Circular dichroism from chiral nanomaterial fabricated by on-edge lithography

Controlling the broadband enhanced light chirality with L-shaped dielectric metamaterials

The inherently weak chiroptical responses of natural materials limit their usage for controlling and enhancing chiral light-matter interactions. Recently, several nanostructures with subwavelength scale dimensions were demonstrated, mainly due to the advent of nanofabrication technologies, as a potential alternative to efficiently enhance chirality. However, the intrinsic lossy nature of metals and the inherent narrowband response of dielectric planar thin films or metasurface structures pose severe limitations toward the practical realization of broadband and tailorable chiral systems. Here, we tackle these problems by designing all-dielectric silicon-based L-shaped optical metamaterials based on tilted nanopillars that exhibit broadband and enhanced chiroptical response in transmission operation. We use an emerging bottom-up fabrication approach, named glancing angle deposition, to assemble these dielectric metamaterials on a wafer scale. The reported strong chirality and optical anisotropic properties are controllable in terms of both amplitude and operating frequency by simply varying the shape and dimensions of the nanopillars. The presented nanostructures can be used in a plethora of emerging nanophotonic applications, such as chiral sensors, polarization filters, and spin-locked nanowaveguides.

Chiral Gold Nanorods with Five-Fold Rotational Symmetry and Orientation-Dependent Chiroptical Properties of Their Monomers and Dimers

Chiral plasmonic nanoparticles have attracted much attention because of their strong chiroptical responses and broad scientific applications. However, the types of chiral plasmonic nanoparticles have remained limited. Herein we report on a new type of chiral nanoparticle, chiral Au nanorod (NR) with five-fold rotational symmetry, which is synthesized using chiral molecules. Three different types of Au seeds (Au elongated nanodecahedrons, nanodecahedrons, and nanobipyramids) are used to study the growth behaviors. Different synthesis parameters, including the chiral molecules, surfactant, reductant, seeds, and Au precursor, are systematically varied to optimize the chiroptical responses of the chiral Au NRs. The chiral scattering measurements on the individual chiral Au NRs and their dimers are performed. Intriguingly, the chiroptical signals of the individual chiral Au NRs and their end-to-end dimers are similar, while those of the side-by-side dimers are largely reduced. Theoretical calculations and numerical simulations reveal that the different chiroptical responses of the chiral NR dimers are originated from the coupling effect between the plasmon resonance modes. Our study enriches chiral plasmonic nanoparticles and provides valuable insight for the design of plasmonic nanostructures with desired chiroptical properties.

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

Nano-fabrication techniques have demonstrated their vital importance in technological innovation. However, low-throughput, high-cost and intrinsic resolution limits pose significant restrictions, it is, therefore, paramount to continue improving existing methods as well as developing new techniques to overcome these challenges. This is particularly applicable within the area of biomedical research, which focuses on sensing, increasingly at the point-of-care, as a way to improve patient outcomes. Within this context, this review focuses on the latest advances in the main emerging patterning methods including the two-photon, stereo, electrohydrodynamic, near-field electrospinning-assisted, magneto, magnetorheological drawing, nanoimprint, capillary force, nanosphere, edge, nano transfer printing and block copolymer lithographic technologies for micro- and nanofabrication. Emerging methods enabling structural and chemical nano fabrication are categorised along with prospective chemical and physical patterning techniques. Established lithographic techniques are briefly outlined and the novel lithographic technologies are compared to these, summarising the specific advantages and shortfalls alongside the current lateral resolution limits and the amenability to mass production, evaluated in terms of process scalability and cost. Particular attention is drawn to the potential breakthrough application areas, predominantly within biomedical studies, laying the platform for the tangible paths towards the adoption of alternative developing lithographic technologies or their combination with the established patterning techniques, which depends on the needs of the end-user including, for instance, tolerance of inherent limits, fidelity and reproducibility.

Hierarchically manufactured chiral plasmonic nanostructures with gigantic chirality for polarized emission and information encryption

Chiral metamaterials have received significant attention due to their strong chiroptical interactions with electromagnetic waves of incident light. However, the fabrication of large-area, hierarchically manufactured chiral plasmonic structures with high dissymmetry factors (g-factors) over a wide spectral range remains the key barrier to practical applications. Here we report a facile yet efficient method to fabricate hierarchical chiral nanostructures over a large area (>11.7 × 11.7 cm2) and with high g-factors (up to 0.07 in the visible region) by imparting extrinsic chirality to nanostructured polymer substrates through the simple exertion of mechanical force. We also demonstrate the application of our approach in the polarized emission of quantum dots and information encryption, including chiral quick response codes and anti-counterfeiting. This study thus paves the way for the rational design and fabrication of large-area chiral nanostructures and for their application in quantum communications and security-enhanced optical communications.

Tunable Chiral Plasmonic Activities Enabled via Stimuli Responsive Micro-Origami

Chiral plasmonic nanomaterials with distinctive circularly polarized light-dependent optical responses over a broad range of frequency have great potential for photonic and biomedical applications. However, it still remains challenging to fabricate 3D plasmonic chiral micro-constructs with readily modulated chiroptical properties over the magnitude of ellipticity, mode frequency, and switchable handedness, especially in the vis-NIR range. In this study, polymeric micro-origami-based 3D plasmonic chiral structures are constructed through self-rolling of gold  nanospheres (AuNSs)-decorated polymeric micro-sheets. Spherical AuNSs are assembled as highly ordered linear chains on 2D rectangular micro-sheets by polydimethylsiloxane-wrinkle assisted assembly. Upon rolling the micro-sheets to micro-tubules, the AuNS chains transform into 3D helices. The AuNS-assembled helices induce collective plasmonic modes propagating in a helical manner, leading to a strong chiral response over the vis-NIR range. The circular dichroism (CD) is measured to be as high as hundreds of millidegree, and the position and sign of CD peaks are actively modulated by controlling the orientated angle of AuNS chains, enabled by tuning the collective plasmonic modes. This micro-origami-based strategy incorporates the incompatible 2D assembly technique with 3D chiral structures, opening up an intriguing way toward constructing chiral plasmonic structures and modulating chiroptical effects based on responsive polymeric materials.

Hot-Electron Dynamics Mediated Medical Diagnosis and Therapy

Surface plasmon resonance excitation significantly enhances the absorption of light and increases the generation of "hot" electrons, i.e., conducting electrons that are raised from their steady states to excited states. These excited electrons rapidly decay and equilibrate via radiative and nonradiative damping over several hundred femtoseconds. During the hot-electron dynamics, from their generation to the ultimate nonradiative decay, the electromagnetic field enhancement, hot electron density increase, and local heating effect are sequentially induced. Over the past decade, these physical phenomena have attracted considerable attention in the biomedical field, e.g., the rapid and accurate identification of biomolecules, precise synthesis and release of drugs, and elimination of tumors. This review highlights the recent developments in the application of hot-electron dynamics in medical diagnosis and therapy, particularly fully integrated device techniques with good application prospects. In addition, we discuss the latest experimental and theoretical studies of underlying mechanisms. From a practical standpoint, the pioneering modeling analyses and quantitative measurements in the extreme near field are summarized to illustrate the quantification of hot-electron dynamics. Finally, the prospects and remaining challenges associated with biomedical engineering based on hot-electron dynamics are presented.

Light-Responsive Supramolecular Nanotubes-Based Chiral Plasmonic Assemblies

We describe the fabrication of dual-responsive (thermo/light) chiral plasmonic films. The idea is based on using photoswitchable achiral liquid crystal (LCs) forming chiral nanotubes for templating helical assemblies of Au NPs. Circular dichroism spectroscopy (CD) confirms chiroptical properties coming from the arrangement of organic and inorganic components, with up to 0.2 dissymmetry factor (g-factor). Upon exposure to UV light, organic molecules isomerize, resulting in controlled melting of organic nanotubes and/or inorganic nanohelices. The process can be reversed using visible light and further modified by varying the temperature, offering a control of chiroptical response of the composite material. These properties can play a key role in the future development of chiral plasmonics, metamaterials, and optoelectronic devices.

Enhanced Diffractive Circular Dichroism from Stereoscopic Plasmonic Molecule Array

Artificial nanostructures with large optical chiral responses have been intensively investigated recently. In this work, we propose a diffractive circular dichroism enhancement technique using stereoscopic plasmonic molecule structures. According to the multipole expansion analysis, the z-component of the electric dipole becomes the dominant chiral scattering mechanism during the interaction between an individual plasmonic molecule and the plane wave at a grazing angle. For a periodical structure with the designed plasmonic molecule, large diffractive circular dichroism can be obtained, which can be associated with the Wood-Rayleigh anomaly. Such a diffractive circular dichroism enhancement is verified by the good agreement between numerical simulations and experimental results. The proposed approach can be potentially used to develop enhanced spectroscopy techniques to measure chiral information, which is very important for fundamental physical and chemical research and bio-sensing applications.

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.

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.

Chiral surface plasmon-enhanced chiral spectroscopy: principles and applications

In this review, the development context and scientific research results of chiral surface plasmons (SPs) in recent years are classified and described in detail. First, the principle of chiral SPs is introduced through classical and quantum theory. Following this, the classification and properties of different chiral structures, as well as the superchiral near-field, are introduced in detail. Second, we describe the excitation and propagation properties of chiral SPs, which lays a good foundation for the application of chiral SPs and their chiral spectra in various fields. After that, we have summarized the recent research results of chiral SPs and their applications in the areas of biology, two-dimensional materials, topological materials, analytical chemistry, chiral sensing, chiral optical force, and chiral light detection. Chiral SPs are a new type of optical phenomenon that have useful application potential in many fields and are worth exploring.

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

The focused ion beam (FIB) is a powerful piece of technology which has enabled scientific and technological advances in the realization and study of micro- and nano-systems in many research areas, such as nanotechnology, material science, and the microelectronic industry. Recently, its applications have been extended to the photonics field, owing to the possibility of developing systems with complex shapes, including 3D chiral shapes. Indeed, micro-/nano-structured elements with precise geometrical features at the nanoscale can be realized by FIB processing, with sizes that can be tailored in order to tune optical responses over a broad spectral region. In this review, we give an overview of recent efforts in this field which have involved FIB processing as a nanofabrication tool for photonics applications. In particular, we focus on FIB-induced deposition and FIB milling, employed to build 3D nanostructures and metasurfaces exhibiting intrinsic chirality. We describe the fabrication strategies present in the literature and the chiro-optical behavior of the developed structures. The achieved results pave the way for the creation of novel and advanced nanophotonic devices for many fields of application, ranging from polarization control to integration in photonic circuits to subwavelength imaging.

Chiral Surface Lattice Resonances

Collective excitation of periodic arrays of metallic nanoparticles by coupling localized surface plasmon resonances to grazing diffraction orders leads to surface lattice resonances with narrow line width. These resonances may find numerous applications in optical sensing and information processing. Here, a new degree of freedom of surface lattice resonances is experimentally investigated by demonstrating handedness-dependent excitation of surface lattice resonances in arrays of chiral plasmonic crescents. The self-assembly of particles used as mask and modified colloidal lithography is applied to produce arrays of planar and 3D gold crescents over large areas. The excitation of surface lattice resonances as a function of the interparticle distance and the degree of order within the arrays is investigated. The chirality of the individual 3D crescents leads to the formation of chiral lattice modes, that is, surface lattice resonances that exhibit optical activity.

Uniform Chiral Gap Synthesis for High Dissymmetry Factor in Single Plasmonic Gold Nanoparticle

Synthesis of chiral plasmonic materials has been highlighted for the last decades with their optical properties and versatile potential applications. Recently reported aqueous-based amino acid- and peptide-directed synthesis of chiral plasmonic gold nanoparticles with 432 point-group symmetry shows exceptionally high chiroptic response within 100 nm scales. Despite its already excellent chiroptic response, a single-nanoparticle dark field scattering study revealed that full chiroptic potential of chiral gold nanoparticle is limited with its overall synthetic uniformity. Based on this knowledge, we present a multi-chirality-evolution step synthesis method for the enhancement of chiroptic response through an increase in particle uniformity. Detailed time variant study and interrelationship study of reaction parameters allowed the systematic construction of design principles for chiral nanoparticles with exceptional chiroptic response. With the application of precisely controlled growth kinetic to two distinct growth regimes, modified chiral gold nanoparticles showed significantly improved uniformity, achieving an improved dissymmetry factor of g = 0.31. We expect that our strategy will aid in precise morphology and property control for chiral nanomaterials, which can be used in various plasmonic metamaterial applications.

Controlling the spin-selective absorption with two-dimensional chiral plasmonic gratings

Spin-selective absorption in a two-dimensional (2D) chiral plasmonic grating is observed by excitation of chiral-dependent plasmonic cavity resonance. For the proposed structures, the incident right-handed circularly polarized light is absorbed with nearly 100% efficiency, whereas the incident left-handed circularly polarized light is reflected with same handedness. Moreover, we show that the location of spin-selective absorption can be controlled flexibly by tuning the plasmonic cavity dimension. The intensity of spin-selective absorption can be enhanced as well as suppressed based on Fabry-Perot interference phase relation. Such 2D chiral plasmonic gratings could find many potential applications in novel photon-spin selective devices, such as circularly polarized light detectors/emitters, chiral sensors, chiral cavities, and spin lasers.

Shape-Dependent, Chiro-optical Response of UV-Active, Nanohelix Metamaterials

We harness a synergy between morphology and the electromagnetic response of semiconducting material to engineer the chiro-optical properties of metamaterials that are active at ultraviolet (UV) wavelengths. Chiral metamaterials have recently ushered in new research directions in fundamental light-matter interactions, while simultaneously opening a range of promising photonics-based applications from polarization control to improved biosensing methods. Despite these recent advances, to date, very little attention has been focused upon engineered large UV-chiro-optical activity, where naturally occurring molecular optical activity bands are most typically encountered. Here, we systematically alter the morphology of titanium dioxide nanohelices, which make up the elements of the chiral metamaterials, to investigate how the nanoparticle shape affects chiro-optical activity across the UV spectrum. When the nanoscale critical dimensions fall within a particular size range, giant chiro-optical activity is observed, which is on the order of the strongest demonstrated in the UV to date and can be tuned by slight alterations of the nanohelices' morphology.

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.

Analyzing the polarization response of a chiral metasurface stack by semi-analytic modeling

We investigate a class of stacked metasurfaces where the interaction between layers is dominated by their respective far-field response. Using a semi-analytic scattering matrix approach, we exploit the Fabry-Perot-type response for different layer distances to show the spectral tunability of the resonant effect. This method presents a faster and more intuitive route to modeling Fabry-Perot-type effects than rigorous numerical simulations. The results are illustrated for a chiral metasurface stack that exhibits asymmetric transmission. Here, the effect of asymmetric transmission is highly sensitive to the layer distance, which is used as a free parameter in our model. To prove our theoretical findings we fabricate two variants of the stack with different layer distances and show that far-field interaction between layers is sufficient to generate the effect while being accessible by semi-analytic modeling. The analyticity of the approach is promising for designing sophisticated layered media containing stacks of arbitrary metasurfaces.

Nanoimprinted Chiral Plasmonic Substrates with Three-Dimensional Nanostructures

We report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. Several structures are successfully fabricated using this method, including L-shaped, twisted arc and trilayer twisted Au nanorod structures, demonstrating its generality. As one typical example, arrays of L-shaped nanostructures, consisting of two layers of orthogonally oriented Au nanorods separated by a Ge dielectric layer in the thickness direction, exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. Electromagnetic simulations show that the optical chirality results from plasmon hybridization between the two orthogonal Au segments. To demonstrate scalability, a 1 cm² chiral substrate is fabricated with uniform chiral optical property. This method combines both high throughput and precise geometrical control and is therefore promising for applications of chiral metamaterials.

Optimized fan-shaped chiral metamaterial as an ultrathin narrow-band circular polarizer at visible frequencies

Chiral metamaterials have the great ability to manipulate the circular polarizations of light, which can be utilized to build ultrathin circular polarizers. Here we build a narrow-band circular polarizer at visible frequencies based on plasmonic fan-shaped chiral nanostructures. In order to achieve the best optical performance, we systematically investigate how different fabrication factors affect the chiral optical response of the fan-shaped chiral nanostructures, including incident angle of vapor depositions, nanostructure thickness, and post-deposition annealing. The optimized fan-shaped nanostructures show two narrow bands for different circular polarizations with the maximum extinction ratios 7.5 and 6.9 located at wavelength 687 nm and 774 nm, respectively.

Three dimensional chiral plasmon rulers based on silver nanorod trimers

The symmetry dependences of plasmon excitation modes are studied in 3D silver nanorod trimers. The degenerate plasmon modes split into chiral modes by breaking the inversion and mirror symmetry of the nanorod trimer through translation and/or rotation of the middle rod. With a translation operation, successive evolution of the circular dichroism (CD) spectrum can be achieved through gradual breaking of the inversion symmetry. An additional rotation operation produces even dramatic spectral changes due to breaking a quasi-mirror symmetry resulted from the same angular distance of the middle rod to the top and bottom rods. Especially, pairs of new chiral modes can be excited due to the contact of the middle rod with the top-bottom rod pair. The spectral changes in the simulations, which are also demonstrated experimentally, envision the 3D chiral nanorod trimer system as plasmon ruler for spatial configuration retrieval and dynamic bio-process analysis at the single molecule level.

Chiral Plasmonic Hydrogen Sensors

In this article, a chiral plasmonic hydrogen-sensing platform using palladium-based nanohelices is demonstrated. Such 3D chiral nanostructures fabricated by nanoglancing angle deposition exhibit strong circular dichroism both experimentally and theoretically. The chiroptical properties of the palladium nanohelices are altered upon hydrogen uptake and sensitively depend on the hydrogen concentration. Such properties are well suited for remote and spark-free hydrogen sensing in the flammable range. Hysteresis is reduced, when an increasing amount of gold is utilized in the palladium-gold hybrid helices. As a result, the linearity of the circular dichroism in response to hydrogen is significantly improved. The chiral plasmonic sensor scheme is of potential interest for hydrogen-sensing applications, where good linearity and high sensitivity are required.

Chiral Metamaterials of Plasmonic Slanted Nanoapertures with Symmetry Breaking

We propose a universal design scheme for a new type of chiral metamaterials based on plasmonic slanted nanoapertures simply milled in a single metal layer. Strong optical chirality is introduced by tilting nanoapertures with almost arbitrary shape along a certain direction to break all the mirror symmetries. As a typical example, chiral metamaterial based on slanted split-ring apertures is demonstrated with giant circular dichroism in transmission (CDT) over 78% at 760 nm. We reveal that the high CDT originates from the circularly dichroic mode coupling process in the slanted nanoapertures induced by spin-dependent field overlap conditions. Furthermore, tunable CDT is presented through the in-plane rotation of nanoapertures to form chiral images with controllable image contrast. Besides, chiral metamaterials with slanted nanoapertures of two other shapes including L-shaped aperture and rectangular aperture are also presented with large circular dichroism. We envision that our demonstrated chiral metamaterials enable promising platforms for a variety of applications in nonlinear optics, chiral imaging and sensing, spectroscopy, and polarization manipulation.

Active Terahertz Chiral Metamaterials Based on Phase Transition of Vanadium Dioxide (VO2)

Compared with natural materials, chiral metamaterials have been demonstrated with orders of magnitude stronger chiroptical response, which provides the basis for applications such as ultracompact polarization components and plasmonic-enhanced biosensing. Terahertz chiral metamaterials that allow dynamic polarization control of terahertz waves are of great practical interest, but remain extremely rare. Here, we show that hybrid metamaterials integrated with vanadium dioxide (VO2) exhibiting phase transition can enable dynamically tunable chiroptical responses at terahertz frequencies. In particular, a circular dichroism of ~40° and a maximum polarization rotation of ~200°/λ are observed around 0.7 THz. Furthermore, our study also reveals that the chiroptical response from the proposed metamaterials is strongly dependent on the phase transition of VO2, leading to actively controllable polarization states of the transmitted terahertz waves. This work paves the way for the development of terahertz metadevices capable of enabling active polarization manipulation.

Chiral plasmonics

We present a comprehensive overview of chirality and its optical manifestation in plasmonic nanosystems and nanostructures. We discuss top-down fabricated structures that range from solid metallic nanostructures to groupings of metallic nanoparticles arranged in three dimensions. We also present the large variety of bottom-up synthesized structures. Using DNA, peptides, or other scaffolds, complex nanoparticle arrangements of up to hundreds of individual nanoparticles have been realized. Beyond this static picture, we also give an overview of recent demonstrations of active chiral plasmonic systems, where the chiral optical response can be controlled by an external stimulus. We discuss the prospect of using the unique properties of complex chiral plasmonic systems for enantiomeric sensing schemes.

Intensity-dependent modulation of optically active signals in a chiral metamaterial

Chiral media exhibit optical phenomena that provide distinctive responses from opposite circular polarizations. The disparity between these responses can be optimized by structurally engineering absorptive materials into chiral nanopatterns to form metamaterials that provide gigantic chiroptical resonances. To fully leverage the innate duality of chiral metamaterials for future optical technologies, it is essential to make such chiroptical responses tunable via external means. Here we report an optical metamaterial with tailored chiroptical effects in the nonlinear regime, which exhibits a pronounced shift in its circular dichroism spectrum under a modest level of excitation power. Strong nonlinear optical rotation is observed at key spectral locations, with an intensity-induced change of 14° in the polarization rotation from a metamaterial thickness of less than λ/7. The modulation of chiroptical responses by manipulation of input powers incident on chiral metamaterials offers potential for active optics such as all-optical switching and light modulation.

Here Rodrigues et al. report an optical metamaterial with tailored chiroptical effects in the nonlinear regime, which exhibits a pronounced shift in its circular dichroism spectrum under a modest level of excitation power.

Optical chiral metamaterials: a review of the fundamentals, fabrication methods and applications

Optical chiral metamaterials have recently attracted considerable attention because they offer new and exciting opportunities for fundamental research and practical applications. Through pragmatic designs, the chiroptical response of chiral metamaterials can be several orders of magnitude higher than that of natural chiral materials. Meanwhile, the local chiral fields can be enhanced by plasmonic resonances to drive a wide range of physical and chemical processes in both linear and nonlinear regimes. In this review, we will discuss the fundamental principles of chiral metamaterials, various optical chiral metamaterials realized by different nanofabrication approaches, and the applications and future prospects of this emerging field.

Heat-enhanced symmetry breaking in dynamic gold nanorod oligomers: the importance of interface control

We reported a surprisingly strong plasmonic circular dichroism (PCD) response in side-by-side (SS) oligomers of gold nanorods (GNRs) just by a simple heat treatment. The maximal anisotropic (g) factor achieved was up to 0.065, one of the largest reported for plasmon-enhanced chiral nanostructures based on a bottom-up strategy. The introduction of chiral thiolated molecules is suggested to guide the symmetry breaking of GNR assemblies and heat treatment provides the necessary energy to assist this process, and thus produces a huge PCD. Furthermore, we first demonstrated the critical role of the non-chiral component (surfactant layer) on the gold nanorod surface in mediating symmetry breaking. Our findings highlight the importance of interface control in the formation of chiral configuration for a plasmonic nanoparticle system. It offers new possibilities for fabricating nanostructures with strong chiroptical activity by the rational design of interface layers.

Multipolar Effects in the Optical Active Second Harmonic Generation from Sawtooth Chiral Metamaterials

Based on the facts that chiral molecules response differently to left- and right-handed circular polarized light, chiroptical effects are widely employed for determining structure chirality, detecting enantiomeric excess, or controlling chemical reactions of molecules. Compared to those in natural materials, chiroptical behaviors can be significantly amplified in chiral plasmonic metamaterials due to the concentrated local fields in the structure. The on-going research towards giant chiroptical effects in metamaterial generally focus on optimizing the field-enhancement effects. However, the observed chiroptical effects in metamaterials rely on more complicated factors and various possibilities towards giant chiroptical effects remains unexplored. Here we study the optical-active second harmonic generation (SHG) behaviors in a pair of planar sawtooth gratings with mirror-imaged patterns. Significant multipolar effects were observed in the polarization-dependent SHG curves. We show that the chirality of the nanostructure not only give rise to nonzero chiral susceptibility tensor components within the electric-dipole approximation, but also lead to different levels of multipolar interactions for the two orthogonal circular polarizations that further enhance the nonlinear optical activity of the material. Our results thus indicate novel ways to optimize nonlinear plasmonic structures and achieve giant chiroptical response via multipolar interactions.

Plasmonic nanostructures for surface enhanced spectroscopic methods

The development within the last five years in the field of surface enhanced spectroscopy methods was comprehensively reviewed.

Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures

Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

Deterministic Assembly of Flexible Si/Ge Nanoribbons via Edge-Cutting Transfer and Printing for van der Waals Heterojunctions

As the promising building blocks for flexible electronics and photonics, inorganic semiconductor nanomembranes have attracted considerable attention owing to their excellent mechanical flexibility and electrical/optical properties. To functionalize these building blocks with complex components, transfer and printing methods in a convenient and precise way are urgently demanded. A combined and controllable approach called edge-cutting transfer method to assemble semiconductor nanoribbons with defined width (down to submicrometer) and length (up to millimeter) is proposed. The transfer efficiency can be comprehended by a classical cantilever model, in which the difference of stress distributions between forth and back edges is investigated using finite element method. In addition, the vertical van der Waals PN (p-Si/n-Ge) junction constructed by a two-round process presents a typical rectifying behavior. The proposed technology may provide a practical, reliable, and cost-efficient strategy for transfer and printing routines, and thus expediting its potential applications for roll-to-roll productions for flexible devices.

An Active Metamaterial Platform for Chiral Responsive Optoelectronics

Chiral-selective non-linear optics and optoelectronic signal generation are demonstrated in an electrically active photonic metamaterial. The metamaterial reveals significant chiroptical responses in both harmonic generation and the photon drag effect, correlated to the resonance behavior in the linear regime. The multifunctional chiral metamaterial with dual electrical and optical functionality enables transduction of chiroptical responses to electrical signals for integrated photonics.

Curved-lattice epitaxial growth of In(x)Al(1-x)N nanospirals with tailored chirality

Chirality, tailored by external morphology and internal composition, has been realized by controlled curved-lattice epitaxial growth of In(x)Al(1-x)N nanospirals. The curved morphology of the spiral segments is a result of a lateral compositional gradient while maintaining a preferred crystallographic growth direction, implying a lateral gradient in optical properties. Individual nanospirals show an asymmetric core-shell structure with curved basal planes. Mueller matrix spectroscopic ellipsometry shows that the tailored chirality is manifested in the polarization state of light reflected off the nanospirals.

The fabrication of three-dimensional plasmonic chiral structures by dynamic shadowing growth

As chiral metamaterials become increasingly more technologically relevant, scalable, yet proficient nanofabrication methods will be needed for their production. Dynamic shadowing growth (DSG) that takes advantage of the vapor shadowing effect during physical vapor deposition is a simple and powerful tool to produce chiral nanostructures. In this report we describe several new DSG strategies for the scalable production of chiral plasmonic thin films with significant optical activity in the visible and near-infrared wavelength region. Specifically, we demonstrate that by use of metal composite (Ti/Ag) and metal/dielectric composite materials (Ag/MgF2), nanoscale helices can be fabricated using DSG at room temperature. Additionally, we show how self-assembled colloidal monolayers of nanospheres can serve as effective templates for the production of a wide variety of highly chiral films. These films can be used to construct chiral metamaterial-based devices for future applications.

Tunable three-dimensional helically stacked plasmonic layers on nanosphere monolayers

We report a simple and scalable method to fabricate helical chiral plasmonic nanostructures using glancing angle deposition on self-assembled nanosphere monolayers. By controlling the azimuthal rotation of substrates, Ag and SiO2 layers can be helically stacked in left-handed and right-handed fashions to form continuous helices. Finite-difference time-domain simulations confirm the experimental results that show that these plasmonic helices exhibit strong chiroptical responses in the visible to near-IR region, which can be tuned by changing the diameter of nanospheres. With such flexibility in the design, helically stacked plasmonic layers may act as tunable chiral metamaterials, as well as serve as different building blocks for chiral assemblies.

Giant chiral optical response from a twisted-arc metamaterial

We demonstrate enormously strong chiral effects from a photonic metamaterial consisting of an array of dual-layer twisted-arcs with a total thickness of ∼ λ/6. Experimental results reveal a circular dichroism of ∼ 0.35 in the absolute value and a maximum polarization rotation of ∼ 305°/λ in a near-infrared wavelength region. A transmission of greater than 50% is achieved at the frequency where the polarization rotation peaks. Retrieved parameters from measured quantities further indicate an actual optical activity of 76° per λ and a difference of 0.42 in the indices of refraction for the two circularly polarized waves of opposite handedness.

Flexible helices for nonlinear metamaterials

The successful fabrication and experimental verification of a novel metamaterial based on flexible metallic helices is reported. The helices undergo compression under the influence of incident radiation, demonstrating a nonlinear chiral electromagnetic response, associated with the power-dependent change in the helix pitch. This design is promising for application to power-dependent polarization rotation of propagating waves.