-metal interface, surface plasmonic mode can still be found at the interface between such atomic media and a dielectric even when both the permittivity and the permeability of the medium are positive. This is in contrast to the conventional plasmonic systems where the signs of the permittivities or permeabilities on the two sides of the interface are opposite. We call this phenomenon an electromagnetically induced plasmon. Additionally, as the chirality and effective refractive index of the atomic medium are dependent on the intensity and phase of the controlling field, we can conveniently manipulate the properties of the surface plasmons.

Chiral metamaterials via Moiré stacking

Chiral metamaterials have attracted strong interest due to their versatile capabilities in spin-dependent light manipulation. Benefiting from advancements in nanofabrication and mechanistic understanding of chiroptical effects, chiral metamaterials have shown potential in a variety of applications including circular polarizers, chiral sensors, and chiroptical detectors. Recently, chiral metamaterials made by moiré stacking, superimposing two or more periodic patterns with different lattice constants or relative spatial displacement, have shown promise for chiroptical applications. The moiré chiral metamaterials (MCMs) take advantage of lattice-dependent chirality, giving cost-effective fabrication, flexible tunability, and reconfigurability superior to conventional chiral metamaterials. This feature article focuses on recent progress of MCMs. We discuss optical mechanisms, structural design, fabrication, and applications of the MCMs. We conclude with our perspectives on the future opportunities for the MCMs.

Surface plasmon modes of nanomesh-on-mirror nanocavities prepared by nanosphere lithography

Metal-insulator-metal (MIM) structures show great potential for numerous photonic applications due to their ability to confine light energy to volumes with deeply sub-wavelength dimensions. Here, MIM structures comprising hexagonal gold nanohole arrays were prepared by nanosphere lithography. Angle-resolved UV-vis-NIR spectroscopy revealed a series of narrow, dispersive and non-dispersive modes, which were attributed to the excitation of surface plasmon polariton (SPP) modes. Applying finite-difference time-domain (FDTD) simulations and analytical diffraction phase-matching theory all resonances can be ascribed to only two SPP modes traveling at the outer gold surface and in the gap layer sandwiched between two metal films. Metamaterial resonances, as reported in the literature for similar structures, are not needed to fully explain the reflectance spectra. Bragg scattering of the symmetric gap SPP mode results in a gap resonance, which is insensitive to the angle of incidence over a broad angular range. The spectral position of this flat band can be controlled by tuning the grating period of the nanohole array as well as the thickness and the refractive index of the dielectric gap.

Five-fold plasmonic Fano resonances with giant bisignate circular dichroism

Chiral metamaterials with versatile designs can exhibit orders of magnitude enhancement in chiroptical responses compared with that of the natural chiral media. Here, we propose an ease-of-fabrication three-dimensional (3D) chiral metamaterial consisting of vertical asymmetric plate-shape resonators along a planar air hole array with extraordinary optical transmission. It is theoretically shown that such chiral metamaterials simultaneously support five-fold plasmonic Fano resonance states and exhibit significant bisignate circular dichroism (CD) with amplitude as large as 0.8 due to the distinctive local electric field distributions. More interestingly, a "bridge" in the proposed double-plate-based architectures can act as a flipped ruler that is able to continuously manipulate optical chirality including the handedness-selective enhancement and the switching of CD signals. Importantly, the proposed designs have been readily fabricated by using a focused-ion-beam irradiation-induced folding technique and they consistently exhibited five-fold Fano resonances with strong CD effects in experiments. The studies are helpful for the understanding, designing and improvement of chiral optical systems towards potential applications such as ultrasensitive biosensing, polarimetric imaging, quantum information processing, etc.