An Active Metamaterial Platform for Chiral Responsive Optoelectronics

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.

Dual-Responsive Fe3O4@Polyaniline Chiral Superstructures for Information Encryption

Incorporating stimuli-responsive mechanisms into chiral assemblies of nanostructures offers numerous opportunities to create optical materials capable of dynamically modulating their chiroptical properties. In this study, we demonstrate the formation of chiral superstructures by assembling Fe3O4@polyaniline hybrid nanorods by using a gradient magnetic field. The resulting superstructures exhibit a dual response to changes in both the magnetic field and solution pH, enabling dynamic regulation of the position, intensity, and sign of its circular dichroism peaks. Such responsiveness allows for convenient control over the optical rotatory dispersion properties of the assemblies, which are further integrated into the design of a chiroptical switch that can display various colors and patterns when illuminated with light of different wavelengths and polarization states. Finally, an optical information encryption system is constructed through the controlled assembly of the hybrid nanorods to showcase the potential opportunities for practical applications brought by the resulting responsive chiral superstructures.

Overcoming Intensity Saturation in Nonlinear Multiple-Quantum-Well Metasurfaces for High-Efficiency Frequency Upconversion

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light-matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixing systems.

Broadband plasmonic chiral meta-mirrors

Chiral meta-mirrors provide a unique opportunity for achieving handedness-selective strong light-matter interaction at the nanometer scale. Importantly, the chiral resonances observed in chiral meta-mirrors arise from the spin-dependent resonant cavity which, however, is generally narrowband. In this paper, by exploiting a genetic algorithm (GA) based optimization method, we numerically validate a chiral meta-mirror with octave bandwidth. In particular, in the wavelength range from 1000 to 2000 nm, the proposed chiral meta-mirror strongly absorbs circularly polarized light of one handedness while highly reflecting the other. A field analysis indicates that the observed broadband chiroptical response can be attributed to the multiple chiral resonances supported by the optimized meta-mirror across the band of interest. The observed broadband chiral response confirms the potential of advanced inverse-design approaches for the creation of chiral metadevices with sophisticated functionalities. Based on the Lorentz reciprocity theorem, we show that the proposed meta-mirror can enable chiral-selective broadband second harmonic generation (SHG). Our study indicates that the application of advanced inverse-design approaches can greatly facilitate the development of metadevices with strong chiral response in both the linear and nonlinear regimes.

Light-driven single-cell rotational adhesion frequency assay

The interaction between cell surface receptors and extracellular ligands is highly related to many physiological processes in living systems. Many techniques have been developed to measure the ligand-receptor binding kinetics at the single-cell level. However, few techniques can measure the physiologically relevant shear binding affinity over a single cell in the clinical environment. Here, we develop a new optical technique, termed single-cell rotational adhesion frequency assay (scRAFA), that mimics in vivo cell adhesion to achieve label-free determination of both homogeneous and heterogeneous binding kinetics of targeted cells at the subcellular level. Moreover, the scRAFA is also applicable to analyze the binding affinities on a single cell in native human biofluids. With its superior performance and general applicability, scRAFA is expected to find applications in study of the spatial organization of cell surface receptors and diagnosis of infectious diseases.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s43593-022-00020-4.

Terahertz Chiral Metamaterials Enabled by Textile Manufacturing

Easy-to-fabricate, large-area, and inexpensive microstructures that realize control of the polarization of terahertz (THz) radiation are of fundamental importance to the development of the field of THz wave photonics. However, due to the lack of natural materials that can facilitate strong THz radiation-matter interactions, THz polarization components remain an undeveloped technology. Strong resonance-based responses offered by THz metamaterials have led to the recent development of THz metadevices, whereas, for polarization control devices, micrometer-scale fabrication techniques including aligned photolithography are generally required to create multilayer microstructures. In this work, leveraging a two-step textile manufacturing approach, a chiral metamaterial capable of exhibiting strong chiroptical responses at THz frequencies is demonstrated. Chiral-selective transmission and pronounced optical activity are experimentally observed. In sharp contrast to smart-clothing-related devices (e.g., textile antennas), the investigated chiral metamaterials gain their THz properties directly from the yarn-twisting enabled microhelical strings. It is envisioned that the interplay between meta-atom designs and textile manufacturing technology will lead to a new family of metadevices for complete control over the phase, amplitude, and polarization of THz radiation.

Linear and nonlinear chiroptical response from individual 3D printed plasmonic and dielectric micro-helices

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques.

Electrical Dynamic Switching of Magnetic Plasmon Resonance Based on Selective Lithium Deposition

Active plasmonic nanostructures have garnered considerable interest in physics, chemistry, and material science due to the dynamically switchable capability of plasmonic responses. Here, the first electrically dynamic control of magnetic plasmon resonance (MPR) through structure transformation by selective deposition of lithium on a metal-insulator-metal (MIM) structure is reported. Distinct optical switching between MPR and surface plasmon polariton (SPP) excitations can be enabled by applying a proper electrical current to the electrochemical cell. Furthermore, the structure transformation through lithium metal deposition indicates the reconfigurable MPR excitation in a full cycling of the charging and discharging process. The results may shed light on electrically compatible self-powered active plasmonics as well as nondestructive optical sensing for electrochemical evolution.

Chiral Nanoceramics

The study of different chiral inorganic nanomaterials has been experiencing rapid growth during the past decade, with its primary focus on metals and semiconductors. Ceramic materials can substantially expand the range of mechanical, optical, chemical, electrical, magnetic, and biological properties of chiral nanostructures, further stimulating theoretical, synthetic, and applied research in this area. An ever-expanding toolbox of nanoscale engineering and self-organization provides a chirality-based methodology for engineering of hierarchically organized ceramic materials. However, fundamental discoveries and technological translations of chiral nanoceramics have received substantially smaller attention than counterparts from metals and semiconductors. Findings in this research area are scattered over a variety of sources and subfields. Here, the diversity of chemistries, geometries, and properties found in chiral ceramic nanostructures are summarized. They represent a compelling materials platform for realization of chirality transfer through multiple scales that can result in new forms of ceramic materials. Multiscale chiral geometries and the structural versatility of nanoceramics are complemented by their high chiroptical activity, enantioselectivity, catalytic activity, and biocompatibility. Future development in this field is likely to encompass chiral synthesis, biomedical applications, and optical/electronic devices. The implementation of computationally designed chiral nanoceramics for biomimetic catalysts and quantum information devices may also be expected.

Plasmonic Chirality and Circular Dichroism in Bioassembled and Nonbiological Systems: Theoretical Background and Recent Progress

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light-interaction cross-sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization-encoded optical communication, polarization-selective chemical reactions, and materials with polarization-dependent light-matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular-plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.

Nonlinear Chiral Meta-Mirrors: Enabling Technology for Ultrafast Switching of Light Polarization

Photonic nanostructures that realize ultrafast switching of light polarization are essential to advancements in the area of optical information processing. The unprecedented flexibility of metasurfaces in light manipulation makes them a promising candidate for active polarization control. However, due to the lack of optical materials exhibiting a fast as well as large refractive index change, photonic metadevices capable of ultrafast polarization switching remain elusive. Here, an ultrathin nonlinear chiral meta-mirror consisting of an array of amorphous silicon (α-Si) split-ring resonators on top of a silver backplane is demonstrated as a feasible platform for picosecond all-optical polarization switching of near-infrared light at picojoule-per-resonator pump energies. This success was made possible by the high-quality-factor resonances of the proposed meta-atoms that enable the mirror to exhibit strong chiro- and enantioselectivity. Experimental results confirm that our meta-mirrors can be used to facilitate high-speed and power-efficient polarization-state modulators.

Strong circular dichroism in chiral plasmonic metasurfaces optimized by micro-genetic algorithm

Strong circular dichroism in absorption in the near-infrared wavelength range is realized by designing binary-pattern chiral plasmonic metasurfaces via the micro-genetic algorithm optimization method. The influence of geometric parameter modifications in the binary-pattern nanostructures on the circular dichroism performance is studied. The strong circular dichroism in absorption is attributed to the simultaneous excitation and field interference of the resonant modes with relative phase delay under linearly polarized incident light. This work provides a universal design method toward the on-demand properties of chiral metasurfaces, which paves the way for future applications in chemical and biological sensing, chiral imaging and spectroscopy.

The study on size dependent dipole-dipole interaction in the self-assembly of twisting nanoribbons with circular polarization activation

The mechanism of self-assembling process of inorganic nanoparticle (NPs) is still an open question due to the various and non-additive interactions between NPs. Kotov et al reported that the semiconductor NPs can be self-assembled by external activation such as irradiation. In this paper, the twisted CdTe nanoribbons were successfully assembled with circular polar light activation based on the chiral selective resonance absorption. The effect of NP size on the morphology of assemblies under circular polar light irradiation is discussed by introducing a new mechanism of photooxidation induced dipole moment which decreases with increasing sizes of the NPs because of the change of band offsets at the CdS/CdTe interface. Moreover, we find that the competition between the dipole-dipole interaction and electrostatic repulsion can be modulated by the size of the NPs and the concentration of dispersion, which are the key points to produce the chiral twisted nanoribbons.

The chiral interfaces fabricated by d/l-alanine-pillar[5]arenes for selectively adsorbing ctDNA

In this paper, the d/l-AP5-interfaces are firstly fabricated by attaching d-alanine-pillar[5]arene and l-alanine-pillar[5]arene (d/l-AP5) onto the gold surface, and they exhibit a significantly different chiral influence on the morphology and the adsorption quantity of the adsorbed ctDNA molecules. The research provides an ideal chiral platform for investigating chiral phenomena in biological systems.

Ultranarrow-band metagrating absorbers for sensing and modulation

Nanostructured plasmonic metamaterials are an excellent platform for narrowband optical absorption, which has wide applications in sensing, filtering, modulation, and emission tailoring. However, achieving a subnanometer absorption bandwidth for optical sensing and dynamical control of light is still challenging. Here, we propose an asymmetric metagrating structure and make use of the propagating surface plasmonic mode that has a small dissipation rate, to achieve perfect optical absorption with a bandwidth of 0.28 nm near the wavelength of 1.55 μm. Our proposed structure can be used in solution environments as a chemical or biological sensor in the visible spectral range just by changing the structural parameters. The sensor possesses a sensitivity of 440 nm/RIU and figure of merit of 1333.33 RIU^-1. In addition, by combining an organic electro-optic material with this metagrating, our device can be reconfigurable with a dynamic range of 15.52 dB. Therefore, our proposed metagrating platform not only works as an ultranarrow-band absorber, but also can be employed for optical sensing and dynamic control of light.

Perfect meta-absorber by using pod-like nanostructures with ultra-broadband, omnidirectional, and polarization-independent characteristics

The on-chip perfect meta-absorber (PMA) is an important optical and thermal energy component in photovoltaics, thermal emitters, and energy harvesting applications. However, most reported PMAs rely on the complicated lithography techniques, which imposed a serious cost barrier on the development of practical applications, especially in the visible to near-infrared (NIR) wavelength range and at very large scales. Importantly, it is hard to realize PMA in the UV wavelength range by using current lithography techniques. In this article, we develop an ultra-broadband PMA by using natural lithography (NL) technique. The morphology of proposed PMA is randomly distributed pod-like nanostructures composed of a nanocomposite (Au/SiO₂) covered a gold layer. It can be formed easily on Si substrate to function as an ultra-broadband, omnidirectional, and polarization-independent PMA by controlling the conditions of sputtering deposition and thermal annealing treatment. We experimentally realized an on-chip ultra-broadband PMA with almost 100% absorption spanned from UV-visible to NIR wavelength ranges. This cost-effective and high-efficiency approach would release the manufacturing barrier for previously reported PMAs and therefore open an avenue to the development of effectively energy harvesting, energy recycling, and heat liberation applications.

Extreme sensitive metasensor for targeted biomarkers identification using colloidal nanoparticles-integrated plasmonic unit cells

Engineered terahertz (THz) plasmonic metamaterials have emerged as promising platforms for quick infection diagnosis, cost-effective and real-time pharmacology applications owing to their non-destructive and harmless interaction with biological tissues in both in vivo and in vitro assays. As a recent member of THz metamaterials family, toroidal metamaterials have been demonstrated to be supporting high-quality sharp resonance modes. Here we introduce a THz metasensor based on a plasmonic surface consisting of metamolecules that support ultra-narrow toroidal resonances excited by the incident radiation and demonstrate detection of an ultralow concertation targeted biomarker. The toroidal plasmonic metasurface was designed and optimized through extensive numerical studies and fabricated by standard microfabrication techniques. The surface then functionalized by immobilizing the antibody for virus-envelope proteins (ZIKV-EPs) for selective sensing. We sensed and quantified the ZIKV-EP in the assays by measuring the spectral shifts of the toroidal resonances while varying the concentration. In an improved protocol, we introduced gold nanoparticles (GNPs) decorated with the same antibodies onto the metamolecules and monitored the resonance shifts for the same concentrations. Our studies verified that the presence of GNPs enhances capturing of biomarker molecules in the surrounding medium of the metamaterial. By measuring the shift of the toroidal dipolar momentum (up to Δω~0.35 cm^-1) for different concentrations of the biomarker proteins, we analyzed the sensitivity, repeatability, and limit of detection (LoD) of the proposed toroidal THz metasensor. The results show that up to 100-fold sensitivity enhancement can be obtained by utilizing plasmonic nanoparticles-integrated toroidal metamolecules in comparison to analogous devices. This approach allows for detection of low molecular-weight biomolecules (≈13 kDa) in diluted solutions using toroidal THz plasmonic unit cells.

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.

Resonant cavity enhanced waveguide transmission for broadband and high efficiency quarter-wave plate

Existing transmission type optical quarter-wave plates based on metallic sub-wavelength structures can hardly realize transmission efficiency above 60%. And their working bandwidths are still very narrow. In this paper, we demonstrate a transmission type quarter-wave plate design with efficiency above 92% over a broad wavelength range (from 1260 nm to 1560 nm). The device proposed is based on a one-dimensional metal-insulator-metal waveguide array buried in silica. Phase difference between transmitted TE and TM components can be tuned continuously. At the same time, transmission efficiency can be kept above 90% in the same spectral range for both the TE and TM incidences. The broad bandwidth and remarkable efficiency are explained with the combination of low dispersion of waveguide modes and the resonant cavity enhanced transmission effect. To give a better understanding of the structure, we also propose a modified effective medium model. The optical response of the structure can be well reproduced with the semi-analytic effective medium model.

Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror

Conventional metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin state of light waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as ∼0.5 is experimentally observed in a near-infrared wavelength band. By imaging a fabricated pattern composed of the enantiomeric unit cells, we directly visualize the two key features of our engineered meta-mirrors, namely the chiral-selective absorption and the polarization preservation upon reflection. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime. Chiral meta-mirrors, exhibiting giant chiroptical responses and spin-selective near-field enhancement, hold great promise for applications in polarization sensitive electro-optical information processing and biosensing.

Electrically Controlled Scattering in a Hybrid Dielectric-Plasmonic Nanoantenna

Electrically tunable devices in nanophotonics offer an exciting opportunity to combine electrical and optical functions, opening up their applications in active photonic devices. Silicon as a kind of high refractive index dielectric material has shown comparable performances with plasmonic nanostructures in tailoring and modulating the electromagnetic waves. However, there are few studies on electrically tunable silicon nanoantennas. Here, for the first time we realize the spectral tailoring of an individual silicon nanoparticle in the visible range through changing the applied voltage. We observe that the plasmon-dielectric hybrid resonant peaks experience blue shift and obvious intensity attenuation with increasing the bias voltages from 0 to 1.5 V. A physical model has been established to explain how the applied voltage influences the carrier concentration and how carrier concentration modifies the permittivity of silicon and then the final scattering spectra. Our findings pave a new approach to build excellent tunable nanoantennas or other nanophotonics devices where the optical responses can be purposely controlled by electrical signals.

High efficiency broadband -90° to 90° arbitrary optical rotation realized with meta reflectarray

We theoretically demonstrate high efficiency broadband -90° to 90° arbitrary optical rotation realized with meta reflectarray composed of a L-shaped silver antenna array, a silica spacer, and a silver ground plane. Co-polarized and cross-polarized components of reflected wave can be manipulated efficiently by adjusting arm length of the L-shaped antenna, and 0° to 90° arbitrary optical rotation with high degree of linear polarization (DoLP) over a broadband can be achieved readily. The phase of cross-polarized field component can be reversed by turning the L-shaped antennas upside down, and 0° to 90° optical rotation can be turned into 0° to -90° rotation. Reflected phase can be shift by π after a 90° rotation of the L-shaped or Γ-shaped antennas, while optical rotation angle remains the same. Thus, rotation angle θ is changed to 180° + θ after the rotation, and we realized 0° to 360° polarization rotation with a step of 60° with the combination of six discrete structure units. In addition, we proposed metamaterial structures for highly efficient generation of vector beams with these units. The high efficiency broadband arbitrary angle optical rotation will profoundly affect a wide range of applications involving optical polarization.

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.

Hybrid metamaterials for electrically triggered multifunctional control

Despite the exotic material properties that have been demonstrated to date, practical examples of versatile metamaterials remain exceedingly rare. The concept of metadevices has been proposed in the context of hybrid metamaterial composites: systems in which active materials are introduced to advance tunability, switchability and nonlinearity. In contrast to the successful hybridizations seen at lower frequencies, there has been limited exploration into plasmonic and photonic nanostructures due to the lack of available optical materials with non-trivial activity, together with difficulties in regulating responses to external forces in an integrated manner. Here, by presenting a series of proof-of-concept studies on electrically triggered functionalities, we demonstrate a vanadium dioxide integrated photonic metamaterial as a transformative platform for multifunctional control. The proposed hybrid metamaterial integrated with transition materials represents a major step forward by providing a universal approach to creating self-sufficient and highly versatile nanophotonic systems.

Electrically Tunable Harmonic Generation of Light from Plasmonic Structures in Electrolytes

An emerging trend in plasmonics is to exploit nanostructured metals as a self-contained electrooptic platform with simultaneously supported electrical and optical functions. When it comes to nonlinear optics, this dual electrical and optical functionality offers an exciting potential to enable electrically controlled wave mixing processes in various nanometallic systems. Here we demonstrate tunable nonlinear generation of light enabled by an electrically active plasmonic crystal in aqueous electrolytic solutions. A modulation depth of ∼150%/V is observed in the second-harmonic signal, thanks to the light concentrating ability of the resonant plasmonic structure as well as the voltage-assisted charge accumulation on the metallic surfaces. The hybrid plasmonic-electrolyte system demonstrated in this work offers the exciting new potential to use plasmonic nanostructures for voltage-controlled nonlinear signal generation and in situ biochemical sensing in an aqueous environment.

Controllable optical activity with non-chiral plasmonic metasurfaces

Optical activity is the rotation of the plane of linearly polarized light along the propagation direction as the light travels through optically active materials. In existing methods, the strength of the optical activity is determined by the chirality of the materials, which is difficult to control quantitatively. Here we numerically and experimentally investigated an alternative approach to realize and control the optical activity with non-chiral plasmonic metasurfaces. Through judicious design of the structural units of the metasurfaces, the right and left circular polarization components of the linearly polarized light have different phase retardations after transmitting through the metasurfaces, leading to large optical activity. Moreover, the strength of the optical activity can be easily and accurately tuned by directly adjusting the phase difference. The proposed approach based on non-chiral plasmonic metasurfaces exhibits large optical activity with a high controllable degree of freedom, which may provide more possibilities for applications in photonics.

Radiative loss-determined circular dichroism of plasmonic nanospirals with bendable stability of chiroptical activity

Plasmonic nanospirals have chiroptical activity dominated by radiative loss, and those deposited on flexible substrates exhibit excellent mechanical stability of their chiroptical activity.